How to "pick up" a thread ?
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  1. #1
    Join Date
    Aug 2004
    Granite, MD USA
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    Hi All,

    I picked up an old driver plate for my SB 9A, and the threads are too tight - will not thread onto the spindle. (I checked the register diameters and they are OK.) I know the basics of thread cutting, but how does one "pick up" an existing thread? A pointer to a good reference would be fine - but I can't find this info in HTRAL, the Atlas book, or Moltrecht.

    Jeff Greenblatt

  2. #2
    Join Date
    Aug 2004
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    Set the cross slide for 29.5dg (for a 60dg thd), load a threading tool on the cross slide. Engage the half nuts (typ on the #1 mark) and let the
    feed screw pull the saddle a bit, then shut down.
    Adjust the cross slide and compound screws so that the cutting point is all the way in the existing thread. Push it in with the cross feed, and tweak the compound. When you get the tool as far in as possible, you can zero out your cross and compound dials.

    Now, you're all set to back out of the cut (cross) move the saddle to the right, restore the cross and advance the compound and take a threading pass. Repeat until it fits [img]smile.gif[/img]

  3. #3
    Join Date
    Jan 2005
    Salem, Ohio
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    Everything Wally said and you will have to turn your compound to the 1 o'clock position at 29.5 degrees. Since this is an internal thread. You can see the whole set up in pics at my yahoo group...Bob

  4. #4
    Join Date
    Feb 2004
    peekskill, NY
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    Because this is an internal thread it will be
    tougher than an external one.

    The compound should be set to the right
    side of the carriage, at 29 degrees, point
    to the *rear*. The best way to imagine this
    is to point the compound straight back to
    the rear, and then swing it towards the
    right side, so it goes through 29 degrees
    of motion. The numbers on the dial won't
    read 29 though.

    The advice so far has been good but because
    it's a bit of a tricky job, I will say it once

    First off mark up the inside of the existing
    threads with a blue sharpie marker so you can
    see exactly where you are cutting.

    Get the threading setup right so it is cutting
    the correct tpi.

    Close the halfnuts and allow the leadscrew to
    take up a strain on the carriage, so you get
    all the slack out of the system. Of course
    at this point you will have the threading tool
    completely clear of the threads, inside the
    bore of the part.

    Stop the motor so the threading tool is
    still inside the threaded portion of the bore.

    Now it is simply a matter of juggling the
    two dials at your disposal (crossfeed and
    compound feed dials) so that the tool engages
    the existing threads as closely as possible.
    If you have the compound set at one particular
    location, as you dial the crossfeed handle out,
    the flanks of the tool will both touch the
    flanks of the existing thread.

    When you think you have it so, set both dials
    to zero.

    When you are cutting, the lash should be loaded
    out of the crossfees screw by unwinding it to
    smaller numbers, and out of the compound screw
    by winding it in to larger numbers.

    Set up a trial cut with the crossfeed at zero,
    and the compound maybe a few thou less than zero.
    Take cuts, feeding the compound a thou at a time
    until you start cutting metal. Then stop and
    see what the bluing looks like in the bore.
    By now you could imagine tweaking the position
    by juggling the two dials to get the cut even.

    If it's off by a thou or so that's probably OK
    because you probably need to take of several
    anyway, as long as both sides are cleaned up
    when you are done it's fine. Don't make the
    threads too tight, cut till the fit is snug
    and then go several more thou.


  5. #5
    Join Date
    Jan 2005
    Salem, Ohio
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    One other thing you can do before you try this is make a small threaded plug the exact size of your spindle thread OD. The best thing to do is mic yours using three wires then make the plug the exact same size. It will come in handy on future projects anyway. Or you can run the carraige to the right with the half nuts still engaged, and carefully unscrew the chuck, turn everything around and see if it fits on your spindle. Its kinda tricky but it works. I just threaded a 40# part and didn't have a threaded plug for my spindle size. And had to turn the whole works around twice. What a job. My dummy spindle from a 9" won't fit my 10k its .006 too small. So if i thread a part to fit the dummy its too tight for my 10k...Bob


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