3 Dimensional machine tool positioning accuracy

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machining centers 3 Dimensional machine tool positioning accuracy Laser vector measurement vs. linear measurement By Charles Wang President Optodyne, Inc. The increasing demand for accuracy of machined parts is being fueled by economics because it reduces assembly, warranty, and ownership costs. Traditionally, manufacturers have ensured accuracy of parts with linear (one-dimensional) calibration of the machine tools used for making them. But linear calibration is inadequate for ensuring accuracy of three dimensional parts. ASME B5.54 and IS0230-6 volumetric machine tool performance measurement standards were introduced. Because of the expense, necessitating the machine to be non-productive for two or three days, manufacturers have been reluctant to adopt volumetric calibration. However, the Laser Vector Technique for 3D volumetric calibration and compensation, developed by Optodyne Inc. using laser Doppler calibration equipment, is becoming popular because it reduces the time factor from two or three days to two or three hours. Relying on linear calibration, one- dimensional measurements parallel to the axis of movement assume that the only possible errors are ballscrew and thermal expansion errors. But this ignores squareness errors, straightness errors, angular errors, and errors sumption, proposes six errors-one displacement error, two straightness errors and three angular errors-in the X, Y, and Z axes. For a 3-axis machine, there are 18 errors plus three for squareness, a total of 21 errors. Therefore to achieve higher positioning accuracy, the angular, straightness, and square- ness errors must be mea- sured and compensated. Using the Laser Vector Technique, only four body diagonal displacement measure- ments are needed to determine 3D volu- metric accuracy. Body diagonal dis- placement errors are sensitive to all the volumetric error components and there- fore make an efficient test of volumetric accuracy. The Laser Vector Technique measures all three displacement errors, three vertical straightness errors, and three horizontal straightness errors with just four setups. The working volume of a typical VMC includes eight body diagonals, a diagonal being defined by starting at one corner of the base plane and mov- ing to the opposite corner at the top plane. These body diagonals are defined by the positive or negative axis movement. The last four body diagonals are the same corners as the first four diagonals, except the directions are reversed. Hence there are only four body diagonal directions with forward movement and reverse movement (bi- directional), and only four setups. For each setup, the machine spindle movement along each of the diagonals is measured by first executing X, Y and, Z portions of the spindle travel. Readouts are taken and recorded at each interme- diate step for three displacement errors, three vertical straightness errors, and three horizontal straightness errors. Laser calibration The two primary systems for linear displacement and volumetric calibration of machine tools and CMMs include the dual aperture laser interferometer system and the single aperture interferometer system. Both systems use lasers and optics, but they differ in how the data is collected and analyzed. The dual aperture laser interferometer system is based on the Michelson caused by non-rigid body motion. In fact, there are many large non-rigid body positioning errors caused by shifting weight and counter weight, etc. Carrying this to the extreme by using Taylor's linear expansion theory, two slope terms in the perpendicular directions can be added. As a result, for a 3-axis machine, there are 45 errors. Of course, not all of these nonrigid body error terms are important. Because positioning accuracy of a machine tool is very complex, it has been simplified with various assumptions. For example, the rigid body as-