Compare to CAD data

The conventional calculation of measurements is also not possible. Free forms are only described by a CAD model. With the colour-coded representation, the form and position deviations of standard geometries can also be easily displayed.

For measurement, the areas of interest of the object are scanned or captured as a point cloud. Measuring software modules then compare the measured values with the CAD model. The result is documented by vectorial or colour-coded representation of the deviations from the CAD model (Fig. 53). This evaluation can be carried out as part of the measurement sequence on the machine or in offline mode at a separate evaluation station. The colours of the measurement points illustrate the deviation between the target and actual values. To include the part tolerances in the display, they are divided into four basic classes:

• positive within tolerance
• negative within tolerance
• positive outside tolerance
• negative outside tolerance.

The optimisation criterion of the second method (Werth ToleranceFit®) is to keep the distance between the measurement point and the tolerance limit as large as possible or, if the measurement point is outside the tolerance limit, to keep the tolerance overrun as small as possible. Figure 54 shows that the object recognised as faulty according to the BestFit method (red areas present), but not actually faulty, can be classified as functional according to the ToleranceFit® method. The contour is checked as with a gauge. The prerequisite is the conversion of the numerical values for form and dimensional tolerance usually specified by design offices into a contour-related tolerance zone system (Fig. 54), which is better adapted to the manufacturing process. The definition of tolerance structures, especially with regard to datum systems, should be agreed between the manufacturer and the customer from the outset if possible. The main advantage of this test procedure lies in the functional measurement. The output is clear and clear in graphic form, so that even operators who are not specially trained can work with it.

In order to incorporate the measured or calculated deviations into the manufacturing process, the specification data can be modified with WinWerth® FormCorrect. To do this, the deviations between the original CAD model and the measured data of a sample workpiece are determined and mirrored on the model. From this, the measurement software generates a corrected CAD model that can be used to compensate for systematic production deviations in the plastic injection moulding process[8] and 3D printing. For high-resolution corrections and for the modification of internal surfaces, the use of coordinate measuring machines with X-ray tomography sensors is recommended (Fig. 55). A similar procedure is possible with the 2D BestFit software. Mould correction can be used both when running in new cutting tools (profile grinding, form milling) and during wire EDM to correct positioning deviations.