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And, after you measure your turned part with the super-duper measuring tool, you still do not know the size...... because the single point tool leaves a "plowed field" finish, and the measurement depends on how hard you "stamp down" the plowed peaks with the measuring tool.
And, of course, the tool you most recently used to cut or finish the bore affects the depth of the "plowing". To get any meaningful result, the surface has to be smooth at least within the tolerance you need on the measurement.
I'd never depend on the corner-to-corner deal, even though it ought to work. It depends too much on the edge finish. Starrett, flung dung, whatever, all same problem. If you were to surface grind the devices before use, to assure a clean corner, then maybe. And at that point, the flung dung will equal the Starrett.
At one time, there were the same thing made with a radius on the top and bottom surfaces, apparently for this exact purpose. I've read about them in textbooks, but never have seen one. Those I would consider to be acceptable.
I even have a dial bore gauge, but it needs to be set up for the size wanted, with the correct extensions, then calibrated etc. A pain for casual use, so I keep using the telescoping gauges. With those, though, you need good ones.
I had a set and consistently they were under size. I thought you folks were all liars, trying to "haze the new guy" by claiming that telescoping gauges actually worked.
Finally I looked at them under magnification..... the ones in the set were so badly finished, or worn, that they had flats on the ends. So they bridged on a chord, and never contacted on the actual diameter. Same problem as calipers. I got rid of them, and found a different set, which seems to work just fine.
And, of course, the tool you most recently used to cut or finish the bore affects the depth of the "plowing". To get any meaningful result, the surface has to be smooth at least within the tolerance you need on the measurement.
I'd never depend on the corner-to-corner deal, even though it ought to work. It depends too much on the edge finish. Starrett, flung dung, whatever, all same problem. If you were to surface grind the devices before use, to assure a clean corner, then maybe. And at that point, the flung dung will equal the Starrett.
At one time, there were the same thing made with a radius on the top and bottom surfaces, apparently for this exact purpose. I've read about them in textbooks, but never have seen one. Those I would consider to be acceptable.
I even have a dial bore gauge, but it needs to be set up for the size wanted, with the correct extensions, then calibrated etc. A pain for casual use, so I keep using the telescoping gauges. With those, though, you need good ones.
I had a set and consistently they were under size. I thought you folks were all liars, trying to "haze the new guy" by claiming that telescoping gauges actually worked.
Finally I looked at them under magnification..... the ones in the set were so badly finished, or worn, that they had flats on the ends. So they bridged on a chord, and never contacted on the actual diameter. Same problem as calipers. I got rid of them, and found a different set, which seems to work just fine.
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