Measurement with CAD Data

The numerical evaluation of scanned contours or surfaces is basically limited to regular geometric features such as cylinders, planes, straight lines, spheres and circles. However, modern methods of production increasingly permit the manufacture of free forms for which regular shapes no longer can be taken into consideration. Free-form parts are created whose form is described only by the CAD model.

The areas of interest on the object are measured via scanning. Then, measuring software modules compare the measured values with the CAD model in off-line mode. Another possibility is to link the CAD software module directly to the measuring machine: In this case, the areas of interest are first selected on the CAD model and then automatically measured using the selected sensor mechanism. The results are documented either by graphic comparison or via graphic display of the deviations from the CAD model (Fig. 49).

The colors of the measured points underscore the deviation between the nominal and the actual shape. Four classes can be distinguished here: “positive in tolerance”, “negative in tolerance”, “positive out of tolerance” and “negative out of tolerance”. The amount of the deviation is indicated by the color. The possibility of changing sensors allows measurement of the entire surface of a part, even in cases where complex forms are involved. By integrating rotating axes in the measuring system, various different views can be included in the measurement and the part can be measured from all sides. Depending on the job requirement involved, the measured results can be displayed either in a previously qualified reference coordinate system (for example, in vehicle coordinates in the automotive industry), or in a coordinate system which has been optimally fitted into selected surface areas. The location of the measured points is optimized here in relation to the CAD model in such a way that the deviations are kept as small as possible.

In the following, two possible best-fit strategies are presented based on the example of a 2-D section. In the first case, the location of the points actually measured is optimized by minimizing the distances from the nominal points (Werth BestFit). Since the tolerances of various different object areas are not taken into consideration, under certain circumstances values exceeding the tolerance are determined even though the tolerance could be maintained by shifting the coordinate system. This method is thus only marginally suitable for quality control. It is, however, often used to correct CAD data in order to improve the quality of the next production step. The optimization criterion of the second method (Werth ToleranceFit®) is to keep the distance between the measured point and the tolerance limit as large as possible or, if the measured point has already exceeded the tolerance limit, to keep the amount of the deviation from the tolerance limit as small as possible. A correct measurement is performed similarly to the way in which a plug gage is used. Figure 50 shows how an object which has been found to be defective according to the BestFit method (red areas present) but is not actually defective can be classified as functional according to the ToleranceFit® method.

Another advantage of the CAD modules integrated in the measuring software is that the CAD information can be used to position the coordinate measuring machine. The entire measuring run can be controlled by selecting the geometric features on the CAD model. The measuring machine automatically moves to the measuring positions thus generated. This operating mode is referred to as the Cad-Online® mode. Technological parameters such as the illumination setting for the image processing sensor can be adjusted via direct interaction between the measuring machine and the workpiece.

The same software configuration can also be operated without the measuring machine on a Cad-Offline® workstation. Here the measurement programs are created and tested on the CAD model. This saves valuable machining time and ensures that the measurement plans are prepared in time before the first workpiece is manufactured. Influencing factors relevant to the workpiece can then be edited during a test run performed in the single-step mode. The chief advantage of the solution described here is that all work can be performed based on a universal operating concept. Consistent use of the same software packages virtually excludes the possibility of incompatibilities. This is not the case if the programming workstations are purchased from a different source than the CMM manufacturer. Figure 51 shows a measurement plan produced in this manner in a two-dimensional graphic window. You can clearly recognize the manner in which the measured points or image processing windows are distributed over the CAD model. A very graphic simulation of the measuring run is thus possible in the off-line mode of operation.