1 Department of Management Engineering, Technical University of Denmark2 University of Padova3 Department of Mechanical Engineering, Technical University of Denmark
The present review outlines the state of the art of the traceability of measuring free form surfaces on coordinate measuring machines (CMMs). A general introduction to free from surfaces and to the way free form surfaces are inspected in industry at present is given.and some problems regarding the traceability of measuring of free form surfaces on CMMs outlined. A newly developed procedure for performance verification on CMMs with respect to free form measurements is described, and some investigations regarding this method are reviewed. The investigations are concerned with two new tests, the Spiral Test and the Cylinder Test developed as a coorperation between the University of Padova and the Technical University of Denmark. Experiments conducted by means of calibrated objects and CAD models created in an independent and external system, using calibrated dimensions are described. The experiments have been carried out on two CMMs with different performances. The investigations have clearly shown that the performance verification procedure must be implemented in the specific software in use for the actual measurements. The Spiral Test showed a clear influence on CMM performances from scanning speed; the Cylinder Test proved the effect of relative workpiece orientation for different measuring strategies. The result of these task-specific performance verification tests can be used to estimate the measuring uncertainty of similar free form measurements on production parts. A newly developed concept of artefact, called the "Modular Free Form Gauge" (MFG) is reviwed. It is based on physical modelling of a given free form surface by a combination of items with regular geometry, well calibrated on their dimensions, form and relative position; this information is used to generate a "calibrated" CAD model as reference for free form measurements. The architecture of the artefact, its collocation in the traceability chain, and the calibration procedure are described. Finally, a procedure for the uncertainty assessment of actual free form measurements is reviewed. An example of implementation of the uncertainty assessment procedure for free form measurements on turbine blades is given. A task-specific Modular Free Form Gauge developed for this application is described.