Rick,
Thanks for posting the article. I recently picked up a set of Starrett Toolmaker inside and outside calipers--these are much smaller than the ones in your article. They are probably no more than 4" in overall length. The small size is one of the main reasons I got them. They are alot easier to use than 6" dial calipers. I also have a set of spacer blocks to practice with and to set the calipers to a specific width. For practice the drill is to put the outside calipers on the blocks, then transfer the measurement to the inside calipers, and then measure the result with a micrometer. Or, in reverse from mic to inside to outside to blocks. It is a bit of an art--getting the alignment right, just the right amount of feel, and measuring with the mic without actually compressing the calipers. And with all due respect to Dave Patterson, it's my understanding that using calipers with a ruler died out at the turn of the last century. I won't claim any
personal knowledge but I was told that they were usually used with gauge blocks, set to the desired dimension and then used to check the progress of machining. Certainly makers sense to me. Since I'm a hobbyist I don't have to work to any greater precision than .001" +/- some tolerance. When I started using dial calipers (Helios, with very sharp needle points) about 25 years ago to measure dental pathologies, I was working to hundredths of a millimeter (0.00039"). We did cross comparisons for repeatability. Once you get to a certain point, the inter- and intraobserver error starts to wash out any alleged gain in precision. So, I guess what I'm trying to say is that unless you have a CMM, you may not get all that much precision from instruments that allegedly deliver it. The person using the instrument is part of the instrument.
Again, thanks for the article and if you run across a set of the dainty, round-legged Starrett calipers, try 'em out.
Jim B.
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