Machining error compensation using artefact based offsets

A method of machining a workpiece using a machine tool comprising a machining head and a workpiece holder moveable relative to each another, the method comprising:

• performing a first machining operation on a workpiece mounted to the workpiece holder according to a first programmed series of movements of the machining head relative to the workpiece holder, the first machining operation having a first machining tolerance;
• performing a second machining operation on the workpiece according to a second programmed series of movements of the machining head relative to the workpiece holder, the second machining operation having a second machining tolerance;
• performing a measurement operation to determine a position of an artefact on the machine tool;
• calculating an offset relative to a corresponding previously stored position of the artefact;
• and
• applying the offset to the second programmed series of movements prior to performing the second machining operation, wherein
• the second machining tolerance is smaller than the first machining tolerance, and
• the machine tool is configured to move the machining head and the workpiece holder relative to each another along mutually orthogonal X, Y and Z axes and to rotate the workpiece holder relative to the machining head about a first rotation axis A, the step of performing the measurement operation comprising measuring, using a probe, pre-determined features on the artefact to determine X, Y and Z coordinates and an orientation about the A axis of the artefact, the step of calculating the offset comprising calculating offsets ΔX, ΔY, ΔZ and ΔA to the X, Y, Z and A axes respectively by comparing said determined X, Y and Z coordinates and orientation about the A axis with previously stored corresponding coordinates and orientation.

A method of machining a workpiece using a machine tool comprising a machining head and a workpiece holder moveable relative to each another, the method comprising:

• performing a first machining operation on a workpiece mounted to the workpiece holder according to a first programmed series of movements of the machining head relative to the workpiece holder, the first machining operation having a first machining tolerance;
• performing a second machining operation on the workpiece according to a second programmed series of movements of the machining head relative to the workpiece holder, the second machining operation having a second machining tolerance;
• performing a measurement operation to determine a position of an artefact on the machine tool;
• calculating an offset relative to a corresponding previously stored position of the artefact;
• and
• applying the offset to the second programmed series of movements prior to performing the second machining operation, wherein
• the second machining tolerance is smaller than the first machining tolerance, and
• the machine tool comprises a machine bed on which the workpiece holder is mounted, the artefact being on the machine bed, the machine tool being configured to move the machining head and the workpiece holder relative to each another along mutually orthogonal X, Y and Z axes, to rotate the machine bed relative to the machining head about a first rotation axis A and to rotate the workpiece holder relative to the machining head about a second rotation axis C orthogonal to the first rotation axis A, the step of performing a measurement operation comprising:
• measuring, using a probe, pre-determined features on the artefact to determine X, Y and Z coordinates and an orientation about the A axis of the artefact;
• measuring, using the probe, features on the workpiece holder to determine an orientation of the workpiece holder about the C axis, and
• the step of calculating the offset comprises calculating offsets ΔX, ΔY, ΔZ, ΔA and ΔC to the X, Y, Z, A and C axes respectively by comparing said determined X, Y and Z coordinates and orientation about the A axis on the artefact and said determined orientation of the workpiece holder about the C axis with previously stored corresponding coordinates and orientations.