The Case for In-Process Gaging - ASCONA

machines, which offer accuracies of ±18
µm at 20º C. The issue is that small FOV
systems have to be scanned across a
sample to capture a complete view. Stage
accuracies are severely hampered by dirt
and temperature. Advancements leading
to large FOV systems that allow a complete
view of a part without stage movement
eliminate that problem. As a result,
the desired precision can be achieved at a
much higher temperature so that deployment
is possible on the manufacturing
floor. Today’s in-process optical systems
have accuracies of ±10 µm across a wide
temperature range.
With regard to dimensional inspection,
the advantage of optical vs. contact-based
inspection systems is largely one of flexibility
and speed. When many different
profiles are extruded, these factors are
critical to the success of an in-process
gage. Flexibility means the ease and speed
to program measurement points to set up
the reference file. Driven by advances in
software engineering, reference file setup
is now a five-minute process vs. 60-plus
minutes for older technologies. In addition,
the cycle time to take the measurements in
process using the latest hardware is now
fewer than 10 seconds; in earlier scanning
systems, more than two minutes were
required. Because the optical system has no
moving parts, both reference file programming
and actual measurement cycle time is
dictated by the software in the system. This
means that the complexity of parts has little
to no effect on inspection cycle time.
However, disadvantages for optical
inspection often include sample preparation
requirements and limitation to 2-D
applications. Because the optical image
taken is from a 2-D perspective, the sample
part must be presented in a specific way.
Depending on the manufacturing process
itself, some form of preparation may be
required. Clearly, not all in-process applications
are appropriate for optical inspection.
Optical imaging techniques utilize four
primary components: camera, optical lens,
light source, and the part fixture or positioning
device.
The world of high-resolution cameras
has grown dramatically in recent
years, leading to more accurate measurements
from optical systems within a much
shorter cycle time. For example, in the last
by Derek Nogiec
18 months, 3.1-megapixel cameras have
evolved to 12.1-megapixel cameras, an
almost 300-percent improvement in precision.
Camera cycle time (i.e., the amount of
time it takes to capture an image) has been
reduced from 60 seconds to one second.
For cameras of any given resolution,
the smaller the area under view, the higher
the measurement resolution. In the past,
capturing an accurate image for measurement
meant using a lens with a small
FOV paired with a high-resolution camera.
This worked well except when parts were
larger than the viewing area of the lens. In
these cases, the optical system had to take
multiple pictures of the part with a moving
camera, introducing more error. With the
advent of higher resolution cameras, wider
FOV lenses could be used. The need for
multiple part images was reduced or eliminated
while still maintaining an acceptable
level of measurement accuracy.
Variations in light source and part
fixtures are typically sample part-specific,
with the major variations stemming from
part size and material.
All of the above-mentioned components
can be purchased off the shelf.
Quality Digest/November 2005 35