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The Ins and Outs of Fixed Body Mechanical Plug Gages
Fixed body mechanical plug gages are one of the best
ways to measure hole diameters. With this type of gage there is no
rocking, as with an adjustable bore gage. It is fast and provides high
performance for tight tolerance holes. Because these gages are easy to
apply, they are usually used to measure high volumes of parts. Even in
cases where the parts are expensive and the tolerance tight, a fixed
body plug may be an economical choice for the application. There simply
may be no other way of measuring the critical part tolerance required.
With a fixed plug type bore gage, the body size of the
gage is very close to the diameter it is made to measure. Therefore, the
plug “locks” the gaging part of the plug in the hole. This requires very
little operator skill and results in virtually no operator influence.
Obviously, there has to be some body clearance between the plug body and
the hole diameter so the plug will fit into the hole being measured, and
control of this clearance is critical for the gage builder. Limited
clearance will provide shorter measuring range but better performance.
More clearance increases measuring range but allows for more
“centralizing error” (actually measuring the chord rather then the
diameter).
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With a fixed plug type bore gage, the
body size of the gage is very close to the diameter it is made to
measure. Therefore, the plug “locks” the gaging part of the plug in
the hole. |
In fact, the plug body acts as a way of holding and
centralizing an internal mechanical transfer mechanism. Typically this
consists of a set of contacts, mounted to some linear spring arrangement
that is in contact with a 90-degree transfer rod (see illustration
below). The transfer rod has a precisely ground vee in it that takes the
diameter change seen at the contacts and transfers it to an indicator
out the rear of the plug. The ground-in vee is made at such an angle
that there will be a one-to-one 90-degree transfer, and no compensation
is needed by the indicating device.
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The plug body holds and centralizes
an internal mechanical transfer mechanism. Typically this consists
of a set of contacts mounted to some linear spring arrangement in
contact with a 90 degree transfer rod. The rod has a precisely
ground vee that transfers the diameter change seen at the contacts
to an indicator out the rear of the plug. |
The rear of the vee rod will either have a spherical or
flat end. It’s important to note the style of the end, as the indicating
device monitoring the rod needs to be the opposite type of contact. For
example, a flat-ended vee rod should be used with an indicator having a
spherical contact and vice-versa. This assures a point-to-point motion
transfer.
The transfer mechanism is also a two-point floating
differential system. This means that once in the hole, the plug may be
moved (albeit slightly) side to side in the direction of the contacts.
When this is done, one contact will move out a little while the other
one moves in. Thus, they cancel each other out and the result is always
the diameter of the hole, regardless of plug position. This
configuration is very important for exploring the bores for various
geometric conditions, including ovality, barrel, taper or hourglass
shapes.
In most cases, tungsten is used for the contacts to
provide best wear characteristics for longest gage life. However, if the
part material is not best served by this type of contact, other
materials such as tool steel, ruby or diamond may be required, and are
readily available.
Finally, the most critical thing to remember about the
fixed bore gage is that it is a dedicated instrument. Specifying the
requirements for the application is really no different than specifying
for an adjustable core gage, snap gage or any other measuring device.
However, unlike these gages, the fixed bore plug is custom made for the
application. Get any one of the parameters wrong and the gage becomes a
very nice looking desk ornament. Always double check the application
when specifying to make sure you get the right custom gage.
Article courtesy of MMS Online.
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