Computer Aided Volumetric Error Calibration ofCo-Ordinate Measuring Machine using the Baseas a Metrological Reference
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Many coordinate measuring machines (CMMs) are equipped with a table which is made of granite or steel, on which work pieces are supported. A new volumetric error calibration technique for the CMM is proposed using the base as a metrological reference. After the flatness calibration has been performed on the CMM base using a rectangular grid technique, and the errors in the locus of the probe stylus is determined by probing the calibrated reference points. These errors are then decomposed into one roll error,two vertical straightness errors. The calibrated reference points on the metrological base are used in conjunction with mechanical artefacts such as squares and straight edges. These artefacts are probed and the reversal procedure is applied in order to eliminate squareness and straightness errors in the artefacts. Probing the mechanical square when located at different reference positions on the base enabled the machine squareness, straightness, pitch and roll about horizontal axes to be measured. A laser interferometer is used to measure the remaining geometric errors: 3 positioning errors, horizontal yaw, vertical yaw and pitch errors. After all parametric errors of the CMM have been measured, a complete volumetric error map is constructed and represented vectorially within the measurement volume. The uncertainty in the 1D, 2D,and 3D length measurement is calculated, and the uncertainty curves are evaluated in a linearised form. The application of micro computer for data acquisition and analysis means that the whole processes such as measurement, analysis, volumetric error construction and representation are performed in a time scale of few hours, with an interactive soft ware which is developed around the IBM PC/XT/AT compatible and the peripherals.
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