Williams uses an articulated arm with a noncontact
probe to measure the profile and the software does
In addition to bending the part, Rockford Process Control uses drilling and machining
processes for various part features. Williams put an idle machining center to use for these steps
(left image). In a case of thinking outside the box, Williams added a drilling station (right
image) next to the machining center. The drilling station uses the Flowdrill® process to make
two holes. The drilling station is stationary; the machining center does the part positioning and
reference, and the locations of the part’s
features would have varied as the tube
length varied from heat to heat.
Verifying the part’s dimensions also
raised some questions. Two obvious choic-
es were a coordinate measuring machine
(CMM) and a test fixture. Williams
felt that an in-process gauge would be
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the more appropriate of the two.
“Some parts are more conducive to a
hard gauge, others to a CMM,” he said.
“Rectangular tubes aren’t perfectly
rectangular, straight tubes aren’t perfectly straight, and bending a tube introduces quite a bit of distortion. A CMM
picks up all of these discrepancies,”
Williams said. “Also, if you use a CMM
and you check different points from one
tube to the next, you’ll get varying
results. In some cases a tube fits into a
test fixture just fine, but the CMM data
says it shouldn’t,” he said (see Figure 2
and Figure 3).
At times like this, Williams refers to
an evaluation that has little to do with
dimensional tolerances and process
control, and a lot to do with instincts.
“Sometimes I ask myself, ‘Does this
pass the look-right test?’” he said. This
test relies more on common sense than
it does on formal manufacturing
principles, but it’s handy for many
situations, especially bend measurement.
And it’s simple.
“If it doesn’t look right, it doesn’t
pass the test,” he said.
RPC prefers test fixtures to verify bent
tubular parts, especially complex parts
with several bends. When RPC does use
a CMM, it checks the dimensions
of straight sections only, and relies on
the software to calculate the bends (see