High-precision roundness measuring system for tungsten ball tips with the diameter of less than 100 µm

Abstract Tungsten balls with a diameter of less than 100 μm can function as probe tips for micro-coordinate measuring machines to measure ultraprecise workpieces with complex features. A high-precision measuring system was developed to evaluate the roundness of self-made tungsten ball tips. The system, based on the two-point method, is composed of two oppositely placed interferometers and a turntable. The turntable rotates the tested ball, and the interferometers measure the variation in radius. A new model based on the minimum zone method is proposed to evaluate roundness. The fabrication principle of tungsten balls is introduced and an analysis is conducted on the maximum permissible contact force and bending stress of tungsten ball tips. The elastic mechanism is designed, based on the analysis. A ruby sphere with a sphericity of 130 nm is measured to verify the system’s effectiveness. Repeated experiments are performed for two tungsten ball tips. The results show that the roundness error of tungsten ball-A is between 550.9 and 586.2 nm with a standard deviation of 11.0 nm while tungsten ball-B’s roundness error is between 898.5 and 959.9 nm with a standard deviation of 21.7 nm. The uncertainty is calculated as 144.6 nm. The developed system can stably measure the roundness of tungsten ball tips.