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Design, prototype via reverse engineering By Nitin K Shankar, Reverse engineering, once considered as something practiced by those who lack original
concepts, has now become an engineering science. Japanese success in new products has led
to reverse engineering being considered as a design process. Many American engineering
colleges have courses in reverse engineering to focus on redesign, instead of original
design, as a problem solving approach. Even the automobile industry uses a variant design
methodology, referred to as direct engineering, to replace more general original design
methods. Originally the Japanese used reverse engineering to improve on competitors' products and, thus, avoid original design effort. The redesign process was initiated by observing and testing a product. Thereafter, it was disassembled and the individual components were analyzed in terms of their form, function, assembly tolerances, and manufacturing process. The intent of this process step was to fully understand the execution of a product. Based on this understanding, an improved product was evolved, either at the subsystem (adaptive) or component (variant) level. In recent years, Americans and Europeans have reverse engineered the reversed engineering process and developed powerful tools to further compress development cycles. Such tools are relevant for industries in those countries where production engineers are faced with the problem of reproducing parts directly from samples. Making spares for obsolete equipment, fabricating copies of old tooling, or redesigning a foreign-licensed product to come up with a new look are examples of how reverse engineering can be successfully employed. Powerful expert software is giving new meaning to reverse engineering. Computers can now be used to capture the geometry of a part, visualize it in 3D form, carry out design changes, test it for engineering performance, and simulate its manufacturing and inspection cycle. In effect, most of the reverse engineering process can be carried out without actually making a prototype. One approach is to use a coordinate measuring machine to probe the surface of the part to be copied. The digital data captured can then be processed by CAD/CAM software to come up with a visual representation of the part as well as the CNC program needed to machine it. Many companies already have the two basic tools needed for reverse engineering, a coordinate measuring machine (CMM) and computer-assisted design (CAD) software. Yet, few companies have the right CMM/CAM interface for the kind of reverse engineering design capabilities needed in today's industrial environment. Selecting hardware As far as using a CMM for digitizing a part is concerned, the focus should be on throughput. Depending on the application, CMM manufacturers offer either single-point or scanning-point heads. The single-point trigger probe, clearly the dominant technology used in entry-level to mid-range CMMs, functions by contacting individual points on a workpiece. Although single-point probing is common, the technique can create significant variations in measuring results due to dispersion of probe points on the workpiece surface. Scanning CMMs make use of control technology to continuously scan along the contour of a workpiece surface, collecting hundreds or even thousands of independent probe points to define a part's true geometry. While a CMM's base accuracy is important, users should know that a true representation
of the part can be achieved by inspecting more points per workpiece. Acquiring a larger
number of data points (e.g. 400 points vs 4 points for diameter) will ensure higher
accuracy because the quality of calculated diameter and position inspections is directly
related to the number of probe points collected. A CMM's ability to reposition and repeat precise probing locations is influenced by the speed at which the system is driven, which also influences the quality of calculated-measuring results. The more points gathered, the greater the repeatability of results. Many factors can determine the measuring speed of a CMM, including acceleration, maximum velocity, probing speed, probing method (single-point or scanning), and the computation power of the CMM's software. Yet, throughput rather than speed is essential for reducing reverse engineering time cycles. There are CMMs that get more throughput via a little intelligence. Computers help them follow an optimal path, avoid obstacles, and speed up between probing points. In the case of reverse engineering applications, throughput includes the time taken to probe the original sample part, write the inspection program for the reverse engineered part, measure this part, and compute the results accurately. Off-line part programming can enable an average-speed CMM to have an acceptable total throughput time. Speeding up the process Companies can often save weeks of development time by scanning a sample on a CMM and the reverse engineering surfaces from the point data instead of modeling surfaces directly on a CAD system. The geometric data from the CMM will be generated in industry-standard formats such as IGES, VDA-FS, ISO G-code, DXF, and delimited ASCII and CAD/CAM; and analysis packages support at least one of these formats. Once the digitized image is captured, users still need the right CAD software to speed up the reverse engineering process. Ideally, CAD software should be equipped to:
Most important, the software should allow the user to visualize the part in a 3D perspective. A 3D model fully defines the shape of the part, eliminating the need for multiple view projection. Designers can rework surface contours, and toolmakers can then machine parts from the electronic mockups. The idea is that the software should accelerate the reverse engineering time cycle by:
Rapid prototyping Of the several CAD packages available, moldmakers may prefer Ann Arbor, MI-based Imageware Corp's Surfacer which generates high quality curve and surface geometry from digitized 3D point data. Point cloud data collected by probing can be visualized on the computer screen in various forms in order to construct surfaces. The process of building and verifying production tooling is time-consuming and expensive. Surfacer helps streamline this critical process by providing quick and complete verification of the most complex, free-form shapes. Users can precisely align scan data with CAD geometry to evaluate the differences between sample and engineered parts. Variations can be calculated and displayed as color-coded plots, clearly illustrating 100% geometric inspection. Surfacer's Rapid Prototyping Module (RPM) can quickly produce prototypes from digitized data or surface geometry from other systems. This shortens the production cycle between digitizing physical prototypes, creating CAD models, then finally generating prototypes. A new Rapid Tooling option to RPM dramatically improves the use of rapid prototyping technology for creating prototype tooling. Working in 3D Other CAD packages may be more suited for product reengineering. One CAD/CAM solution which might become the industry standard is CATIA. Developed by Dassault Systèmes of France, CATIA enables designers and engineers to work simultaneously on design problems and manufacturing details without actually building prototypes. Designers are able to create virtual prototypes to determine which one would best meet the needs of the real world. The idea is to do more work on simulating manufacturing techniques before actually making the part on the machine. Quality is part of the process Once the part is made, it has to be inspected. If different brands of CMMs are being used, an inspection package that works to an open standard is a must. One such program is PC-DMIS, a PC-based software package that works within the Windows operating system. The software provides a bidirectional link between the CMM and the CAD software that enables the CAD model to be used to scan the workpiece. In effect, the CMM's direct computer control (DCC) is driven by PC-DMIS and the measured results are compared with the CAD model. Another package is VW-Gedas' audimess which creates programs in the DMIS (Dimensional Measuring Interface Standard) format, a standard for exchanging data between CMM software solutions and CMMs. Such a standard makes the program CMM-independent. The audimess system operates within CATIA and uses CATIA data to create, modify, and simulate the inspection program. Since the programming is also done off-line, it can be modified on the screen at any time. This reduces the costs of inspection programming as well as enabling CMMs to be used for actual inspection tasks only.
Working on the CAD model, the user aligns the part, determines the probe path and selects the features to be inspected. Then audimess generates the DMIS program which is interpreted by a post-processor to convert the program for the required CMM. The whole program created by the audimess user can be simulated and graphically shown on the monitor screen. The starting point of simulation has to be selected and the probe will move along the programmed part on the screen. Simultaneously, a small window shows the actual DMIS code. The availability of all the above software should be enough to convince any user about the advantages to be obtained though a CAD/CAM systems interface. The key lies in making the transition from old-fashioned 2D CAD to the latest 3D techniques. Here engineering management has to focus on the correct application of 3D design to speed up reverse engineering processes. For more information from Brown & Sharpe, N Kingstown, RI, http://www.brownandsharpe .com Circle 389. For more information from Imageware Corp, Ann Arbor, MI, http://www.iware.com Circle 390. [tooling/incl/99tp.htm]Originally published in the May1999 issue |
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A virtual prototype
