Mixing technology, intuition, and art, problems arise in developing automotive body parts from the fact that the most difficult design challenges are almost always resolved with a clay model rather than on the computer. Then the design engineers face the hurdle of updating the computer-aided design (CAD) model to reflect the changes made in clay. Capturing points with a coordinate measuring machine (CMM) is difficult because it took designers at one aftermarket manufacturer too long to capture enough points to completely define the surfacegoing back and forth between the CAD file and models until everything looked right.
Recently, those developers have switched to a laser scanner from NVision that quickly captures millions of points from the surface of the clay model, enough to completely characterize the model. The point cloud can then be imported into the CAD model and used to fully characterize the new surface. Besides the substantial time savings, engineers have more confidence than in the past that they have a perfect match between the clay and CAD models.
Difficulties of characterizing clay models
"The computer provides enormous advantages as a body design tool compared to the days when the entire process was carried out in clay," said Martin Meade, a studio engineer formerly with Porsche Engineering Services. "But when things get really tough, which typically happens several times during the life of any design project, the tools in the computer become too limiting and the designers utilize a clay model in order to gain the freedom they need. The subtle changes that are required to make the highlights and forms look right can be much more easily and accurately made and visualized on a physical model rather than on the computer screen. A typical example comes when you have a wheel flare running into a rear bumper. It's very hard to get the surfaces to blend together and look just right. Usually the best way is to smear some clay and start scraping until it looks good. But then you face the very difficult challenge of mathematically characterizing the physical model."
In the past, designers normally used the touch probe on a CMM to mathematically define the clay model geometry. They manually moved the probe around the clay model, capturing one point at a time. The biggest problem with this approach is that a very large number of points are often needed to define a complex surface. It's almost impossible to determine which points are needed. So, designers typically begin by capturing a number of points, then moving the resulting point cloud into the CAD program. They use these points to define a surface and compare it to the physical model. Typically it doesn't look right. So they must then go back to the clay model and capture additional points in the areas that need improvement
Often this process needs to be repeated many times before the CAD model looks like the physical model. Even when they are done, designers are sometimes uncertain as to whether they have truly captured the physical geometry.