On 2020-07-09, Charlie+ wrote:
On Wed, 8 Jul 2020 09:55:15 -0700 (PDT), wrote
as underneath :

On Friday, November 14, 2003 at 10:03:37 PM UTC+5, surftom wrote:
How do I *accurately* (n.nnnn) measure very small hole diameters?

I can poke them with a numbered drill but that would only get me close by
"feel" and only if the hole was a standard size. e.g. measureing a .0431
hole is kind of hard with a vernier or mic

No real reason - enquiring minds just want to know

Tom

Hi Tom,
you can easily measure min 0.1 from vernier caliper and +/- 0.001 mm to +/- 0.02 mm with micro meter.
Source: a href="https://ahmedtrader.com/kendo-tapes-kobe-spray/"ahmedtrader.com/a

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
1/10,000.

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 a

0.175" 4.44mm
0.300" 7.62mm

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.

Enjoy,
DoN.

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