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  • #16
    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.
    CNC machines only go through the motions

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    • #17
      Originally posted by J Tiers View Post
      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.
      Speaking of calipers. I seem to recall one of the more serious YouTubers setting up a pair of calipers with round carbide contact points for measuring. It might have been Robin Renzetti. I might have the details all wrong. At the time I was thinking I would never destroy an expensive tool like that, but now I just might. LOL.
      *** I always wanted a welding stinger that looked like the north end of a south bound chicken. Often my welds look like somebody pointed the wrong end of a chicken at the joint and squeezed until something came out. Might as well look the part.

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      • #18
        Originally posted by J Tiers View Post
        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.
        Good point, most here dont have machines that will repeat down to .0002 of an inch. 400 paper will get that last .0004 tenths out. Then you have thermal coefficient of expansion to deal with. Working in the barn @ 88° F you have to re- calculate your measurements. 68° F is the standard here in the States. Lapping will also get those corn rows out.

        My goto machines were Hardinge lathe and Houser Jig bore.
        On these machines you could get down to .0001 tenth of an inch. Probably the finest iron I ever work on!

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        • #19
          I have a Slocomb mic with spherical ends.... is an OD mic, but will measure tubing thickness accurately

          Originally posted by Fasturn View Post

          Good point, most here dont have machines that will repeat down to .0002 of an inch. 400 paper will get that last .0004 tenths out. Then you have thermal coefficient of expansion to deal with. Working in the barn @ 88° F you have to re- calculate your measurements. 68° F is the standard here in the States. Lapping will also get those corn rows out.
          ..............
          You likely do not need to recalculate too much if everything is steel... the mic will grow as much as the part. More important that everything is at the same temp. For the ultimate, or for "certified" measurements, your room should be at the "agreed correct" temperature.
          CNC machines only go through the motions

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          • #20
            Bob: I have had a set of the imports such as you describe, probably twins to yours. I have grown to really like them for jobs such as shallow diameters, think counterbores such as seal counterbores. They proved to be accurate and dependable.
            Sarge41

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