Method and apparatus for detecting flaw on threads of male screw

An apparatus for detecting a flaw on threads of a male screw, which comprises:

• a male screw rotating means for rotating a male screw at a prescribed circumferential speed around the axis thereof;
• a light source for uniformly applying a light onto substantially the entire surface of threads of said male screw in the axial direction thereof and at least part of the surface of said threads of said male screw in the circumferential direction thereof;
• a photoelectric converter for continuously measuring, during one turn of said male screw around the axis thereof, electric signal values corresponding to variations in brightness of the surface of said male screw in the axial direction thereof, which are caused by tops, roots and flanks of said threads of said male screw onto which said light is applied;
• and
• a flaw-determining mechanism for comparing an average value of said electric signal values corresponding to said variations in brightness of said male screw in the axial direction thereof, as measured by said photoelectric converter, with a previously set reference values, to detect a flaw on said threads of said male screw, said flaw-determining mechanism comprising a memory for continuously storing, during said one turn of said male screw, said electric signal values corresponding to said variations in brightness of said male screw in the axial direction thereof, as measured by said photoelectric converter;
• and a flaw-detecting means for obtaining said average value of said electric signal values corresponding to said variations in brightness of said male screw in the axial direction thereof, as continuously stored in said memory during said one turn of said male screw, and for comparing the thus obtained average value of said electric signal values, with said previously set reference value, to detect a flaw on said threads of said male screw;
• characterized in that:
• said light source (4) is provided at a position from which said light is fully applied onto the tops, the roots and the flanks of said threads (2) of said male screw (1);
• said memory (11) of said flaw-determining mechanism (6) includes a means for continuously determining, during said one turn of said male screw (1), electric signal values corresponding to variations in brightness of said male screw (1) in the circumferential direction thereof, on the basis of said electric signal values corresponding to said variations in brightness of said male screw (1) in the axial direction thereof;
• and
• said flaw-detecting means of said flaw-determining mechanism (6) comprises:
• a first moving average calculating circuit (10) for moving-averaging, during said one turn of said male screw (1), said electric signal values corresponding to said variations in brightness of said male screw (1) in the axial direction thereof, as measured by said photoelectric converter (5), the thus moving-averaged electric signal values being entered into said memory (11);
• a second moving average calculating circuit (12) for moving-averaging, during said one turn of said male screw (1), said electric signal values corresponding to said variations in brightness of said male screw (1) in the circumferential direction thereof, as determined by said memory (11);
• a difference calculating circuit (13) for calculating, during said one turn of said male screw (1), values of differences between said electric signal values corresponding to said variations in brightness of said male screw (1) in the circumferential direction thereof, as moving-averaged by said second moving average calculating circuit (12), on one hand, and said electric signal values before said moving-averaging corresponding to said variations in brightness of said male screw (1) in the circumferential direction thereof, as determined by said memory (11), on the other hand;
• a mean square calculating circuit (14) for obtaining a mean square value of said electric signal values as said average value of said electric signal values, from said values of differences as calculated by said difference calculating circuit (13);
• and
• a comparing circuit (15) for comparing said mean square value of said electric signal values obtained by said mean square calculating circuit (14) with said previously set reference value.