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Threading 16 tpi on a 9x20 lathe

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  • Threading 16 tpi on a 9x20 lathe

    I am doing some restorations on my 19" LeBlond, and need to thread a piece 1" x 16 tpi.

    I made the piece on my 9x20 lathe, and threaded it per the chart. When I went to put the original nut on, it wouldn't fit. I checked the change gears and am sure I used the correct tumbler. all ok! So I setup a dial indicator and measured the carriage travel for 4 spindle turns, and divided the numbers. 16.8xxx tpi!!! so I did a bit more calculation with the numbers, and found it matched metric 1.5mm thread. I checked the threading chart again, and found the same change gears are used for 16 tpi and
    1.5mm!!! how can this be?

    Did I do something stupid here, or is the threading on this lathe really whacked? Has anyone run across this kind of problem?


  • #2
    You may want to join the 9x20 yahoo group and ask them.


    • #3
      I'm not exactly sure how that lathe works but generally, when switching between English and metric threads, a transposing gear (compound) is used. The same settings on the gearbox are used for both threads but the compound convers between the two systems. So the gearbox will (may) be marked for both but will cut only one at a time.

      Some math: 1.5mm = 0.059055" If a 120/127 compound gear is used, that becomes 0.059055 x 127 / 120 = 0.0625" And 1/0.0625" = 16 TPI. So your LeBlond is obviously intended to use a 120/127 compound transposing gear to do the "other" system's threads.

      I say "other" system because I don't know if you have an English or metric leadscrew. You seem to be cutting metric threads now. Look and see if there is a transposing compound in the gear chain now. If so, then your leadscrew is English. If not, it's probably metric. It may not be 120/127. It could be one of the approximate ratios like 37/47 which approximates 100/127 combined with another compound (6/5 ratio like 30/25 or 48/40) to change the 100 to an effective 120.

      Paul A.
      Paul A.
      SE Texas

      Make it fit.
      You can't win and there IS a penalty for trying!


      • #4
        I have a Grizzly 9x20. I have done quite a bit of threading on it and it has always been right on. I'll look at the chart on it for 16 tpi and 1.5mm tomorrow and get back to you on this thread.


        • #5
          I think from memory to get 1.5mm pitch from a 16tpi setting on the QC gear box you change the 120tooth gear for a 127 tooth gear...
          Precision takes time.


          • #6
            Grizz 9x 20----

            Feed rate selector: Position 1.

            Gear positions A and B: Each get a 30 tooth.

            For 16 tpi: Gear A drives gear B via 120 tooth.

            For 1.5mm: Gear A drives 127 tooth, 127 tooth is 1:1 with 120 tooth and 120 tooth drives Gear B.




            • #7
              On my Grizzly chart: for 16 tpi, 30t(A position) driving 127t idler (this is a simple idler and can have any number of teeth), driving 30t(B position) on gear box, gear box lever in position 1.

              For 1.5mm: 30t(A position), driving 127t which is keyed on the idler axis to the 120t gear( this is a compound idler and the number of teeth must be exact), 120t of idler driving 30t(B position) on gear box(you must move the spacer from in front of the 30t(B position) gear to behind the gear so it will engage the 120t gear), gear box lever in position 1.

              This lathe has a 16 tpi leadscrew, so to cut 16 tpi you must gear the spindle to leadscrew in a 1 to 1 ratio. The 30t A gear driving through a simple idler to a 30t B gear gives you a 1 to 1 ratio. The lever position #1 on the gear box is also 1 to 1 ( it has to be to get an overall 1 to 1 ratio spindle to leadscrew).

              The 127/120 compound idler gives a ratio of: 127/30 = 4.233 to 1, the 30/120 gives a ratio of .25 to 1, multiplied together = 1.058333 to 1.

              The .0625 pitch of the leadscrew divided by 1.058333 = .059055 divided by .03937 (inches per mm) = 1.500003mm pitch cut on the work piece.
              THAT OLD GANG 'O MINE


              • #8
                Neonman, I went through the same thing the other day with my 9x20. The chart on the front of my lathe doesn't show the switch between gear B but it has to be switched from the 127 tooth gear to the 120 tooth gear. Carl gives a good explanation of this. I had everything else right but couldn't figure out what I was doing wrong until I did some research on the web and found some info on it. One last note, always check pitch after the first skim pass of the tool with a thread pitch gage to keep from scrapping the whole part.

                [This message has been edited by japcas (edited 03-26-2005).]
                Jonathan P.


                • #9
                  Thanks for the help guys! I took a closer look at the chart, and it's subtle, but the picture actually shows using the 127 tooth for english, and the 120 for metric.

                  I now have a nice 1 x 16 thread.