Hole center detecting apparatus, straightness measuring apparatus, and residual torsion measuring apparatus

A hole center detecting apparatus comprising:

• an object having a round hole;
• coordinate detecting means for detecting the coordinates of two points at which the inner circumference of said hole and a virtual straight line intersect;
• coordinate computing means for computing the coordinates of the center of said hole from the detected coordinates of the two points;
• an insertion member having a leading end to be inserted into said hole;
• moving means for moving the object and the leading end of the insertion member relative to each other so that the leading end of the insertion member may be positioned at the computed coordinates of the center;
• a mark formed in advance on the object and which has a predetermined coordinate relation with respect to said hole;
• auxiliary detecting means for detecting said mark;
• a mark formed in advance on the object and which has a predetermined coordinate relation with respect to said hole;
• and
• auxiliary detecting means for detecting said mark.