Gage Pin vs Arbor Precision

Another situation unique to CL Mfg.

I was recently asked about gage pin precision compared to arbor precision.
The question was prompted by differing collet runout readings with the
two. Most notable was that the gage pin had more runout than the arbors.
And the gage pin runout was different depending on how the pin was held
in the collet while closing it. How could that be? One explanation could be
the plastic arbors are more “forgiving” and do not respond to minor defects
in the collet. This means they are not as hard as a gauge pin, so any defects
in the collet will not effect the run-out. A new collet should give the same
readings from a gage pin to an arbor.

Many of you know that we give away certified gage pins at the lens
manufacturer meetings. The 12.7 mm diameter gage pins are used to validate
the lathe collets. This has been a nice promotion that has real value. It fits in
with our mission to apply measurement to the CL manufacturing process.

We looked at the certification of these pins and found that it has to do only
with the diameter and straightness, not with the ends. So, when a gage pin is
inserted onto a collet which has a bottom ground face, the pin will either fit
against the face or the diameter of the collet. If it is against the face it may
not be straight in the collet.

This problem is partly due to the traditional use of collets from the machine
tool industry. That is, metal turning is usually done with long (meters) bar
stock where the collet is straight through. One part is made and the rod
is pushed through to a stop for the next part until the rod is used up. Step
collets are rarely used and usually only for second operations.

If a machinist wants to validate a collet, a gage pin is inserted fully engaging
the collet and a reading is taken. There is no face or shoulder to worry about.
The CL industry uses step collets or bottom stop collets to hold the base
curve or arbor. So, this requires an end of the arbor that is perpendicular to
the shank of the arbor. This squareness is a result of how the arbors are made
and is usually very good. This explains the better readings with a good arbor
versus a gage pin.

So what can we do with the gage pin? Since the pins are about 50 mm long,
I cut one in half and ground the ends perpendicular to the diameter. This was
a bit of work, but it is what is required for a validation tool.

One creative lab tech asked about locating off of the nose of the collet, thus
avoiding the gage pin end problem. To my knowledge, collet noses are not
precision ground (unless specified) and should not be used for precision
location. A better approach is to use the face of the spindle for a flanged
arbor location which is about as precise as it gets.

So there are always details that present themselves that we have to address.

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