A motion error of a machine tool in a coordinate system having its origin at an arbitrary position is identified by means of error data measured by a commonly-used method. An X-axis feed mechanism, a Y-axis feed mechanism, and a Z-axis feed mechanism are operated in a three-dimensional space of a machine coordinate system to measure translational errors, angular errors, and perpendicularity errors thereof, and error data for translational error parameters, angular error parameters, and perpendicularity error parameters in a three-dimensional space of a set coordinate system having its origin at a preset reference position X.sub.a, Y.sub.a, Z.sub.a are derived based on the measured actual error data. Subsequently, a relative motion error between a spindle and a table in the three-dimensional space of the set coordinate system is derived based on the derived error data.

A motion error of a machine tool in a coordinate system having its origin at an arbitrary position is identified by means of error data measured by a commonly-used method. An X-axis feed mechanism, a Y-axis feed mechanism, and a Z-axis feed mechanism are operated in a three-dimensional space of a machine coordinate system to measure translational errors, angular errors, and perpendicularity errors thereof, and error data for translational error parameters, angular error parameters, and perpendicularity error parameters in a three-dimensional space of a set coordinate system having its origin at a preset reference position X.sub.a, Y.sub.a, Z.sub.a are derived based on the measured actual error data. Subsequently, a relative motion error between a spindle and a table in the three-dimensional space of the set coordinate system is derived based on the derived error data.

A machine tool that attaches and detaches workpieces to and from a spindle in a shorter time includes: a machining mechanism part including a spindle, a tool rest, a first-axis feed mechanism moving the spindle in a a first axis extending along a center axis of the spindle, and a second-axis feed mechanism relatively moving the spindle and the tool rest in a second axis orthogonal to the first axis; a loading mechanism part including holding parts for holding a workpiece, and a third-axis feed mechanism moving the holding parts in a third axis orthogonal to the first axis to position them at the transfer position; and a numerical controller. The numerical controller operates to move the spindle toward the transfer position and to position the holding parts at the transfer position such that the operations at least partially overlap.

A tool center path is compensated so that the number of collisions with a workpiece in positions on cutter tips complies with use frequencies in cutter tip information, based on tool information including the diameter of a tool and the number of cutter tips attached to the tool, a tool center movement path specified by a machining program, workpiece shape data, and cutter tip information.