The paper proposes and realizes a new discriminance for the compatibility of screw fastener based on random Hough transform(RHT). In noncontact screw measurement via computer vision

the three-dimensional entity of thread was transformed into the two-dimensional orthogonal projection of thread using a CCD camera system

then the one-dimensional contour was extracted from the projectional image

and the projectional pitch diameters were calculated from the contour of the thread portion by random Hough transform. The paper also analyses the probability histograms of projectional pitch diameters in order to judge the compatibility of the screws. In comparison with the template matching method

the expensive experiment has proved that the RHT method is both correct and effective

which has laid a good foundation for automatic testing screw fasteners.