We develop a technology for detecting the inner wall (not an "edge portion") of a small hole in machine parts such as a die, a nozzle, and a gauge directly and measuring the inner diameter (usually, the inner diameter is 0.3 to 10.0 mm and the length of hole is 1 to 20 mm). Especially, we discuss theoretically and experimentally the measurement error in a gap larger than 0.3 mm of a quadrilateral hole sample using an optical noncontact measuring machine. By projecting a spot image onto the sample's inside wall and detecting the reflected image with a CCD camera, the position of the inside wall of the sample can be measured. The inner diameter of the sample can be measured from the distance between the positions of each inside wall. Experimental results show that the measurement error of the gap varies with the gap size of the sample. In a comparison with theoretical simulation results, it is verified that the spherical aberrations of the lens in the measuring machine is the cause of this error. With the maximum spherical aberration of the lens at 50 μm, the maximum measurement error of the gap is +1.0 μm for a 0.3×0.3-mm square hole, and is -0.30 to -0.35 μm for a quadrilateral hole with a gap larger than 0.6 mm. These theoretical results are in good agreement with the experimental results. The measurement error is always negative when the spherical aberration is assumed to be 0.
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