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A turning mystery

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  • A turning mystery

    I've turned a short cylindrical piece - like a fat washer - 2.5" OD, 1.5" ID, 1" thick. I find I need to bring the OD down a few thou, and in particular for one 90 deg segment.

    I mount it with the internal jaws on the 4-jaw, and I dial it in so that the part that needs to lose material is, let's say, at jaw 1 (jaws count clockwise looking at the chuck). This segment, over jaw 1, is dialing up as ten thou proud of round. So when I skim a little off the OD, this bit, the proud/high part at jaw 1 should lose material first.

    I'm using my standard dial indicator, I know which way is high and which way is low, and I double check it several times in any case.

    I turn it a little, just a few thou off the 'OD'.

    I stop the lathe. The material has come off the segment that leads jaw 1, off the segment over jaw 4. The tool has not even touched the segment that should have been proud, the 90 deg segment over jaw 1.

    I can put a crowbar under the chuck. There's no - less than a thou - movement unless I lean on the thing. I can't find any play, and the whole machine is tight. The cross slide is locked down. It's a very good condition D1-3 6" 4-jaw. I use a sturdy Dickson toolpost, with carbide insert tooling. There's no out of balance, and I'm using 300 rpm, fast enough for the carbide to be happy, but slow enough to avoid any imbalance (which there isn't anyway). I'm taking a very light cut, and the cut is clean - no chatter, no polishing without cutting, and this is a low radius insert that is quite new and sharp - it's my finishing insert in fact.

    I can't figure out what's going on at all. I no longer have the problem - emery saw to that - but I still have the mystery and the uncertainty. Up till now I'd approached turning even eccentrics with confidence.

    Any ideas ?
    Richard - SW London, UK, EU.

  • #2
    Tool

    How sharp is sharp? You mention you are using an insert and there is a 'feed/ depth of cut' cutting window for all inserts. The tool may refuse to cut during certain section of the cut.
    Last edited by J. R. Williams; 06-04-2012, 12:14 PM.

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    • #3
      So .. you turned a piece and after you turned it the piece was no longer round?

      Also, just a suggestion, next time you do something like this, before you take a cut spin up the lathe and just touch the surface with a piece of chalk. Then turn the lathe off and look where the chalk touched. It's easy to get things backwards, which is what I rather suspect you've done.

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      • #4
        Originally posted by metalmagpie
        So .. you turned a piece and after you turned it the piece was no longer round?

        Also, just a suggestion, next time you do something like this, before you take a cut spin up the lathe and just touch the surface with a piece of chalk. Then turn the lathe off and look where the chalk touched. It's easy to get things backwards, which is what I rather suspect you've done.
        +1 on that. I have misread indicators before, in spite of having used them thousands of times. Indicators usually don't lie.

        Brian
        OPEN EYES, OPEN EARS, OPEN MIND

        THINK HARDER

        BETTER TO HAVE TOOLS YOU DON'T NEED THAN TO NEED TOOLS YOU DON'T HAVE

        MY NAME IS BRIAN AND I AM A TOOLOHOLIC

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        • #5
          The fact that you are 90 degrees off may be a clue. DIs can stick and this can throw your centering off if you only rotate the part one way while doing the offset. A small chip or dried oil or coolant or whatever. With just a bit of latency in the reading you could easily be off by one or two jaws. I like to rotate both ways to check where a high spot is. Back and forth in decreasing amounts until I am sure I am on the true high spot.
          Paul A.

          Make it fit.
          You can't win and there is a penalty for trying!

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          • #6
            So please keep in mind this is from a "rookie" and a "tenth" is a bit of a pipe dream for me on the lathe I have...but I think the principle is the same regardless of measurement.

            If I understand the orientation of jaws numbering, in moving the part up that amount, have you not also, moved the center line/axis of the part up as well?
            So that, to the lathe trying to turn a circle, that spot (which was the correct diameter at the initial turning) is in effect "high"? And so it was removed...

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            • #7
              You need to put a piece of round material in the chuck, turn it down to a light press for your washer ID, push on your washer using the tail stock and turn the OD. In other words - make a mandrel.

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              • #8
                It could be that the operator was a bit "heavy handed" with the chuck handle and pushed the part "out-of-round" and when it was turned and released the tension eased and the part returned to the stress-relieved state.

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                • #9
                  Originally posted by oldtiffie
                  It could be that the operator was a bit "heavy handed" with the chuck handle and pushed the part "out-of-round" and when it was turned and released the tension eased and the part returned to the stress-relieved state.
                  Yup, easily done even on a heavy walled tube. Even lightly gripping can get it out of round.
                  Amount of experience is in direct proportion to the value of broken equipment.

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                  • #10
                    For small eccentrics it can be easier and more reliable to use the three jaw chuck with shim under one of the jaws.

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                    • #11
                      no cutting??

                      is material steel? maybe their is a hard spot, and carbide isnt as sharp as you think,carbide does chip easily too, or steel "hard spot" is abrasive too.make sure indicator is contacting part the full 360 degrees, i have made that mistake a time or 2, indicator not touching part the full 360.i agree , best way to skim off a few thou is with a mandrel of some kind, use tailstock to push part against chuck, or soft steel or AL mandrel.good luck.
                      Last edited by madokie; 06-07-2012, 02:22 AM.
                      FORD BEATING JAP CRAP SINCE 1941!! CAROLYN JONES(1930-1983 actress)may this lady never be forgotten.

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