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Thread: What would cause this?

  1. #1
    Join Date
    May 2010
    Posts
    14

    Default What would cause this?

    What would cause my SB 9 lathe to turn a 0.010" taper over a 7inch piece of bar stock using a live center?

    Thanks Steve

  2. #2
    PeteF Guest

    Default

    Offset tailstock.

  3. #3
    Join Date
    Jun 2004
    Posts
    295

    Default

    Twist in the bed.

  4. #4
    Join Date
    Oct 2005
    Location
    British Columbia
    Posts
    3,449

    Default

    As PeteF said offset tailstock.
    Or perhaps loose bearings in your live center.

  5. #5
    Join Date
    Dec 2009
    Location
    Owensboro KY
    Posts
    3,970

    Default

    all of the above plus an off center center hole, a very dull tool, or a too loose live center.

    To help diagnose the problem you need to turn a piece between centers.

  6. #6
    PeteF Guest

    Default

    Quote Originally Posted by Dr Stan
    To help diagnose the problem you need to turn a piece between centers.
    Isn't that what he has just done?

    The easiest way to check the tailstock is correctly centred is to chuck a rod with a DTI in the chuck. Bring the tailstock up to the DTI and ensure the tailstock is locked to the bed and the friction clamp on the quill is locked. Swing the DTI around the tailstock quill. The DTI should read precisely the same left to right, but will read slightly different top to bottom even on a perfectly adjusted lathe. It is the side to side reading you should be concerned about at this point. If the 2 readings aren't exactly the same you will turn a taper between centres. To adjust, unlock the base and adjust the screws on the base of the tailstock. Re-lock and check. This is an easy check and the most common reason for turning a taper between centres.

    Pete

  7. #7
    Join Date
    May 2010
    Posts
    14

    Default

    Quote Originally Posted by PeteF
    Isn't that what he has just done?

    The easiest way to check the tailstock is correctly centred is to chuck a rod with a DTI in the chuck. Bring the tailstock up to the DTI and ensure the tailstock is locked to the bed and the friction clamp on the quill is locked. Swing the DTI around the tailstock quill. The DTI should read precisely the same left to right, but will read slightly different top to bottom even on a perfectly adjusted lathe. It is the side to side reading you should be concerned about at this point. If the 2 readings aren't exactly the same you will turn a taper between centres. To adjust, unlock the base and adjust the screws on the base of the tailstock. Re-lock and check.

    Pete

    Thanks Pete, I'll give it a try.

  8. #8
    Join Date
    May 2010
    Posts
    14

    Default

    Pete, I think your right. The tail stock seems to be out by about 0.008. I'll try to fine tune it and see if thats the issue. thanks again. Steve

  9. #9
    PeteF Guest

    Default

    Let us know how you go. I'd suggest some of the suggestions above won't in fact cause a taper, but one that may is loose bearings in the live centre. There is a certain gap in bearings, albeit small, that allow them to turn. The pressure of the tool will cause the piece to be forced back and cause a taper (again small unless the bearings are totally crap) just as if the tailstock was slightly off. If you're after ultimate precision you probably want to turn between 2 dead centres and drive the work with a dog. Personally I think it's nit-picking, but the way the lathe geometry is set up that's just the way it works.

    Pete

  10. #10
    Join Date
    Jan 2003
    Location
    Chilliwack, B.C.
    Posts
    9,207

    Default

    -offset headstock-

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