# How to measure really small hole diameter

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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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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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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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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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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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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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