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Lathe Test Bar - Someone's in error

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  • #16
    Setting the tailstock set-over just right
    is just a standard part of setup when
    the tolerance of the job calls for a job
    that has to be really straight, without
    any taper. Usually the tailstock set-over
    is adjusted with the actual workpiece
    set up in the lathe, in context.
    In the grinding world, the table is always
    adjusted for set whenever the tailstock
    is moved. Parts can be changed between
    centers, but if you move the tailstock,
    you must re-adjust the table set.
    With a lathe, it is tolerance dependent
    to tweak tailstock set-over or not.
    Using a tramming bar thingy and hoping
    to set it and forget it, is a false dream
    of running a lathe.

    --Doozer
    DZER

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    • #17
      Originally posted by lynnl View Post
      .......

      (added) I have a couple of really nice chromed piston rods from a pair of McPherson struts that serve such purposes nicely. Far more precise than any I'm likely to produce.
      Of course, with a test bar for that purpose, it is the centers which determine accuracy as much as any other part. They need to be on-center and accurately round in order to have the bar work as intended.

      So, if you put in one or both of the centers, you "made" the test bar. It will be as accurate between centers as your positioning of the centers. That is one point that is very much in favor of a bar of the type that requires a test cut, because the centers will be on-center to the degree that the machine you use turns accurately and concentrically. Once you have a condition of same size at both ends (and middle), THEN you have concentricity, and you can switch over to using it with an indicator.
      Last edited by J Tiers; 11-06-2018, 06:11 PM.
      CNC machines only go through the motions.

      Ideas expressed may be mine, or from anyone else in the universe.
      Not responsible for clerical errors. Or those made by lay people either.
      Number formats and units may be chosen at random depending on what day it is.
      I reserve the right to use a number system with any integer base without prior notice.
      Generalizations are understood to be "often" true, but not true in every case.

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      • #18
        It would be good to also check the height of the test bar at both extremes. It won't have as much effect on turning a given diameter, but it could cause stress problems if you have a rod in the chuck and supported by a live center.
        http://pauleschoen.com/pix/PM08_P76_P54.png
        Paul , P S Technology, Inc. and MrTibbs
        USA Maryland 21030

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