04-18-2012, 12:18 AM
Many do have a 0.2000 diameter. And some have a 0.5000 diameter. In fact, I haven't seen one that the main contact diameter is 0.3750. I have a Starrett (or 3) with a 0.3750 body that has a 0.2000 contact diameter and a short section immediately above it that is 0.3750, and have used that part before, but it is a very short (around 1/8" I'd guess) and awkward to use.
The issue with stopping when it runs true is that you can't really see when it is. As you approach an edge with it "wobbling", yes it runs truer and truer as you get closer, but if you try to declare a perfectly true point with your eyes, you won't know whether you are right on the edge or maybe a few thousandths away. The physics behind it require the contact diameter to "pop" off to the side require very little overtravel. Probably in the 0.0002 range. That can be considered negligible for most work, and I suspect far closer than you could get by eye as you approach the edge and declare a true running edge finder.
The issue with stopping when it runs true is that you can't really see when it is. As you approach an edge with it "wobbling", yes it runs truer and truer as you get closer, but if you try to declare a perfectly true point with your eyes, you won't know whether you are right on the edge or maybe a few thousandths away. The physics behind it require the contact diameter to "pop" off to the side require very little overtravel. Probably in the 0.0002 range. That can be considered negligible for most work, and I suspect far closer than you could get by eye as you approach the edge and declare a true running edge finder.