Development of an on-machine micro measuring technique

Dimensional and geometric accuracy measurement of a micro part or microstructure, especially on-machine measuring, is tedious and extremely difficult. By modifying the discharge circuit of the table top high precision CNC machine developed recently in our laboratory an on-machine micro measuring technique is developed. A properly designed probe is fabricated by the wire electro-discharge grinding (WEDG) after a micro part made of conductive material is machined. It is driven via precision stages of the machine until it is in critical contact with the micro part. The position is recorded at the instant a short circuit is detected. The dimension and contour of a micro part are determined based on the positions of all measured points by software. To improve the measuring accuracy a three steps feed strategy is proposed. In addition, an approach to reduce measuring time is adopted. To realize the proposed approach, experiments are conducted and the optimal reference voltage to ensure the proper contact of probe-part, and the working voltage are determined. Measurement of a grade-A gauge block is taken to verify the developed system. The errors of the straightness and the width are within 0.5 μm and 0.3 μm, respectively. Hence, achievable measuring accuracy of the developed micro measuring technique is within 1 μm. Based on the proposed technique, on-machine measuring of the straightness and flatness of a micro part, and the circularity and cylindricity of a micro hole has been accomplished.