Portable CMMs and software make reverse engineering useful for modifications, new designs, and duplicating parts without drawings.
Lake Mary, Fla.
If you can measure an object, you can reverse engineer it. The key is measuring with sufficient accuracy to capture the degree of detail necessary for a faithful reproduction. The concept behind reverse engineering (RE) is simple. First, measure the object or part. Then transcribe dimensions into a digital or CADcompatible format as an image of dots, streaming lines, or wire frames. The image can be enhanced for end use by engineering programs that deal with surfacing, stress analysis, human factors, ergonomics, plant layout, or product flow.
Several examples of reverse engineering show its usefulness. For instance, engineers in a steel mill needed to replace an aging 7,500-hp motor. The problem: The new motor would not fit on the old mount. Reverse engineering let engineers design an adapter so the old mount could accept the new motor.
RE also recreates drawings for old parts. For example, a blade on the impeller of an air compressor broke off after years of service. But the compressor manufacturer asked for eight months to make a new one. Plant engineers decided to reverse engineer a new one from the original, and measured the dimensions to digitally capture blade locations. Shaft and bearing dimensions were also recorded. This data was loaded into a CAM program to generate a machining plan. The new impeller was milled from an aluminum blank with the toughness and corrosion resistance at least equal to the original. From start to finish, the project took only three weeks.
Tool making and product testing also benefit from reverse engineering. Using a physical model, dimensions can be taken to create everything from molds to fixtures for robotic welders. The same process is used to adjust tooling to dial-in specifications. For complex automotive assemblies, fabricators have cut almost a year off the time to qualify so-called first-article parts.
A part recreated in CAD can also become a test object. For instance, the digital part can be stress analyzed, checked for fluid flow, and reproduced on rapid-prototyping equipment for ergonomic studies.
In the past, measuring a complex shape required novel techniques. One was stick building. Points on an object were measured using calipers, rules, or depth gages and a model was built with sticks, each representing an individual measurement. Model accuracy depended entirely on the skill of the model maker and the process usually required weeks to get right.
Eventually, engineers began using conventional CMMs to digitally capture objects. Accuracy greatly improved but the process remained fairly slow because the CMM had to be programmed for each different shape.
RE's tool today is a device originally developed for quality-assurance departments to make fast, inprocess quality checks. The portable CMM is based on an articulating arm, with joints housing optical encoders. They reproduce the X-Y-Z location and I-J-K orientation of a stylus to an accuracy of up to±0.0002 in. The arm moves freely within a sphere defined by its own reach. It captures data rapidly, with a measuring potential for hundreds of points per minute. The arm records measurements as individual points or streaming lines for CAD-compatible software.
Software for portable CMMs also simplifies data gathering. Most parts are composed of prismatic shapes such as arcs, circles, spheres, and rods. Users specify a standard shape from a menu, measure several points, and let the software complete the shape. Nonprismatic shapes can be digitized freehand, or by using one of several "locked-plane" scan techniques in which a part is traced and the software sections the trace as parallel planes, radial sections, or concentric circles.
Although the software is not intended as a design medium, it provides ways to manipulate images by reversing, doubling, or repositioning them. When a part is symmetrical, standard practice is to digitize half, then duplicate and reverse or mirror it in software, and join the two halves. An automotive model shop uses the technique to cut its digitizing work in half. It digitizes one fender then uses the software to create the mirror image.
A FEW APPLICATIONS FOR REVERSE ENGINEERING
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COMMON SOFTWARE PACKAGES
Wire frame (low-density images)
Polygonal shapes (high-density images)
CAM software and NC programs
Wire frame (low density)
Polygonal shapes (high density)
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Rendering software (for precise visual recreations)
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External rendering software
After capturing a basic image by a portable CMM, engineering programs transform the image into more useful shapes. A few compatible formats include CAM2 Measure X, Catia, Pro/E, STEP, and VDA.