A three-probe system for measuring the parallelism and straightness of a pair of rails for ultra-precision guideways

This paper describes a three-probe system that can be used to measure the parallelism and straightness of a pair of rails simultaneously. The parallelism is measured using a modified reversal method, while the straightness is measured using a sequential two-point method. The measurement algorithms were analyzed numerically using a pair of functionally defined rails to validate the three-probe system. Tests were also performed on a pair of straightedge rails with a length of 250 mm and a maximum straightness deviation of 0.05 μm, as certified by the supplier. The experimental results demonstrated that the parallelism measurement algorithm had a cancellation effect on the probe stage motion error. They also confirmed that the proposed system could measure the slope of a pair of rails with an accuracy of about 0.06 μrad. Therefore, by combining this technique with a sequential differential method to measure the straightness of the rails simultaneously, the surface profiles can be determined accurately and the stage error eliminated. The measured straightness deviation of each straightedge was less than 0.05 μm, consistent with the certified value.