Laser scanning fits internal components into tight packages
Accuracy can be improved, saving valuable cost and time-to-market.
Laser scanning can play a key role in engineering internal components to fit within tight surroundings. The problem with verifying the fit of components produced by rapid prototyping is that their tolerances are typically much looser than the finished parts. So they provide only a rough idea of how the assembly fits together, which often isn’t enough for a groundbreaking design. If inaccuracies in the rapid prototype parts cause engineers to make bad decisions, they could be forced to make additional $50,000 die-casting molds and, worse yet, delay the product introduction by four to six weeks, which could cost millions in revenues.
Some companies have overcome this problem by using a service bureau to reverse-engineer the rapid prototype parts to an accuracy of 0.001 in., making it easy to distinguish between problems caused by prototype inaccuracies and problems with the design. Based on the prototype measurements, engineers make changes to the design and adjust tolerances. Laser scanning makes it possible to validate rapid prototype parts to the master solid model and use them to accurately evaluate design intent. Once a critical area is identified, a laser scanning service bureau can zoom in for an extremely close look that helps get the product to market weeks faster with a perfect fit.
The compact size of many portable products creates numerous mechanical engineering challenges. In many cases, the greatest challenge is fitting all the components within the case. The task’s complexity increases when the case’s geometry is complex, with numerous contoured 3D surfaces, so it’s difficult to measure. In some cases, there may be numerous features that need to mate up with internal components.
Engineers may build up an assembly model of the entire product during the design process and find that everything seems to fit together just fine. But there’s no way they would consider investing in injection-molding or die-casting tooling without having actual parts they can put together and make sure work right. The problem is that the tolerances involved in the stereolithography process are rather large, typically about 0.002 in. The production process has much tighter tolerances, around 0.005 in. So when they make the prototypes and put them together, they have no way of knowing if the interferences they see are due to the prototypes being beyond the production tolerances or problems with the design. They also can’t be sure whether other areas that fit together fine are actually fitting, only because the prototypes were out of tolerance and would no longer fit once they went into production.
Tooling for die-cast and injection-molded parts can easily cost $50,000 each and take four to six weeks to build. The danger is that if you build the tooling and it turns out to be wrong, you would have to modify the tools, which would probably cost at least $10,000 and take a few weeks. The worst-case situation would arise if the tools were so far off that they had to be rebuilt from scratch. That would cost another $50,000 and delay the introduction by four to six weeks. The potential cost of the delay is actually much greater than the tooling cost because the product lifecycle in many fast-changing portable markets is typically only 18 to 24 months. Any delay in getting the product to market sacrifices a significant chunk of revenues that would otherwise be generated by the product over its lifetime and reduces the chances of beating competitors to market. So it’s essential to get the design right the first time.
One alternative is taking measure-ments on a coordinate-measuring machine, but that isn’t very helpful in the common case where there are no flat surfaces on the part so there really aren’t any reference points to measure against. Laser scanning represents a viable solution because it can replicate the complete geometry of a complex part in the form of a surface model to a high level of accuracy, typically about 0.001 in. Then the model can be superimposed on the original design geometry to determine exactly where they differ. Laser scanning systems work by projecting laser light onto surfaces, while cameras continuously triangulate the changing distance and profile of the laser as it sweeps along, enabling the object to be accurately replicated. Laser scanners are able to quickly measure large parts while generating far greater numbers of data points than touch probes without the need for templates or fixtures. Because there’s no probe on a laser scanner that must physically touch the object, the problems of depressing soft objects and measuring small cavities are eliminated.
Most portable-system designers have a relatively small number of parts that needed scanning every year. Hence, it’s probably not a smart idea to purchase a laser scanner. The cost is high, and the company would face the challenge of training operators and maintenance staff. There’s also the risk that the machine could become obsolete before it pays for itself. Fortunately, service bureaus have arisen that offer laser scanning services on a project basis. The best of these service bureaus can achieve a high level of accuracy, fast turnaround, and reasonable prices. Just as important, their people will explain the capabilities and limitations of the laser scanning process and make sure their customers get everything they need to make design decisions.
In a typical example, designers shipped the pre-production prototypes of a portable projector’s critical components. The projector packs a lot of functionality into a small package. A long, thin form factor makes it possible to fit it into places that a regular mobile projector wouldn’t. Within a couple of days, the projector designers received computer-aided design files that defined their complete geometry. Each individual point was accurate to within 8 microns and surfaces generated from the point cloud were accurate to at least 0.001 in. The designers identified several problem areas and asked the scanning house to zoom in and rescan at a higher accuracy level.
Judy Dancer is an engineering services manager at InFocus. She holds an AS degree in medical technology and a BS in business administration. Based in Wilsonville, OR, the company can be reached at (800) 294-6400, (503) 685-8888, or www.infocus.com.
