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Using a tap flute to internally thread oversize hole

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  • Using a tap flute to internally thread oversize hole

    I have a need to internally thread a 15mm hole with 24TPI thread. I'm trying to make some jam nuts for a shaft that's the above mentioned size/thread. I'm curious if I could use a 3/8-24 tap and feed it into the hole with a tool holder so a single flute makes the thread. Seems like I could set the lathe to 24TPI and set the tap into a holder so just one flute cuts the thread??
    Thanks!

  • #2
    If you have a lathe then single point it. The lathe doesn't care what diameter a hole is.

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    • #3
      Yes, that will work. Do not expect to cut too heavily, just regard the tap as being a substitute for grinding up a screwcutting tool yourself. Incidentally, IF you have a tap you can use in this way, but your lathe will not cut that thread you CAN, with care get the tap alone to cut the thread, you lightly feed the carriage by hand and once you get the tap feeding at its own pitch it will follow correctly for the following cuts. ( This is not for the faint of heart nor the heavy handed brigade, but if done carefully may get you out of a jam!!!)
      Regards David Powell.

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      • #4
        Have done that a lot . It is a superb way to make threaded bores larger. I know David said to use hand power but let me explain another method I use to enlarge previous threaded holes
        ( for new holes, I single point )
        The "secret" to using a tap for re-threading a hole is backlash control. So my friend has a motorcycle and his replacement transmission part had a 1-1/8 x 16 threaded bore that was too small.
        Chucked it up and set the quick change GB to 16 TPI and engaged the half nut and forget using the Tool-Slide. This is Carriage and Cross-slide job only..
        Turn on power briefly while holding the carriage back using the carriage handwheel., This takes out all Carriage travel backlash as the carriage moves towards the headstock
        Turn the Cross-slide handle retracting the Cross-slide while pushing the tool holder away from you . This removes backlash in the cross-slide.
        Mount a 3.8 x16 tap in your holder and with one flute at centerline and the tap flutes fully engaged in the existing threads, clamp the tap without moving it, in the tool holder and Zero out the dial.
        So move the cross-slide towards the center to dis-engage the tap ,retract the Carriage (reverse) and bring back to "0" and you are ready to start.
        Do not disengage the half-nut and use the Lathes forward/reverse to make additional passes to enlarge the hole.
        So my friend had his part reworked in less than 10 minutes and it fit, and did not require a custom tap.
        Rich

        PS Unlike single point threading, you are cutting on both sides of the thread "V's so you do not need the tool-slide set at any angle
        However ( !) , the cutting load for the tap is not balanced (as it would be in a normal sized hole) and there is stress on the tap body, so do not get heavy handed with the cross-feed
        Green Bay, WI

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        • #5
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ID:	1876053 Thanks folks. I decided to cut the threads with a single point tool. I screwed up the first time by moving the compound. It was set at about 45* so the next cut just obliterated the previously cut threads. Fwiw I used only the cross slide to cut the threads. It's such a fine and shallow thread I took lightish cuts by moving only the cross slide. Second attempt was perfect. I did have to do about four spring passes.
          ​​​​​​​Next I used a four jaw to cut two opposing flats on the threaded cylinder. I'm trying to figure out how to get get the other two sets of flats turned using the lathe but I can't figure out how to do this so I'm taking a break. Thanks!

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          • #6
            I learned a lot in a small country shop in Wales in summer holidays from school ajnd beyond. We did all our threading without using the half angle method, for example we often fed in 5 thous( then with the top slide set straight) moved it 1 or 2 forward. then another 5 of depth . It made hitting thread depth easy and almost foolproof. You had to be very careful keeping proper pitch in machining multistart threads !!!! Regards David Powell

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            • #7
              Feeding with the topslide isn't unusual - just set it to the thread's flank angle (usually 60 deg, 55 for Whitworth). Then apply feed with the topslide. Ian
              All of the gear, no idea...

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              • #8
                about 4:00

                https://www.youtube.com/watch?v=OqLsNQc2It8

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                • #9
                  Originally posted by dian View Post

                  Dang! Why don't we all do our threading this way? Seems like it is a way easier way to thread than single point.
                  Thanks

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                  • #10
                    Well, Not really. With single point ,you only need one toolbit for a variety of threads.

                    The above video shows it pretty good, but note that for male threads ,you use the rear of the tap flute, while the normal tap front flute cutting edge is used for internal threads.

                    Rich
                    Green Bay, WI

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                    • #11
                      I should have added that when cutting male threads with a tap-- that the helix angles are opposite between the Tap and the work piece and that could present a relief problem
                      This does not occur when doing a female thread
                      Rich
                      Green Bay, WI

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                      • #12
                        Years ago, before lathes commonly had lead screws or change gears, threads were cut by hand, using hand-held chasers. Much like a wood turning lathe, the tool was held against the work and the rest, and advanced at the rate of the pitch required. I've collected a few - - here's a pic of some pre-war British thread chasers, all in Whitworth form to 40 TPI. Notice the internal threaders too.

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                        25 miles north of Buffalo NY, USA

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                        • #13
                          You may collect those old thread chasers, I chuck them in the bin.

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                          • #14
                            Originally posted by old mart View Post
                            You may collect those old thread chasers, I chuck them in the bin.
                            if you knew what I paid for those, you wouldn't be binning them
                            25 miles north of Buffalo NY, USA

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                            • #15
                              What exactly do you want those antiques for, are you starting a museum?

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