System integration for on-machine measurement using a capacitive LVDT-like contact sensor

In this study, an air-bearing capacitive linear variable differential transformer (LVDT)-like contact sensor with a rounded diamond tip was mounted to a desktop machine tool to construct an on-machine (OM) measuring system. The measuring system was capable of decoding the digital signals of linear encoders mounted on the machine tool and acquiring the analog signal of the contact sensor. To verify the measuring system, experimental examinations were performed on an oxygen-free copper (OFC) convex aspheric mold with a diameter of 5 mm and a curve height of 0.46 mm. The acquired signals were processed by the implemented Gaussian regression filter (GRF), removing the tilt of measured profile, and compensating for the radius of probe tip. The profile obtained was compared to that measured using a commercially available device, and a maximum deviation of 14.6 mu m was found for the rough cutting. The compensation cutting was then performed according to the form error of OM measurement. As a result, the PV form error compared with the designed profile was reduced from 19.2 mu m over a measured diameter of 4 mm to 9.7 mu m over a measured diameter of 3.1 mm, or a percentage improvement of 35.4% in form accuracy. Through the examination for aspheric machining, the effectiveness of the implemented OM measuring system was demonstrated, and the technical details of system implementation were presented. Further improvement was suggested to reduce the diameter of probe tip and measuring force.