Contact detecting apparatus

In a machine tool including a machine body, a work table mounted on said machine body for mounting thereon a workpiece to be machined, a spindle head mounted on said machine body, a tool spindle rotatably supported in said spindle head for removably supporting a tool, a clamp bar received in said tool spindle in coaxial relationship therewith and engageable with said tool for clamping the same relative to said tool spindle, spring means interposed between said tool spindle and said clamp bar urging said clamp bar in a direction to clamp said tool, unclamping means for moving said clamp bar against said spring means to unclamp said tool, and control means for effecting relative movement between said work table and said tool spindle, a contact detecting apparatus comprising:

• a brush disposed in contact engagement with said clamp bar in coaxial alignment therewith and electrically connected to said spindle head;
• an AC power source;
• a coil mounted on said spindle head and connected to said AC power source for generating an induced current in a looped secondary circuit including the workpiece, work table, machine body, spindle head, brush, tool spindle and tool, when said workpiece and said tool are moved relatively into contact with each other;
• and
• means connected between said AC power source and said coil for detecting a change in an electric signal caused by the contact between said workpiece and said tool.