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Cutting a 27TPI thread on an Atlas 10F

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  • Cutting a 27TPI thread on an Atlas 10F

    Hi all;

    I have a 27TPI thread to cut on an Atlas 10F ... I have a quick change gear box installed on the lathe, but it has no setting for 27 TPI

    I have a quick change gear set and a bracket (I have always called it a banjo...) BUT am missing some of the gears required to get the correct ratio (2x32 tooth gears) that and the bracket position A (the one to be used of course) has been welded several times & I can see a crack behind one of the welds - I don't have much faith in it holding together.

    There are already about 6-8 gears on the bracket for the quick change box to be able to engage the lead screw forward/reverse control and engage the spindle. (of course none of them were 32 teeth!!)


    Does anyone know if I can swap out the gears in place now (to engage the quick change box) with different sizes to be able to change the ratio and get that 27TPI?

    -thanks
    -sean

  • #2
    Try this.

    https://www.hobby-machinist.com/thre...r-table.71526/

    Page 32
    Last edited by Stu; 03-10-2019, 10:11 PM.

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    • #3
      27 is missing on a lot of lathes.. 'cos it's hard to get gears to do it. Prime numbers and all that stuff

      Read this : https://www.practicalmachinist.com/v...-lathe-343738/
      Last edited by lakeside53; 03-11-2019, 12:20 AM.

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      • #4
        Originally posted by Stu View Post
        You need to create an account to access that. I found another source here:
        http://www.machinistweb.com/forum/showthread.php?t=2711
        While you still need to create an account there, they don't ask for a bunch of personal information like hobby-machinist does.
        Just a user name, a password, and an email address.
        Last edited by RichR; 03-11-2019, 10:22 AM.
        Location: Long Island, N.Y.

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        • #5
          I agree with mf205i, if they are pipe threads, 1/8 npt is 27tpi, and 1/2 bsp is 28tpi, blame the Americans, they had to change for the sake of change.

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          • #6
            Got metric gears?
            Some oddball combination might be enough close

            My imperial Kerry would do 0.9375mm pitch that would be 27.09 TPI

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            • #7
              I've done this on my Sheldon. I will need to know the configuration of your end gearing and QCGB. Then I can tell you what gears to use.

              Best Regards,
              Bob

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              • #8
                Originally posted by mf205i
                If you are cutting pipe threads, cut the threads at 28tpi. Do the math and you will see that the pitch error is .0013 per inch and that’s only a couple of tenths pitch error over the length of a typical pipe thread.
                Mike
                Wait a minute.. not to suggest 28tpi won't work, but 27tpi = ..037037 pitch; 28tpi = .035714 pitch.
                The difference between the two is .001323. That's the error over the distance of one thread, not per inch. ...isn't it?
                So over, say 1/2" or 14 threads engagement, the error is 14 X .001323 = .018519, by the time you reach the last thread.
                Lynn (Huntsville, AL)

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                • #9
                  Originally posted by Stu View Post
                  Thanks!

                  That is precisely what I am looking for - the lathe is a different model with a different QCGB but the setup may be the same I can start looking for a manual that matches my model.

                  thanks!!
                  -sean

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                  • #10
                    Originally posted by lynnl View Post
                    Wait a minute.. not to suggest 28tpi won't work, but 27tpi = ..037037 pitch; 28tpi = .035714 pitch.
                    The difference between the two is .001323. That's the error over the distance of one thread, not per inch. ...isn't it?
                    So over, say 1/2" or 14 threads engagement, the error is 14 X .001323 = .018519, by the time you reach the last thread.
                    I may be able to fudge it like that as the number of threads that are engaged is 4 -5 ... HOWEVER the part is a retaining screw for an 1873 Springfield Trapdoor sear bar. kind of the 'most critical' pieces in the action (lock) and quite precise fit. I'd like to try to get it 'right' before trying something else (and not damaging the lock plate threads in case someone comes along with an original screw - the metal is VERY soft)

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                    • #11
                      Success!

                      Thanks guys - you all helped out a little.... found the appropriate supplement here:

                      http://vintagemachinery.org/pubs/51/16337.pdf

                      it lists the setup for the change gears and my QCGB together

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