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Re-using
geometry for new filter product
Veins carry
blood from all organs of the body back to the heart. There are a number
of conditions that can occur as a result of localized inflammation and
clotting of veins, particularly in the legs. Deep vein thrombosis is the
term often used for a blood clot that develops in the deep veins of the
legs. This is a potentially dangerous condition because there is some
risk that a part of the clot can break loose and travel in the blood
stream to lodge in the lung, which is a condition known as a pulmonary
embolus. The mainstay of treatment is anticoagulant therapy using coumadin or heparin. However, in selective circumstances, a venous
filter is placed to prevent blood clots from reaching the lung.
The geometry of
the new filter is based on the design of an arterial filter that has
been on the market for some time and has been proven to be both safe and
effective. Engineers wanted to take advantage of this experience by
duplicating much of the flow path geometry from the earlier arterial
filter design in the new venous filters. This can be done easily by
scanning the existing flow path geometry and extracting IGES curves from
the triangulated point cloud in the form of an STL file for use as a
template in creating the new filter geometry.
Challenge of
validating CAD model with as-built product
But the
engineers realized that the complexity of the flow path geometry meant
that they could not be certain that the computer aided design model that
they intended to re-use in the venous filter actually matched the molded
part that had been proven in the field. They had complete confidence in
the molded part because it worked fine in the market. But the geometry
was so complex that there was no way that they could be sure that it
matched the original CAD model. They thought about inspecting the part
with a coordinate measuring machine (CMM) but realized that the number
of points that would be required to fully validate the 3D surfaces was
far more than it would be practical to capture using one-point-at-a-time
contact methods.
Fortunately, the
technology of laser scanning has emerged in recent years to present a
viable alternative. Laser scanners work by projecting a line of 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. They collect thousands of points
every second at a high level of accuracy so they are able to accurately
digitize complicated parts. The elimination of the need to maintain
contact with the workpiece also means that the results are independent
of the skill of the operator.
With a
relatively small number of parts that need scanning every year,
engineers felt that it didn’t make sense to rush into a purchase of a
machine. So they looked for a service bureau that could provide the high
accuracy and fast turnaround the company needed on nearly every project.
They talked to three different companies and selected the GKS Inspection
Services division of Laser Design
Inc because they not only understand laser scanning technology but they also
understand part inspection and were ready, able and willing to help them
through the entire process.
Laser
scanning provides fast and economical solution
The engineers
emailed the CAD geometry and shipped the physical part to GKS Inspection
Services.
GKS then scanned the part, generating a point cloud with the
millions of points needed to accurately define the complex surface
geometry of the part. GKS then generated a graphical comparison
of the manufactured part vs. the CAD model, while coding differences
between the two in colors that indicate the magnitude of the variation
from the design intent. Being able to see the differences in 3D made it
much easier and faster to compare the parts. The engineers were able to
instantly visualize the differences between the model and the molded
part. They asked GKS to rescan several areas that were critical
at a higher level of resolution. Based on these scans, engineers made a
number of modifications to their CAD model to match it more perfectly to
the actual as-built part. The entire process of scanning the part and
developing the difference map took only about one day plus the time of
shipping the part to GKS. The cost for the Inspection Process
of scanning and comparing back to the CAD model was minimal and the turn
around was much faster than normally expected for conventional CMM touch
probe based measuring yielding uncertain results.
When you are
re-using existing design geometry, it’s a good time to stop and ask
whether or not the geometry that you are re-using actually matches the
product that has been proven in the field. This question is particularly
important when you are dealing with medical devices. This application
demonstrates that laser scanning provides a fast, accurate and
economical method of validating the geometry of even the most complex
geometries. The new venous filter has since gone into production and
has proven just as safe and effective as its predecessors.
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