Patent ID: 7104866

Claim:
A machining device, comprising: a rotatable conductive workpiece holder that holds a conductive workpiece; a rotatable conductive tool holder that holds a conductive tool for machining the workpiece; a first conductive outer peripheral portion formed to cover at least a part of an outer peripheral surface of the tool holder; a second conductive outer peripheral portion formed to cover at least a part of an outer peripheral surface of the workpiece holder; a first non-contact bearing formed by elevating the tool holder from an inner peripheral surface of the first outer peripheral portion; a second non-contact bearing formed by elevating the workpiece holder from an inner peripheral surface of the second outer peripheral portion; a conductor that electrically connects the first outer peripheral portion with the workpiece and the first outer peripheral portion with the second outer peripheral portion; a closed-circuit formed when the workpiece contacts the tool in machining, the closed-circuit connecting in order the workpiece, the tool, the tool holder, the first outer peripheral portion, the conductor, the second outer peripheral portion and the workpiece holder; an AC power supplier that feeds an alternating-current to the closed-circuit; p 1 a detector that detects the alternating-current passing through the closed-circuit; and a monitoring/controlling unit that monitors an output value of a signal based on the alternating-current detected by the detector according to a predetermined monitoring condition, wherein the monitoring condition includes a light/heavy contact-determining threshold for determining whether a contact state between the workpiece and the tool is a light contact or a heavy contact, and wherein the monitoring/controlling unit controls the contact state between the workpiece and the tool so that the output value of the signal constantly stays within a light contact region relative to the light/heavy contact-determining threshold.