Design News for Mechanical and Design Engineers

City of Industry, CA--For Scott Knight, president of Scott's Hammer Works, building antique car components has traditionally been a tedious and painstaking manual process. Now, desktop photogrammetry enables Knight to build these components to a much higher level of accuracy than was possible in the past. The result: parts that fit better with other components, and a car body that more closely matches the appearance of the original.

Because this process is so difficult, Knight now uses 3D Builder Pro photogrammetric software (3D Construction Company; Elizabethton, TN) to convert a photograph of the part being made into a digital image. He manipulates the image in a computer graphics program, defining edges and points that constitute critical areas. He then converts the image to a computer model and prints section drawings that serve as templates for cutting plywood stations. These are used to make "bucks" that serve as guides to form the sheet metal to its final shape.

"A key advantage of this program is that I can enter measurements to set any dimensions or coordinates equal to physical measurements," explains Knight. To accurately model smooth surfaces, he creates artificial reference points on the vehicle being photographed and measures their distance against hard points. Once he enters the information in the program, it automatically adjusts the image so all measurements are maintained at 100% accuracy in the finished model.

Another benefit for Knight is that 3D Builder Pro does not require full frame prints. "While full frame prints are desirable, the program can analyze any image and determine its center with very good accuracy. The only limitation is that photos cannot be greatly cropped," he says. Photographs from 35-mm, digital, or video cameras are acceptable, as well as images from scanners and photo CDs. The program can even make a good estimate of the focal length of the camera which must be known in order to perform photogrammetry, he adds.

Perhaps the biggest advantage, says Knight: considerable ease of use. The entire process comprises only three steps. First, create a digital image of your photograph. Then locate points, lines, or faces of interest in the object. Finally, identify photo relationships by marking the points in a picture that correspond to other points in other pictures. The software then generates a 3-D model of your objects. And it can export the resulting 3-D object file into your rendering or animation program.

A simple photo can be converted to a model in 10 minutes. Building accurate antique car parts is a lengthier process, requiring the use of multiple photographs and a considerable number of reference points to enhance the accuracy of the model. Despite that fact, says Knight, photographs of a typical fender can easily be converted into an accurate 3-D model in about six hours. "That's a dramatic reduction in time over the previous methods. And, with the new approach, accuracy on the order of +0.025 inch can be achieved. It was usually necessary to settle for +0.250 inch with the previous method," he notes.

Once the model is complete, Knight imports it into SolidWorks 97 (SolidWorks Corp.; Concord, MA) where he connects the points with curves and adds lofted surfaces. He can also view the model from various angles and with different light sources, making it possible to fine tune the appearance of the fender and smooth any rough edges. This process typically takes about two hours, including the creation of section cuts that are used to guide the construction of the mockup. "The sheet-metal parts formed from the mock-up provide an appearance far superior to traditional pieces and do not require any fitting," says Knight. One additional benefit of the photogrammetric process: It significantly reduces the cost of producing these car components.