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Challenge of
first article inspection
Most of these
very complex geometry-shaped components are designed to be injection
molded, requiring costly and time-consuming tooling. Headsets often have
many components with very intricate geometry that must fit together
perfectly at the assembly stage. Furthermore, maintaining the geometric
integrity of all components is critical to successful, time-to-market
delivery of the company's cutting-edge styling demanded by the customer.
The inspection at this stage is very critical.
In the past,
first article inspection was a very difficult and labor intensive
process. Engineers first generated 10 to 30 datum points, picking points
that seem to be key in defining the overall geometry. Inspectors then
moved the touch probe of a coordinate measuring machine (CMM) around the
parts to measure the geometry of each of these individual points. The
measured datum points were then compared to the points captured by the
CMM. If the points matched with the part tolerances then the first
article was approved. If the points were out of tolerance, then the
supplier was informed of the discrepancy and asked to produce a new
first article.
There were
several challenges with this approach. First of all, the previous method
was an incomplete inspection of the part. The possibility existed that
the datum points could be within tolerances on the first article while
other points on the part might be out of tolerance. Normally, the datum
points characterized all the features that were critical in the assembly
match-up and envelope considerations but there was no guarantee that
this was always the case. Engineers could increase the number of datum
points to increase the probability that they fully represented
the part geometry but this would have increased the time required to
perform the inspection.
Laser scanning
provides alternative
Company engineers
looked for an alternative method of performing the first article
inspection. They heard about laser scanning systems that work by
projecting a line of laser light onto surfaces while cameras
continuously triangulate the changing distance and profile of the laser
line as it sweeps along, enabling the object to be accurately
replicated. The laser probe computer translates the video image of the
line into 3D coordinates, providing real-time data renderings that give
the operator immediate feedback on areas that might have been missed.
Laser scanners are able to quickly measure large parts while generating
far greater numbers of data points than probes without the need for
templates or fixtures. Since there is no contact tip on a laser scanner
that must physically touch the object, the problems of depressing soft
objects, measuring small details, and capturing complex free form
surfaces are eliminated.
Instead of
collecting points one by one, the laser scanner picks up tens of
thousands of points every second. Special software, Geomagic Qualify
Inspection Software can be used to compare original design geometry to
the actual physical part, generating an overall graduated color error
plot that shows in a glance where, and by how much, surfaces deviate
from the original design.
Company engineers
evaluated various types of laser scanning equipment. They selected the
Surveyor DS 3D laser scanner from Laser Design Inc., Minneapolis,
Minnesota, because it offers an exceptionally high level of accuracy
based upon a highly automated, CMM machine base of granite and steel
composition that positions the laser probe with a very high level of
accuracy. The laser probe scans parts from all directions then rotates
the data back into a common coordinate system. The standard scale
resolution is 0.00004 inches and 0.00002 inch resolution is available as
an option. Linear accuracy of the table is 0.000220 inches plus 0.000010
inches per inch and repeatability is 0.00024 inches. Various models are
available to handle maximum part sizes ranging from 20” X 20” X 15” to
93” X 105” X 156”.
New process
for first article inspection
The next step was
developing a new process for first article inspection based on the use
of laser scanning technology. With a laser scanner there is no need to
select datum points because the scanner captures the complete part
geometry. Instead inspectors simply mount the part on the table of the
laser scanner and start the scanner to automatically scan the entire
part geometry. Typically, they turn the part once during this process
and the scanner software assembles the different views of the part into
a single point cloud that defines the part’s 3D geometry in high
resolution and complete detail. The inspector then asks the Geomagic
Qualify Inspection software to compare the point cloud scan data to the
part’s original CAD model. The result is a Color Error Map that quickly
shows any discrepancy between the scanned part and the original design
intent of the part.
“Laser scanning
has substantially improved our ability to inspect first articles,” the
Mechanical Inspector concluded. “As our company places more emphasis on
style-driven consumer markets, the complexity of the geometry of our
first articles has greatly increased. Touch probe inspection takes so
long that we were only able to inspect a limited number of points, not
enough to provide a global comparison. Laser scanning, on the other
hand, enables us to completely inspect the geometry of every first
article. Another advantage is that the first article inspection process
can be completed in about four hours, about half the time that was
required in the past. The time that we save in first article inspection
translates directly into reduced time to market since we can now provide
a faster response to our suppliers that enables them to get our parts
into production.” |
