There are devices that look rather like a large jeweller's loup that
are used to measure things like that. They often have interchangable
reticles gradiated in millimeters or thousanths of an inch etc. They
are called "optical comparators" and "scale loups"
Gently enter a standard taper pin (available in a wide range of
sizes)into the hole. Mic the diameter where it stops or mic the large
end diameter and calculate the entry diameter from the taper distance
- 1/4" per foot for imperial taper pins, 1 in 50 for metric pins.
Gauge pins can be bought in .00005 inch increments. Deltronic makes
them. Also, air gauges and their electronic counterparts can be used.
Air gauges work by letting air out a hole. This relieves the pressure
some what in the system. This pressure either moves a dial or floats
an indicator in a graduated tube. The hole in the gauging member will
be close to the inside of the hole. If the hole is the same size as
the gauge then the pressure will be high. If the hole is much oversize
then the pressure will be lower because the side of the hole will be
farther away from the hole in the gauge thus letting more air out.
There are other ways as well.
The problem with pins is you need to allow for clearance. A .1250 pin will
not fit into a .1250 hole. It will fit into a .1255/.126 hole but not a
.1250 hole. Last place I worked had a hard time understanding this concept.
Air gauges are the most accurate and fastest way to check small holes.
What size is a very small hole ? We jig holes .02900 Diameter
holding .0001 tol. on the diameter and .0001 on location.We check them using
lapped gage pins on a Moore inspection machine.The hole has a very good grind
finish but not good enough for checking with the gage head that reads in
.00001+or-Our coustmer checks them to + or- .000035 This is a series of holes
with a pin in them.This pin has a hole in it and thats what is held to the
.0001 tolarance. These are gages to check pin locations on gold plated plugs
that are used in a wepon system. Ray Mueller
On Wed, 8 Jul 2020 09:55:15 -0700 (PDT), firstname.lastname@example.org wrote
as underneath :
Short of accurate plug gauge, optical projectors used to be the way in
instrument engineering, you could do it as a bodge with a macro lens
photo and calibrating from your nearest fit drill at identical distance
+ simple maths etc. But there will always be an error envelope down at
Beware that the shank of a drill bit is typically a little
smaller than the hole size which it drills -- at least until it gets
burred by slipping in the chuck. :-)
For measuring in a fairly narrow range, and assuming that the
hole does not have a beveled edge, there are devices for the purpose.
They push a cone into the hole, and measure how far in it goes.
The one which I have is a "Hole Check" by SPI. It comes with a
steel plate with three holes, each marked in both Imperial and metric
units. Only one of the three holes matches the tool which I have. It
is marked 0.070" and 1.78mm. It has a dial indicator with a custom
scale on it to match the point. The range is from 0.030" up to 0.127".
The other two holes are:
If you don't have one of these, a set of wire gauges for the
smaller sizes, or pin gauges for those a bit larger. They come in
0.001" steps in size, and they are not bothered by a beveled edge hole,
while the tapered pin and dial gauge is.
If it is a through hole, you could perhaps measure it by how
much air can flow through it at a given pressure.
For fairly small holes (say down to about 0.100" or so), the
split "small hole gauges" slip in, you turn a knob at the top until they
drag slightly, pull it out, and measure the size with a micrometer.
The big end of a typical set is around 0.500" max, IIRC. Starrett makes
them, Lufkin used to, and likely a bunch of other names by now. But you
need to develop a feel for this the drag on the ID of the hole vs the
drag on the anvils of the micrometer.
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