Measurement and diagnostics of machine tool errors during circular contouring motions

Abstract

This paper deals with the measurement and diagnosis of the machine tool errors by application of the ball link bar system. It includes the development of a new link bar, the in-plane and out-of-plane measurement techniques and the machine error analysis system. The new type of ball link incorporates an idealized three-point kinematic contact principle between the balls and sockets of the link bar, and provides the following features: (a) the minimum length of the link corresponds to the minimum length of a standard LVDT transducer, (b) the link is of light weight, (c) the accuracy and the repeatability of the link corresponds to that of the LVDT (linear variable differential transformer) transducer and (d) the link is mounted between two reference balls by using the spring force of the LVDT transducer and thereby eliminating the need for magnets as in the case of the conventional double-ball bar.

With the in-plane and out-of-plane measurements under different test conditions, the machine errors, such as set-up errors, closing errors, backlash errors, dynamic backlash errors, positional errors, pitch motion errors, transient errors around the start point of the test path, vibration due to too high a gain setting, stick-slip errors, squareness errors and servo mis-match errors, can be analysed by the application of the machine error analysis software.

Practical measurements on the TAKISAWA CNC (computerized numerical control) machining centre were carried out and the machine tool errors were effectively evaluated.

Keywords

CNC machine tools machine errors machine accuracy

References

BS 4656: Part 30: 1992 Accuracy of Machine Tools and Methods of Test, Part 30: Specification for machining centres and computer numerically controlled milling machines, horizontal and vertical spindle types, 1992 (British Standards Institution, London).

ISO 230-4: 1994 Acceptance Code for Machine Tools–Part 4: Determination of circular hysteresis, circular and radial deviation of circular movements of numerically controlled machine tools, 1994 (International Standardization Organization, Geneva, Switzerland).

ISO 230-1: 1986 Acceptance Code for Machine Tools–Part 1: Geometric accuracy of machines operating under no-load or finishing conditions, 1986 (International Standardization Organization, Geneva, Switzerland).

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