Method and apparatus for inspecting workpieces

A method for measuring production workpieces on a dimensional measuring apparatus, the method comprising:

• measuring a master artefact on the measuring apparatus, the master artefact having a plurality of features, the size and shape of which approximate a production workpiece of a first series of nominally identical workpieces produced by a production process;
• obtaining calibration values for the master artefact, from a source external to said measuring apparatus;
• comparing the calibration values with the measurement of the master artefact to produce one or more master artefact correction values;
• using said master artefact correction values to populate or further populate an error map or lookup table or to calculate or recalculate an error function for calibrating the measuring apparatus;
• measuring one or more nominally identical production workpieces of the first series produced by the production process on the measuring apparatus;
• correcting the measurements of the nominally identical workpieces of the first series using said master artefact correction values or the error map or lookup table or the error function;
• measuring one or more second workpieces on the measuring apparatus, wherein the one or more second workpieces are different from, or are differently located on the apparatus from, the nominally identical workpieces of said first series of workpieces;
• and
• correcting the measurements of the one or more second workpieces using correction values derived by interpolation or extrapolation from said error map or lookup table or derived from said error function, wherein the error map or lookup table or error function has been produced using the master artefact correction values.

A method of further calibrating a dimensional measuring apparatus which is calibrated by an initial error map or error function,

• the method comprising:
• measuring a production workpiece on the measuring apparatus, the production workpiece being one of a first series of nominally identical workpieces produced by a production process;
• comparing the measurements of the production workpiece with calibration values for the production workpiece, obtained from a source external to said measuring apparatus, to produce one or more error values;
• determining one or more updated error maps or error functions which combine some or all of the error values with all or part of the initial error map or the initial error function;
• determining whether one or more of the updated error maps or error functions gives better correction of measurement errors than the initial error map or error function;
• and
• if an error map or error function is determined to give better correction, then selecting that error map or error function for use in correcting the measurements of one or more further workpieces.