A Comparative Measurement Strategy to Implement Precise On-Machine Measurement for Free-Form Surfaces

Accurate on-machine measurement (OMM) is crucial for enhancing machining accuracy and enabling adaptive machining operations. However, OMM accuracy is constrained by multiple error sources, including machine tool geometric errors and probe errors. To mitigate OMM accuracy limitations, this study proposes a comparative measurement strategy that enhances measurement reliability. In this method, the geometry of a machined workpiece is obtained by comparing it with a reference workpiece of the same nominal shape. The reference workpiece is calibrated using a coordinate measuring machine (CMM), and its measurement data serve as the truth surface. To achieve high repeatability, the comparative measurement process is conducted under symmetric clamping with rotary table assistance. The calibrated reference surface is reconstructed using B-spline surface fitting based on CMM-measured points. The deviations between the OMM points and the reconstructed reference surface are calculated using a point-to-surface distance approach. The calibrated surface is formed by assigning the deviations to the reference piece's design surface. Finally, the geometric form error of the test piece is then determined by calculating the distance between its OMM points and the calibrated reference surface. The viability of the proposed measurement strategy and the impact of clamping errors are analyzed via a numerical study. The accuracy of the strategy is demonstrated experimentally for both 3-axis and 5-axis OMM systems. Experimental results show that the proposed strategy reduces the measurement error from 36.7 μm to 9.1 μm for the 3-axis system, and from 63.3 μm to 20.2 μm for the 5-axis system, compared to the direct OMM strategy.