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Thread: Using the lathe threading dial

  1. #1
    mklotz mklotz's Avatar
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    Using the lathe threading dial

    Some folks are confused by which marks to use on the threading dial on their
    (assumed Imperial) lathe. Of course, you're always safe restarting on the
    same dial mark on which you started but that means you'll spend a lot of time
    'waiting for your number to come up'.

    It's pretty easy to think through. Most dials have four numbered marks
    labeled, unsurprisingly, 1-4. Between these marks are smaller, unnumbered
    marks.

    [Aside: A consequence of the four numbered mark dial is the fact that the
    gear that meshes with the feed screw will have a number of teeth which is four
    times the tpi of the feed screw, i.e., an 8 tpi lead screw will have a 32
    tooth gear on its thread dial. This is worth knowing if you intend to build a
    threading dial for a lathe that lacks one. Since there's no load on this
    gear, a perfect match to the helix angle of the lead screw is not needed. Any
    old gear with the right number of teeth can be pressed into service.]

    The numbered marks almost always correspond to a carriage movement of one
    inch. (However, you should confirm this by actual measurement on your lathe.)
    By deduction, the unnumbered lines must then correspond to a carriage movement
    of one-half inch.

    Now, suppose I'm cutting an even-numbered thread (e.g., 32 tpi). If I move the
    carriage by one-half inch, the tool will reenter the thread perfectly. If I
    move the carriage one inch, the tool will also reenter. Therefore, I can
    reengage the half-nuts on ANY line on the threading dial.

    If I'm cutting an odd-numbered thread (e.g., 13 tpi), the tool will not reenter
    the thread if I move one-half inch - I have to move one inch. Therefore, I
    must reengage the half-nuts on ANY NUMBERED LINE on the dial. (If I started
    the thread on an unnumbered line, I would then reengage on unnumbered lines but
    that is generally too difficult to remember in the 'heat' of thread cutting and
    should be avoided.)

    If I'm cutting a half-fractional thread (e.g., 11-1/2 tpi), the tool will only
    reenter the thread every two inches. Therefore I must reengage the half-nuts
    on ONLY THE ODD NUMBERS ON THE DIAL (1 and 3) or on the even numbers (2 and
    4). It's a good idea to decide on one of these options and always stick with
    it. I always start the thread on 1 and then use either 1 or 3 to reengage.

    Rarely, a lathe can cut a 1/4 thread (e.g., 1-1/4 tpi). This thread will only
    repeat every four inches so, in cutting such a thread, one would reengage the
    half-nuts on the same mark one used to start the thread.
    ---
    Regards, Marv


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  2. The Following 3 Users Say Thank You to mklotz For This Useful Post:

    Carlos B (03-23-2017), threesixesinarow (07-19-2018), Toolmaker51 (02-19-2017)

  3. #2
    Toolmaker51's Avatar
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    As we cut both Imperial and Metric almost daily; I always have to stop and think this through before engaging that half-nut. Otherwise I'd be the half-nut and unemployed to boot, crashing a machine or expensive custom part.
    Sincerely,
    Toolmaker51
    ...we'll learn more by wandering than searching...

  4. #3
    mklotz mklotz's Avatar
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    While you're thinking it through, you may want to make a copy and add a copy of this...

    Threading setup chart

    to it. Together they make a good bit of lathe catechism.
    ---
    Regards, Marv


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  5. #4
    mklotz mklotz's Avatar
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    How did you arrive at 16 teeth for the gear?

    I'm not familiar with the layout of the Atlas dial. Is it not the typical 1-4 layout? Could you describe it or provide a picture?


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    ---
    Regards, Marv


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