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Prediction and reduction of cycle time for five-axis CNC machine tools

University of British Columbia

The growing demand for high speed machining in aerospace, automotive and die and mold industry has directed the interest of research community towards prediction and reduction of machining cycle time. In this thesis, a cycle time prediction scheme is proposed for milling operations based on identified CNC machine dynamics in exact-stop and continuous mode. Various system identification techniques are utilized to identify the implemented trajectory generation and corner smoothing technique and feed drive dynamics of the CNC system. An analytical approach for predicting cycle time based on the identified CNC system dynamics and given part program is presented. It is shown that the cycle time of NC machining process is predominantly affected by trajectory generation and corner smoothing techniques implemented on CNC systems. The closed-loop feed drive dynamics does not have much influence on the cycle time, since the tracking delay is insignificant in position control servos. The proposed algorithm is validated in experiments and experimental results has shown that the cycle time prediction error remains within 5% for various 2-axis, 3-axis and 5-axis toolpaths. In the later half of the thesis, a new decoupled approach for five-axis corner smoothing is presented to reduce the cycle time of milling operations. Toolpath position and orientation are smoothed by inserting quintic and normalized septic micro-splines, respectively between the adjacent linear toolpath segments. Optimal control points are calculated for position and orientation splines to achieve C³ continuity at the junctions between the splines and the linear segments while respecting user-defined corner position tolerance and orientation tolerance limits. Synchronization of position and orientation splines is carried out. After geometrical modification of the toolpath, feedrate planning is performed using C³ continuous cubic acceleration feedrate profile to preserve jerk continuity in toolpath motion. The proposed C³ continuous toolpath motion is compared against the unsmooth and C² continuous motion in experiments and simulations to show improvements in cycle time, tracking accuracy and smoothness throughout the toolpath.

Text

2014-11-25

Attribution-NonCommercial-NoDerivs 2.5 Canada

http://hdl.handle.net/2429/51195

Mechanical Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/2.5/ca/