mklotz
Well-Known Member
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).
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).