If you check one of the standard mechanical engineering handbooks
(Machinery's Handbook is one) you will find that a shaft (pin) that has
to fit into a hole has three general tolerance zones.  If the pin is
0.0002" smaller than the hole it is called a slip fit.  If the pin is
the same size as the hole it is called an interference fit.  If the pin
is 0.0002" greater in diameter than the hole it is called a press fit.
The difference between an interference fit and a slip fit is the amount
of force required to get the pin into the hole.

Keeping all of this in mind, the tolerance on a "minus" set of pin
gauges is -0.0002" to -0.0000" (from the Meyers pin gauge catalogue)
which implies that you should use a minus set to measure holes.  Using
the example of an hole specified at 0.040" +/- 0.003", if a 0.037" minus
gauge fits into the hole the diameter is not too small.  If the 0.037"
pin doesn't fit then the hole is out of tolerance.  If a 0.044" minus
gauge fits into the hole it is out of tolerance.  If a 0.043" diameter
pin fits into the hole it is in tolerance.

For critical (beyond the scope of printed boards) hole diameters, I
suppose that the best procedure would be to use both minus and plus
gauges to perform the measurements.  In the above example, the minimum
(0.037") diameter would remain the same, but if a 0.043" plus gauge fit
into the hole then the hole would be out of tolerance.  With this method
the insertion force for the plus pin would be recorded .... you see what
I mean.

Considering other comments regarding drill wander and having each
circular segment in tolerance while a pin gauge won't fit into the hole,
I prefer to consider the board from my customer's view.  If a pin gauge
that falls inside the acceptable tolerance range doesn't fit into the
hole when it is approximately perpendicular to the board, then the
component won't fit into the board either.  When all is said and done
the purpose behind the specifications is to deliver a functional
component to the customer.

Along with the other people responding to your question I am not aware
of an industry specification that directly addressees your question.
Sorry about the delay for this response, but I was on vacation for a few
days and am digging through 172 Technet messages.  I hope this helped
answer your question.

Don Vischulis

[log in to unmask] wrote:
>
> I remember reading in the machinist hand book in the 1960's that pin gauges
> were 0.0002 inch smaller than the hole you wanted to measure.  I  believe it
> is also in the old quality control handbooks that used to be around when we
> had "quality control"
>
> Phil Hinton