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lathe re-centering a longish part

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  • lathe re-centering a longish part

    I need to bore a hole in a longish part. The whole part is 110mm long, and I I can clamp in the 4 jaw about 40mm. Of course centering to +/- thou is no problem. but if I center the end, the base is off, if I center the base, the end is off. Like 10-20 thou. - thats too much.

    What is your trick to centering a longish part?

    thanks for the tips - it has really help me get to a repeatable thou.



  • #2
    put dial indicator on the toolpost and indicate it in, bump it around with a mallet until it is true

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    • #3
      Isn't that what center rests are for?
      Location: Saskatoon, Saskatchewan, Canada

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      • #4
        Originally posted by Arcane View Post
        Isn't that what center rests are for?
        Sure! I am looking for techniques for setting it up to have it true from one end to the other. Thanks!
        Last edited by JW1942; 05-05-2020, 11:27 PM.

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        • #5
          If Ringo's idea doesn't work I think I might suggest a piece of scrap maybe half or a bit longer than the part you need to bore. Chuck it up and bore out a hole that is a really close fit. Then relieve the middle by a couple or three thou leaving collars at each end which will be a snug press fit but which you can do with the tail stock ram. Press it, drill and bore. If you miss and it's too loose use some CA glue to temporarily hold the bar. Heat it up to a little more than the temp of boiling water and you can press it out and clean it up.

          Working with a piece held in the chuck without moving it at all would ensure a good axial hold. Automatic trueness. I'd also want it to be around 50 to 60 mm long.

          Think of making this "inside arbor" as practice for boring the long hole in your part....

          EDIT- You might want to check for slop, play or runout in your four jaw chuck jaws. They should not cause that much wobble over that length. I know I would not be at all happy if my four jaw had that much wobble.
          Chilliwack BC, Canada

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          • #6
            The solution is NOT to grip on 40mm of the part. Set a stop behind the part so you can only grip 5 or 6mm. Then use the suggestions above: set the part zero at the chuck and tap the tail end until it's zeroed. Re-set the chuck end to zero, then tap the tail end again. Repeat until acceptable TIR at both ends.

            Alternatively, you could grip it deeper but wrap a copper wire around the part to act as a pivot point. Then use the same method to zero the part. I HIGHLY suggest use of a stop behind the part if you use either method. And I hope it's obvious, but you don't want to be using this method for heavy roughing unless you like wearing your workpieces implanted in your body... rough first, then reset in this manner.
            Last edited by eKretz; 05-05-2020, 11:58 PM.

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            • #7
              grip 5 mm? are you sure he needs a wiggler? i understand op wants to do this without tail.

              there are two possibilities: either the part is not straight, then you tap it, or your jaws are not straight, then you tap it, but without success. copper pads might facilitate both. (yes there is a third possibility.)

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              • #8
                thanks guys - I will give it a go.

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