US Patent 7194329 Numerical control apparatus and numerical control system

An NC apparatus controls a position of a tool by driving a plurality of shafts with a plurality of servomotors. The shafts includes at least one clamp shaft that can be mechanically clamped or non-clamped. A clamp determining unit determines whether a state of the clamp shaft and outputs a signal based on the state. A correcting unit corrects a position instruction value or a position detection value of at least one shaft other than the clamp shaft based on the signal and a correction value so as to cancel an error in a position of the tool due to movement of the tool performed by the clamp shaft.