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Study of Accuracy of CNC Machine Tools

ABSTRACT The dimensional accuracies as well as surface finishes of parts produced by a CNC machine tool are strongly dependent on the motion accuracy of each axis of the machine. The overall accuracy of the machine tool is decided by the mechanical characteristics of the machine as well as the characteristics if the control system driving the individual axes. A CNC Machine is programmed to travel along a predetermined or contour, and any deviation from the programmed path is referred as the contouring error. A typical test that is used is a circular test in which the machine is programmed to travel along a circle, and the difference between the programmed path and the measured actual path is compared. The purpose of this study is to identify the various contributors to this contouring error, and in particular estimate the error due to stick slip motion using analytical techniques. It is planned to measure the errors experimentally making use of capacitance probes. The study is intended to develop an appreciation for the sources of different errors produced in a machine tool and as such forms a module in a typical semester long class devoted to manufacturing processes.

INTRODUCTION

Computer controlled machine tools are expected to perform within the specified limits of accuracy, especially for the contouring operations. With the increasing demands placed on high precision combined with high contouring accuracy a careful study of the accuracy degrading mechanisms arising during machine operation needs to be considered. A CNC machine is programmed to travel along a predetermined contour, and any deviation of the actual path as compared to the programmed path is the contouring error. Most contouring tests use a circular path, and the contouring error is the difference between the programmed circle and the actual circular (a distorted one) path described by the machine tool. This is what constitutes the circular test as described by Knapp (1983). The specific focus of this work is to analytically determine the magnitude of such an error in a circular test due primarily to the stick slip friction.

CONTRIBUTORS TO CONTOURING ERROR If the two axes of a CNC machine are not perpendicular to one another then an oval path will result. In addition backlash in lead screws, axis straightness, pitch and yaw errors can all affect contouring accuracy. These errors constitute geometric errors. In particular, backlash is a positioning error that occurs when the spindle approach direction is reversed and in general it is due to the tolerance of the ball lead screw or the guide way. The other set of errors arise due to control system, and are the interpolation errors, errors due to hysteresis, and those due to servo mismatch and master slave changeover.

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Chattopadhyay, S. (2008, June), Study Of Accuracy Of Cnc Machine Tools Paper presented at 2008 Annual Conference & Exposition, Pittsburgh, Pennsylvania. 10.18260/1-2--4470