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Laser
Scanning Helps Fit Internal Components in Small Digital Projector
Laser scanning played a key role in engineering internal components to fit
within a 2.05 inch by 3.69 inch by 9.75 inch portable digital projector.
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 wasn’t enough for a groundbreaking design like the under
2 pound projector. If inaccuracies in the rapid prototype parts had caused
engineers to make bad decisions, they could have been forced to make
additional $50,000 die casting molds and, worse yet, delayed the product
introduction by 4 to 6 weeks which would have cost millions in revenues.
The engineers overcame this problem by using GKS Inspection Services’ service
bureau to reverse engineer the rapid prototype parts to an accuracy of
0.001 inch, making it easy to distinguish between problems caused by
prototype inaccuracies and problems with the design. Based on the
prototype measurements, engineers made several changes to the design and
also adjusted tolerances in several areas. “Laser scanning made it
possible to validate our rapid prototype parts to the master solid model
and use them to accurately evaluate our design intent,” said the
manufacturer’s Product Manager. “Once we identified a critical area,
the laser scanning service bureau zoomed in for an extremely close look
that helped us get our product to market weeks faster with a perfect
fit.”
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The
projector’s compact size created numerous mechanical engineering
challenges. The greatest challenge was fitting all of the components
within the die-cast magnesium alloy case. The complexity of the task was
increased by the fact that the geometry of the case is so complex, with
numerous contoured 3D surfaces, so it’s very difficult to measure.
Furthermore, there are numerous features on the case that need to mate up
with internal components. “We built up an assembly model of the entire
projector during the design process and everything seemed to fit together
just fine,” the Product Manager said. But there was no way that we would
consider investing in die casting tooling without having actual parts that
we could put together and make sure they worked right. The problem that we
ran into, and have faced many times in the past, is that the tolerances
involved in the stereolithography process are rather large, typically
about 20 thousandths of an inch. The production process has much tighter
tolerances, around 5 thousandths. So when we made the prototypes and put
them together we had no way of knowing if the interferences we were seeing
were due to the prototypes being out of the production tolerances or
problems with our design. We also wondered if other areas that fit
together fine were actually fitting only because the prototypes were out
of tolerance and would no longer fit once we went into production.”
“We
took some measurements on a coordinate measuring machine but this was not
very helpful because there are no flat surfaces on the part so there
really aren’t any reference points to measure against,” the Product
Manager continued. “Fortunately, we had faced this same problem on
previous, if not quite as congested designs, and developed a very viable
solution. Laser scanning can replicate the complete geometry of a complex
part in the form of a surface model to a high level of accuracy, typically
about 1 thousandth. Then the model can be superimposed upon the original
design geometry to determine exactly where they differ.”
The
pre-production prototypes of the critical components were shipped to GKS
Inspection Services and within a couple of days the projector manufacturer 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 inch. The projector
manufacturer identified several problem areas and asked GKS Inspection
Services to
zoom in and rescan them at a higher level of accuracy. “The as-built
geometry provided by GKS Inspection Services made it possible to compare the geometry
of the prototypes to the design intent and determine exactly where they
differed. That made it possible to determine which fit problems we saw
were caused by the prototypes being out of tolerance and which were
actually problems with the design. We also checked the prototypes for
areas where being out of tolerance made the prototypes fit together better
than we could have achieved with the actual production parts. Based on the
results, we made several design changes that addressed interference issues
that, if they hadn’t been fixed, could have caused delays and additional
tooling costs. In the end, we saved a considerable amount of time because
we were able to zero in on the real problems and not waste any effort on
the issues that were caused by prototype inaccuracies. The first
production parts produced with the original tooling fit together
perfectly, making it possible to beat competitors to market with a product
that is substantial smaller and lighter than existing products.
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