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  • #31
    Some serious thoughts on the basic question.

    First, you should be able to take a mike reading without scratching the shaft. If you can't, practice. At no time should the mike be supported by the shaft as some of the comments may be implying. As stated above, a gentle, consistent touch is needed.

    OK, if you want to use something between the mike anvils and the shaft, DO consider that if you are using a mike then tenths DO matter. Perhaps you do not need +/- 0.0001" but most micrometers can have their reading vary by several tenths by normal differences in pressure and you can achieve a difference of a full thousandth or more by heavy force.

    So in reference to the various plastics and felt tips suggested, try a bit more pressure when you are zeroing them. You will be amazed at the variation: it could easily be several thousandths with just moderate extra force. Do see if you can get repeatable readings with the ratchet or friction device on the mike. I will bet you can't. And consider that when you are re-zeroing it the faces are in full contact over all of their area but when you are measuring a shaft they will only contact on a narrow line due to the radius of that shaft. The soft materials WILL compress more due to that and the readings will be several tenths less. Exactly how many tenths is a matter of the material and of the radius of the shaft. Oh, and if the material you use is adhesive backed, the adhesive will also compress by an unknown and variable amount under pressure.

    Frankly, I would NOT use any of the materials suggested. If you must use anything, I would suggest a foil of a softer metal, perhaps aluminum. Clean everything completely and calibrate the mike with two thicknesses of aluminum foil. If possible use the same two pieces of foil to make the measurements. Use as little force as possible because the foil will also cold flow under the pressure but it should be less than almost any plastic will. Use fresh foil for additional measurements and do zero the mike each time.
    Paul A.
    SE Texas

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

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    • #32
      Another serious suggestion, perhaps a bit radical, would be to use a very fine stone to round the edges of the mike's anvils. Do take precautions to prevent abrasive from getting into the mike and to prevent any cutting on the faces of the anvils. I would wrap the entire moving end of the mike in a plastic bag and tape the edges of that bag completely to the frame and moving anvil, leaving only non operating parts exposed. Actually just the two anvils would be exposed. We are talking about a radius of only a few thousandths on those edges, just barely enough to prevent scratching.

      It is best to try this on an older mike if you have more than one.
      Paul A.
      SE Texas

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

      Comment


      • #33
        Actually, L.S. Starrett Co. of Athol Massachusetts still makes a "standard stock special" No.436-3 1/2 (edp#65493) "Automotive Crankshaft Micrometer" with a range of 1.500" to 3.500". There's also a metric version, the No.436M-88 (edp#65600.)

        Those micrometers have a thimble with a 2" (50mm) range, and extra-long solid anvils to afford reaching into the crank webs and still be able to see the reading on the thimble. Hardened and lapped steel anvil and thimble surfaces rather than carbide help prevent marring a crankshaft surface.

        The crankshaft micrometers look similar to this one, but their anvil & thimble wouldn't meet as this one does. This might be good for smaller (under 1") journal diameters.

        http://www.ebay.com/itm/Starrett-Spe...-/280611630166
        Last edited by PixMan; 12-20-2011, 05:39 PM.

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        • #34
          I tried the felt pads, then the balsa wood pads. I thought that would work while the lathe was turning, like the Doc suggested.

          Now my lathe's on fire ! What should I do ? There's a stream of burning flakes of balsa being flung off the work and I can't get at the switch. Oh, and the micrometer is spining round and smashing into the toolpost every revolution.

          I'm just glad I didn't try the plastic coating. That would've been worse.
          Richard - SW London, UK, EU.

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          • #35
            Another serious response.

            Back in the day I used to work for a shop that used quite a lot of bronze and brass in our work. It is very possible to leave a "polished spot" where the measurement was taken. If the surface was polished to size with 400 to 600 grit you can leave a small burnished spot or on rare occasions if you slide the mic off the surface a small shave of material can happen. I have one mic, a 0-1 that as Paul suggested, has had the edges of the anvil slightly dehorned to prevent it digging if slid over soft surfaces.

            Certainly any burnished mark left by properly measuring, using a skilled feel for tension, would be easily removed with a gentle pass of the abrasive paper. The sheen of the burnished mark will be removed without any significant removal of material.

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            • #36
              Originally posted by rohart
              ...the micrometer is spining round and smashing into the toolpost every revolution.
              Not good. It may affect its accuracy.

              First of all, relocate the mic so it's not in front of the toolpost. Then use a second mike at 180 degree to serve as a counterweight and balance the shaft. Otherwise, the readings may be slightly off.

              And make sure that the mics are attached ahead of the tool bit, so you cut toward them. The metal there is not hot, so you won't worry about thermal expansion skewing your readings. Besides, you won't need to re-attach your mics after each cut. Just keep cutting and reading the mics (with two mics you will be able to register readings twice per revolution).
              Last edited by MichaelP; 12-21-2011, 11:53 AM.
              Mike
              WI/IL border, USA

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              • #37
                Two Mic's is a great idea. But for best accuracy they should be from two different manufacturers to average out manufacturing tolerance. Best results will be if you use a Starrett and a Mitutoyo. We don't want to use any cheep import stuff here and mess up your precision work.
                Oh no Captain O!! That can't be true

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