# Novice tool for measuring small hole diameters



## mklotz (Jan 13, 2008)

Measuring the diameter of small holes is a challenge for the novice with few tools - as it was for me when I was starting out.

Vernier calipers are poor tools for this job because most of them have slight flats on the inside jaws. A much better tool is an inside micrometer but most of them can't measure holes smaller than about 0.2" because of their jaw size.

The right tool(s) for the job is a set of pin gages but they can be pricey for someone in the early stages of equipping a shop.

While I use pin gages today, here's how I dealt with the problem in the earlier, poorer days.







The tool consists of a handle with a flat base, a through hole that's a nice sliding fit on the measuring rod and a setscrew to lock the measuring rod in place when a measurement is being made.

The measuring rod has a nice gentle taper turned on one end and has a flat for the setscrew to bear on. Note that the flat doesn't extend to the end of the rod. This prevents the rod from falling out when the setscrew is loosened.

Here's a picture of the two parts assembled and ready for use.






Using the tool is simple. The gage is set over the hole and the setscrew loosened so the cone can descend into the hole as far as it will go. The setscrew is locked to capture the measurement. The diameter of the hole is then determined by measuring the projection of the flat end of the rod from the top of the gage.

Calibrate the tool by first drilling and reaming a hole of known diameter in brass. Measure the projection of the rod when inserted into this hole. Write these two data (hole size & rod projection) down and keep the note with the tool. Now, when measuring a hole of unknown diameter it's a simple matter of a little ratio math to get the size of the unknown hole from the measured projection.

While it should be obvious, I guess I should mention that the exact angle of the cone is irrelevant if the tool is used in the way I've described.

And now for a MAJOR CAVEAT...

This is most definitely NOT a precision tool. The accuracy of the measurement will be affected by the hole edge quality. Deburred or chamfered holes will cause inaccurate measurements from this tool. IF YOU WANT TO ACCURATELY MEASURE SMALL HOLES, GET SOME PIN GAGES.

In fact, given that caveat, I hesitated to show this tool. Nevertheless, for the tool poor novice who wants to get a close estimate, this is something to consider (if you keep the caveat in mind when using it).


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## BobWarfield (Jan 13, 2008)

Nice little gadget, Marv.

Your gadget is not unlike a Starret gage I have: http://catalog.starrett.com/catalog/images/objects/900/870.jpg

They're not too expensive on eBay. 

Small holes are indeed a bear. In fact, I will argue any bore is difficult to measure really accurately. There's a whole raft of different professional tools made to do the job: small hole gages (finicky like snap gages), the Starret gage I mention, inside micrometers, pin gages, dial bore gages, Intramiks (those odd 3 footed inside micrometers), and so on. 

The other standby is to use drill bits--not the cutting end, the other end--as a poor man's pin gage. You can at least check them with your micrometer to see their size.

Cheers,

BW


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## mklotz (Jan 14, 2008)

Bob,

Yes, when I made this I was aware of Starrett's taper gages. However, for the novice, calibration of such a tool will be difficult if he elects to make one whereas my conical gage can be 'read' with nothing more than calipers and needs no (incised) calibration.

Drill bits work but they suffer from two problems. First, they come only in predetermined sizes whereas the gage fits any size hole within its range. Secondly, not every amateur is aware that drills are tapered slightly and that he needs to mike the portion he's using for measurement.

Dial bore gages and Intramikes are pricey and unlikely to exist in a novice's shop.

As I said, this is a stopgap tool meant for the novice's use until he can afford the exotica. It's simple to make but must be used with the knowledge of its inherent limitations. (Those limitations also apply to Starrett's taper gages.)

For holes smaller than about 0.2", there's no real substitute for pin gages. Besides, pin gages will tell you if the hole is non-round or tapered which this tool can't determine.

In my original post, I neglected to mention that, for the mathematically challenged, the tool can be used in a different mode that's simpler but less accurate. Once the cone position is locked in with the setscrew, simply withdraw the entire tool from the hole and measure the base of the exposed portion of the cone with the caliper jaws laid flat against the base of the tool body. Quick and dirty but the caveat about hole edge problems still applies.


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