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Cutting a 6 or 8 tpi thread

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  • Cutting a 6 or 8 tpi thread

    I had an idea for a project/product. But it requires a thread far more coarse than my little 7x12 is capable of, and wider than most taps I've seen.

    Looks like it's about a 1.25" major diameter and 6 tpi. (Don't have the mating part handy to measure.)

    Is that just a matter of digging in somebody's die catalog? The host material is most likely going to be brass and I only need to thread about 3/4" of the length.
    ----
    Proud machining permanoob since September 2010

  • #2
    1-1/4"-7 is standard NC thread, but after you purchase a die and die holder you are going to have to find a way to hold the workpiece while threading. It takes quite some force to cut that thread in one pass with a hand die.

    Why isn't your 7X12 capable of cutting that thread?
    Jim H.

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    • #3
      I see a dozen gear combinations on the threading web app (tank.com) for 8 tpi. One of them should work. There are some good threading charts on the web for 7x10 class lathes.

      Dan
      At the end of the project, there is a profound difference between spare parts and left over parts.

      Location: SF East Bay.

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      • #4
        Consider starting with a standard bolt? Machine the head as needed, press fit into something else if you require more material.
        Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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        • #5
          I see. My naivete shines brightly. I was just judging only by the gearing chart in the manual.

          Never occurred to me I might be able to get ahold of an existing bolt that size. It's something close to 1.25" thick. (3-liter bottle opening.) But cutting down an existing bolt (assuming I could find one) would certainly be the easiest solution.

          Thanks for the pointer o/

          Time to noodle around with threading on the lathe for the first time.
          ----
          Proud machining permanoob since September 2010

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          • #6
            Cutting a 6 or 8 on a 7x lathe will probably break something.

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            • #7
              Like most things, There are is no reason the 7x12 lathe can not cut a coarse thread. You may need to make more, shallower passes, but that is already a familiar way to work when using small lathes.

              I have done 6 tpi before.

              Dan
              At the end of the project, there is a profound difference between spare parts and left over parts.

              Location: SF East Bay.

              Comment


              • #8
                If your lathe can't handle it, I invite you to take the 3 hour ride to where I am and I'll let you do it on my lathe.

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                • #9
                  On my end of the world of fasterners... 8tpi for all things above 1" is the normal...

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                  • #10
                    Originally posted by cuemaker View Post
                    On my end of the world of fasterners... 8tpi for all things above 1" is the normal...


                    Yep in the energy biz and most others, >1" is 4, 8, or 12. But if the thread is that bottle, that's a 2 start thread.

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                    • #11
                      National Coarse thread chart.
                      http://www.motalia.com/Html/Charts/unc_chart.html

                      1"-8TPI is standard, that is the only standard 8TPI National Coarse. There is an 8TPI series, but they are not standard threads.
                      Jim H.

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                      • #12
                        Originally posted by macona View Post
                        Cutting a 6 or 8 on a 7x lathe will probably break something.
                        I've done 1 1/4" x 7 threads on mine with no problem. It's just a matter of getting the right gears.

                        p.s. With a little engineering you can add more gears.
                        Last edited by elf; 09-17-2012, 09:35 PM.

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                        • #13
                          Originally posted by JCHannum View Post
                          National Coarse thread chart.
                          http://www.motalia.com/Html/Charts/unc_chart.html

                          1"-8TPI is standard, that is the only standard 8TPI National Coarse. There is an 8TPI series, but they are not standard threads.
                          Just to be a bit picky..but who says 8UN is not standard??..IT is a standard. Its a standard for a LOT of ASTM specifications...

                          Edit to add: To seem less feisty, 8UN is uncommon in your every day applications....So JC is right in spirit...
                          Last edited by cuemaker; 09-17-2012, 09:59 PM.

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                          • #14
                            One thing to consider at such low TPI the cutting feed will be very fast. I have seen some people put cranks on there spindle to hand crank the spindle at low RPM's because there small lathe did not have a low enough backgear (or enough torque at low speed with variable speed DC drives) to handle low TPI threads safely (Have to disengage really quick when turning towards a shoulder)
                            Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                            • #15
                              "(Have to disengage really quick when turning towards a shoulder)"

                              Or mount the tool upside down and backwards and cut the thread away from the shoulder.

                              I have hand cranked my fair share though...

                              doug.

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