Too Many Indicators!

There is a thing called Segal's Law which states, "A man with one watch knows what time it is. A man with two watches is never sure." It's about the limits of certainty when measuring, and also a caution about trusting an instrument without verification. That Wikipedia link has some really daunting math to back it up.

Well, while quarantined by COVID-19, what better time to check various test indicators to see what I have? And I actually had a good reason: my Sherline lathe is not giving a great finish to the cuts, and I wanted to test it out. I gathered up four different indicators to get a consensus measurement.


When I first started with machine work, I had a Harbor Freight dial indicator that measured 0.001" (one thousandth) per division on the dial. Compared to my woodworking rulers and tape measures, I thought I was now hyper-precise! But experience told me I needed more. I now use a 0.001" indicator for only the most casual and coarse measurements - which are most of them in MY shop!

I bought an inexpensive dial test indicator from Shars that has a resolution of 0.0005" per division - half a thou! Now I'm accurate, right? Well... anyway, I started with that dial test indicator because I had learned to trust it. It's probably best to click each picture to view full sized if you want to try it yourself.

I chucked a piece of drill rod (assumed to be "straight") in the chuck, and manually rotated the chuck. I counted the number of division between the high and low points as I rotated. In this case, I counted about 3 divisions, which I then multiplied by the resolution circled on the face of the indicator. You'll note I used the "~" character that is usually used in mathematics to mean "about." It's hard to interpolate when reading "between the lines." TIR means "Total Indicated Runout" - in this case, about 1.5 thousandths. Not great, especially for a precision lathe like the Sherline. I'll have to investigate further.

But with that baseline, I also wanted to test my other indicators. I have a pair that Kelly W. gave me that were pro grade instruments. They were discarded by Kelly's employer because their calibration certificates had expired. I started with this Hamilton dial indicator that has a resolution of 0.0001" per division - one ten-thousandth. Yes, it's the same parent company as the Hamilton watches we used to wear.

By my interpolation, there were 15.5 divisions times 0.0001" per division = 0.00155 TIR. That's impressive - the two indicators vary only by 5 hundred-thousandths of an inch!

But there's more. This dial test indicator by Brown & Sharpe (one of the oldest and most trusted names in the business) has a resolution of 0.00005" per division - that's truly microscopic, equaling 1.27 microns. On that one, I counted just shy of 30 divisions, which also multiplies out to 0.0015" TIR. Wow, that's impressive!

One more indicator, though - one I just recently bought used on Craigslist. It's the same Shars indicator as the first one. I didn't even want it, but I wanted the high-quality stand it was mounted on, and the indicator was part of the deal and was really cheap.

When I got it home, I learned why the price was so low - it was clear that indicator had been dropped. The bezel was loose and the round scale with the indications printed on it was floating, causing the indicator to bind. I still wanted to give it a try, and my rough interpolation said 3.8 divisions, for a TIR reading of 0.0019.

Object lesson of the day: don't drop your indicators! At least I wasn't the meathead that dropped it. Subsequent testing gave varying results, so that indicator is still on probation.

Fortunately, I do have three that I feel I can really trust, with the B&S indicator being the Queen of the shop. Now the Queen and I are going to do some more testing on this Sherline lathe...