Uncertainty evaluation of an on-machine chromatic confocal measurement system

On-machine optical measurement receives growing attention as a viable quality assurance technique to satisfy the increasingly critical requirements for dimensional accuracy and surface finish upon precision components. It preserves the temporal and spatial consistency between manufacturing and measurement activities, allowing for a substantial productivity improvement and a great opportunity for on-line quality control. However, additional uncertainty can be introduced under the harsh on-machine measuring environment degrading the measurement quality. The explicit research on the on-machine measurement uncertainty is therefore essential but rarely carried out to date. To fill this gap, this paper presents the uncertainty investigation of an on-machine areal measuring instrument based on the chromatic confocal scanning principle. The vibration transmitted from neighboring machine tools is found to strengthen the drive vibration and to cause an overestimation of the roughness in the static noise test. The motion-induced disturbance and subdivisional error are recognized as artefacts in the flatness map influencing the evaluation of the flatness deviation, though they can be attenuated by the notch filter. Based on further evaluation on the roughness parameters, the uncertainty contributions from the noise and the corrected flatness deviation are found to be 18 nm and 50 nm respectively in.... for the rough surface (pulse energy=250 mu J). An uncertainty budget concerning all these influencing factors enables a systematic uncertainty quantification for the on-machine measurement.