Patent ID: 7490411

Claim:
A screw measuring method comprising: an internal-dimension measuring step of using a coordinate measuring machine provided with a screw measuring probe that has a stylus-tip sphere having a diameter selected according to the pitch of a female screw to measure internal-dimension information D w equal to the diameter of a cylinder having a center axial line identical with the axial line of the female screw and having a wall that passes through the center of the stylus-tip sphere when the stylus-tip sphere is butted against the screw groove of the female screw, based on coordinate information measured while the stylus-tip sphere is butted against the screw groove of the female screw; and an effective-diameter calculating step of obtaining effective-diameter information D e of the female screw from the internal-dimension information D w measured in the internal-dimension measuring step, diameter information 2r of the stylus-tip sphere, and thread angle information α and pitch information P of the female screw, by using an effective-diameter calculation expression D e =D w +2r×cosec(α/2)−(P/2)×cot(α/2).