GENERALIZED MODELING OF METAL CUTTING MECHANICS by MUSTAFA KAYMAKCI M.Sc. Koc University, Turkey, 2007 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in The Faculty of Graduate Studies (Mechanical Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) August 2009 Β© Mustafa Kaymakci, 2009 Abstract Metal cutting is the most commonly used manufacturing process for producing parts with final dimensions. The aim of engineering science is to model the physics of the process which allows the simulation of part machining operations ahead of costly trials. There is a need to develop generalized models of cutting process which is applicable to various tool geometries and cutting processes in order to simulate machining of industrial parts in virtual environment. This thesis presents a generalized mathematical model which can be used to predict turning, drilling, boring and milling processes. The tool geometry is adopted from ISO 13399 standards. The rake face of the tool is mathematically modeled from ISO13399 model by considering tool geometry, engagement with the workpiece, feed and speed directions of cutting motion. Various geometric features of the tool, such as chamfer, nose radius, and cutting edge angles, are considered in developing coordinate transformation models between the machine motion and tool coordinate systems. The cutting forces on the rake face are defined in the direction of chip flow and perpendicular to the rake face. The cutting force coefficients in the two directions are either identified mechanistically by conducting experiments specific to the tool geometry, or using orthogonal to oblique transformation of shear angle, average friction angle and shear stress. The friction and normal forces on the rake face are transformed to both stationary and rotating tool coordinate systems defined on the machine tool. ii Table of Contents Abstract .............................................................................................................................. ii Table of Contents ............................................................................................................... ii List of Tables ..................................................................................................................... v List of Figures ................................................................................................................... vi Acknowledgements......................................................................................................... xiii Nomenclature ..................................................................................................................... x 1. Introduction.................................................................................................................. 1 2. Literature Survey ......................................................................................................... 4 2.1 Overview ............................................................................................................ 4 2.2 Mechanics of Metal Cutting ............................................................................... 4 2.3 Cutting Force Models......................................................................................... 6 2.3.1 Orthogonal Cutting ................................................................................ 6 2.3.2 Forces in Turning ................................................................................... 7 2.3.3 Forces in Milling .................................................................................. 10 2.4 Inserted Cutters ................................................................................................ 12 2.5 Generalized Mechanics of Machining ............................................................. 13 3. ISO Cutting Tool Geometryβ¦ .................................................................................. 14 3.1 Overview .......................................................................................................... 14 3.2 Planes ............................................................................................................... 14 3.3 Points and Angles............................................................................................. 16 3.4 Dimensional Quantities .................................................................................... 20 3.5 Insert Shapes .................................................................................................... 21 3.5.1 Equilateral Equiangular Insert.............................................................. 21 3.5.2 Equilateral Nonequiangular Insert ....................................................... 22 ii 3.6 3.7 3.5.3 Nonequilateral Equiangular Insert ....................................................... 22 3.5.4 Nonequilateral Nonequiangular Insert ................................................. 22 3.5.5 Round Insert ......................................................................................... 23 Tool Coordinate Frames and Transformations ................................................ 24 3.6.1 Turning Operations .............................................................................. 25 3.6.2 Transformation of Rotary Tools........................................................... 31 Summary .......................................................................................................... 38 4. Generalized Geometric Model of Inserted Cutters .................................................... 39 4.1 Overview .......................................................................................................... 39 4.2 Mathematical Modeling of an Insert ................................................................ 40 4.3 Tool Coordinate Frames and Transformations ................................................ 42 4.3.1 4.3.2 4.3.3 4.4 4.5 4.6 Frame ππππ to Frame ππππ Rotation.......................................................... 43 Frame ππππ to Frame ππππ Rotation.......................................................... 44 Frame ππππ to Frame ππππ Rotation.......................................................... 45 Mathematical Relationships between Angles .................................................. 46 4.4.1 Normal Rake Angle ............................................................................. 46 4.4.2 True Cutting Edge Angle ..................................................................... 47 4.4.3 Inclination (Helix) Angle ..................................................................... 48 Examples .......................................................................................................... 51 4.5.1 Rectangular Insert with Corner Radius ................................................ 51 4.5.2 Rectangular Insert with Corner Chamfer ............................................. 59 4.5.3 Rhombic Turning Insert ....................................................................... 63 Summary .......................................................................................................... 67 5. Generalized Mechanics of Metal Cutting .................................................................. 68 5.1 Overview .......................................................................................................... 68 5.2 Rake Face Based General Force Model ........................................................... 68 5.3 Identification of Specific Cutting Coefficients ................................................ 72 iii 5.4 5.3.1 Orthogonal to Oblique Transformation ................................................ 72 5.3.2 Mechanistic Identification .................................................................... 79 Cutting Force Simulations and Validations ..................................................... 91 5.4.1 Turning Process Simulations................................................................ 91 Rhombic (ISO Style C) Insert: ........................................................................................ 95 5.4.2 5.5 Milling Process Simulations .............................................................. 102 Summary ........................................................................................................ 115 6. Conclusions.............................................................................................................. 116 Bibliography .................................................................................................................. 118 iv List of Tables Table 3-I: Summary of Angles for Definition of Orientation of Cutting Edge and Rake Face. ................................................................................................................................. 20 Table 3-II: Insert Shapes Defined in ISO 13399. ............................................................ 23 Table 4-I: Inputs Used in the Model. ............................................................................... 51 Table 4-II: Inputs Used in the Analysis of the Insert with Chamfer. .............................. 59 Table 4-III: Inputs for the Rhombic Turning Insert......................................................... 63 Table 5-I: Cutting Coefficients and Tool Geometry Used in the Analysis. .................... 86 Table 5-II: Comparison of the Methods for the Solution of Oblique Shear Parameters. 88 Table 5-III: Orthogonal Turning Validation Experiments Cutting Conditions. .............. 93 Table 5-IV: Tool Geometry and Cutting Conditions for Rhombic Insert. ...................... 95 Table 5-V: Tool Geometry and Cutting Conditions for Square Insert. ......................... 100 v List of Figures Figure 2-1: Orthogonal Cutting Process [5]. ..................................................................... 4 Figure 2-2: Illustration of Cutting Force Components [12][5]. ......................................... 5 Figure 2-3: Turning Operation Using a Tool with a Nose Radius and Colwellβs Approach............................................................................................................................ 8 Figure 2-4: Different Approaches for Predicting Chip Flow Direction: (a) Colwell, (b) Okushima and Minato, (c) Young et al. ............................................................................ 9 Figure 3-1: Planes Defined in ISO 3002 and ISO 13399. ............................................... 16 Figure 3-2: Illustration of the Cutting Reference Point (CRP). ....................................... 17 Figure 3-3: Demonstration of Tool Angles in ISO Standards for a Turning Tool. ......... 19 Figure 3-4: Dimensional Quantities Defined in the ISO Standards. ............................... 21 Figure 3-5: Mechanics of Oblique Cutting Process. ........................................................ 25 Figure 3-6: RTA and the Machine Coordinate Systems on a Lathe. ............................... 27 Figure 3-7: Cutting Edge Coordinate System along with the RTA Coordinate System. 28 Figure 3-8: Rake Face Coordinate System (System 3): Rotation around the ππππππ Axis by the Amount of Rake Angle πΈπΈπΈπΈ. ....................................................................................... 29 Figure 3-9: Chip Flow Coordinate System (System 4): Rotation around the πΏπΏπΏπΏπΏπΏπΏπΏ Axis by the Amount of Chip Flow Angle πΌπΌ. ................................................................................. 30 Figure 3-10: Illustration of System * on a Rotary Tool with Four Flutes. ...................... 32 Figure 3-11: Transformation between System 1 and System *. ...................................... 34 Figure 3-12: Transformation between System 1 and System 2 for Rotary Tools. .......... 35 Figure 3-13: Transformation between Rake Face Coordinate System (System 3) and Cutting Edge Coordinate System (System 2). ................................................................. 36 Figure 3-14: Representation of the Chip Flow Coordinate System on a Milling Tool. .. 37 Figure 4-1: Local Coordinate Systems and Cutting Reference Points on Different Inserts. ......................................................................................................................................... 40 Figure 4-2: Control Points on a Parallelogram Insert. ..................................................... 41 Figure 4-3: Frame ππππ to Frame ππππ Rotation. ................................................................. 43 Figure 4-4: Frame ππππ to Frame ππππ Rotation. ................................................................ 44 vi Figure 4-5: Frame ππππ to Frame ππππ Rotation. ................................................................. 45 Figure 4-6: Derivation of Normal Rake Angle. ............................................................... 47 Figure 4-7: Derivation of the True Cutting Edge Angle.................................................. 48 Figure 4-8: Definition of Frame ππππ and Frame ππππ to Frame ππππ Rotation. .................... 49 Figure 4-9: Definition of the Inclination (Helix) Angle. ................................................. 50 Figure 4-10: Illustrations of the Insert and the Cutter Body [55]. ................................... 51 Figure 4-11: Analytical Model of an Insert with a Corner Radius and Wiper Edge. ...... 52 Figure 4-12: Control Points of the Insert before the Rotations. ...................................... 54 Figure 4-13 : Control Points of the Insert after the Rotations. ........................................ 55 Figure 4-14: CAD Model of the Insert. ........................................................................... 56 Figure 4-15: Change of Local Radius along the Tool Axis. ............................................ 57 Figure 4-16: Change in Normal Rake Angle along the Tool Axis. ................................. 58 Figure 4-17: Change in Helix Angle along the Tool Axis............................................... 58 Figure 4-18: Catalogue Figure of the Insert Taken from Sandvik Coromant [55]. ......... 59 Figure 4-19: Illustration of Control Points and Dimensions for an Insert with Chamfer. 60 Figure 4-20: Change in the Local Radius along the Tool Axis for a Chamfered Insert. . 61 Figure 4-21: Change in Normal Rake Angle along the Tool Axis. ................................. 62 Figure 4-22: Change in Helix Angle along the Tool Axis............................................... 62 Figure 4-23: Catalogue Figures of the Turning Insert [55]. ............................................ 63 Figure 4-24: Control Points of the Rhombic Turning Insert. .......................................... 64 Figure 4-25: Position of the Cutting Edge for a Rhombic Turning Insert. ...................... 65 Figure 4-26: Change in the Normal Rake Angle along the Cutting Edge. ...................... 66 Figure 4-27: Change in the Helix Angle along the Cutting Edge.................................... 66 Figure 5-1: Mechanics of Oblique Cutting [11]. ............................................................. 69 Figure 5-2: Summary of the Proposed Mechanistic Approach. ...................................... 71 Figure 5-3: Shear Plane Area Comparison: Orthogonal Cutting (Left) and Oblique Cutting (Right). ................................................................................................................ 73 Figure 5-4: Cutting Forces, Velocities, and Angles in Oblique Cutting [11]. ................. 74 Figure 5-5: Illustration of angle π½π½π½π½. (ππππ, ππππππ, and ππππππ Are Projections of Forces onto Normal Plane). ................................................................................................................. 75 Figure 5-6: Summary of the Classical Approach in Cutting Coefficient Identification. . 78 vii Figure 5-7: Block Diagram of the Iterative Method to Enhance the Theoretical Models. ......................................................................................................................................... 84 Figure 5-8: Results of Helix Angle Predictions for Ti6Al4V.......................................... 86 Figure 5-9: Results of Helix Angle Predictions for Al7075. ........................................... 87 Figure 5-10: Theoretical Identification Procedure. ......................................................... 89 Figure 5-11: Effect of Chip Thickness on the Cutting Coefficients. ............................... 90 Figure 5-12: Effect of Chip Thickness on the Chip Flow Angle. .................................... 90 Figure 5-13: Workpiece and 3- Component Dynamometer Fixed to Turret for Cutting Tests. ................................................................................................................................ 92 Figure 5-14: Sample Output Screen in CutPro 8.0 for Cutting Forces. ........................... 92 Figure 5-15: Forces simulated and measured for an orthogonal tool with 75Β° cutting edge angle for V=100 m/min. .................................................................................................. 94 Figure 5-16: Forces simulated and measured for an orthogonal tool with 75Β° cutting edge angle for V=150 m/min. .................................................................................................. 94 Figure 5-17: Illustration of Insert and the Holder Body [55]. ......................................... 95 Figure 5-18: Geometric Control Points of a Rhombic Turning Insert............................. 97 Figure 5-19: Comparison of Measured and Calculated Forces for the Rhombic Tool while the Width of Cut is 2 mm....................................................................................... 99 Figure 5-20: Comparison of Measured and Calculated Forces for the Rhombic Tool while the Width of Cut is 4 mm....................................................................................... 99 Figure 5-21: Illustration of Insert and the Holder Body [55]. ....................................... 101 Figure 5-22: Comparison of Measured and Calculated Forces for the Square Insert while the Width of Cut is 2 mm. ............................................................................................. 101 Figure 5-23: Comparison of Measured and Calculated Forces for the Square Insert while the Width of Cut is 4 mm. ............................................................................................. 102 Figure 5-24: Workpiece and 3-Component Dynamometer Fixed to Machine Table for Cutting Tests. ................................................................................................................. 103 Figure 5-25: Illustration of the Experimental Setup for Measurement of Cutting Forces. ....................................................................................................................................... 104 Figure 5-26: 25 mm Diameter Shoulder End Mill from Sandvik Coromant [55]. ........ 104 Figure 5-27: Illustration of Control Points for the Shoulder Milling Insert. ................. 106 viii Figure 5-28: Measured and Predicted Forces for Al7050 Half Immersion Up-Milling. 107 Figure 5-29: Measured and Predicted Forces for Al7050 Slot Milling. ........................ 108 Figure 5-30: 20 mm Diameter Bull-Nose Mill from Sandvik Coromant [55]. ............. 109 Figure 5-31: Measured and Predicted Forces for Al7050 Half Immersion Up-Milling. 110 Figure 5-32: Measured and Predicted Forces for Al7050 Slot Milling. ........................ 111 Figure 5-33: 20 mm Diameter Ball-End Mill from Sandvik Coromant [55]. ............... 112 Figure 5-34: Control Points of a Round Milling Insert ................................................. 113 Figure 5-35: Measured and Predicted Forces for Al7050 Half Immersion Up-Milling. 114 Figure 5-36: Measured and Predicted Forces for Al7050 Slot Milling. ........................ 115 ix Nomenclature ππππ : Orthogonal shear angle π½π½ππ : Friction angle π΄π΄π π : Shear plane area πΉπΉ : Resultant force πΉπΉππ : Feed force πΉπΉπ₯π₯ : Force in ππ direction πΉπΉπ§π§ : Force in ππ direction ππππ : Normal shear angle πππ π : Shear Stress ππππ , ππππ , ππππ : Oblique angles πΉπΉπ‘π‘ : Tangential force πΉπΉππ : Radial force πΉπΉπ¦π¦ : Force in ππ direction πΉπΉπ’π’ : Friction force on the rake face πΉπΉπ£π£ : Normal force on the rake face πΉπΉππ : Normal force on the shear plane πΎπΎππππ : Radial cutting coefficient πΎπΎπ‘π‘π‘π‘ : Tangential edge coefficient πΎπΎππππ : Axial edge coefficient πΎπΎπ£π£π£π£ : Normal cutting coefficient πΎπΎπ£π£π£π£ : Normal edge coefficient πΎπΎππ : Feed cutting pressure πΉπΉπ π : Shear force on the shear plane πΎπΎπ‘π‘π‘π‘ : Tangential cutting coefficient πΎπΎππππ : Axial cutting coefficient πΎπΎππππ : Radial edge coefficient πΎπΎπ’π’π’π’ : Friction cutting coefficient πΎπΎπ’π’π’π’ : Friction edge coefficient πΎπΎπ‘π‘ : Tangential cutting pressure x πΎπΎππ : Radial cutting pressure β : Uncut chip thickness ππ : Width of cut ππ : Feed rate πππ‘π‘ : Cutting power ππ1 & ππ2 : Cutting constants ππ ππππππππ ππππ : Depth of cut : Effective width of cut : Cutting speed πππ΄π΄ππ : Differential chip load ππππ : Differential cutting edge length ππππ : Number of cutting edges ππ : Immersion angle π΄π΄Μ ππ : Average chip load ππππ : Differential cutting edge area πΎπΎ : Total number of discrete points along the cutting edge πππ π π π : Tool entry angle π π ππ : Cutting edge angle ππππ : Approach angle Ξ ππ : Corner chamfer angle πΎπΎππ : Radial rake angle ππ : Chip flow angle ππππ : Tool reference plane ππππππ : Tool exit angle ππππ : Tool included angle πππ π : Inclination (oblique) angle πΎπΎππ : Normal rake angle πΎπΎππ : Axial rake angle ππΜ : Average chip flow angle ππππ : Assumed working plane xi ππππ : Tool back plane ππππ : Cutting edge normal plane π΄π΄πΎπΎ : Rake face π·π·ππ : Cutting Diameter πππ π : Wiper edge length πππ π : Tool cutting edge plane ππππ : Wiper edge normal plane πΆπΆπΆπΆπΆπΆ : Cutting reference point πΏπΏ : Insert length ππππ : Insert width ππππ : Corner radius π’π’ οΏ½β : Unit vector from center of the arc to circumference πΆπΆ : Center of the arc ππππβ : Corner chamfer length οΏ½οΏ½β ππππ : Control Points πποΏ½β : Unit vector perpendicular to arc οΏ½οΏ½οΏ½οΏ½οΏ½οΏ½οΏ½β ππ1.2 : Point on the tool axis xii Acknowledgements I am indebted to my supervisor, Prof. Yusuf Altintas, for the considerable guidance, support, and encouragement he has provided me throughout the duration of my studies at the University of British Columbia. I also wish to thank Dr. Doruk Merdol, who cosupervised much of this research, for his time and for teaching me many things. I also would like to thank Dr. Farrokh Sassani and Dr. Steve Feng for reading this thesis and involving in my thesis committee. It is an honor to work in an exceptionally professional environment in the Manufacturing Automation Laboratory. I have enjoyed every moment I spent in and out of the lab with my colleagues. They are not just colleagues, but also good friends. I have lots of good memories I will remember for the rest of my life. I also would like to thank my friends in Vancouver for their support and friendship. Finally, I would like to thank my brother Orkun Kaymakci, my father Emin Kaymakci, and especially my mother Hamidiye Ozberk for their life-long love, for the kind of encouragement and support they have provided me throughout my entire education life, and for their absolute confidence in me. This thesis as well as all my previous success is dedicated to my family. xiii Chapter 1. Introduction 1. Introduction In terms of operations that take place in industry, metal cutting is the most common manufacturing method in producing final shapes of mechanical parts with tight tolerances and complex shapes. The main metal cutting processes can be listed as turning, milling, boring, and drilling. Metal cutting operations can be conducted manually or automatically by the help of Computer Numerical Control (CNC) tools. The motion of the machine tool on a CNC is controlled by the NC commands which are generated on computer β aided design/ computer β aided manufacturing (CAD/CAM) systems. The productivity and the output quality depends on the preparation of NC programs, process planner, cutting conditions, workpiece material, cutter geometry, cutter material, machine tool rigidity and the performance of the CNC system. Economical and efficient manufacturing in metal cutting operations are vital in order to produce parts with desired accuracy and low cost. Understanding the mechanics of the metal cutting process assists in avoiding damages to the tool, machine and workpiece while improving productivity and accuracy. Cutting tools with replaceable inserts are widely used in machining industry. Insert geometries can vary depending on the cutting operation and workpiece material. While all turning tools are inserts, milling cutters use either inserts distributed on the cutter body or they are ground from solid carbides. Due to the geometric complexities of inserts with poorly defined rake face geometry, it is difficult to accurately model mechanics of the cutting operation. As a result, an accurate cutting force model which can include various has been a challenge. Current literature focuses to determine separate mechanics models for each cutting process and even for each different cutter. In other words, researchers aimed to develop dedicated cutting models for turning, boring, drilling, and milling operations having different tool geometries. However, by understanding the fundamentals of metal cutting mechanics, it must be possible to develop a unified-generalized process model that can 1 Chapter 1. Introduction predict the cutting forces for a wide spectrum of machining operations practiced in industry. This thesis focuses on developing a generalized cutting mechanics model that can be applicable to most cutting processes. Cutting forces are first modeled on the rake face of the tool as friction and normal loads. The friction and normal forces are transformed to tool coordinate system. An improved and enhanced inserted cutter model has been developed based on the previous studies of Engin and Altintas [1]. Analytical model for all insert geometries defined in ISO 13399 standards have been derived. The forces are further transformed from the tool to cutting operation coordinate system to predict cutting forces, torque and power for various machining operations and cutting conditions. The thesis is organized as follows; Chapter 2 presents necessary background and literature review on metal cutting research. Cutting mechanics, previous models for chip thickness, chip flow models, as well as prediction of cutting forces for different operations are discussed. Previous geometric models of inserted cutters are summarized and generalization of metal cutting mechanics is reviewed. The geometric parameters of the cutting tools are defined according to ISO 13399 standards in Chapter 3. The reference planes and points required to describe the cutting angles are presented. Angular and dimensional quantities are considered to model an extensive variety of cutting tool geometries. The coordinate transformations needed to describe the cutting forces on rake face of the tool are presented. The geometric modeling of inserted cutters is presented in Chapter 4. Using the geometric identities described in Chapter 3, geometric control points are calculated analytically at the local coordinate system of the insert, which is placed on the cutter body using the orientation angles. Physical angles of the cutter required for cutting mechanics models are calculated with sample illustrations. The generalized modeling of cutting forces using friction and normal forces on the rake face of the cutter is presented in Chapter 5. The proposed mechanics model is 2 Chapter 1. Introduction compared with the current force models found in the literature. In addition to cutting forces, material model and calibration methods are also discussed and the transformations between the models are summarized. Using the geometric modeling of inserted tools which is described in Chapter 4, experimental validations of the model are presented. Simulations and measurements are presented for turning and milling operations for various types of inserted cutters and cutting conditions. The thesis is concluded in Chapter 6 with the summary of contributions and future research. 3 Chapter 2. Literature Survey 2. Literature Survey 2.1 Overview A literature review of past research on modeling of cutting tool geometry and me- chanics are presented. The distribution of chip along the cutting edge is discussed, and the corresponding approaches in predicting the cutting forces are presented. 2.2 Mechanics of Metal Cutting Mechanics of metal cutting has been a subject of research for the last 60 years [2][3] [4] [5]. The process of mechanics are affected by parameters such as feed rate, depth of cut, cutting speed, cutting edge angle, rake angle, helix angle, and workpiece material [6]. Researchers have been trying to establish a relationship between these parameters and process mechanics. The early work done by Merchant [7][8] has been a foundation used in the modeling of cutting forces [9][10]. Merchantβs model is based on the concept of a steady process in which a chip is produced by shearing a strip of uncut metal continuously and uniformly, and the deformation of the chip takes place along a shear plane. Figure 2-1: Orthogonal Cutting Process [5]. 4 Chapter 2. Literature Survey As shown in Figure 2-1, the uncut material approaches to the tool, sheared, and leaves parallel to the rake face of the tool with a new chip thickness. The width of the chip is assumed to be constant throughout the process. When the face of the tool is perpendicular to the plane of cutting (Figure 2-1), it is called orthogonal cutting, otherwise the process is considered to be oblique. Cutting forces occur in three directions in oblique cutting as shown in Figure 2-2 [11]. The component of the force acting on the rake face of the tool, normal to the cutting edge, is the tangential cutting force. The force component, acting in the radial direction, tending the push the tool away from the workpiece, is called the radial force. The third component is acting on the tool in the horizontal direction, parallel to the direction of feed, is referred as the feed force [12][13][5]. Figure 2-2: Illustration of Cutting Force Components [12][5]. Researchers [14] attempted to improve the models developed by Merchant. They included sophisticated mathematical formulations of frictional behavior on the tool rake face, high strain rate, work hardening of the workpiece material, and high temperature. Endres et al. [15] developed a cutting force model incorporating parameters of tool 5 Chapter 2. Literature Survey geometry. Lee and Shaffer [16] developed a more sophisticated model by introducing plasticity of the workpiece material into the solution. 2.3 Cutting Force Models Cutting force models using the tool geometry have been developed starting with simple orthogonal geometries and extended to the general turning and to milling processes. Following sections present a background on various cutting force models. 2.3.1 Orthogonal Cutting Orthogonal cutting provides the simplest geometry to study metal cutting mechanics, and models of the process have been advanced in the form of shear angle solutions. The best known approach is presented by Merchant [7] who defined the rake face contact as elastic with a constant coefficient of friction. The shear angle solution of this model is derived from minimum energy principle as: ππππππππππ βππππππ = ππ πΎπΎππ π½π½ππ + β 2 4 2 (2.1) where πΎπΎππ and π½π½ππ are the normal rake angle and friction angle respectively. Lee and Shaffer [16] proposed the following shear angle relationship using a slip-line field approach: πππΏπΏπΏπΏπΏπΏ = ππ + πΎπΎππ β π½π½ππ 4 (2.2) The validity of the shear angle relationships have been evaluated [17], and improved models have been presented [18][19][20]. Assuming there is only sliding friction on the rake face and there is a thin primary shear deformation area, the above models lead to the following expressions for the magnitudes of the tangential force and feed force: πΉπΉπ‘π‘ = β ππ οΏ½πππ π cos(π½π½ππ β πΎπΎππ ) οΏ½ sin ππππ cos(ππππ + π½π½ππ β πΎπΎππ ) sin(π½π½ππ β πΎπΎππ ) οΏ½ πΉπΉππ = β ππ οΏ½πππ π sin ππππ cos(ππππ + π½π½ππ β πΎπΎππ ) 6 (2.3) Chapter 2. Literature Survey where β is the uncut chip thickness, ππ is the width of cut and πππ π is the shear yield stress of the workpiece material. Since it is difficult to predict the shear and the friction angles, a simplified mechanistic model of cutting forces has been developed. The normal approach in practice is to combine the effects of shear angle, rake angle, and friction angle under a parameter called as the specific cutting pressure: πΉπΉπ‘π‘ = βπππΎπΎπ‘π‘ πΉπΉππ = βπππΎπΎππ (2.4) where πΎπΎππ is the specific cutting pressure of direction ππ. Another approach to calcu- late the forces is using an exponential force model in which specific cutting force pressures have been expressed as an exponential function of the chip thickness. Sabberwal and Koenigsberger [21][22] used this approach and obtained specific cutting coefficients experimentally. Their cutting force equation has been stated by: πΉπΉπ‘π‘ = πΎπΎπ‘π‘ ππβππ 1 πΉπΉππ = πΎπΎππ ππβππ 2 (2.5) where πΎπΎπ‘π‘ , πΎπΎππ , ππ1 , and ππ2 are experimentally calibrated empirical constants. It is possible to account for the edge forces by linearizing the cutting force expression. Linearization leads to a formulation of total cutting forces that are proportional to the undeformed chip cross sectional area and ploughing forces that are proportional to the length of the active cutting edge: πΉπΉπ‘π‘ = πΎπΎπ‘π‘π‘π‘ ππβ + πΎπΎπ‘π‘π‘π‘ ππ πΉπΉππ = πΎπΎππππ ππβ + πΎπΎππππ ππ (2.6) In this thesis, a linear cutting force model with edge coefficients is used. The advantage of linear cutting coefficient model is that it is more compatible with other process models, i.e. stability calculations require constant cutting coefficient in order to solve the differential equations. With the nonlinear coefficient model, the differential equations in stability calculation will be nonlinear. 7 Chapter 2. Literature Survey 2.3.2 Forces in Turning Most turning tools have oblique geometry with a nose radius (ππππ ), a cutting edge an- gle (π π ππ ), and an inclination (oblique) angle (πππ π ). It is possible to extend the orthogonal cutting model by introducing the concept of equivalent chip thickness [23]. The equivalent chip thickness combines the effects of nose radius and cutting edge angle on the cutting forces. At constant velocity, it has been found that the cutting forces can be expressed as a function of equivalent chip thickness [23][24]. In this model, the direction of the in-plane feed force is described by the chip flow angle ππ, using the following equations (Figure 2-3): πΉπΉπ¦π¦ = πΉπΉππ sin ππ πΉπΉπ§π§ = πΉπΉππ cos ππ (2.7) Figure 2-3: Turning Operation Using a Tool with a Nose Radius and Colwellβs Approach. The chip flow angle (ππ) has been modeled mostly empirically without considering the mechanics of the process. Nevertheless, these models are fairly successful in predicting chip flow angle, provided they are used at certain cutting conditions. Colwell [25] suggested that, without obliquity, the feed force is perpendicular to the line connecting the two end points of the active cutting edge. Okushima and Minato [26] proposed that the average chip flow (ππΜ ) is the summation of elemental flow angles over the entire length of cutting edge: 8 Chapter 2. Literature Survey ππΜ = β« ππ(π π )ππππ β« ππππ (2.8) where ππ(π π ) is the direction of each unitβs elemental surface normal and π π is the arc length along the cutting edge. For the case of a straight oblique cutting edge, Stabler [27] stated that the chip flow angle is equal to the inclination angle. Young et al. [28] published a combined approach which assumed Stablerβs chip flow rule was valid for infinitesimal chip widths and summed the directions of the elemental friction forces in order to obtain the direction of chip flow. Dividing the tool β chip interface into small elements and calculating the force contribution of each element in ππ and ππ directions, they estimated the direction of chip flow as (Figure 2-4): ππΜ = tanβ1 οΏ½ β« sin ππ(π π ) ππππ οΏ½ β« cos ππ(π π ) ππππ (2.9) Figure 2-4: Different Approaches for Predicting Chip Flow Direction: (a) Colwell, (b) Okushima and Minato, (c) Young et al. Wang [29] improved Young et al.βs method [28] by incorporating tool inclination angle and normal rake angle. In Wangβs model, cutting region is also divided into many local cutting elements and he assumed that the local chip flow for each element is collinear with that elementβs friction force, i.e. , Stablerβs chip flow rule is applied for each element. The above methods for obtaining the direction of chip flow and feed force are empirical methods and cannot be applicable to all cutting operations. Usui et al. [30][31] have proposed and upper bound model for oblique cutting with a non-straight cutting edge. 9 Chapter 2. Literature Survey The first step in this thesis is to develop a simplified but complete cutting force model for oblique non-straight cutting edges. The model uses the mechanics on the rake face of the tool to account for friction and shear forces. This model is described in Chapter 5. 2.3.3 Forces in Milling The milling process differs from the turning process, because the generated chips, hence the forces are discontinuous, and periodic. Cutting forces occurred during milling is one of the most important parameters in order to improve the productivity and part quality, because deflection, tool breakage, surface quality, and form errors are mainly influenced by cutting forces. Determination of chip formation is the first step in mechanistic modeling of cutting forces. Early study of Martelotti [32][33] showed that the path of the tool is trochoidal, rather than circular because of the combined rotation and translation of the tool towards the workpiece. Martelotti also claimed that when the feed per tooth is much smaller than the tool radius, circular tool path assumption is valid and the error is negligible: β(ππ) = ππ sin ππ (2.10) where β is the instantaneous chip thickness, ππ is the feed rate, and ππ is the immer- sion angle of a tooth. Milling force models in the literature can be classified into two categories. In mechanistic models, the focus is to derive a relationship between cutting forces and process parameters such as tool geometry, workpiece material, and cutting conditions. Early work by Koeingsberger and Sabberwal [22] used experimentally determined cutting coefficients and related chip load to calculate cutting forces: πΉπΉ = πΎπΎπ π ππβ πΎπΎπ π = πΆπΆβ π₯π₯ (2.11) where πΎπΎπ π is the cutting pressure, ππ is the radial depth of cut, β is the instantaneous chip thickness, πΆπΆ and π₯π₯ are empirical constants. This model is considered as the first complete model in this category. However, calibration method used in this study is far from the physics of the process, because an empirical curve fitting technique was used 10 Chapter 2. Literature Survey instead of employing cutting laws. Tlusty and McNeil [34], Kline et al. [35], Sutherland and DeVor [36], and Altintas and Spence [37] have improved and adopted the empirical method in their models. Armarego and Deshpande [38] proposed linear cutting force model by introducing edge force components: πΉπΉπ‘π‘ = πΎπΎπ‘π‘π‘π‘ ππβ + πΎπΎπ‘π‘π‘π‘ ππ πΉπΉππ = πΎπΎππππ ππβ + πΎπΎππππ ππ (2.12) πΉπΉππ = πΎπΎππππ ππβ + πΎπΎππππ ππ In Eq. (2.12), indices ππ and ππ represent the edge and cutting force components, re- spectively. Specific cutting coefficients and specific edge coefficients can be determined by applying linear regression to average cutting forces measured at different feed rates and this method is widely used in literature [39]. Mechanistic approach has limitations on milling with complex tools which have va- riable geometry along the axis of tool. Therefore, specific cutting coefficients are calculated as functions of shear stress, shear angle and friction angle [40] [41]. This method is called mechanics of milling model. In mechanics of milling model, oblique cutting force model is implemented to calculate the cutting coefficients [42][43]. This model is useful for general application to different cutters and it is applicable to cutters with variable geometry. Although orthogonal cutting database preparation is a time consuming process, it is very effective and accurate in calculating the cutting force coefficients [44]. In this thesis, since the aim is to obtain a generalized cutting force model that is capable of covering different cutting processes and cutter geometries, mechanics of milling approach is used to calculate the cutting coefficients. Adaptation of this approach into the proposed cutting force model is demonstrated in Chapter 5. It is shown that the model can accurately predict the cutting forces for different cutter geometries and processes. 11 Chapter 2. Literature Survey 2.4 Inserted Cutters Inserted (indexable) cutters are widely used in industry. In turning operations, most of the tools use inserts with wide selection of shapes and geometries. In milling, inserted mills with large diameters are widely used for machining operations such as rough and finish machining. Compared to solid type cutters, inserted end mills have the following advantages: β’ Higher material removal rate β’ More stable machining without chatter β’ Large cutter diameter availability β’ Better chip extraction performance β’ Longer tool life β’ Lower setup costs There have been only few studies available in modeling indexed cutters. Fu et al. [45] presented a model for inserted face milling cutters. They included corner radius, calculated equivalent axial and radial rake angles, and used experimentally identified cutting coefficients in their force model, which is improved in [46]. Various optimization methods with feed rate scheduling and surface error models have been studied by implementing mechanistic models in milling. Kim et al. [47] presented a feed rate scheduling algorithm for indexable end mills. Gu et al. [48] presented a model to predict the surface flatness in face milling. Ko and Altintas [49] developed a model for plunge milling operation using inserted cutters. Choudhury and Mathew [50] adopted nonuniform pitch angles to their face milling model. Most of the work done on inserted cutters is for certain types of cutters and very few researchers attempted to generalize a cutting model which can be applied to all cutters used in industry. Engin and Altintas [1] presented a cutting force model for indexable end milling. They generalized the envelope of the indexable end mill, and predicted cutting forces to evaluate surface roughness and chatter stability. They considered the insert edge on the cutter envelope to be straight line and used average cutting coefficients varying along the axial disk elements. Also, the uncut chip thickness was calculated from a geometric model. This is the first approach to generalized inserted 12 Chapter 2. Literature Survey cutter geometries. However, this model cannot account for corner radius, chamfer edge, and wiper edge. In addition, insert shapes used in this model are not capable of resembling all insert shapes currently being used in industry. 2.5 Generalized Mechanics of Machining As described throughout this chapter, there are various successful cutting force models to determine the cutting mechanics for different operations. However, these models are mostly limited by specific machining process, cutter geometry and cutting conditions. There is a lack of a fundamental approach that can combine different models by using the mechanics of metal cutting. Armarego [51][52] proposed a unified cutting modeling approach by combining oblique cutting operations and defining the cutting forces in shear and friction directions. These unique studies to generalize the cutting models are the first and only approach so far, but he concluded his work by defining individual empirical equations for different cutting processes to predict the machining parameters as functions of tool geometry and cutting conditions. The aim of this thesis is to determine the cutting forces by using fundamentals of metal cutting mechanics. The cutting forces are expressed as friction and normal forces on the rake face, and by using the cutter geometry, forces are transformed to machine coordinates. This model can be applied to different metal cutting processes, such as, turning, milling, boring, and other cutting operations. 13 Chapter 3. ISO Cutting Tool Geometry 3. 3.1 ISO Cutting Tool Geometry Overview In this chapter, definitions of the cutting tool geometry elements stated in ISO 3002 [53] and ISO 13399 [54] standards are summarized. These standards are published by the Organization for Standardization (ISO) and include the definitions of reference planes, major and minor cutting edge geometries, tool holders, connection elements, etc. Although the illustrations and figures in this chapter are mostly for turning tools and inserted cutters, they are applicable to the geometry of different individual tools, such as single-point tools, drills, and milling cutters. In the following sections, standard definitions are summarized and outlined in four different parts. In the first part, reference planes which are used to define the cutting angles are defined. The planes used in standards are defined for a selected point on the cutting edge. Since the most tool angles are designated by planes, so that the angles are defined for the same selected point on the cutting edge. In the second part, tool angles and other angular quantities are defined to describe the orientation of the cutting edge. In the third part, dimensional quantities necessary to describe the cutting edge are defined and finally, definitions of insert shapes, which are highly used in this thesis, are presented. For the sake of simplicity, definitions of angular and dimensional quantities related only to major cutting edge are given. 3.2 Planes Tool Reference Plane (ππππ ): A plane through the selected point on the cutting edge, so chosen as to be either parallel or perpendicular to a plane or axis of the. It is generally oriented perpendicular to the assumed direction of primary motion. For an ordinary turning system, it is a plane parallel to the base of the tool. For a milling cutters and drills, it is a plane containing the tool axis. 14 Chapter 3. ISO Cutting Tool Geometry Assumed Working Plane (ππππ ): A plane through the selected point on the cutting edge and perpendicular to the tool reference plane ππππ and so chosen as to be either parallel or perpendicular to a plane or an axis of the. It is generally oriented parallel to the assumed direction of feed motion. For ordinary lathe tools it is a plane perpendicular to the tool axis. For drills, facing tools, and parting-off tools, it is plane parallel to the tool axis. For milling cutters, it is a plane perpendicular to the tool axis. Tool Back Plane (ππππ ): Tool back plane is a plane through a selected point on the cutting edge and perpendicular both to the tool reference plane ππππ and to the assumed working plane ππππ . Tool Cutting Edge Plane (πππ π ): Tool cutting edge plane is a plane tangential to the cutting edge at the selected point and perpendicular to the tool reference plane ππππ . Cutting Edge Normal Plane (ππππ ): Cutting edge normal plane is perpendicular to the cutting edge at the selected point on the cutting edge. Wiper Edge Normal Plane (ππππ ): Wiper edge normal plane is a plane through the intersection of the reference planes ππππ and ππππ perpendicular to the wiper edge. All the planes defined above are illustrated in Figure 3-1. 15 Chapter 3. ISO Cutting Tool Geometry Figure 3-1: Planes Defined in ISO 3002 and ISO 13399. 3.3 Points and Angles Cutting Reference Point (πΆπΆπΆπΆπΆπΆ): Cutting reference point is the theoretical point of the tool from which the major functional dimensions are taken. For the calculation of this point the following cases are applied. Figure 3-2 shows the cutting reference point for different types of cutting edges: Case 1: Cutting Edge Angle (π π ππ ) β€ 90Β° - the point is the intersection of: the tool cutting edge plane πππ π , the assumed working plane ππππ , and the tool rake plane. Case 2: Cutting Edge Angle (π π ππ ) β₯ 90Β°- the point is the intersection of: the as- sumed working plane ππππ , a plane perpendicular to assumed working plane and tangential to the cutting corner, and the tool rake plane. Case 3: ISO tool styles D and V (55Β° and 35Β° rhombic inserts respectively) with only axial rake. The point is the intersection of: a plane perpendicular to the primary feed 16 Chapter 3. ISO Cutting Tool Geometry direction and tangential to the cutting edge (tangential point), a plane parallel to the feed direction through the tangential point, and the tool rake plane. Case 4: Round inserts - a) feed direction parallel to the tool axis, primary used for turning tools. The point is the intersection of: a plane perpendicular to the primary feed direction and tangential to the cutting edge, a plane parallel to the feed direction through the tangential point, and the tool rake plane; b) feed direction perpendicular to the tool axis, primarily used for milling tools. The point is the intersection of: a plane perpendicular to the primary feed direction and tangential to the cutting edge, a plane parallel to the feed direction through the tangential point, and the tool rake plane. Figure 3-2: Illustration of the Cutting Reference Point (CRP). 17 Chapter 3. ISO Cutting Tool Geometry Tool Radial Rake Angle (πΎπΎππ ): Tool rake angle is the angle between rake face and tool reference plane ππππ measured in assumed plane ππππ . It is also called side rake angle. Tool Axial Rake Angle (πΎπΎππ ): Tool axial rake angle is the angle between rake face and tool reference plane ππππ measured in the tool back plane ππππ . It is also called back rake angle. Tool Normal Rake Angle (πΎπΎππ ): Tool normal rake is the angle for major cutting edge between the rake face and the reference plane ππππ measured in plane ππππ . Tool Included Angle (ππππ ): It is the angle between the tool cutting edge plane πππ π , and tool minor cutting edge plane. Simply, it is the angle between the major and minor cutting edges of a cutting item. Tool Cutting Edge Angle (π π ππ ): Tool cutting edge angle is the angle between πππ π and plane ππππ measured in the reference plane ππππ . In other words, it is the angle between major cutting edge and the direction of major feed. Tool Approach Angle (ππππ ): The angle between the tool cutting edge plane πππ π and the tool back plane ππππ measured in the tool reference plane ππππ . Tool approach angle is only defined for the major cutting edge. Thus at any selected point on the major cutting edge, the following equation is valid: π π ππ + ππππ = 90Β° (3.1) Tool Cutting Edge Inclination Angle (πππ π ): It is the angle between the cutting edge and the tool reference plane ππππ measured in the tool cutting edge plane πππ π . Tool cutting edge inclination angle is also referred as oblique angle or helix angle in milling tools. Corner Chamfer Angle (Ξ ππ ): Corner chamfer angle is the angle of a chamfer on a corner measured from the major cutting edge. Angles defined in this section are shown in Figure 3-3 and Figure 3-4 and the definitions of these angles are summarized in Table 3-I. 18 Chapter 3. ISO Cutting Tool Geometry Figure 3-3: Demonstration of Tool Angles in ISO Standards for a Turning Tool. 19 Chapter 3. ISO Cutting Tool Geometry Table 3-I: Summary of Angles for Definition of Orientation of Cutting Edge and Rake Face. Definition ANGLE Angle Between Measured In Plane Orientation Of the Cutting Edge πΏπΏππ Tool Cutting Edge Angle πππ π ππππ ππππ ππππ Tool Cutting Edge Inclination πππ π ππππ πππ π ππππ Tool Approach Angle πππ π πΊπΊππ Tool Included Angle πππ π ππππ πππ π ππππ Orientation of the Rake Face (π¨π¨πΈπΈ ) πΈπΈππ Tool Normal Rake π΄π΄πΎπΎ ππππ ππππ πΈπΈππ Tool Back Rake π΄π΄πΎπΎ ππππ ππππ πΈπΈππ Tool Side Rake 3.4 ππππ π΄π΄πΎπΎ Dimensional Quantities ππππ ππππ Dimensional quantities are illustrated in Figure 3-4. Cutting Diameter (π·π·ππ ): Cutting diameter is the diameter of a circle created by a cutting reference point (πΆπΆπΆπΆπΆπΆ) revolving around the tool axis of a rotating tool item. Insert Length (πΏπΏ): Insert length is the theoretical length of the cutting edge of a cutting item over sharp corners. Wiper Edge Length (πππ π ): Wiper edge length is the measure of the length of a wiper edge of a cutting item. Insert Width (ππππ): Insert width is the distance between two sides of an insert when the inscribed circle cannot be used because of the shape of the insert. Corner Chamfer Length (ππππβ): Corner chamfer length is the nominal length of a chamfered corner measured in the ππππ plane. Corner Radius (ππππ ): Corner radius is the nominal radius of a rounded corner meas- ured in the ππππ plane. 20 Chapter 3. ISO Cutting Tool Geometry Figure 3-4: Dimensional Quantities Defined in the ISO Standards. 3.5 Insert Shapes In ISO 13399 standard, insert shapes are categorized in five main different shapes [54]. These categories are equilateral equiangular, equilateral nonequiangular, nonequilateral equiangular, nonequilateral nonequiangular and round inserts. For each main category, there are different shapes of inserts are defined. Table 3-II summarizes the definitions stated below with sample figures. 3.5.1 Equilateral Equiangular Insert Equilateral equiangular insert is a type of cutting item of regular geometric shape with sides of equal length and equal tool included angles. This category contains inserts with the ISO shape codes: T, S, P, O, and H. 1. Triangular Insert (T): Insert with three equal sides and three equal internal angles with the included angle (ππππ ) of 60Β°. 2. Square Insert (S): Insert with four equal sides and four equal internal angles with the tool included angle (ππππ ) of 90Β°. 3. Pentagonal Insert (P): Insert with five equal sides and five equal internal angles with the included angle (ππππ ) of 108Β°. 21 Chapter 3. ISO Cutting Tool Geometry 4. Hexagonal Insert (H): Insert with six equal sides and six equal internal angles with the tool included angle (ππππ ) of 120Β°. 5. Octagonal Insert (O): Insert with eight equal sides and eight equal inter- nal angles with the tool included angle (ππππ ) of 135Β°. 3.5.2 Equilateral Nonequiangular Insert It is the type of cutting item of regular geometric shape with sides of equal length and non-equal tool included angles. This category contains inserts with the following ISO shape codes: C, D, E, M, and V for rhombic (diamond) inserts and W for trigon inserts. 1. Rhombic (Diamond) Insert (C, D, E, M, V): Insert with two cutting corners, four sides of equal length and four internal angles none of which are equal to 90Β°. 2. Trigon Insert (W): Insert with a generally triangular shape with enlarged tool included angles. The edges between the corners may be curved or straight. 3.5.3 Nonequilateral Equiangular Insert It is the type of cutting item of regular geometric shape with sides of non-equal length and equal tool included angles. This category contains inserts with the ISO shape code L. Rectangular Insert (L): Insert with four sides and four equal internal angles with the tool included angle (ππππ ) of 90Β°. Opposing sides are in equal length but adjacent sides are not equal in length. 3.5.4 Nonequilateral Nonequiangular Insert It is the type of cutting item of regular geometric shape with sides of non-equal lengths and non-equal tool included angles. This category contains inserts with the following ISO shape codes: A, B, and K. 22 Chapter 3. ISO Cutting Tool Geometry 1. Parallelogram Insert (A, B, K): Insert with four sides and four internal angles none of which are equal to ninety degrees. Opposing sides are parallel and equal in length. 3.5.5 Round Insert It is the type of cutting item with circular edges. This category contains inserts with the ISO shape code R. Table 3-II: Insert Shapes Defined in ISO 13399. Symbol Shape S T C D E F M V W H O P L A B N/K R Insert Shape Square Triangular Rhombic (Diamond) Trigon Hexagonal Octagonal Pentagonal Rectangular Parallelogram Round 23 Nose Angle 90 60 80 55 75 50 86 35 80 120 135 108 90 85 82 55 - Chapter 3. ISO Cutting Tool Geometry 3.6 Tool Coordinate Frames and Transformations Coordinate transformations play an important role in this study, hence they are re- quired to define and transform the cutting mechanics from machine coordinate system to the rake face of the cutting tool. Machine coordinate system is defined by axes of the machine tool or the directions of the primary feed motion and primary cutting direction. On the other hand, chip flow coordinate system is defined in two axes; one is on the rake face and aligned in the direction of chip flow, and the second one is normal to the rake face and perpendicular to the chip flow axis. By using these transformations, it is possible to remove the effects of tool geometry from the mechanics of cutting, and eliminate the complexity of different cutting operations such as, turning, milling etc. Two different coordinate frame definitions and rotations are considered in this thesis. The following procedure is designed to transform the cutting forces or specific cutting force coefficients from machine coordinate system to the rake face. The mechanics of oblique cutting in turning operations is illustrated in Figure 3-5. In order to perform the transformations, chip flow angle (ππ), which is not defined in ISO standards, is used. 24 Chapter 3. ISO Cutting Tool Geometry Figure 3-5: Mechanics of Oblique Cutting Process. 3.6.1 Turning Operations Machine Coordinate System (πΏπΏππ ππππ ππππ ) Machine coordinate system (System 0) is given by the convention of the CNC system. The orientation of the machine coordinate system depends on the specific CNC tool, or the orientation of the dynamometer used to measure the cutting forces. In the most common approach, the ππ 0 axis is collinear with the main spindle axis and the ππ 0 and ππ 0 axes are perpendicular to the ππ 0 axis. On a lathe, the ππ 0 axis is the one on which the tool post moves to and away from the workpieceβs axis of rotation. The sign conven- tion for this coordinate system is determined such that the movement of the tool in a positive direction leads to a growing measurement of the workpiece. Figure 3-6 shows the machine coordinate system on a turning tool. 25 Chapter 3. ISO Cutting Tool Geometry Radial β Tangential β Axial Coordinate System (πΉπΉπΉπΉπΉπΉ) (πΏπΏπ°π° πππ°π° πππ°π° ) π π π π π π coordinate system is defined according to the directions of the cutting forces in orthogonal cutting. The forces in orthogonal turning are in the radial and the tangential directions, whereas the tangential direction is parallel to the direction of primary motion and the radial direction is perpendicular to the primary cutting edge and tool cutting edge plane πππ π . Both tangential and radial directions point towards the tool and the coordinate frameβs origin is on the cutting edge. The axial direction is perpendicular to both of these directions, pointing away from the tool shaft. In turning, geometrical transformation from the machine coordinate system (ππ 0 ππ 0 ππ 0 ) to π π π π π π coordinate system (ππ πΌπΌ ππ πΌπΌ ππ πΌπΌ ) can be performed by rotating the machine coordinate system around ππ 0 axis by amount of cutting edge angle (π π ππ ) in negative direction. Figure 3-6 illustrates the machine coordinate system and the π π π π π π coordinate system on a turning tool. The transformation to the machine coordinate system can be expressed by a linear rotation matrix πΆπΆ01 , so that: ππ0 = πΆπΆ01 ππ1 (3.2) where vector ππ0 is defined in System 0 and vector ππ1 is defined in System 1. Rota- tion matrix πΆπΆ01 can be expressed as follows: cos π π ππ πΆπΆ01 = οΏ½ 0 sin π π ππ ππ β ππππ οΏ½ 0 cos οΏ½ β‘ 0 β sin π π ππ 2 β’ 1 0 οΏ½=β’ 0 1 0 cos π π ππ β’ sin οΏ½ππ β ππ οΏ½ 0 ππ β£ 2 sin ππππ 0 β cos ππππ 1 0 πΆπΆ01 = οΏ½ 0 οΏ½ cos ππππ 0 sin ππππ 26 ππ β sin οΏ½ β ππππ οΏ½β€ 2 β₯ 0 β₯ ππ cos οΏ½ β ππππ οΏ½ β₯β¦ 2 (3.3) Chapter 3. ISO Cutting Tool Geometry Figure 3-6: RTA and the Machine Coordinate Systems on a Lathe. Similarly, the inverse transformation from System 0 to System 1 can be determined by calculating the inverse of πΆπΆ01 : πΆπΆ10 cos π π ππ =οΏ½ 0 βsin π π ππ β1 ππ πΆπΆ10 = πΆπΆ01 = πΆπΆ01 0 1 0 sin π π ππ sin ππππ 0 οΏ½=οΏ½ 0 cos π π ππ βcos ππππ Cutting Edge Coordinate System (πΏπΏπ°π°π°π° πππ°π°π°π° πππ°π°π°π° ) 0 1 0 cos ππππ 0 οΏ½ sin ππππ (3.4) Figure 3-7 illustrates the cutting edge coordinate system (System 2, (ππ πΌπΌπΌπΌ , ππ πΌπΌπΌπΌ , ππ πΌπΌπΌπΌ )). The ππ πΌπΌπΌπΌ axis of the cutting edge coordinate system lies along the cutting edge of the tool, ππ πΌπΌπΌπΌ axis is collinear with π π axis, and ππ πΌπΌπΌπΌ axis is defined perpendicular to ππ πΌπΌπΌπΌ and ππ πΌπΌπΌπΌ axes. This coordinate frame is also used for the definition of the oblique cutting mechanics in Altintas [5]. In order to transform the π π π π π π coordinate system (System 1) to cutting edge coordinate system (System 2), π π π π π π coordinate system needs to be rotated about π π axis by the amount of inclination (helix) angle (πππ π ) and additionally, a 90Β° rotation around the π΄π΄ axis and 90Β° rotation around the π π axis need to be performed, in order to get the axes pointing the proper directions. 27 Chapter 3. ISO Cutting Tool Geometry Figure 3-7: Cutting Edge Coordinate System along with the RTA Coordinate System. Therefore, the transformation matrix is: πΆπΆ21 0 = οΏ½οΏ½1 0 0 0 1 1 cos πππ π 0οΏ½ οΏ½ sin πππ π 0 0 β sin πππ π cos πππ π 0 0 β1 0 0οΏ½οΏ½ = οΏ½0 1 1 cos πππ π β sin πππ π 0 sin πππ π cos πππ π οΏ½ 0 (3.5) And the resultant transformation from machine coordinate system can be accomplished by Eq. (3.6). 28 Chapter 3. ISO Cutting Tool Geometry πΆπΆ20 πΆπΆ20 = πΆπΆ21 πΆπΆ10 β cos ππππ sin πππ π = οΏ½β cos ππππ cos πππ π sin ππππ cos πππ π β sin πππ π 0 Rake Face Coordinate System (πΏπΏπ°π°π°π°π°π° πππ°π°π°π°π°π° πππ°π°π°π°π°π° ) sin ππππ sin πππ π sin ππππ cos πππ π οΏ½ cos ππππ (3.6) Rake face coordinate system (System 3, (ππ πΌπΌπΌπΌπΌπΌ ππ πΌπΌπΌπΌπΌπΌ ππ πΌπΌπΌπΌπΌπΌ )) is a transitionary coordinate system used in this study. The ππ πΌπΌπΌπΌπΌπΌ axis of the rake face coordinate system is defined on the cutting edge and ππ πΌπΌπΌπΌπΌπΌ axis is defined on the rake face of the cutting tool. The transformation from cutting edge coordinate system (System 2) to the rake face coordinate system (System 3) can be accomplished by rotating the cutting edge coordinate system around ππ πΌπΌπΌπΌ axis by the amount of normal rake angle (πΎπΎππ ). Figure 3-8 shows the orientation of the cutting edge coordinate system and rake face coordinate system, and the transformation matrix is shown in Eq. (3.7). Figure 3-8: Rake Face Coordinate System (System 3): Rotation around the πππ°π°π°π° Axis by the Amount of Rake Angle πΈπΈππ . 29 Chapter 3. ISO Cutting Tool Geometry πΆπΆ32 cos πΎπΎππ =οΏ½ 0 sin πΎπΎππ 0 1 0 Chip Flow Coordinate System (πΌπΌπΌπΌ) β sin πΎπΎππ 0 οΏ½ cos πΎπΎππ (3.7) Chip Flow Coordinate System (System 4, (ππ πΌπΌπΌπΌ ππ πΌπΌπΌπΌ ππ πΌπΌπΌπΌ )) can be obtained by rotating the rake face coordinate system (System 3) by its ππ πΌπΌπΌπΌπΌπΌ axis by the amount of chip flow angle (ππ). With this transformation operation, ππ(ππ πΌπΌπΌπΌ ) axis of the chip flow coordinate system becomes parallel to the friction force (πΉπΉπ’π’ ) and the ππ (ππ πΌπΌπΌπΌ ) axis becomes collinear with the normal force (πΉπΉπ£π£ ) acting on the rake face. ππ or ππ πΌπΌπΌπΌ axis has no physical mean- ing and will be omitted in this study. Figure 3-9 shows the relationship between the rake face coordinate system and the chip flow coordinate system. The transformation between rake face coordinate system and the chip flow coordinate system can be described by the following equation: 0 ππ οΏ½ οΏ½=οΏ½ ππ 1 β sin ππ 0 πΌπΌπΌπΌπΌπΌ cos ππ πππΌπΌπΌπΌπΌπΌ οΏ½ οΏ½ππ οΏ½ 0 ππ πΌπΌπΌπΌπΌπΌ (3.8) Figure 3-9: Chip Flow Coordinate System (System 4): Rotation around the πΏπΏπ°π°π°π°π°π° Axis by the Amount of Chip Flow Angle πΌπΌ. 30 Chapter 3. ISO Cutting Tool Geometry Furthermore: πΆπΆ41 πΆπΆ41 = πΆπΆ43 πΆπΆ32 πΆπΆ21 cos πΎπΎππ cos ππ = οΏ½ sin πππ π sin ππ + cos πππ π sin πΎπΎππ cos ππ β cos πππ π sin ππ + sin πππ π sin πΎπΎππ cos ππ β sin πΎπΎππ β1 ππ cos πππ π cos πΎπΎππ οΏ½ = πΆπΆ41 sin πππ π cos πΎπΎππ (3.9) Finally, general transformation can be calculated by multiplying the transformation matrices from machine coordinate system to chip flow coordinate system. The resulting transformation is: π΄π΄ = οΏ½ 3.6.2 0 1 βsin ππ 0 cos πΎπΎππ cos ππ οΏ½οΏ½ 0 0 sin πΎπΎ ππ 0 1 0 ππ ππ οΏ½ οΏ½ = π΄π΄ Γ οΏ½ππ οΏ½ ππ ππ βsin πΎπΎππ 0 0 οΏ½ οΏ½0 cos πΎπΎππ 1 Transformation of Rotary Tools cos πππ π β sin πππ π 0 (3.10) sin πππ π sin ππππ cos πππ π οΏ½ οΏ½ 0 cos ππππ 0 0 1 0 βcos ππππ 0 οΏ½ sin ππππ As in milling and drilling, instead of workpiece, cutting tool rotates and rotary tools may have more than one cutting edge. Therefore, for each flute, following transformations should be defined individually. Machine Coordinate System (πΏπΏπΏπΏπΏπΏ) Similar to a turning machine, the ππ axis of the machine coordinate system (System 0, (ππππππ)) is parallel to the axis of the spindle. In milling machines, the ππ axis is the main axis parallel to the working surface, and the ππ axis can be found accordingly. Radial β Tangential β Axial Coordinate System (πΉπΉπΉπΉπΉπΉ) π π π π π π coordinate system (System 1, (ππ πΌπΌ ππ πΌπΌ ππ πΌπΌ )) is defined similar to a turning tool. Since the cutting edge on a rotary tool is positioned differently relative to the machine coordinate system (System 0), there is a slight difference between transformations in turning and other operations. 31 Chapter 3. ISO Cutting Tool Geometry In order to perform the transformations from machine coordinate system to RTA coordinate system, an additional coordinate system (System *) is introduced for each flute. System * has its origin located at the selected point on the cutting edge and rotates with the tool. The angle between the machine coordinate system and this coordinate frame is the immersion angle (ππ). Figure 3-10 illustrates the introduced coordinate system. Immersion angle ππ is measured clockwise between the positive ππ axis and the flutes of the cutting tool. The axes of the System * are aligned so that they are parallel to ππππππ for a cutting flute positioned on the ππ axis, in which the immersion angle is equal to 90Β° (Flute 2 in Figure 3-10). For instance, when ππ is equal to 0Β° (Flute 1 in Figure 3-10) the transformation between System 0 and System * is a rotation around the ππ axis by an amount of 90Β°. Figure 3-10: Illustration of System * on a Rotary Tool with Four Flutes. 32 Chapter 3. ISO Cutting Tool Geometry Consequently, the transformation from tool coordinate system (System *) to machine coordinate system (System 0) is a combination of two basic rotations: cos ππ πΆπΆβ0 = οΏ½ sin ππ 0 β sin ππ 0 cos 90Β° sin 90Β° cos ππ 0οΏ½ οΏ½β sin 90Β° cos 90Β° 0 0 0 1 sin ππ cos ππ 0 πΆπΆβ0 = οΏ½βcos ππ sin ππ 0οΏ½ 0 0 1 0 0οΏ½ 1 (3.11) In π π π π π π coordinate system, ππ axis is defined parallel to the primary rotational mo- tion of the tool, π π axis is aligned towards the axis of the tool while being normal to the cutting edge, and π΄π΄ axis is defined on the cutting edge. The direction of π π axis is represented with cutting element position angle (180Β° β ππππ ). So, the transformation from π π π π π π coordinate system (System 1) to System * is a positive rotation around the ππ β axis by an amount of π π ππ + 90Β°. Figure 3-11 shows the transformation and the resulting transformation matrix is: cos(π π ππ + 90Β°) πΆπΆ1β = οΏ½ 0 βsin(π π ππ + 90Β°) cos(180Β° β ππππ ) πΆπΆ1β = οΏ½ 0 βsin(180Β° β ππππ ) 0 1 0 0 1 0 β sin π π ππ sin(π π ππ + 90Β°) 0 οΏ½=οΏ½ 0 cos π π ππ cos(π π ππ + 90Β°) β cos ππππ sin(180Β° β ππππ ) 0 0 οΏ½=οΏ½ sin ππππ cos(180Β° β ππππ ) 33 0 1 0 0 1 0 cos π π ππ 0 οΏ½ β sin π π ππ sin ππππ 0 οΏ½ β cos ππππ (3.12) Chapter 3. ISO Cutting Tool Geometry Figure 3-11: Transformation between System 1 and System *. Thus, the resultant transformation from machine coordinate system to π π π π π π coordi- nate system can be found as the following: πΆπΆ10 = πΆπΆ1β πΆπΆβ0 πΆπΆ10 β sin π π ππ =οΏ½ 0 cos π π ππ βsin ππ sin π π ππ = οΏ½ βcos ππ βsin ππ cos π π ππ 0 1 0 cos π π ππ sin ππ 0 οΏ½ οΏ½βcos ππ β sin π π ππ 0 βcos ππ sin π π ππ sin ππ βcos ππ cos π π ππ Cutting Edge Coordinate System (πΏπΏπ°π°π°π° πππ°π°π°π° πππ°π°π°π° ) cos ππ sin ππ 0 cos π π ππ 0 οΏ½ β sin π π ππ 0 0οΏ½ 1 (3.13) For rotary tools, cutting edge coordinate system (System 2, (ππ πΌπΌπΌπΌ ππ πΌπΌπΌπΌ ππ πΌπΌπΌπΌ )) is defined the same as turning operations. The transformation from cutting edge coordinate system (System 2) and the π π π π π π coordinate system (System 1) is stated in Eq. (3.14). The coordinate system is demonstrated in Figure 3-12. 34 Chapter 3. ISO Cutting Tool Geometry πΆπΆ21 0 = οΏ½0 1 cos πππ π β sin πππ π 0 sin πππ π cos πππ π οΏ½ 0 (3.14) Figure 3-12: Transformation between System 1 and System 2 for Rotary Tools. Rake Face Coordinate System (πΏπΏπ°π°π°π°π°π° πππ°π°π°π°π°π° πππ°π°π°π°π°π° ) As it can be seen from Figure 3-13, rake face coordinate system (System 3) is defined same as the turning operations, which is described in Section 0. As a result, πΆπΆ32 cos πΎπΎππ =οΏ½ 0 sin πΎπΎππ 35 0 1 0 β sin πΎπΎππ 0 οΏ½ cos πΎπΎππ (3.15) Chapter 3. ISO Cutting Tool Geometry Figure 3-13: Transformation between Rake Face Coordinate System (System 3) and Cutting Edge Coordinate System (System 2). Chip Flow Coordinate System (πΌπΌπΌπΌ) Chip flow coordinate system (System 4, (ππ πΌπΌπΌπΌ ππ πΌπΌπΌπΌ ππ πΌπΌπΌπΌ )) for rotary tools is defined same as turning. It has only two axes: ππ axis is on the rake face, aligned with the direction of chip flow, and ππ axis is normal to the rake face. Figure 3-14 shows the representation of chip flow coordinate system on a rotary tool. In this figure, ππ axis has no physical meaning and is shown only to complete the coordinate system. 36 Chapter 3. ISO Cutting Tool Geometry Figure 3-14: Representation of the Chip Flow Coordinate System on a Milling Tool. The transformation between rake face coordinate system (System 3) and chip flow coordinate system (System 4) is defined by the following matrix: πΆπΆ43 = οΏ½ 0 1 β sin ππ 0 37 cos ππ οΏ½ 0 (3.16) Chapter 3. ISO Cutting Tool Geometry 3.7 Summary If the transformation operations in turning and rotating tools are compared, it can be observed that the transformation matrices from π π π π π π coordinate system (System 1) to ππππ coordinate system (System 4) are identical. The only difference between these operations comes from the rotation of multiple flutes in rotary tools. This difference is explained in Section 0. By performing these transformations, it is possible to describe the kinematics of the cutting process and cutting forces on the rake face of a cutter regardless of the operation. Thus, cutting forces measurable in machine coordinate system can be described as the friction and normal forces which are in the chip flow coordinate system. 38 Chapter 4. Generalized Geometric Model of Inserted Cutters 4. Generalized Geometric Model of Inserted Cutters 4.1 Overview In this chapter, a generalized model for inserted cutters is presented. Inserted cutters are widely used in turning and milling processes, both in roughing and finishing operations because of their low cost advantage on solid cutters. When tool wear or breakage occurs, instead of replacing the cutter body, replacing the insert is sufficient. Due to their popularity and availability in different types of applications, many different inserts are available in terms of shapes and dimensions. As described in Section 3.5, 17 different insert shapes are defined in ISO 13399 standard. Details of these shapes can be seen in Table 3-II of Chapter 3. Since this thesis aims to develop a generalized model, it covers not only insert shapes defined in ISO 13399, but also any arbitrary insert geometry defined by control points. Modeling of inserted cutters is more complicated than solid body cutters, because number of parameters used to define the insert geometry as well as to place on the cutter body is significantly higher. The output of this model is the axial locations, radii, helix / inclination angles, normal rake angles, and cutting edge angles of the cutting edge(s) at each specified point along the cutter axis. By using this data, it is possible to determine the cutting forces, tool vibrations, static deflections, and stability lobes for almost any kind of machining operations, such as, turning, boring, milling, etc. In the model, first of all, geometry of the insert is defined insertβs local coordinate system analytically and it is placed on the cutter body using the orientation angles, i.e. cutting edge angle π π ππ , axial rake angle πΎπΎππ , and radial rake angle πΎπΎππ . By using these angles, cutting edge positions are transformed to global coordinate system which is located at the cutter tip. After the transformation, for cutting mechanics which is described in Chapter 5.2, normal rake angle πΎπΎππ and helix angle πππ π are calculated using the position of the cutting edge(s). Sample examples are presented at the end of this chapter. 39 Chapter 4. Generalized Geometric Model of Inserted Cutters Inputs required to define the cutting edge on an insert are defined in Chapter 3 with illustrative figures, along with the coordinate axes used in this model. 4.2 Mathematical Modeling of an Insert In mathematical modeling, a similar approach to Engin and Altintasβ study [1] has been used with certain modifications and improvements. Firstly, using the inputs, mathematical model of one insert was developed on the local coordinate system (ππ 0 ππ 0 ππ 0 ) positioned at the cutting reference point (πΆπΆπΆπΆπΆπΆ) of the insert. Figure 4-1 illustrates the local coordinate system and the cutting reference point on two different inserts. The aim of this model is to calculate the control points that are sufficient to define the features (nose radius, corner chamfer, wiper edge etc.) on an insert. Figure 4-1: Local Coordinate Systems and Cutting Reference Points on Different Inserts. 40 Chapter 4. Generalized Geometric Model of Inserted Cutters In ISO 13399 standard, there are various shapes defined for inserts. Therefore, for this study, for each insert shape, control points were formulated for both corner chamfer and nose radius cases. Control points include the positions of the start and the end points of a feature on the cutting edge. For ISO type inserts, these control points are all in π₯π₯ 0 π§π§ 0 plane, therefore they were all assumed to be flat inserts. For instance, Figure 4-2 shows the control points and dimensions of a parallelogram insert with a wiper edge and a corner radius. In this Figure, points π΄π΄, π΅π΅, π·π·, and πΈπΈ are the control points on the cutting edge, πΆπΆ and πΌπΌ used to locate the insert center and the center of corner radius, πΆπΆπΆπΆπΆπΆ is the cutting reference point which is the origin for specific dimensions and rotations, point πΉπΉ is the theoretical sharp point of the insert. Figure 4-2: Control Points on a Parallelogram Insert. 41 Chapter 4. Generalized Geometric Model of Inserted Cutters Cutting reference point was selected as the origin of the local coordinate system. Locations of each control point as well as insert and radius centers were calculated analytically. These control points are rotated according to the given axial and radial rake angles, and transformed to the global coordinate system which is located at the tip of the cutter body. Finally, for modeling of mechanics, normal rake angle, helix angle, and true cutting edge angle were calculated for each point on the cutting edge. In the following sections, transformations and calculations of angles are described and the whole procedure is applied as examples for two milling inserts with different corner modifications and a turning insert. 4.3 Tool Coordinate Frames and Transformations Since the cutting reference point (πΆπΆπΆπΆπΆπΆ) is used to define all the angular and dimen- sional values to define an insert, all of the coordinate axesβ origins used in this model to orient and place the insert on the cutter body are located at the πΆπΆπΆπΆπΆπΆ. Four different coordinate systems were defined to accomplish the required rotations for the orientation of an insert on the cutter body. Initial coordinate frame πΉπΉ 0 is located with its origin at πΆπΆπΆπΆπΆπΆ and the ππ axis (ππ 0 ) along the primary feed direction as seen in Figure 4-3. ππ axis (ππ 0 ) of the πΉπΉ 0 frame is parallel to the cutter body rotation axis and positive ππ 0 is pointing towards the cutter body. In πΉπΉ 0 frame, ππ 0 ππ 0 plane corresponds to tool reference plane ππππ , and ππ 0 ππ 0 plane corresponds to assumed working plane ππππ . Final coordinate frame πΉπΉ 3 has its ππ axis (ππ 3 ) directed along the cutting edge and the ππ axis (ππ 3 ) on the rake face of the insert. In order to transform the πΉπΉ 0 frame to πΉπΉ 3 frame, two intermediate coordinate frames were defined. 42 Chapter 4. Generalized Geometric Model of Inserted Cutters 4.3.1 Figure 4-3: Frame ππππ to Frame ππππ Rotation. Frame ππππ to Frame ππππ Rotation The first step is to align the ππ 1 axis with the wiper edge or parallel land in a coordi- nate frame πΉπΉ1 by rotating the frame πΉπΉ 0 around ππ 0 axis by an amount of radial rake angle in the reverse direction (βπΎπΎππ ). As a result, the rotation matrix π π 01 from frame πΉπΉ 0 to frame πΉπΉ1 becomes: π π 01 πππ§π§01 = βπΎπΎππ cos(βπΎπΎππ ) = οΏ½ sin(βπΎπΎππ ) 0 43 β sin(βπΎπΎππ ) cos(βπΎπΎππ ) 0 0 0οΏ½ 1 (4.1) Chapter 4. Generalized Geometric Model of Inserted Cutters 4.3.2 Frame ππππ to Frame ππππ Rotation The second step is to align the ππ 2 axis of the frame πΉπΉ 2 with the rake face of the in- sert by rotating the frame πΉπΉ1 around ππ 1 axis by an amount of axial rake angle also in reverse direction (βπΎπΎππ ). Consequently, the rotation matrix π π 12 which defines the rotation from frame πΉπΉ1 to frame πΉπΉ 2 becomes: π π 12 1 = οΏ½0 0 πππ₯π₯12 = βπΎπΎππ 0 cos(βπΎπΎππ ) sin(βπΎπΎππ ) 0 β sin(βπΎπΎππ )οΏ½ cos(βπΎπΎππ ) Figure 4-4: Frame ππππ to Frame ππππ Rotation. 44 (4.2) Chapter 4. Generalized Geometric Model of Inserted Cutters 4.3.3 Frame ππππ to Frame ππππ Rotation The final step is to align the ππ 3 axis of the frame πΉπΉ 3 with the cutting edge of the in- sert by rotating the frame πΉπΉ 2 around the ππ 2 axis by an amount of cutting edge angle in the reverse direction (βπ π ππ ). So that the rotation matrix π π 23 defining the rotation from frame πΉπΉ 2 to frame πΉπΉ 3 is: π π 23 πππ¦π¦23 = βπ π ππ cos(βπ π ππ ) 0 = οΏ½ β sin(βπ π ππ ) 0 1 0 sin(βπ π ππ ) 0 οΏ½ cos(βπ π ππ ) Figure 4-5: Frame ππππ to Frame ππππ Rotation. 45 (4.3) Chapter 4. Generalized Geometric Model of Inserted Cutters Finally, in order to complete the rotation matrix between frames πΉπΉ 0 and πΉπΉ 3 can be calculated as the following: π π 03 = π π 01 π π 12 π π 23 where; cos πΎπΎππ cos π π ππ + sin πΎπΎππ sin πΎπΎππ sin π π ππ π π 03 = οΏ½sin πΎπΎππ cos π π ππ + cos πΎπΎππ sin πΎπΎππ sin π π ππ cos πΎπΎππ sin π π ππ 4.4 (4.4) sin πΎπΎππ cos πΎπΎππ β cos πΎπΎππ sin π π ππ + sin πΎπΎππ sin πΎπΎππ cos π π ππ β sin πΎπΎππ cos πΎπΎππ cos π π ππ cos πΎπΎππ cos πΎπΎππ sin πΎπΎππ sin π π ππ + cos πΎπΎππ sin πΎπΎππ cos π π ππ οΏ½ (4.5) Mathematical Relationships between Angles Next task in mathematical modeling of insert geometry is to derive the angles used in mechanics of metal cutting which are normal rake angle πΎπΎππ , helix / inclination gle πππ π , and cutting edge angle π π ππ . These angles were derived according to the definitions summarized in Chapter 3. 4.4.1 Normal Rake Angle The normal rake angle πΎπΎππ can be described as the angle between the direction of primary motion vector (βππ 0 ) and the normal of the rake face (ππ 3 ) measured in the cutting edge normal plane ππππ (ππ 3 ππ 3 plane). In order to determine the normal rake angle using the tool design angles (πΎπΎππ and πΎπΎππ ), a vector (π£π£0 ) directed along the direction of primary motion was defined in frame πΉπΉ 0 . Vector π£π£0 later can be described in frame πΉπΉ 3 with the inverse of the rotation matrix π π 03 . Therefore: π£π£3 = οΏ½ 0 β1 ππ π£π£0 = οΏ½β1οΏ½ β π£π£3 = π π 03 π£π£0 = π π 03 π£π£0 0 cos πΎπΎππ cos π π ππ β cos πΎπΎππ sin πΎπΎππ sin π π ππ β cos πΎπΎππ cos πΎπΎππ β sin πΎπΎππ sin π π ππ β cos πΎπΎππ sin πΎπΎππ cos π π ππ 46 π£π£3π₯π₯ οΏ½ = οΏ½π£π£3π¦π¦ οΏ½ π£π£3π§π§ (4.6) Chapter 4. Generalized Geometric Model of Inserted Cutters In order to measure the normal rake angle πΎπΎππ in ππππ plane, π₯π₯ component of the π£π£3 vector (π£π£3π₯π₯ ) was set to zero. From Figure 4-6, it can be observed that: π£π£3π§π§ βπ£π£3π§π§ ) = atan( ) πΎπΎππ = atan( π£π£3π¦π¦ βπ£π£3π¦π¦ sin πΎπΎππ sin π π ππ + cos πΎπΎππ sin πΎπΎππ cos π π ππ ) πΎπΎππ = atan( cos πΎπΎππ cos πΎπΎππ (4.7) Figure 4-6: Derivation of Normal Rake Angle. 4.4.2 True Cutting Edge Angle True cutting edge π π ππβ is defined as the angle between the cutting edge (ππ 3 ) and the assumed working plane ππππ (ππ 0 ππ 0 plane) measured in the tool reference plane ππππ (ππ 0 ππ 0 plane). The mathematical relationship between the true cutting edge angle π π ππβ and the tool design angles (πΎπΎππ , πΎπΎππ and π π ππ ) can be calculated by defining a vector, π£π£ππππππ ππ3 , along the cutting edge in frame πΉπΉ 3 and using the rotation matrix π π 03 to describe the vector in πΉπΉ 0 coordinate frame. In order to measure the angle in tool reference plane ππππ , π£π£ππππππ ππ0π¦π¦ component of the vector π£π£ππππππ ππ3 was set to zero. As a result, true cutting edge angle π π ππβ can be determined as the angle between the cutting edge vector π£π£ππππππ ππ0 and the ππ 0 axis. 47 Chapter 4. Generalized Geometric Model of Inserted Cutters π£π£ππππππππ 0π₯π₯ 1 π£π£ππππππππ 3 = οΏ½0οΏ½ , π£π£ππππππππ 0 = π π 03 π£π£ππππππππ 3 = οΏ½π£π£ππππππππ 0π¦π¦ οΏ½ π£π£ππππππππ 0π§π§ 0 cos πΎπΎππ cos π π ππ + sin πΎπΎππ sin πΎπΎππ sin π π ππ π£π£ππππππππ 0 = οΏ½β sin πΎπΎππ cos π π ππ + cos πΎπΎππ sin πΎπΎππ sin π π ππ οΏ½ cos πΎπΎππ sin π π ππ π π ππβ = atan οΏ½ π£π£ππππππππ 0π§π§ οΏ½ π£π£ππππππππ 0π₯π₯ cos πΎπΎππ sin π π ππ οΏ½ π π ππβ = atan οΏ½ cos πΎπΎππ cos π π ππ + sin πΎπΎππ sin πΎπΎππ sin π π ππ (4.8) (4.9) Figure 4-7: Derivation of the True Cutting Edge Angle. 4.4.3 Inclination (Helix) Angle The inclination angle πππ π is defined as the angle between the cutting edge (ππ 3 ) and the tool reference plane ππππ (ππ 0 ππ 0 plane) measured in tool cutting edge plane πππ π . In order to relate the tool design angles, a new coordinate frame must be defined such that the frame πΉπΉ π π has its ππ π π axis aligned with the tool cutting edge plane πππ π , and its ππ π π axis aligned with ππ 0 axis of the coordinate frame πΉπΉ 0 . Figure 4-8 illustrates the frame πΉπΉ π π . ππ Since a rotation of β οΏ½ β π π ππ οΏ½ around the ππ 0 axis is needed to align the ππ π π axis with the 2 plane πππ π , the rotation matrix between frames πΉπΉ0 and πΉπΉπ π becomes: 48 Chapter 4. Generalized Geometric Model of Inserted Cutters π π 0π π cos(βπ π ππ ) 0 =οΏ½ β sin(βπ π ππ ) 0 1 0 sin(βπ π ππ ) cos(π π ππ ) 0 οΏ½=οΏ½ 0 cos(βπ π ππ ) sin(π π ππ ) 0 1 0 β sin(π π ππ ) 0 οΏ½ cos(π π ππ ) (4.10) Figure 4-8: Definition of Frame ππππ and Frame ππππ to Frame ππππ Rotation. As a result, vector π£π£ππππππ πππ π can be calculated as: ππ π£π£ππππππ πππ π = π π 0π π π π 03 π£π£ππππππππ 3 π£π£ππππππ πππ π π£π£ππππππ πππ π π π = οΏ½π£π£ππππππ πππ π π π οΏ½ π£π£ππππππ πππ π π π cos 2 π π ππ cos πΎπΎππ + cos π π ππ sin πΎπΎππ sin πΎπΎππ sin π π ππ + sin2 π π ππ cos πΎπΎππ β sin πΎπΎππ cos π π ππ + cos πΎπΎππ sin πΎπΎππ sin π π ππ =οΏ½ οΏ½ 2 β sin π π ππ cos πΎπΎππ cos π π ππ β sin π π ππ sin πΎπΎππ sin πΎπΎππ + cos π π ππ cos πΎπΎππ sin π π ππ (4.11) Following that, the inclination angle πππ π can be defined as the angle between the ππ π π axis in ππππ plane and the edge vector π£π£ππππππ πππ π with π£π£ππππππ πππ π π π component equal to zero. 49 Chapter 4. Generalized Geometric Model of Inserted Cutters πππ π = atan οΏ½ π£π£ππππππππ _π π π π οΏ½ π£π£ππππππππ _π π π π β sin πΎπΎππ cos π π ππ + cos πΎπΎππ sin πΎπΎππ sin π π ππ οΏ½ πππ π = atan οΏ½ 2 cos π π ππ cos πΎπΎππ + cos π π ππ sin πΎπΎππ sin πΎπΎππ sin π π ππ + sin2 π π ππ cos πΎπΎππ (4.12) Figure 4-9: Definition of the Inclination (Helix) Angle. Calculation of these angles is important to determine cutting forces along the cutting edge. In most cases, radial and axial rakes are constant along the cutting edge of the insert, and cutting edge angle changes with the edge modification; it becomes a different constant value along the chamfered corner, and quadratically changes along the round edges. Using the vectors and formulations derived in this chapter, it is possible to calculate the angles at each selected point on the cutting edge. In the next section, as case examples, several different inserts were selected and modeled using the proposed model. 50 Chapter 4. Generalized Geometric Model of Inserted Cutters 4.5 Examples 4.5.1 Rectangular Insert with Corner Radius The first selected sample case is a rectangular milling insert with a corner radius. Insert and cutter body geometry were taken from Sandvik Coromant [55]. These dimensions are listed in Table 4-I and illustrated in Figure 4-10. Table 4-I: Inputs Used in the Model. Inputs π³π³ (Insert Length) 11 mm πΏπΏππ (Cutting Edge Angle) 90Β° πΈπΈππ (Axial Rake Angle) 10Β° πΈπΈππ (Radial Rake Angle) ππππ (Insert Width) 6.8 mm πππΊπΊ (Corner Radius) 0.4 mm πΊπΊππ (Tool Included Angle) ππππ (Wiper Edge Length) 0.5 mm π«π«ππ (Cutting Diameter) 9Β° 90Β° 20 mm Figure 4-10: Illustrations of the Insert and the Cutter Body [55]. After obtaining the dimensions and the angles required for the placement of the insert on the cutter body, insert center is calculated in (ππ 0 ππ 0 ) plane. In Figure 4-11, insert center is defined by point πΌπΌ(π₯π₯, π§π§): 51 Chapter 4. Generalized Geometric Model of Inserted Cutters πΌπΌπ₯π₯ = πΌπΌπ§π§ = πΏπΏ cos π π ππ ππππ ππππ sin π π ππ π·π·ππ + β + ππππ sin π π ππ (1 β cos π π ππ ) β 2 sin π π ππ 2 2 + ππππ cot π π ππ β πππ π cos 2 π π ππ ππππ cos π π ππ βπΏπΏ sin π π ππ + πππ π sin π π ππ cos π π ππ β ππππ cos 2 π π ππ + ππππ cos π π ππ β 2 2 Figure 4-11: Analytical Model of an Insert with a Corner Radius and Wiper Edge. 52 (4.13) Chapter 4. Generalized Geometric Model of Inserted Cutters The second step is to calculate locations of control points which define the cutting edge. In this case, these points are π΄π΄, π΅π΅, π·π·, and πΈπΈ. Wiper edge is on the feed plane and defined by a between points π΄π΄ and π΅π΅. The locations of these points can be calculated as: π΄π΄π₯π₯ = βπππ π + π΅π΅π₯π₯ = ππππ π·π·ππ + ππππ cot π π ππ , π΄π΄π§π§ = 0 β 2 sin π π ππ ππππ π·π·ππ + ππππ cot π π ππ , π΅π΅π§π§ = 0 β 2 sin π π ππ (4.14) (4.15) Corner edge is also defined by two points; points π΅π΅ and π·π·. Similarly, location of point π·π· can be calculated by the following equations: π·π·π₯π₯ = ππππ cos π π ππ ππππ π·π·ππ , π·π·π§π§ = |ππππ (β1 + cos π π ππ )| + + ππππ sin π π ππ β sin π π ππ sin π π ππ 2 (4.16) Since there are only two symmetrical edges on a rectangular insert (Figure 4-10), corner modification is present only at two corners, thus main cutting edge is defined by points π·π· and πΈπΈ. The position of point πΈπΈ can be calculated as: πΈπΈπ₯π₯ = ππππ π·π·ππ + (βπππ π + ππππ cot ππππ ) cos 2 π π ππ + ππππ sin π π ππ β sin π π ππ 2 ππππ β ππππ cot ππππ β πππ π sin π π ππ cot ππππ οΏ½ cos π π ππ + οΏ½πΏπΏ β ππππ sin π π ππ + sin π π ππ (4.17) πΈπΈπ§π§ = |βπΏπΏ sin π π ππ + πππ π cot ππππ + ππππ sin π π ππ cot ππππ + (βππππ β πππ π cot ππππ ) cos 2 π π ππ + (ππππ + πππ π sin π π ππ β ππππ sin π π ππ cot ππππ ) cos π π ππ | After the calculation of control points and the cutting edges between these points, cutting edge is defined in (ππ 0 ππ 0 ) plane in two dimensions. Rotation about ππ 0 axis by π π ππ has been already implemented in these equations in order to make sure that the wiper edge is parallel to (ππππ) plane. Moreover, these locations were defined in global ππππππ coordinate frame with the tool tip as origin, therefore, these points must be translated to cutting reference point then the rotations presented in Section 4.3 must be performed. 53 Chapter 4. Generalized Geometric Model of Inserted Cutters Transformations from a fixed frame is not commonly used in robotics, however, placement of the insert on the cutter body is a fixed frame transformation. Fixed frame transformation matrix can be calculated by taking the inverse of the current frame transformation using the same rotations [56]. After the rotations, all control points were transformed back to global ππππππ coordinate frame. Following figures show the locations of the control points before and after the transformations in global coordinate system. Figure 4-12: Control Points of the Insert before the Rotations. 54 Chapter 4. Generalized Geometric Model of Inserted Cutters Figure 4-13 : Control Points of the Insert after the Rotations. Following the rotations of the control points, lines and arcs between the consecutive points were defined in three dimensions. Since the performed rotations were solid body rotations, the relationship between any consecutive points would be same. However, these lines and arcs were no longer in (ππ 0 ππ 0 ) plane; therefore, parametric equations were defined to obtain the location of each selected point on the cutting edge. For linear cutting edges: For arcs: ππ = πποΏ½βππβ1 + οΏ½πποΏ½βππ β πποΏ½βππβ1 οΏ½ π‘π‘ , 0 β€ π‘π‘ β€ 1 ππ = π π cos π‘π‘ π’π’ οΏ½β + π π sin π‘π‘ πποΏ½β Γ π’π’ οΏ½β + πΆπΆ, 0 β€ π‘π‘ β€ 55 (4.18) ππ 2 (4.19) Chapter 4. Generalized Geometric Model of Inserted Cutters where ππ is any point between πποΏ½βππ and πποΏ½βππβ1 which are two consecutive control points, π π is the radius of the arc, π’π’ οΏ½β is a unit vector from the center of the arc to any point on the circumference, πποΏ½β is a unit vector perpendicular to the plane of the arc, and πΆπΆ is the center vector of the arc. Note that these equations are parametric equations, in order to calculate the positions along the cutter axis, the relationship between any axial point and corresponding parameter value should be calculated. During the calculation of geometric control points, cutting edge shape between any two consecutive points are stored and during the general model, corresponding parametric equation is called automatically. The radius of any point on the cutting edge can be calculated by a point β line distance equation in space. Hence: ππ = οΏ½(ππβ2 β ππβ1 ) Γ (ππβ1 β πποΏ½β)οΏ½ οΏ½ππβ2 β ππβ1 οΏ½ (4.20) where ππ is the local radius, ππβ1 and ππβ2 are any two points along the cutter axis. Using these formulations and rotation matrix developed in section 4.3, it is possible to calculate the local radius, normal rake angle, and helix angle of any point on the cutting edge of the insert. Figure 4-14: CAD Model of the Insert. 56 Chapter 4. Generalized Geometric Model of Inserted Cutters Figure 4-15 shows the local radii of the insert along the cutter axis with comparison of the local radii extracted from the CAD part of the insert which is shown in Figure 4-14. As stated before, this model is valid for flat inserts; therefore, due to the complexity of the insert geometry, mathematical model and CAD data does not match perfectly. However, the error is acceptable; since the maximum error between the model and the insert is around 50 microns. Figure 4-16 and Figure 4-17 show the change in the normal rake angle and helix angle along the cutter axis respectively. Figure 4-15: Change of Local Radius along the Tool Axis. 57 Chapter 4. Generalized Geometric Model of Inserted Cutters Figure 4-16: Change in Normal Rake Angle along the Tool Axis. Figure 4-17: Change in Helix Angle along the Tool Axis. 58 Chapter 4. Generalized Geometric Model of Inserted Cutters 4.5.2 Rectangular Insert with Corner Chamfer Similar procedure was applied to an insert with a chamfered corner. Table 4-II and Figure 4-18 show the angles and dimensions of the insert. Table 4-II: Inputs Used in the Analysis of the Insert with Chamfer. Inputs π³π³ (Insert Length) 11 mm ππππ (Insert Width) π²π²πΊπΊ (Corner Chamfer) 90Β° πΈπΈππ (Axial Rake Angle) 5Β° πΈπΈππ (Radial Rake Angle) 5Β° 1 mm x45Β° πΊπΊππ (Tool Included Angle) 90Β° 11.5 mm ππππ (Wiper Edge Length) πΏπΏππ (Cutting Edge Angle) 1.5 mm π«π«ππ (Cutting Diameter) 63 mm Figure 4-18: Catalogue Figure of the Insert Taken from Sandvik Coromant [55]. 59 Chapter 4. Generalized Geometric Model of Inserted Cutters Analytical model of the insert can be seen in Figure 4-19. Insert center is defined by point πΌπΌ. Coordinates of this point in (ππ 0 ππ 0 ) plane is: πΌπΌπ₯π₯ = πΌπΌπ§π§ = 1 |π·π· + ππππ cos(ππππ + π π ππ ) csc ππππ 2 ππ + cos π π ππ (πΏπΏ β 2πππ π csc ππππ sin(ππππ + π π ππ )) β 2ππππβ cot π π ππ csc ππππ sin(ππππ + π π ππ ) sin(π π ππ β πΎπΎππ )| (4.21) 1 οΏ½csc ππππ οΏ½πΏπΏ sin ππππ sin π π ππ 2 + sin(ππππ + π π ππ ) (ππππ β 2πππ π sin π π ππ β 2ππππβ sin(π π ππ β πΎπΎππ ))οΏ½οΏ½ Figure 4-19: Illustration of Control Points and Dimensions for an Insert with Chamfer. For this type of insert, control points are points π΄π΄, π΅π΅, πΆπΆ, and π·π·. Locations of these points can be calculated as the following: 60 Chapter 4. Generalized Geometric Model of Inserted Cutters π΄π΄π₯π₯ = βπππ π + π·π·ππ + π΅π΅π΅π΅π΅π΅ sin πΎπΎππ cot π π ππ β π΅π΅π΅π΅π΅π΅ cos πΎπΎππ , π΄π΄π§π§ = 0 2 π·π·ππ + π΅π΅π΅π΅π΅π΅ sin πΎπΎππ cot π π ππ β π΅π΅π΅π΅π΅π΅ cos πΎπΎππ , π΅π΅π§π§ = 0 2 π·π·ππ + π΅π΅π΅π΅π΅π΅ sin πΎπΎππ cot π π ππ , πΆπΆπ§π§ = π΅π΅π΅π΅π΅π΅ sin πΎπΎππ πΆπΆπ₯π₯ = 2 π·π·ππ β πΏπΏ cos π π ππ + πππ π cos(ππππ + π π ππ ) csc ππππ sin π π ππ π·π·π₯π₯ = οΏ½πππ π β 2 π΅π΅π₯π₯ = + ππππβ cot π π ππ csc ππππ sin(ππππ + π π ππ ) sin(π π ππ β πΎπΎππ )οΏ½ (4.22) (4.23) (4.24) (4.25) π·π·π§π§ = |πΏπΏ sin π π ππ β csc ππππ sin(ππππ + π π ππ ) (πππ π sin π π ππ + ππππβ sin(π π ππ β πΎπΎππ ))| After the calculations of the locations of control points in (ππ 0 ππ 0 ) plane, same pro- cedure explained in the previous example is applied. As a result, tool angles and local radii were calculated along the cutting edge of this insert. Following figures illustrate these results. Figure 4-20: Change in the Local Radius along the Tool Axis for a Chamfered Insert. 61 Chapter 4. Generalized Geometric Model of Inserted Cutters Figure 4-21: Change in Normal Rake Angle along the Tool Axis. Figure 4-22: Change in Helix Angle along the Tool Axis. 62 Chapter 4. Generalized Geometric Model of Inserted Cutters 4.5.3 Rhombic Turning Insert As stated before, geometric model of inserted cutters can be applied not only for milling inserts, but also turning inserts. For that purpose, a rhombic (diamond) shaped insert was selected for analysis. Table 4-III and Figure 4-23 show the insert geometry. Table 4-III: Inputs for the Rhombic Turning Insert. Inputs π³π³ (Insert Length) 15 mm ππππ (Wiper Edge Length) - ππππ (Insert Width) πππΊπΊ (Corner Radius) πΏπΏππ (Cutting Edge Angle) 93Β° πΈπΈππ (Axial Rake Angle) -10Β° πΈπΈππ (Radial Rake Angle) - 1.6 mm πΊπΊππ (Tool Included Angle) -8Β° 55Β° Figure 4-23: Catalogue Figures of the Turning Insert [55]. In the analysis of turning tools, similar procedure to previous examples was applied with some certain modifications. Since turning tools do not rotate, π·π·ππ (Cutting Diameter) is not defined. Moreover, wiper edge was also not considered for turning tools. As a result, instead of tool axis tip, intersection between cutting edge and feed plane (Point π΄π΄ in Figure 4-24) was selected as the origin of the global coordinate system. Figure 4-24 shows the variable and control points used in the analysis of this type of insert. 63 Chapter 4. Generalized Geometric Model of Inserted Cutters Figure 4-24: Control Points of the Rhombic Turning Insert. Location of insert center with respect to the point π΄π΄ can be calculated as: (βπΏπΏ + 2ππππ sin ππππ + πΏπΏ cos 2 ππππ ) sin π π ππ πΌπΌπ₯π₯ = 2 sin ππππ + (β2ππππ + πΏπΏ sin ππππ β 2ππππ cos ππππ + πΏπΏ sin ππππ cos ππππ ) cos π π ππ 2 sin ππππ ππππ sin π π ππ πΏπΏ sin π π ππ (1 + cos ππππ ) β β ππππ sin π π ππ cot ππππ β ππππ cos π π ππ πΌπΌπ§π§ = ππππ + sin ππππ 2 πΏπΏ cos π π ππ β πΏπΏ cos 2 ππππ cos π π ππ + 2 sin ππππ 64 (4.26) Chapter 4. Generalized Geometric Model of Inserted Cutters The locations of control points π΄π΄, π΅π΅, and π·π· can be described as the following equa- tions. The results of the model are shown in the following figures. π΄π΄π₯π₯ = 0 , π΄π΄π§π§ = 0 π΅π΅π₯π₯ = ππππ sin π π ππ , π΅π΅π§π§ = ππππ (1 β cos π π ππ ) π·π·π₯π₯ = ππππ sin π π ππ + οΏ½πΏπΏ β ππππ β ππππ cot ππππ οΏ½ cos π π ππ sin ππππ ππππ sin π π ππ β ππππ sin π π ππ cot ππππ β ππππ cos π π ππ π·π·π§π§ = ππππ + πΏπΏ sin π π ππ β sin ππππ Figure 4-25: Position of the Cutting Edge for a Rhombic Turning Insert. 65 (4.27) (4.28) (4.29) Chapter 4. Generalized Geometric Model of Inserted Cutters Figure 4-26: Change in the Normal Rake Angle along the Cutting Edge. Figure 4-27: Change in the Helix Angle along the Cutting Edge. 66 Chapter 4. Generalized Geometric Model of Inserted Cutters 4.6 Summary In this chapter, the analytical calculations needed to define and orient the cutting edge on a cutter or tool holder is presented. Variables used in the equations have been obtained from ISO standards ISO 3002 and ISO 13399. It is shown that for any type of insert geometry, it is possible to define the cutting edge on the tool axis and to convert from design angles to physical angles. From the definitions and transformation equations, it can be observed that normal rake angle πΎπΎππ and helix (inclination) angle πππ π are functions of the cutting edge angle π π ππ , radial rake angle πΎπΎππ , and axial rake angle πΎπΎππ . As a result, any change in one of these angles along the cutting edge alters others. However, in this study, axial and radial rake angles are kept constant; therefore, changes observed in helix and normal rake angles in the presented figures are due to the cutting angle changes along the corner modifications. For instance, for an insert with -10Β° axial rake and -8Β° radial rake, as the cutting edge angle changes from 0Β° to 93Β°, normal rake angle changes from -13Β° to -9Β° and the helix angle changes from +6Β° to -10Β° along 1.6 mm corner radius. In the next chapter, generalized mechanics of metal cutting is presented. A new cutting force model which is applicable to different cutting processes is introduced and simulations are presented with experimental verifications. 67 Chapter 5. Generalized Mechanics of Metal Cutting 5. Generalized Mechanics of Metal Cutting 5.1 Overview In this chapter, generalized modeling of cutting forces using the mechanics on the rake face, the friction force πΉπΉπ’π’ and the normal force πΉπΉπ£π£ , is presented. The generalized model is compared with the current cutting force models in the literature, and the relationships between the models are presented. In addition to cutting force models, calibration and material models are summarized and their transformations to the proposed model are presented. With this information, it is possible to convert most of the models currently being used in the literature to the proposed model. 5.2 Rake Face Based General Force Model The normal and friction forces on the rake face are used as the base in predicting the cutting forces in the machining system. Figure 5-1 shows a sample oblique cutting process with all the variables used in this study. In Figure 5-1, πΉπΉβ is the total resultant force, οΏ½οΏ½οΏ½β πΉπΉπ’π’ is the friction force on the rake face, οΏ½οΏ½οΏ½β πΉπΉπ£π£ is the normal force on the rake face, οΏ½οΏ½οΏ½β πΉπΉπ π and οΏ½οΏ½οΏ½β πΉπΉππ are the shearing and normal forces acting on the shear plane respectively, πΎπΎππ is the rake angle, πππ π is the inclination / helix angle, ππ is the chip flow angle, ππππ is the normal shear angle, π½π½ππ is the friction angle, and finally, ππ and β are the width of cut and uncut chip thickness respectively. 68 Chapter 5. Generalized Mechanics of Metal Cutting Figure 5-1: Mechanics of Oblique Cutting [11]. For a differential chip load (πππ΄π΄ππ ) in an engagement with a selected point on the cut- ting edge of the tool, the differential friction (πππΉπΉπ’π’ ) and the differential normal force (πππΉπΉπ£π£ ) acting on the rake face can be expressed as, πππΉπΉπ’π’ = πΎπΎπ’π’π’π’ πππ΄π΄ππ + πΎπΎπ’π’π’π’ ππππ πππΉπΉπ£π£ = πΎπΎπ£π£π£π£ πππ΄π΄ππ + πΎπΎπ£π£π£π£ ππππ (5.1) where πΎπΎπ’π’π’π’ and πΎπΎπ£π£π£π£ are the friction and normal cutting coefficients and πΎπΎπ’π’π’π’ and πΎπΎπ£π£π£π£ are the related edge coefficients. These specific cutting coefficients depend on the tool β workpiece combination. For a specific workpiece, these cutting coefficients can be described as a function of chip thickness, rake angle, and the cutting speed (ππππ ), they are called size, geometry and speed effects, respectively. The identification of the cutting 69 Chapter 5. Generalized Mechanics of Metal Cutting force coefficients are explained in the following sections. In Eq. (5.1), πππ΄π΄ππ and ππππ are differential chip area and differential cutting edge length can be calculated with the following; πππ΄π΄ππ = β ππ ππππ = ππππππππ = ππ sin π π ππ (5.2) Once the differential friction (πππΉπΉπ’π’ ) and differential normal (πππΉπΉπ£π£ ) cutting forces are evaluated through the use of Eq. (5.1), they can be transformed into machine coordinate system (ππππππ) with procedure described in Chapter 4.3. Since the tool geometry (rake, helix, cutting edge angles) may change along the cutting edge of the tool, these transformations must be repeated for each differential part of the cutting edge(s). After the transformation process, the differential forces are summed to determine the total cutting forces acting on the machine coordinates (πΉπΉπ₯π₯ , πΉπΉπ¦π¦ , and πΉπΉπ§π§ ), as: ππππ πΎπΎ πππππ₯π₯ πΉπΉπ₯π₯ οΏ½πΉπΉπ¦π¦ οΏ½ = οΏ½ οΏ½ οΏ½πππππ¦π¦ οΏ½οΏ½ πΉπΉπ§π§ ππ=1 ππ=1 πππππ§π§ ππ,ππ (5.3) where ππππ is the total number of cutting edges and πΎπΎ represents the total number of discrete points along the cutting edge ππ. Proposed cutting force model is summarized in Figure 5-2. 70 Chapter 5. Generalized Mechanics of Metal Cutting Figure 5-2: Summary of the Proposed Mechanistic Approach. 71 Chapter 5. Generalized Mechanics of Metal Cutting 5.3 Identification of Specific Cutting Coefficients Accurate determination of cutting force coefficients is critical to cutting force pre- diction. There are several parameters influencing the cutting force coefficients and they can be estimated either mechanistically or using classical orthogonal to oblique transformation method. The cutting force coefficients depend on: - Cutting Method : constant or varying chip volume removal (Turning vs. Milling) - Cutting Conditions: feed rate, depth of cut, cutting speed, use of coolant; - Workpiece Material: chemical composition; - Tool: tool material, chip breaker, tool wear. Since it is not possible to account for all of these variables simultaneously, only the effects of chip thickness, cutting speed, and normal rake angle have been considered which are most dominant. In the following sections, both mechanistic approach and classical approach are explained in detail to determine the specific cutting coefficients in friction (πΎπΎπ’π’ ) and normal (πΎπΎπ£π£ ) directions. 5.3.1 Orthogonal to Oblique Transformation Orthogonal cutting tests can be used to calculate the shear stress (πππ π ), shear angle (ππππ ), and the friction angle (π½π½ππ ) as a function of chip thickness, rake angle, and cutting speed. However, cutting edge(s) are usually not orthogonal to the cutting velocity, but inclined with helix (πππ π ) or inclination (ππ) angle. Thus, an orthogonal to oblique transformation needs to be applied. The same concept has been implemented stated in Shamoto and Altintas [11], except the cutting coefficients are calculated in normal and friction directions instead of radial, tangential, and axial directions. In orthogonal cutting, the rectangular shear plane area can be calculated as (Figure 5-3): π΄π΄π π ,ππππππ βππππππππππππ = ππ β sin ππππ (5.4) However, due to the helix angle (πππ π ) in oblique cutting, the shear plane area is a pa- rallelogram and its area can be calculated by: 72 Chapter 5. Generalized Mechanics of Metal Cutting π΄π΄π π ,ππππππππππππππ = ππ β cos πππ π sin ππππ (5.5) Where ππππ is the normal shear angle, i.e. shear angle measured in the normal plane ππππ . In this study, using the same assumptions stated in Altintas [5], normal shear angle (ππππ ) has been assumed equal to the shear angle in orthogonal cutting (ππππ ). Figure 5-3: Shear Plane Area Comparison: Orthogonal Cutting (Left) and Oblique Cutting (Right). In oblique cutting, shear force is equal to: πΉπΉπ π = πππ π π΄π΄π π = πππ π ππ β cos πππ π sin ππππ (5.6) Detailed illustration of the oblique cutting geometry is shown in Figure 5-4 [11]. When the shearing force is transformed to Cartesian coordinates, following equations can be obtained: πΉπΉππ πΌπΌπΌπΌ = πΉπΉπ π cos ππππ cos ππππ cos(ππππ + ππππ ) πΉπΉππ πΌπΌπΌπΌ = πΉπΉπ π cos ππππ sin ππππ cos(ππππ + ππππ ) πΉπΉππ πΌπΌπΌπΌ = βπΉπΉπ π sin ππππ 73 (5.7) (5.8) (5.9) Chapter 5. Generalized Mechanics of Metal Cutting Figure 5-4: Cutting Forces, Velocities, and Angles in Oblique Cutting [11]. In order to use Eq. (5.7), Eq. (5.8), and Eq. (5.9), the unknown angles ππππ and ππππ must be expressed with known angles. ππππ is the angle between cut surface and the resultant cutting force πΉπΉ. Due to the simplified geometry in Figure 5-5: ππππ = π½π½ππ β πΎπΎππ 74 (5.10) Chapter 5. Generalized Mechanics of Metal Cutting Similarly, the tangent of ππππ is equal to the following: πΉπΉπ¦π¦πΌπΌπΌπΌ tan ππππ = πΉπΉπ π π π (5.11) where πΉπΉπ π π π is the projection of shear force πΉπΉπ π onto normal plane ππππ . Figure 5-5: Illustration of angle π½π½ππ . (πππ΅π΅ , ππππππ , and ππππππ Are Projections of Forces onto Normal Plane). In Figure 5-4, πΉπΉππ can be expressed as, πΉπΉππ = πΉπΉπ’π’ cos ππ πΉπΉπ π cos ππππ = sin π½π½ππ cos(ππππ + ππππ ) πΉπΉπ π sin ππππ πΉπΉπ’π’ = sin ππ (5.12) Combining equations (5.10) and (5.12) gives the following relationship: tan ππππ = tan ππ sin π½π½ππ cos(ππππ + π½π½ππ β πΎπΎππ ) 75 (5.13) Chapter 5. Generalized Mechanics of Metal Cutting And furthermore: cos ππππ = sin ππππ = cos(ππππ + π½π½ππ β πΎπΎππ ) οΏ½cos 2 (ππππ + π½π½ππ β πΎπΎππ ) + tan2 ππ sin2 π½π½ππ tan ππ sin π½π½ππ (5.14) οΏ½cos 2 (ππππ + π½π½ππ β πΎπΎππ ) + tan2 ππ sin2 π½π½ππ Combining equations (5.7), (5.8), (5.9), and (5.10) with (5.14) results in: πΉπΉππ πΌπΌπΌπΌ = πΉπΉπ π = ππβ οΏ½ οΏ½cos 2 (ππππ + π½π½ππ β πΎπΎππ ) + tan2 ππ sin2 π½π½ππ cos(π½π½ππ β πΎπΎππ ) πππ π οΏ½ cos πππ π sin ππππ οΏ½cos 2 (ππππ + π½π½ππ β πΎπΎππ ) + tan2 ππ sin2 π½π½ππ πΉπΉππ πΌπΌπΌπΌ = πΉπΉπ π = ππβ οΏ½ cos(π½π½ππ β πΎπΎππ ) sin(π½π½ππ β πΎπΎππ ) (5.15) οΏ½cos 2 (ππππ + π½π½ππ β πΎπΎππ ) + tan2 ππ sin2 π½π½ππ sin(π½π½ππ β πΎπΎππ ) πππ π οΏ½ cos πππ π sin ππππ οΏ½cos 2 (ππππ + π½π½ππ β πΎπΎππ ) + tan2 ππ sin2 π½π½ππ Similarly, by substituting equations (5.6) and (5.16) into Equation (5.8): πΉπΉππ πΌπΌπΌπΌ = βπΉπΉπ π tan ππ sin π½π½ππ οΏ½cos 2 (ππππ + π½π½ππ β πΎπΎππ ) + tan2 ππ sin2 π½π½ππ tan ππ sin π½π½ππ πππ π = ππβ οΏ½β οΏ½ cos πππ π sin ππππ οΏ½cos 2 (ππππ + π½π½ππ β πΎπΎππ ) + tan2 ππ sin2 π½π½ππ (5.16) Corresponding specific cutting coefficients in ππ πΌπΌπΌπΌ , ππ πΌπΌπΌπΌ , and ππ πΌπΌπΌπΌ directions can be eva- luated as: πΎπΎππ πΌπΌπΌπΌ = οΏ½ cos(π½π½ππ β πΎπΎππ ) πππ π οΏ½ cos πππ π sin ππππ οΏ½cos 2 (ππππ + π½π½ππ β πΎπΎππ ) + tan2 ππ sin2 π½π½ππ πΎπΎππ πΌπΌπΌπΌ = οΏ½β πΎπΎππ πΌπΌπΌπΌ = οΏ½ tan ππ sin π½π½ππ πππ π οΏ½ cos πππ π sin ππππ οΏ½cos 2 (ππππ + π½π½ππ β πΎπΎππ ) + tan2 ππ sin2 π½π½ππ sin(π½π½ππ β πΎπΎππ ) πππ π οΏ½ cos πππ π sin ππππ οΏ½cos 2 (ππππ + π½π½ππ β πΎπΎππ ) + tan2 ππ sin2 π½π½ππ 76 (5.17) Chapter 5. Generalized Mechanics of Metal Cutting The cutting coefficients are transformed from the cutting edge coordinate system (System 2) to chip flow coordinate system (System 4) to calculate the cutting coefficients πΎπΎπ’π’ and πΎπΎπ£π£ . It is possible to accomplish this task in two different ways. Firstly, these coefficients in the cutting edge coordinate system can be geometrically trans- formed into the chip flow coordinate system as described in Chapter 4.3. The transformation matrix required for this task πΆπΆ42 can be obtained by: πΆπΆ42 = (πΆπΆ23 πΆπΆ34 )ππ Hence: οΏ½ As a result: πΎπΎππ πΌπΌπΌπΌ πΎπΎπ’π’ οΏ½ οΏ½ = πΆπΆ42 οΏ½πΎπΎππ πΌπΌπΌπΌ οΏ½ πΎπΎπ£π£ πΎπΎππ πΌπΌπΌπΌ sin πΎπΎππ cos ππ πΎπΎπ’π’ οΏ½=οΏ½ πΎπΎπ£π£ cos πΎπΎππ β sin ππ 0 πΎπΎππ πΌπΌπΌπΌ cos πΎπΎππ cos ππ οΏ½ οΏ½πΎπΎππ πΌπΌπΌπΌ οΏ½ β sin πΎπΎππ πΎπΎππ πΌπΌπΌπΌ πΎπΎπ’π’ = sin πΎπΎππ cos ππ πΎπΎππ πΌπΌπΌπΌ β sin ππ πΎπΎππ πΌπΌπΌπΌ + cos πΎπΎππ cos ππ πΎπΎππ πΌπΌπΌπΌ πΎπΎπ£π£ = cos πΎπΎππ πΎπΎππ πΌπΌπΌπΌ β sin πΎπΎππ πΎπΎππ πΌπΌπΌπΌ (5.18) (5.19) (5.20) Alternatively, specific cutting coefficients in chip flow coordinate system can be calculated by evaluating the resultant force: πΉπΉ = οΏ½πΉπΉππ2πΌπΌπΌπΌ + πΉπΉππ2πΌπΌπΌπΌ + πΉπΉππ2πΌπΌπΌπΌ (5.21) By assuming that the average friction angle (π½π½ππ ) is equal to the friction angle (π½π½) in orthogonal cutting [5]. πΉπΉ sin π½π½ππ ππβ πΉπΉ cos π½π½ππ πΎπΎπ£π£ = ππβ πΎπΎπ’π’ = 77 (5.22) Chapter 5. Generalized Mechanics of Metal Cutting Figure 5-6 illustrates and summarizes the procedure outlined above. As result, specific cutting coefficients πΎπΎπ’π’ and πΎπΎπ£π£ on friction and normal directions can be evaluated with the following equations: πΎπΎπ’π’ = πΎπΎπ£π£ = πππ π οΏ½1 β tan2 ππ sin2 π½π½ππ cos πππ π sin ππππ οΏ½cos 2 (ππππ + π½π½ππ β πΎπΎππ ) + tan2 ππ sin2 π½π½ππ πππ π οΏ½1 β tan2 ππ sin2 π½π½ππ cos πππ π sin ππππ οΏ½cos 2 (ππππ + π½π½ππ β πΎπΎππ ) + tan2 ππ sin2 π½π½ππ sin π½π½ππ (5.23) ππππππ π½π½ππ Or applying the geometrical transformation in Equation (5.20): πΎπΎπ’π’ = πππ π ππππππππ(sin πΎπΎππ cos(π½π½ππ β πΎπΎππ ) + tan2 ππ sin π½π½ππ + cos πΎπΎππ sin(π½π½ππ β πΎπΎππ )) πΎπΎπ£π£ = cos πππ π sin ππππ οΏ½cos 2 (ππππ + π½π½ππ β πΎπΎππ ) + tan2 ππ sin2 π½π½ππ πππ π cos πΎπΎππ cos(π½π½ππ β πΎπΎππ ) (1 β tan πΎπΎππ tan(π½π½ππ β πΎπΎππ )) cos πππ π sin ππππ οΏ½cos 2 (ππππ + π½π½ππ β πΎπΎππ ) + tan2 ππ sin2 π½π½ππ Shear Parameters From Orthogonal Cutting Data Cutting Conditions (5.24) ππππ , πππ π , π½π½ππ (5.15) ππ, β Tool Geometry And Assumed Chip Flow Direction (5.6) πΉπΉπ π (5.16) πΉπΉπππΌπΌπΌπΌ πΉπΉπππΌπΌπΌπΌ πΉπΉπππΌπΌπΌπΌ (5.19) πΎπΎπ’π’ πΎπΎπ£π£ πππ π , πΎπΎππ , ππ Figure 5-6: Summary of the Classical Approach in Cutting Coefficient Identification. In summary, cutting coefficients in friction and normal directions can be evaluated using orthogonal shear parameters by Eq. (5.24). After obtaining the cutting coefficients, cutting force model explained in Section 5.2 can be applied. The edge coefficients πΎπΎπ’π’π’π’ and πΎπΎπ£π£π£π£ can be calculated by transforming the edge coefficients from π π π π π π coordinate system to ππππ coordinate system. Alternatively, they can be added when the cutting forces are calculated and transformed back to the π π π π π π coordinate system or machine coordinate system. In the simulations and validations shown in the following sections, second method has been used. 78 Chapter 5. Generalized Mechanics of Metal Cutting 5.3.2 Mechanistic Identification Currently, the most commonly used approach to determine the cutting coefficients is the mechanistic identification. Although this approach cannot provide the detailed microscopic effects of the machining process, such as shearing and chip flow, it allows predicting the cutting forces without extensive turning tests. However, in mechanistic modeling, accurate determination of these cutting coefficients over a wide range of cutting conditions is time consuming, since it requires a large number of experiments with many parameters such as cutting tool β workpiece material combination, cutting tool geometry, cutting speed, and depth of cut. In the following sections, two different cases to identify the cutting coefficients πΎπΎπ’π’ and πΎπΎπ£π£ are discussed. Empirical Approach In the first case, calculations that are needed to identify the cutting coefficients from experiments are presented. By using this method, it is also possible to calculate the true chip flow angle (ππ). Since edge forces have no effect on shearing, they must be calculated and subtracted from total cutting forces in radial, tangential, and axial directions [5]. Thus, it is possible to reduce the cutting force equations as the following: πΉπΉππππ = πΎπΎππ π΄π΄ππ πΉπΉπ‘π‘π‘π‘ = πΎπΎπ‘π‘ π΄π΄ππ (5.25) πΉπΉππππ = πΎπΎππ π΄π΄ππ where πΉπΉππππ is the cutting force in ππ β (ππ, π‘π‘, ππ) direction, πΎπΎππ is the specific cutting coefficient in ππ direction, and π΄π΄ππ is the chip load. In order to evaluate the coefficients in normal and friction directions, the transformation equations derived in Chapter 4.3 are applied. πΎπΎππ = (πΎπΎπ’π’ cos πΎπΎππ cos ππ β πΎπΎπ£π£ sin πΎπΎππ ) πΎπΎπ‘π‘ = (πΎπΎπ’π’ sin Ξ»s sin ππ + πΎπΎπ’π’ cos πππ π sin πΎπΎππ cos ππ + πΎπΎπ£π£ cos πππ π cos πΎπΎππ ) πΎπΎππ = (βπΎπΎπ’π’ cos πππ π sin ππ + πΎπΎπ’π’ sin πππ π sin πΎπΎππ cos ππ + πΎπΎπ£π£ sin πππ π cos πΎπΎππ ) 79 (5.26) Chapter 5. Generalized Mechanics of Metal Cutting In Equation (5.26), πΎπΎππ , πΎπΎπ‘π‘ , and πΎπΎππ can be obtained from cutting tests, normal rake (πΎπΎππ ) and oblique / helix angle (πππ π ) can be obtained from tool geometry, and πΎπΎπ’π’ , πΎπΎπ£π£ , and chip flow angle (ππ) are unknown. There are three equations and three unknowns, however, chip flow angle (ππ) is nonlinear. To overcome this problem, two new variables, πΎπΎπ’π’1 and πΎπΎπ’π’2 were introduced: πΎπΎπ’π’ 1 = πΎπΎπ’π’ cos ππ πΎπΎπ’π’ 2 = πΎπΎπ’π’ sin ππ (5.27) Substituting Equation (5.27) into Equation (5.26): where πΎπΎππ = πΆπΆπ’π’π’π’ 1 πΎπΎπ’π’ 1 + πΆπΆπ£π£π£π£ πΎπΎπ£π£ πΎπΎπ‘π‘ = πΆπΆπ’π’π’π’ 1 πΎπΎπ’π’ 1 + πΆπΆπ’π’π’π’ 2 πΎπΎπ’π’ 2 + πΆπΆπ£π£π£π£ πΎπΎπ£π£ πΎπΎππ = πΆπΆπ’π’π’π’ 1 πΎπΎπ’π’ 1 + πΆπΆπ’π’π’π’ 2 πΎπΎπ’π’ 2 + πΆπΆπ£π£π£π£ πΎπΎπ£π£ (5.28) πΆπΆπ’π’π’π’ 1 = cos πΎπΎππ , πΆπΆπ£π£π£π£ = βsin πΎπΎππ , πΆπΆπ’π’π’π’ 1 = cos πππ π sin πΎπΎππ , πΆπΆπ’π’π’π’ 2 = sin Ξ»s , πΆπΆπ£π£π£π£ = cos πππ π cos πΎπΎππ πΆπΆπ’π’π’π’ 1 = sin πππ π sin πΎπΎππ , πΆπΆπ’π’π’π’ 2 = βcos πππ π , πΆπΆπ£π£π£π£ = sin πππ π cos πΎπΎππ These coefficients are only functions of tool geometry, and they are constant for a specific experiment. As a result, there are 3 linear equations with three unknowns. In matrix form: πΆπΆπ’π’π’π’ 1 πΎπΎππ οΏ½ πΎπΎπ‘π‘ οΏ½ = οΏ½ πΆπΆπ’π’π’π’ 1 πΎπΎππ πΆπΆπ’π’π’π’ 1 that: 0 πΆπΆπ’π’π’π’ 2 πΆπΆπ’π’π’π’ 2 πΆπΆπ£π£π£π£ πΎπΎπ’π’ 1 πΆπΆπ£π£π£π£ οΏ½ οΏ½πΎπΎπ’π’ 2 οΏ½ πΆπΆπ£π£π£π£ πΎπΎπ£π£ (5.29) Solving these equations in Eq. (5.29) results in πΎπΎπ’π’1 , πΎπΎπ’π’2 , and πΎπΎπ£π£ . It should be noted πΎπΎπ’π’ = οΏ½πΎπΎπ’π’ 12 + πΎπΎπ’π’ 22 ππ = tanβ1 πΎπΎπ’π’ 2 πΎπΎπ’π’ 1 (5.30) (5.31) Consequently, a set of simple experiments with known cutting conditions and tool geometry are sufficient to calculate the cutting coefficients and chip flow angle. Since most of the chip flow models in the literature are based on empirical equations, and the 80 Chapter 5. Generalized Mechanics of Metal Cutting theoretical models are difficult to apply and generalize, this method provides a quick way to determine the chip flow angle. During milling, chip thickness varies continuously as tool motion is a trochoidal motion. In order to apply proposed mechanistic identification method, cutting force coefficients can be obtained in terms of the average chip thickness defined as [45]: οΏ½οΏ½οΏ½ππ = π΄π΄ ππππππ ππππππ 1 οΏ½ π΄π΄ (ππ) ππππ β πππ π π π πππ π π‘π‘ ππ (5.32) where πππ π π π and ππππππ are tool entry and exit angles, respectively. Theoretical Approach In the second case, it has been assumed that the cutting coefficients in radial, tangential, and axial directions as well as tool geometry are known, but the objective is to calculate the shear parameters (shear stress πππ π , shear angle ππππ , and friction angle π½π½ππ ) only using this set of experiments. There have been two fundamental approaches for the solution of oblique cutting parameters, namely maximum shear stress principle and the minimum energy principle. Although these principles are iterative models, it is possible to obtain an approximate solution using Stablerβs chip flow assumption (ππ = πππ π ). Maximum Shear Stress Principle Maximum shear stress principle states that the resultant cutting force (πΉπΉ) makes an angle amount of (ππππ + π½π½ππ β πΎπΎππ ) with the shear plane, and the angle between the maximum shear stress and the principal stress is 45Β° [57]. So: πΉπΉπ π = πΉπΉ(cos ππππ cos(ππππ + ππππ ) cos ππππ + sin ππππ sin ππππ ) πΉπΉπ π = πΉπΉ cos 45Β° (5.33) Moreover, another statement of this principle dictates that the projection of the resultant force to the shear plane coincides with the shear direction. This results in: πΉπΉ(cos ππππ cos(ππππ + ππππ ) sin ππππ β sin ππππ cos ππππ ) = 0 Eq. (5.33) and Eq. (5.34) are used to derive the following relationships: 81 (5.34) Chapter 5. Generalized Mechanics of Metal Cutting ππππ = sinβ1 οΏ½β2 sin ππππ οΏ½ ππππ = cos β1 οΏ½ tan ππππ οΏ½ β ππππ tan ππππ (5.35) (5.36) Figure 5-4 illustrates the oblique cutting mechanics. In order to avoid dividing by zero for orthogonal cases, Eq. (5.36) can be rewritten as: ππππ = cos β1 οΏ½ 1 cos ππππ οΏ½ β ππππ β2 cos ππππ (5.37) Using Eq. (5.35), Eq. (5.37), and the tool geometry (πΎπΎππ , πππ π ), new chip flow angle (ππππ ) can be calculated using the velocity relation: ππππ = tanβ1 οΏ½ tan πππ π cos(ππππ β πΎπΎππ ) β cos πΎπΎππ tan ππππ οΏ½ sin ππππ (5.38) In order to calculate the true chip flow angle, following interpolation algorithm is applied by using new and previous chip flow angles: ππ(ππ) = ππ ππ(ππ β 1) + (1 β ππ) ππππ (5.39) Using the above equations, true chip flow angle can be calculated iteratively. In this equation, ππ is the convergence parameter. When the iteration is completed, the shear stress can be calculated as the following: πΉπΉ = οΏ½πΉπΉπ’π’2 + πΉπΉπ£π£2 = ππβοΏ½πΎπΎπ’π’2 + πΎπΎπ£π£2 πΉπΉπ π = πππ π π΄π΄π π = πππ π οΏ½ πππ π = οΏ½ β ππ οΏ½ = πΉπΉπΉπΉπΉπΉπΉπΉ 45Β° οΏ½οΏ½ cos πππ π sin ππππ πΎπΎπ’π’2 + πΎπΎπ£π£2 cos πππ π sin ππππ 2 (5.40) (5.41) (5.42) Results and discussions about this approach are given in the following sections. 82 Chapter 5. Generalized Mechanics of Metal Cutting Minimum Energy Principle In this approach work done by Shamoto and Altintas [11] is used. In their study, resultant cutting force is expressed as: πΉπΉ = πππ π ππβ [cos(ππππ + ππππ ) cos ππππ cos ππππ + sin ππππ sin ππππ ] cos πππ π sin ππππ (5.43) And the cutting power (πππ‘π‘ ) required during cutting can be expressed as: πππ‘π‘ = πΉπΉ(cos ππππ cos ππππ cos πππ π + sin ππππ sin πππ π ) ππ (5.44) where ππ is the cutting speed. Non-dimensional cutting power (πππ‘π‘β² ) can be derived by substituting Eq. (5.43) into Eq. (5.44). πππ‘π‘β² = cos ππππ tan ππππ tan πππ π πππ‘π‘ = πππππ π ππβ [cos(ππππ + ππππ ) cos ππππ + tan ππππ sin ππππ ] sin ππππ (5.45) Minimum energy principle states that the cutting power drawn must be minimal for a unique shear angle solution [11]. Therefore: πππππ‘π‘β² =0 ππππππ πππππ‘π‘β² =0 ππππππ (5.46) Using Stablerβs chip flow angle rule and maximum shear stress equations for the initial guess, shear angles (ππππ and ππππ ) are iterated to minimize the non-dimensional cutting power πππ‘π‘β² using the following iterative equations: οΏ½ ππππ (ππ) ππ (ππ β 1) Ξππβ² βΞππππ οΏ½ = οΏ½ ππ οΏ½ β ππ οΏ½ π‘π‘β² οΏ½ ππππ (ππ) ππππ (ππ β 1) Ξπππ‘π‘ βΞππππ (5.47) In this method, convergence depends on the step size ππ and the marginal increase during the perturbation (Ξππππ,ππ ). After the iterations are completed, shear stress can be calculated by the following equations: πΉπΉπ π = πΉπΉ[cos(ππππ + ππππ ) cos ππππ cos ππππ + sin ππππ sin ππππ ] πππ π = οΏ½πΎπΎπ’π’2 + πΎπΎπ£π£2 [cos(ππππ + ππππ ) cos ππππ cos ππππ + sin ππππ sin ππππ ] cos πππ π sin ππππ 83 (5.48) Chapter 5. Generalized Mechanics of Metal Cutting Enhancement of the Theoretical Models Stablerβs rule of chip flow is used as the initial guess in most theoretical models. However, since it is possible to obtain true chip flow angle using maximum shear stress and minimum energy principles, an iterative method was adapted to improve the calculation of chip flow angle and cutting coefficients. Block diagram of this enhancement is illustrated in Figure 5-7. Stablerβs rule (ππ = πππ π ) was used as the initial value and cutting coefficients in ππππ coordinate system were calculated by using geometrical transforma- tions. Then using the minimum energy or maximum shear stress principle, new chip flow angle was calculated and the procedure was repeated until the difference between the new and previous chip flow angle is less than the specified tolerance value. πΎπΎππ , πΎπΎπ‘π‘ , πΎπΎππ πΎπΎππ , πππ π Geometrical πΎπΎπ’π’ , πΎπΎπ£π£ Transformation Max. Shear Stress Min. Energy (5.39), (5.47) ππ Figure 5-7: Block Diagram of the Iterative Method to Enhance the Theoretical Models. By using the proposed method, it is possible to determine the inclination angle πππ π of the tool used to obtain the mechanistically determined cutting, πΎπΎππ , πΎπΎπ‘π‘ , and πΎπΎππ . In order to accomplish this task, following procedure was applied: 1. Cutting coefficients in radial, tangential, and axial directions were transformed to friction and normal coefficients πΎπΎπ’π’ and πΎπΎπ£π£ using the geometrical transformation equations described in Chapter 4.3. In order to complete the transformations, an arbitrary value for πππ π was selected. Since πππ π is unknown, this procedure must be repeated for each value of πππ π in a predetermined range. 2. Using the minimum energy principle, shear parameters (πππ π , ππππ , and π½π½ππ ) as well as true chip flow angle ππ were calculated. 84 Chapter 5. Generalized Mechanics of Metal Cutting 3. Following that, πΎπΎππ , πΎπΎπ‘π‘ , and πΎπΎππ were calculated back using the orthogonal to oblique transformation using the following equations [5]: πΎπΎππ = sin(π½π½ππ β πΎπΎππ ) πππ π sin ππππ cos πππ π οΏ½cos 2 (ππππ + π½π½ππ β πΎπΎππ ) + tan2 ππ sin2 π½π½ππ πΎπΎπ‘π‘ = πΎπΎππ = cos(π½π½ππ β πΎπΎππ ) + tan πππ π tan ππ sin π½π½ππ πππ π sin ππππ οΏ½cos 2 (ππππ + π½π½ππ β πΎπΎππ ) + tan2 ππ sin2 π½π½ππ (5.49) cos(π½π½ππ β πΎπΎππ ) tan πππ π β tan ππ sin π½π½ππ πππ π sin ππππ οΏ½cos 2 (ππππ + π½π½ππ β πΎπΎππ ) + tan2 ππ sin2 π½π½ππ 4. Calculated and original values of the cutting coefficients were compared and the errors for each πππ π were calculated using Eq. (5.50). % πΈπΈπΈπΈπΈπΈπΈπΈπΈπΈ = οΏ½πΎπΎππππππππππππππππππππ β πΎπΎππππππππππππππππ οΏ½ Γ 100 πΎπΎππππππππππππππππ (5.50) It can be observed that for a unique oblique angle, percentage error for axial cutting coefficient πΎπΎππ is minimal. Mechanistically determined cutting coefficients for two different workpiece mate- rials are used as inputs. Although the tool geometry (rake and helix angles) was known, helix angle (πππ π ) has been assumed to be unknown to check the validity of the proposed method. The first analysis was made on Ti6Al4V workpiece. Results of Budakβs cutting coefficient identification [58] were used. Tool used for the mechanistic calibration has 15Β° rake angle (πΎπΎππ = 15Β°) and 30Β° helix angle (πππ π = 30Β°). In order to apply the model, helix angle is assumed to be unknown and all angles between 0Β° and 40Β° were scanned in the analysis and errors were calculated using the procedure outlined above. Table 5-I presents the cutting coefficients and tool workpiece used in both cases. 85 Chapter 5. Generalized Mechanics of Metal Cutting Figure 5-8: Results of Helix Angle Predictions for Ti6Al4V. Table 5-I: Cutting Coefficients and Tool Geometry Used in the Analysis. Material Ti6Al4V Al7075 πΎπΎππ [MPa] πΎπΎπ‘π‘ [MPa] πΎπΎππ [MPa] Helix πππ π [Β°] Rake πΎπΎππ [Β°] 340 1630 608 30 15 788.83 1319.41 48.751 5 0 Second analysis was performed using the coefficients calculated in Enginβs study [44]. Workpiece material has been Al7075, and the tool used in mechanistic identification has been a bull-nose end mill with 5/4β diameter, 0Β° rake angle, and 5Β° helix angle. Cutting coefficients are listed in Table 5-I and error results are presented in Figure 5-9. 86 Chapter 5. Generalized Mechanics of Metal Cutting Figure 5-9: Results of Helix Angle Predictions for Al7075. It can be observed from Figure 5-8 and Figure 5-9 that πΎπΎππ is very sensitive to the changes in the helix angle and it is possible to accurately determine the helix angle by applying the proposed method. On the other hand, theoretical proof of this method is rather difficult, because chip flow angle also depends on the helix angle and the equations used in minimum energy principle is rather complex and they can be solved only using numerical methods. 87 Chapter 5. Generalized Mechanics of Metal Cutting Results and Summary Block diagram of the procedures explained in this section is illustrated in Figure 5-10. As a case study, to validate the models discussed in this section, mechanistically determined average cutting coefficients in π π π π π π coordinate system were transformed to ππππ coordinate system and shear parameters were calculated using both maximum shear stress principle and minimum energy principle. Table 5-II summarizes the results: Table 5-II: Comparison of the Methods for the Solution of Oblique Shear Parameters. Maximum Shear Stress Principle Cutting Tool CTGP SCGP MTGN Orthogonal Rake Angle πΎπΎππ [Β°] 5 3.5 -5 5 Oblique Angle πππ π [Β°] 0 3.5 -5 0 Friction Angle π½π½ππ [Β°] 26.9 32 20.3 28.1 Shear Angle ππππ [Β°] 23 16.5 17.1 31.2 Shear Stress πππ π [MPa] 508 459.2 477.8 549 Chip Flow Angle ππ [Β°] 0 3.3 -5.4 0 Comp. Time [ms] # of Iterations 6.6 ms 6.4 ms 7.4 ms - 2 31 14 - Comp. Time [ms] # of Iterations 13.2 14.5 14.4 - 140 115 124 - Minimum Energy Principle Cutting Tool CTGP SCGP MTGN Orthogonal Rake Angle πΎπΎππ [Β°] 5 3.5 -5 5 Oblique Angle πππ π [Β°] 0 3.5 -5 0 Friction Angle π½π½ππ [Β°] 26.9 32 20.3 28.1 Shear Angle ππππ [Β°] 33.1 30.1 31.6 31.2 Shear Stress πππ π [MPa] 573.8 596 600 549 Chip Flow Angle ππ [Β°] 0 1.72 -3.1 0 Workpiece used in these experiments was AISI Steel 1045, and Kennametal tools were used. Cutting speed (ππ) was 150 m/min and the width of cut (ππ) was 3.05 mm. Highlighted row is the experimentally measured orthogonal reference data. From Table 5-II, it can be observed that the minimum energy principle gives more accurate results than the maximum shear stress principle, since maximum shear stress principle is more conservative approach. However, minimum energy principle needs more iteration to calculate the shear parameters than the maximum shear stress principle. 88 Chapter 5. Generalized Mechanics of Metal Cutting Input Angles π½π½ππ Force Relation (5.35) & (5.36) πΎπΎππ πππ π ππ (5.39) Velocity Relation (5.38) ππππ , ππππ Max. Shear Stress (5.35) & (5.37) Minimum Energy (5.45) & (5.46) ππππ , ππππ Figure 5-10: Theoretical Identification Procedure. In mechanistic modeling, it is possible to determine the cutting coefficients by using a single test. Therefore, it provides an opportunity to observe the effect of chip thickness on the cutting coefficients. On the other hand, theoretical methods use average cutting coefficients, in which one set of coefficients can be obtained by conducting tests with different chip thicknesses. Figure 5-11 demonstrates this argument with SCGP tool. Tool geometry and cutting conditions are listed above. In Figure 5-11, it can be observed that specific cutting coefficients increase exponentially as the chip thickness decreases. This phenomenon is called size effect. Figure 5-12 shows the comparison of chip flow angle of different methods. Stabler rule states that the chip flow angle is equal to the inclination angle πππ π . It can be seen that according to the mechanistic approach, there is also an exponential relationship between chip thickness and chip flow angle, similar to the size effect. 89 Chapter 5. Generalized Mechanics of Metal Cutting Figure 5-11: Effect of Chip Thickness on the Cutting Coefficients. Figure 5-12: Effect of Chip Thickness on the Chip Flow Angle. 90 Chapter 5. Generalized Mechanics of Metal Cutting 5.4 Cutting Force Simulations and Validations Proposed insert geometry and mechanics models discussed in previous chapter are combined to simulate cutting forces in turning and milling using different tools. Several different insert geometries are considered for each type of cutting process. 5.4.1 Turning Process Simulations To validate the model in turning, two different cases are selected. First case is orthogonal cutting with a triangular insert, and the second case is oblique cutting with a square insert. Experimental Setup Validation experiments for turning process have been performed on Hardinge Superslant lathe. The workpiece material was AISI Steel 1045 bar with 255HB hardness. The diameter of the workpiece was 38.1 mm. Kistler three component dynamometer (Model 9121) and charge amplifier were used to measure the cutting forces. The dynamometer was mounted on the turret as shown in Figure 5-13. Displaying and recording of the measured data were performed with data acquisition software, MalDAQ module of CutPro 8.0. Figure 5-14 shows a sample output screen of MalDAQ. 91 Chapter 5. Generalized Mechanics of Metal Cutting Figure 5-13: Workpiece and 3- Component Dynamometer Fixed to Turret for Cutting Tests. Figure 5-14: Sample Output Screen in CutPro 8.0 for Cutting Forces. 92 Chapter 5. Generalized Mechanics of Metal Cutting Orthogonal Cutting Tests A carbide triangular insert (TPG322) from Kennametal and a shank style tool holder (CTGP) with 5Β° normal rake angle (πΎπΎππ ), 75Β° cutting edge angle (π π ππ ) and 0Β° inclination angle (πππ π ) was used in orthogonal cutting tests. The influence of nose radius was avoided by using a tube with a wall thickness of 3.05 mm. Orthogonal cutting parameters were used to calculate cutting coefficients in friction and normal directions at each selected point on the cutting edge. The tests were performed at different cutting speeds and feed rates. Cutting conditions for orthogonal cutting validation experiments are summarized in Table 5-III. As the cutting speed changes along the cutting edge, cutting coefficients become different at each differential disk on the tool. Therefore, at each differential disk, cutting coefficients πΎπΎπ’π’ and πΎπΎπ£π£ are calculated by using the following equations: ππππ = ππ (0.6123 ββ0.6988) π½π½ππ = ππ (β0.1515 ββ0.5453) (5.51) πππ π = ππ 0.0417 ββ0.0079 ππ+6.2938 The model is validated by comparing the predicted forces against new experimental results as shown in Simulated and experimental forces are plotted versus chip thickness in Figure 5-15 and Figure 5-16. Table 5-III: Orthogonal Turning Validation Experiments Cutting Conditions. Tool Geometry π³π³ (Insert Length) 12 mm πΏπΏππ (Cutting Edge Angle) 75Β° ππππ (Insert Width) ππππ (Wiper Edge Length) πππΊπΊ (Corner Radius) πΈπΈππ (Radial Rake Angle ) - πΈπΈππ (Axial Rake Angle) - 0Β° 0Β° πΊπΊππ (Tool Included Angle) 60Β° - Cutting Conditions Width Of Cut 3.05 mm Feed Rate 0.1 β 0.3 mm/rev Cutting Speed 100 β 150 m/min Workpiece Material AISI 1045 Steel 93 Chapter 5. Generalized Mechanics of Metal Cutting Figure 5-15: Forces simulated and measured for an orthogonal tool with 75Β° cutting edge angle for V=100 m/min. Figure 5-16: Forces simulated and measured for an orthogonal tool with 75Β° cutting edge angle for V=150 m/min. 94 Chapter 5. Generalized Mechanics of Metal Cutting Oblique Cutting Tests After obtaining satisfactory validation results in orthogonal cutting validation tests, mechanics model has been tested for oblique tools with nose radius. Several tests were performed with different tools and the results are presented in the following sections. Rhombic (ISO Style C) Insert: A rhombic insert with a tool included angle of 80Β° with -8Β° rake angle and -8Β° inclination angle was simulated and tested. The tool holder was Sandvik DCKNL 2020K 12, and the insert was Sandvik CNMA 12 04 08 rhombic insert with 0.8 mm nose radius. Insert geometry and cutting conditions are listed in Table 5-IV. Table 5-IV: Tool Geometry and Cutting Conditions for Rhombic Insert. Tool Geometry π³π³ (Insert Length) 12 mm ππππ (Wiper Edge Length) - ππππ (Insert Width) πππΊπΊ (Corner Radius) πΏπΏππ (Cutting Edge Angle) 75Β° πΈπΈππ (Radial Rake Angle ) - πΈπΈππ (Axial Rake Angle) -8Β° -8Β° 0.8 mm πΊπΊππ (Tool Included Angle) 80Β° Cutting Conditions Width Of Cut 2 β 4 mm Feed Rate 0.1 β 0.24 mm/rev Cutting Speed 150 m/min Workpiece Material AISI 1045 Steel Figure 5-17: Illustration of Insert and the Holder Body [55]. 95 Chapter 5. Generalized Mechanics of Metal Cutting The equations for the control points of this type of insert are given in Section 4.5.3. Hence, the locations of the geometric control points are (Figure 5-18): Global coordinate system origin (Point A): π΄π΄π₯π₯ = 0, π΄π΄π§π§ = 0 (5.52) Insert center (Point I): πΌπΌπ₯π₯ = β3.359 , πΌπΌπ§π§ = 8.076 (5.53) Insert cutting reference point (Point CRP): πΆπΆπΆπΆπΆπΆπ₯π₯ = 0.613 , πΆπΆπΆπΆπΆπΆπ§π§ = 0 (5.54) Control Points (Points A, B, and D): π΄π΄π₯π₯ = 0, π΅π΅π₯π₯ = 0.772, π·π·π₯π₯ = 3.651, π΄π΄π§π§ = 0 π΅π΅π§π§ = 0.592 (5.55) π·π·π§π§ = 11.336 Using the orientation angles (π π ππ = 75Β°, πΎπΎππ = πΎπΎππ = β8Β°), control points are trans- formed and described in the global coordinate system by implementing the procedure presented in Section 4.3. After the transformations, the locations of the control points become: π΄π΄π₯π₯ = 0, π΄π΄π¦π¦ = 0, π΄π΄π§π§ = 0 π΅π΅π₯π₯ = 0.753, π΅π΅π¦π¦ = β0.189, π΅π΅π§π§ = 0.587 π·π·π₯π₯ = 3.396, π·π·π¦π¦ = β2.070, π·π·π§π§ = 11.226 96 (5.56) Chapter 5. Generalized Mechanics of Metal Cutting Figure 5-18: Geometric Control Points of a Rhombic Turning Insert. For the arc between points A and B, following parametric arc equation is employed: πππ΄π΄π΄π΄ = π π cos π‘π‘ π’π’ οΏ½β + π π sin π‘π‘ πποΏ½β Γ π’π’ οΏ½β + πΆπΆ, 0 β€ π‘π‘ β€ π π = 0.8, ππ 2 0.019 β0.138 β0.015 π’π’ οΏ½β = οΏ½ 0.138 οΏ½, πποΏ½β = οΏ½β0.980οΏ½ , πΆπΆ = οΏ½β0.110οΏ½ β0.99 β0.139 0.792 0.015 cos π‘π‘ + 0.792 sin π‘π‘ β 0.015 πππ΄π΄π΄π΄ = οΏ½0.110 cos π‘π‘ β 0.111 sin π‘π‘ β 0.110οΏ½ 0.792 β 0.792 cos π‘π‘ 97 (5.57) Chapter 5. Generalized Mechanics of Metal Cutting For the linear edge between points B and D, linear parametric equation was used: πππ΅π΅π΅π΅ = πποΏ½βπ΅π΅ + οΏ½πποΏ½βπ·π· β πποΏ½βπ΅π΅ οΏ½ π‘π‘ , 0 β€ π‘π‘ β€ 1 3.396 0.753 οΏ½β οΏ½β πππ΅π΅ = οΏ½β0.189οΏ½, πππ·π· = οΏ½β2.070οΏ½ 11.226 0.587 πππ΅π΅π΅π΅ (5.58) 2.642 π‘π‘ + 0.753 = οΏ½β1.881 π‘π‘ β 0.189οΏ½ 10.639 π‘π‘ + 0.587 By changing the parameter π‘π‘ from 0 to 1 for each equation, the location of any point on the cutting edge can be calculated. Moreover, using the equations presented in Section 4.4, physical angles along the cutting edge were calculated. Finally, orthogonal parameters shown in (5.51) were used to calculate cutting coefficients πΎπΎπ’π’ and πΎπΎπ£π£ . Cutting forces were calculated using the rake face based force model which is described in Section 5.2. Simulated and experimental cutting forces for rhombic insert are shown in Figure 5-19 and Figure 5-20. The simulated forces showed good agreement with the experimental forces. 98 Chapter 5. Generalized Mechanics of Metal Cutting Figure 5-19: Comparison of Measured and Calculated Forces for the Rhombic Tool while the Width of Cut is 2 mm. Figure 5-20: Comparison of Measured and Calculated Forces for the Rhombic Tool while the Width of Cut is 4 mm. 99 Chapter 5. Generalized Mechanics of Metal Cutting Square (ISO Style S) Insert: Next tool used to validate the cutting force model in turning was a Sandvik DSBNL 2020K 12 holder with 75Β° cutting edge angle, -6Β° rake and inclination angle and Sandvik SNMA 12 04 08 insert with 0.8 mm corner radius. Geometry inputs and experiment conditions are listed in Table 5-V. Tool geometry is illustrated in Figure 5-21. Similar to rhombic insert 2 sets of experiments were conducted with 2 mm and 4 mm widths of cut. Using the same procedure described in the previous example, position of each selected point is calculated with the associated physical angles. Orthogonal cutting parameters shown in Eq. (5.51) and same cutting force model used to simulate the cutting forces. Although the validations are not as accurate as orthogonal cutting, results are satisfactory. The main reason behind the error is that cutting coefficients were determined using a positive rake tool, however the tools used in oblique cutting have negative rake angles. Following figures can be seen to compare the simulations and measured forces. Table 5-V: Tool Geometry and Cutting Conditions for Square Insert. Tool Geometry π³π³ (Insert Length) 12 mm ππππ (Wiper Edge Length) - ππππ (Insert Width) πππΊπΊ (Corner Radius) πΏπΏππ (Cutting Edge Angle) 75Β° πΈπΈππ (Radial Rake Angle ) - πΈπΈππ (Axial Rake Angle) -6Β° -6Β° 0.8 mm πΊπΊππ (Tool Included Angle) 90Β° Cutting Conditions Width Of Cut 2 β 4 mm Feed Rate 0.12 β 0.24 mm/rev Cutting Speed 150 m/min Workpiece Material AISI 1045 Steel 100 Chapter 5. Generalized Mechanics of Metal Cutting Figure 5-21: Illustration of Insert and the Holder Body [55]. Figure 5-22: Comparison of Measured and Calculated Forces for the Square Insert while the Width of Cut is 2 mm. 101 Chapter 5. Generalized Mechanics of Metal Cutting Figure 5-23: Comparison of Measured and Calculated Forces for the Square Insert while the Width of Cut is 4 mm. 5.4.2 Milling Process Simulations Experimental Setup The experiments for milling force validations have been performed on Mori Seiki NMV5000DCG 5 β axis machining center with 20000 rpm spindle. Various inserted milling tools were used from Sandvik Coromant. The workpiece material was aluminum blocks (Al7050-T451) with dimensions of 160x100x100 mm. Kistler three component dynamometer (Model 9257B) and a charge amplifier was used to measure the cutting forces. The dynamometer was mounted on the machine table using fixtures and the aluminum block was attached to the dynamometer as seen in Figure 5-24. Similarly, displaying and recording the measured data was performed with data acquisition software MalDAQ module of CutPro 9.0. The complete actual testing environment is illustrated in Figure 5-25. 102 Chapter 5. Generalized Mechanics of Metal Cutting Various tests were performed for validation of the proposed force model. Differential cutting edge disk height and the differential rotation angle in simulations were selected to be 0.01 mm and 4Β° respectively which adequately resembled the actual cutting conditions for the mathematical cutting force model. The applied conditions and the results of the performed milling tests are described throughout this section. The cutting coefficients were described as functions of rake angle, cutting speed and chip thickness. As a result, for each differential cutting edge element, corresponding orthogonal parameter were calculated and converted to the cutting coefficients in normal and friction directions individually. The list of shear parameters used in the mathematical model is given as: ππππ = 19.4004 + 42.0174 β + 0.02 ππ + 0.3842 πΎπΎππ πππ π = 266.8047 + 174.1289 β β 0.0437 ππ + 0.8961 πΎπΎππ (5.59) π½π½ππ = 25.8772 β 1.2837 β β 0.0075 ππ + 0.1818 πΎπΎππ Figure 5-24: Workpiece and 3-Component Dynamometer Fixed to Machine Table for Cutting Tests. 103 Chapter 5. Generalized Mechanics of Metal Cutting Figure 5-25: Illustration of the Experimental Setup for Measurement of Cutting Forces. Shoulder End Mill with Rectangular Insert A two fluted inserted cutter with built-in HSK holder and 25 mm diameter was selected. The cutter body is Sandvik R790-025HA06S2-16L and the inserts are R790160408PH-PL with 16 mm insert length, 1 mm wiper edge, and 0.8 mm corner radius. Cutter body has 10Β° axial rake angle, 8Β° radial rake angle, and 90Β° cutting edge angle. Details of the cutter body can be seen in Figure 5-26. Tool 25 mm End Mill w/ 0.8 mm Nose Radius Axial Rake: 10Β° Radial Rake: 8Β° Insert Rake: 0Β° Insert Helix: 0Β° Half Immersion Full Immersion Width of Cut: 12.5 mm 25 mm Depth Of Cut: 4 mm 4 mm Material: Al7050-T541 Al7050-T541 Spindle Speed: 1500 rpm 2000 rpm Feed Rate : 0.08 mm/tooth 0.1 mm/tooth Figure 5-26: 25 mm Diameter Shoulder End Mill from Sandvik Coromant [55]. 104 Chapter 5. Generalized Mechanics of Metal Cutting Control Points in global coordinate system (A, B, D, and E) (Figure 5-27): π΄π΄π₯π₯ = 11.2, π΅π΅π₯π₯ = 11.7, π·π·π₯π₯ = 12.5, πΈπΈπ₯π₯ = 12.5, π΄π΄π§π§ = 0 π΅π΅π§π§ = 0 π·π·π§π§ = 0.8 (5.60) πΈπΈπ§π§ = 16 Using the orientation angles (π π ππ = 90Β°, πΎπΎππ = 10Β°, πΎπΎππ = 8Β°), control points are trans- formed using Eq. (4.5). After the transformations, the locations of the control points become: π΄π΄π₯π₯ = 11.212, π΄π΄π¦π¦ = β0.180, π΄π΄π§π§ = 0 π΅π΅π₯π₯ = 11.707, π΅π΅π¦π¦ = β0.111, π΅π΅π§π§ = 0 π·π·π₯π₯ = 12.480, π·π·π¦π¦ = 0.137, π·π·π§π§ = 0.787 (5.61) πΈπΈπ₯π₯ = 12.113, πΈπΈπ¦π¦ = 2.751, πΈπΈπ§π§ = 15.756 In this example there are two linear edges (|π΄π΄π΄π΄| and |π·π·π·π·|) and one arc (|π΅π΅π΅π΅|). Therefore, two linear and one arc parametric equations are used: πππ΄π΄π΄π΄ = πποΏ½βπ΄π΄ + οΏ½πποΏ½βπ΅π΅ β πποΏ½βπ΄π΄ οΏ½ π‘π‘ , 0 β€ π‘π‘ β€ 1 πππ·π·π·π· = πποΏ½βπ·π· + οΏ½πποΏ½βπΈπΈ β πποΏ½βπ·π· οΏ½ π‘π‘ , 0 β€ π‘π‘ β€ 1 πππ΄π΄π΄π΄ πππ·π·π·π· 0.495 π‘π‘ + 11.212 = οΏ½ 0.069 π‘π‘ β 0.180 οΏ½ 0 β0.367 π‘π‘ + 12.480 = οΏ½ 2.613 π‘π‘ + 0.137 οΏ½ 14.969 π‘π‘ + 0.787 105 (5.62) Chapter 5. Generalized Mechanics of Metal Cutting Figure 5-27: Illustration of Control Points for the Shoulder Milling Insert. πππ΅π΅π΅π΅ = π π cos π‘π‘ π’π’ οΏ½β + π π sin π‘π‘ πποΏ½β Γ π’π’ οΏ½β + πΆπΆ, 0 β€ π‘π‘ β€ π π = 0.8, ππ 2 0.024 0.137 11.688 π’π’ οΏ½β = οΏ½β0.172οΏ½, πποΏ½β = οΏ½β0.975οΏ½ , πΆπΆ = οΏ½ 0.026 οΏ½ β0.984 0.173 0.787 (5.63) 0.019 cos π‘π‘ + 0.792 sin π‘π‘ + 11.688 πππ΅π΅π΅π΅ = οΏ½0.111 cos π‘π‘ β 0.137 sin π‘π‘ + 0.0262οΏ½ 0.787 β 0.787 cos π‘π‘ Using πππ΄π΄π΄π΄ , πππ΅π΅π΅π΅ , and πππ·π·π·π· , local radii and physical angles of each selected point on the cutting edge were calculated and cutting forces were simulated with the same force model used in turning simulations. Validations of cutting forces were performed for half immersion up milling and full immersion slot cutting tests. In simulations, orthogonal parameters shown in Eq. (5.59) are used. The tests were conducted for different feed rates and spindle speeds. Cutting conditions for validation tests are summarized also in Figure 5-26. Simulated and experimental cutting forces are plotted versus time for two revolution of the tool in Figure 5-28 and Figure 5-29 for 4 mm axial depth of cut. 106 Chapter 5. Generalized Mechanics of Metal Cutting Although there was a forced vibration during cutting tests due to the vibration of the dynamometer, simulated cutting force patterns and their magnitudes showed very good agreement with the measurements. The tool was two fluted end mill, as a result two peaks can be observed in a single rotation of the tool. For consistency, cutting force of two rotations (720Β°) of the tool is presented for each width of cut. Figure 5-28: Measured and Predicted Forces for Al7050 Half Immersion Up-Milling. 107 Chapter 5. Generalized Mechanics of Metal Cutting Figure 5-29: Measured and Predicted Forces for Al7050 Slot Milling. 108 Chapter 5. Generalized Mechanics of Metal Cutting Bull-Nose End Mill with Parallelogram Insert In the second case, same workpiece (Al7050 β T451) was used to validate the cutting force model using a 20 mm diameter Sandvik R216-20B25-050 cutter with two flutes and two parallelogram inserts (Sandvik R216-20 T3 M-M 1025) with 10 mm nose radius. Cutter body has 10Β° axial rake angle and 5Β° radial rake angle. For the sake of simplicity, control points and parametric equations are not shown in this insert; hence cutting edges are very similar to the shoulder end mill. Only difference between two inserts is that bull nose insert has a bigger corner radius. One slot milling and one half immersion up-milling tests were performed with different spindle speeds, feed rates, and depth of cuts. Cutting conditions are listed in Figure 5-30. Comparison of the measured and calculated cutting forces can be seen Figure 5-31 and Figure 5-32. It can be observed from the figures that the simulations and the experimental values are in good agreement; this proves that the cutting force model proposed in this chapter is valid not only for straight but also for round cutting edges. Tool 20 mm Bull Nose Mill w/ 10 mm Nose Radius Axial Rake: 12Β° Radial Rake: 5Β° Insert Rake: 0Β° Insert Helix: 0Β° Half Immersion Full Immersion Width of Cut: 10 mm 20 mm Depth Of Cut: 2 mm 3 mm Material: Al7050-T541 Al7050-T541 Spindle Speed: 2000 rpm 1000 rpm Feed Rate : 0.1 mm/tooth 0.05 mm/tooth Figure 5-30: 20 mm Diameter Bull-Nose Mill from Sandvik Coromant [55]. 109 Chapter 5. Generalized Mechanics of Metal Cutting Figure 5-31: Measured and Predicted Forces for Al7050 Half Immersion Up-Milling. 110 Chapter 5. Generalized Mechanics of Metal Cutting Figure 5-32: Measured and Predicted Forces for Al7050 Slot Milling. 111 Chapter 5. Generalized Mechanics of Metal Cutting Ball-End Mill with a Circular Insert In the final case of milling validation tests, a ball end mill was selected with a circular (round) insert. Cutter body was Sandvik RF216F-20A20S-038, a 20 mm ball-end mill cutter. Unlike the other cutters presented in this section, this cutter has some unique properties. First of all, although it is a two fluted cutter, it has only one flat insert that cuts with both sides. Therefore, it does not have any axial or radial rake. The insert used with this cutter was Sandvik R216F-20 50E-L P10A circular insert with diameter of 20 mm. Tool 20 mm Ball End Mill Axial Rake: 0Β° Radial Rake: 0Β° Insert Rake: 0Β° Insert Helix: 0Β° Half Immersion Full Immersion Width of Cut: 10 mm 20 mm Depth Of Cut: 3 mm 3 mm Al7050-T541 Al7050-T541 Material: Spindle Speed: 5000 rpm Feed Rate : 0.1 mm/tooth 2000 rpm 0.08 mm/tooth Figure 5-33: 20 mm Diameter Ball-End Mill from Sandvik Coromant [55]. Control points in global coordinate system (A and B) (Figure 5-34): π΄π΄π₯π₯ = 0, π΄π΄π¦π¦ = 0, π΄π΄π§π§ = 0 π΅π΅π₯π₯ = 10, π΅π΅π¦π¦ = 0, π΅π΅π§π§ = 10 (5.64) In this inserted cutter, since all orientation angles are 0Β°, the transformation matrices will be identity; therefore, the final positions of the control points do not change. There is only one parametric equation used in this type of insert: 112 Chapter 5. Generalized Mechanics of Metal Cutting πππ΄π΄π΄π΄ = π π cos π‘π‘ π’π’ οΏ½β + π π sin π‘π‘ πποΏ½β Γ π’π’ οΏ½β + πΆπΆ, 0 β€ π‘π‘ β€ π π = 10, ππ 2 0 0 0 π’π’ οΏ½β = οΏ½ 0 οΏ½, πποΏ½β = οΏ½β1οΏ½ , πΆπΆ = οΏ½ 0 οΏ½ 0 10 β1 πππ΄π΄π΄π΄ = οΏ½ (5.65) 10 sin π‘π‘ οΏ½ 0 10 β 10 cos π‘π‘ Figure 5-34: Control Points of a Round Milling Insert By changing parameter π‘π‘ from 0 to 1, local radii and physical angles along the cut- ting edge can be determined. Similar to the previous milling examples, one half immersion and one full immersion slot milling tests were conducted with different spindle speeds and feed rates. Cutter geometry and tests conditions can be seen in Figure 5-33. Figure 5-35 and Figure 5-36 show the comparison of simulated values and experimental values. It can be noted that the measured and predicted cutting forces are found to be in good agreement. 113 Chapter 5. Generalized Mechanics of Metal Cutting Figure 5-35: Measured and Predicted Forces for Al7050 Half Immersion Up-Milling. 114 Chapter 5. Generalized Mechanics of Metal Cutting Figure 5-36: Measured and Predicted Forces for Al7050 Slot Milling. 5.5 Summary In this chapter, generalized cutting mechanics and mathematical cutting force model for inserted cutters are presented. Mechanics model calculates the cutting forces on the rake face along the normal and friction directions. By doing so, it is possible to model any type of cutting process and tool geometry. This model also allows using any type of ISO insert as well as any insert with a custom shape. Proposed cutting force model is verified both in turning and milling operations using various tool geometries and cutting conditions. 115 Chapter 6. Conclusions 6. Conclusions The aim of this thesis has been to develop a generalized mathematical modeling of metal cutting mechanics which allows prediction of cutting forces for a variety of machining operations. The cutting forces are used to analyze torque, power and stiffness requirements from a machine tool. They are also primary variables in simulating and optimizing machining operations in virtual environment. The proposed generalized model has three fundamental steps: 1) generalized geometric model of cutting tool based on ISO standards; 2) kinematic transformation of force vectors in machining systems; 3) and modeling of two principal cutting forces acting on the rake face of the tool. The two principal forces, namely the friction and normal forces on the rake face, are transformed to both cutting tool and machine tool coordinate systems. The generalized transformations allow the use of same material properties and rake face forces in predicting the loads in a variety of machining processes such as drilling, turning, boring, milling and other operations conducted with defined cutting edges. The main contributions of the thesis can be summarized as follows: β’ Instead of developing dedicated and tool geometry and cutting operation specific cutting force models as reported in the literature, an integrated geometric-mechanic and kinematic model of the process is presented. The model can be used to predict forces in various cutting operations with general tool geometry. β’ A generalized geometric model of inserts and their placement on the tools have been developed using ISO standards for cutting tool geometry. The cutting edge coordinates, where the force is generated, are analytically evaluated from the common geometric model. The model allows the use of multiple cutting edges mounted on the cutter body. The required normal and oblique angles, which are needed in mechanics models, are evaluated analytically using general geometric model. 116 Chapter 6. Conclusions β’ The normal and friction forces on the rake face are used as the principal cutting forces. They are predicted either from orthogonal parameters (shear stress, shear angle and average friction coefficient) or empirical cutting force coefficients calibrated from mechanistic experiments. The principal cutting forces are transformed to both stationary and rotating tool coordinates depending on machining operations. β’ Generalized coordinate transformation models are developed for both stationary and rotating tools. The cutting forces acting on the rake face are transformed to feed, normal and axial directions of the machine tool motion. β’ The proposed general mechanics and geometric models are experimentally validated in turning and milling experiments with inserts having complex geometries. The generalized mechanics model allow prediction of cutting forces, torque and power in a number of cutting operations conducted with tools having arbitrary geometries. The proposed model improves the computational efficiency and accuracy in simulating process physics and optimizing the operations in virtual machining of parts. The most important limitation to the generalized cutting force model is the accuracy of the material model. Since the physical angles and cutting conditions significantly change along the cutting edge, material model (shear parameters or mechanistically determined cutting coefficients) should cover these changes. 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