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adjusting a Central Tools micrometer

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  • adjusting a Central Tools micrometer

    This should be an easy one.
    does anyone know how to adjust this type of micrometer? Usually I see a set screw, but not on this type. I checked with my A+ Jo blocks and it reads any stack I make a tenth over.
    Attached Files

  • #2
    If it's anything like mine, you should be able to get a pin spanner in that hole and twist the barrel until the hash marks line up like they should

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    • #3
      That grubscrew is unusual, I would try unscrewing it before using the c-spanner in the hole in the other side. Normally it is simply friction from a close fit that stops the tube from moving by itself. Unless the readings are very far out you should not disturb the end with the ratchet, it is on a steep tapered joint on the spindle might as well be retightened first if the taper is still tight.
      Last edited by old mart; 10-18-2021, 04:48 PM.

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      • #4
        I had that tiny set screw out , it seemed to not hold anything, but i'll try the spanner there or look for another spot where it ought to go. The mic is plenty usefull now, but on the last job i was chasing down tenths and just had to keep in mind to think down one tenth.

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        • #5
          It reads a tenth over? Before you go adjusting it, be sure you check it again after completely cleaning the anvils.
          I have an older, analog micrometer and just the oils that accumulate from my fingers on the anvils will increase the zero reading by 0.0001". This is a repeatable situation.

          I like to tighten the anvils with the ratchet on a piece of clean, cotton rag (old undershirt) and then pull the rag out. I do that twice before zeroing the scales. The zero has held on my micrometer for over 15 years now and I do not baby it.

          And if tenths count, you should get a digital. Most of them read down to 0.00005" and you can zero them in an instant. I have a Fowler (<$50) and it checks 100% against my shop blocks.

          If you did not completely clean the anvils on that last job, you may have been subtracting that tenth for no reason and your adjusted readings may be too low. I say this because the oil film over the entire surface of the anvils, which are lapped, will more easily effect the zero reading than the reading on something that does not contact the anvil 100%. This lesser contact can happen with a spherical or cylindrical object. But it can also happen with a flat surface if it is not polished. The peaks on a rough surface (just off the lathe, mill, or even the surface grinder) can easily penetrate the oil film and reach the surface of the anvil so such surfaces will not show the increase in their readings as an anvil-to-anvil reading will. You need to be constantly aware of what is going on when you are down to the tenths level of lower.
          Last edited by Paul Alciatore; 10-18-2021, 08:14 PM.
          Paul A.
          SE Texas

          And if you look REAL close at an analog signal,
          You will find that it has discrete steps.

          Comment


          • #6
            It looks like it may be a collet style. If so, that back piece that the washer bears against is a tapered collet style bushing. It's probably bound up tight from sitting without adjustment for so long. Give it some twisting back and forth or a gentle rap with a piece of nylon or something similar, might come loose. After it's loose you just spin the barrel where you need it then tighten that ratchet back up again to pinch the collet down again.

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            • #7
              Don't use a Cspanner in the hole where the grubscrew sits, you might damage the threads, there is a dedicated hole showing in your first picture. Getting repeatability of only one tenth is difficult, and it is vital to get the anvils absolutely clean, and then there is temperature to think about and matching the clamping forces, plus the surface finish and material you are measuring are all factors. I have mics which run to microns which are even more picky.

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              • #8
                I'm with Paul on the idea of double checking the cleaning as your first step. Even clean with a strong solvent like acetone or cheap lacquer thinner to remove any more stubborn films that might be there. You can then re-oil it with a finer oil.

                All my mic's came with a small pin spanner that goes into the small hole to be used for adjusting the final zero. My metric mics are imports and have the same set screw which MIGHT be a lock for the barrel. You'd need to check to see if the torque needed for moving the barrel with the pin spanner actually changes. But the pin spanner is the proper tool. The two Mitutoyo mics I have for imperial sizes have no set screw, just the pin spanner holes.
                Chilliwack BC, Canada

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                • #9
                  eKretz nailed it, the barrel is obviously retained by the split bush which is a neat fit inside the barrel and locates on a taper on the end of the spindle. When the ratchet and washer are assembled and tightened down the bush expands slightly and retains everything. A "C" type pin wrench is definitely not used on this type of micrometer. They are not easy to set up and with .0001 error, I would leave it alone.

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                  • #10
                    Yet the little hole for the pin wrench is still there. If it came with the little wrench then I suspect that's what it's for.
                    Chilliwack BC, Canada

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                    • #11
                      There are quite a few different styles. The small spanner may work to turn the fixed part of the barrel and it may not. It's worth a try. It does look like it has the tapered collet setup on the thimble to me though. I have seen at least 5 or 6 different ways that different manufacturers handle this. There are different methods of handling thread wear also. One of the most interesting is in some of the older Lufkin mics. They have a sliding section of OD thread that is spring loaded and acts as an anti-backlash mechanism.

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                      • #12
                        Never owned a Central Tool micrometer, mostly Mitutoyo and B&S.

                        Mitutoyo and B&S employ different methods of attaching the spindle to the thimble, one uses a set screw and the other a taper.

                        I was wrong, I do own a Central ball anvil micrometer that someone gave me 30 years ago that has never been used, it's engraving is crude at best.

                        The black framed B&S tools from 30+ years ago are excellent, Swiss Made at the time. If you can find them on the used market buy them all.
                        This picture is 5 years old.
                        Last edited by Bented; 10-20-2021, 09:12 PM.

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                        • #13
                          Originally posted by Bented View Post
                          The black framed B&S tools from 30+ years ago are excellent, Swiss Made at the time. If you can find them on the used market buy them all.
                          They are among my favorites also.

                          Comment


                          • #14
                            Thanks for all the responces. I did wipe off the anvils and the blocks too, although I'll clean them with acetone and pull the rag through the anvils too. I'll give it another shot when I get back to the shop this weekend.
                            I got the mic used and without any wrenches. No I don't think it adjusts with a c-spanner.

                            It's a great mic otherwise if I'm not measuring to the last tenth. Certainly repeatable and plenty better than a C-clamp. The job I put it to was sneaking up to half a thousanth with emery cloth going for a press fit or shrink fit.

                            Comment


                            • #15
                              Originally posted by Tillie's in a bottle View Post
                              It's a great mic otherwise if I'm not measuring to the last tenth. Certainly repeatable and plenty better than a C-clamp. The job I put it to was sneaking up to half a thousanth with emery cloth going for a press fit or shrink fit.
                              Is it off by the same amount throughout the 0 to 1" range? Does it zero properly? Are the anvils scratched or otherwise damaged?

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