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UBC Theses and Dissertations

Modeling and compensation of machine tool volumetric errors for virtual CNC environment Bal, Evren

Abstract

Machine tools produce positioning errors due to thermal and structural deformations of the structural elements, geometric errors due to inaccuracies in the manufacturing and assembly of their components, and computer numerical control errors caused by the bandwidth of their servo drives. This thesis presents the measurement, modeling and compensation of geometric errors of three axis machine tools in order to simulate and improve their volumetric accuracy in virtual environment. The geometric accuracy of each axis is measured using a laser interferometer. The displacement, pitch, yaw and backlash errors of each axis element are measured and mapped to the machine coordinates using rigid body kinematic transformations of the system. The identified errors are curve fitted to the position of each drive in the machine coordinates. The algorithm allows prediction of relative positioning error between the tool and workpiece within the working volume of the machine tool, or pre-compensates the errors by adding the estimated positioning errors to Numerical Control (NC) program in Virtual Environment before the machining takes place. The prediction and compensation of geometric errors are experimentally demonstrated on a three axis, vertical CNC machining center. Standard ISO geometric profiles, a circle and a diamond, slot milled on the machine. The machined profiles are measured using a coordinate measuring machine, and the lengths and offsets from the command profiles are estimated from the measurements. The measurement results and the predicted geometric errors are compared. The contributions of CNC and tool deflections to the total errors are estimated, and the remaining errors are correlated to geometric errors of the machine tool. Although, it was not possible to account all the machine tool errors, the proposed prediction and correction method in virtual environment improved the compensation of geometric errors significantly. The overall model is integrated to UBC Manufacturing Automation Laboratory's Virtual CNC system for use in industry.

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