How to measure really small hole diameter

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
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Pin gauges.

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Reminds me of how carburettor jets are measured - by timing how long it takes for a known amount of liquid to pass through by gravity.
Bob Swinney wrote:

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wrote:

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"
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Depending on the material, you may need to specify the temperature at which you are measuring if you need to have 4 decimal accuracy.
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wrote:

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.
jim
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wrote:

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. ERS
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surftom writes:

Machinist microscope, if you have a few spare $1Ks.
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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.

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woodsy wrote:

What about a .1251 hole? Would it fit into that or not fit until you have .1255 or .1260?
michael
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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
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On Friday, November 14, 2003 at 10:03:37 PM UTC+5, surftom wrote:

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>
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On Wed, 8 Jul 2020 09:55:15 -0700 (PDT), snipped-for-privacy@googlemail.com 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 1/10,000.
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    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:
    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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