Measuring Inside Diameters

I just recently received issue 328 here in North America and enjoyed reading the Artful Dodge #8 article on inside measuring.

Many years ago when I was being trained to use a jig grinder by the seasoned operator he gave me hints on inside measuring. The toolroom did not have sophisticated three-point gauges so accurate hole measurement too large for pin gauges was done by telescoping gauges with expected accuracies in the tenths thousands of an inch.

The process he demonstrated for me has served me well. His instructions were to set the telescoping gauge a little larger than the hole, and to tighten the locking knob enough to keep it from springing open, but not fully locked. It was then positioned vertically in the hole and tilted so the ends touched. Now for the trickier part. Pull the handle down to drag the arms of the gauge through the diameter once, AND ONLY ONCE. Pay attention to the feel of the drag as the gauge is pulled through.

Now begin checking with the appropriate micrometer by waggling the gauge between the mic anvils while slowly closing it to achieve the same drag resistance feel you had in the hole you're checking. If you can get consistent readings with two or three tests and consistent drag inside and out, you can use telescoping gauges to measure to tenths.

That is similar to how I use one of those gauges. If the hole diameter is important, I will take up to three measurements, and hopefully they will be close to identical. I would not expect to be closer than 0.005", but that isgood enough for government work.

Similar to how I was taught to use T gauges. But we were taught to rock the T gauge from side to side a little before dragging it through to make sure it was situated across the very largest part of the circle. Getting within .001" is easy enough, especially if you are doing it all the time. Would take a lot of careful practice to get down to a tenth.

I much prefer ball gauges on anything under about half an inch.

When I started my apprenticeship, one of the training exercises was to be measuring a bearing on the outside, and then using a telescopic gauge and micrometer, until I got the correct dimensions. My last measuring exercise was to measure the bore of a ring gauge , not seeing it's exact size, and then writing down on paper those sizes of the measurements. Then measure a gauge block without seeing it's length and writing down those sizes, all under observation for the test.

It was an exercise well worth doing. In those days we had a temp controlled measuring and inspection room. It taught me 2 things, when dimensions get bigger than 100mm, temp of the part and measuring gear matters. As well as the materials they are made from etc. The other was, when using a telegauge to measure a bore, take two measurements at 45 deg to each other. Otherwise you can not tell if the part is slightly oval or lobed.

Proper bore micrometers in a workshop , will pay for themselves just in the measuring time savings alone in a year, depending on how many and how often a bore is required to be measured etc.