Development of a Surface Roughness Measurement System in a Narrow Borehole

In industrial fields, it is frequently necessary to measure surface roughness in confined spaces such as boreholes and grooves in workpieces. However, the surface roughness of a narrow borehole can be measured only up to a few millimeters from its end when using a small stylus. Alternatively, destructive measurements must be performed. We previously proposed a novel surface roughness measurement sensor. To make the sensor sufficiently small, we used a stylus with a cylindrical mirror and a lensed fiber instead of a conventional inductive pick-up. The proposed sensor converts the signal used for measuring the surface roughness of a borehole into an optical signal, which is transferred outside the borehole by an optical fiber. The experimental results demonstrate that this system has a measurement range of 8 μm and a sensitivity of 19 nm. In this paper, we propose a carriage that supports the stylus when measuring the surface roughness in a small borehole. The proposed carriage has two degrees of freedom: displacement along the borehole axis and rotation around the borehole axis. In experiments, the surface roughness of standard test pieces was measured using the proposed method and the conventional method. The measurement results obtained by these methods were found to be very similar. Furthermore, a borehole with 2.4 mm diameter was measured. The measurement result included the characteristic wave that was obtained by the conventional method in places. The experiments also revealed some problems of the proposed system. For example, the setting procedure of the measured surface in the 8 μm measurement range was difficult. Consequently, a large measurement range or a null method is required.