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What's wrong with my threading technique?

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  • What's wrong with my threading technique?

    Okay guys, I'm stumped here. I single point threaded a shaft to fit a "nut" with a known size and thread pitch (I made it, and used a drill and tap to create the internal threads). I first turned the shaft down to the specified OD, then set up the gears on my lathe for the correct thread pitch. I threaded in reverse using an internal threading tool on the back side. I got good looking threads, cleaned them up and tried to thread the two parts together.... only got a thread or two, so I took another pass of a few thou. After that, I could only get 3-4 threads. I continued to cut deeper and deeper unto I could get 10-12 threads, but then the first several threads were very loose, then it got tight and stopped before I reached the bottom of either of my threads. It's as if the threads were cut on a taper, like pipe threads, only a very small one. What am I doing wrong?

    For reference, I can thread the tap back into my female part by hand, so I'm pretty sure toffee threads are right, it's just that my male threads start lose and get tight.

    Sent from my SAMSUNG-SM-G891A using Tapatalk

  • #2
    With boring type tools, which an internal threading tool would presumably be, there is a tendency for a taper to develop. many theories on that, but it happens. You take "spring cuts" to correct that.

    Aside from that issue, if between centers, are you sure things were lined up right so the work was centered all along the length?

    Did you take any "spring cuts"?
    CNC machines only go through the motions

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    • #3
      Try threading missionary style, and report what happens. .

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      • #4
        Appears you were maybe cutting a taper. Did you have the shaft between centers? The cutting force can bend an unsupported shaft somewhat, particularly if you extend beyond, say, 4 times the diameter of the shaft.

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        • #5
          Did you check the diameter after turning the shaft to the major OD? If no taper was made, and threading was done with similar radial force, it should not have caused much of a taper.
          http://pauleschoen.com/pix/PM08_P76_P54.png
          Paul , P S Technology, Inc. and MrTibbs
          USA Maryland 21030

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          • #6
            An internal threading tool has a different profile than an external one. You should have used an external insert for an external thread.

            Examine the illustration. The external thread has crests that are narrower than the crests of an internal thread. By the same token, the external thread is shallower than the root internal one.

            Last edited by danlb; 03-26-2018, 11:43 PM.
            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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            • #7
              To confirm if your change gears are right, did you hold up your tap against the cut threads to confirm pitch ?

              Is your OD to size or slightly under ? ( On a 1/2 -13. the normal OD is .500, but make your .002 under that using a file before the last pass (removes filing burrs))

              Is your tail-stock aligned if between centers ?

              Did you run several "cleanup passes" before the trying the nut to confirm finsihing cuts

              Are you lubricating ?

              Rich
              Green Bay, WI

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              • #8
                Are you feeding the tool with the compound, or with the cross slide? If with the cross slide what angle is it set to? Feeding the tool straight in, as with the cross slide, will make the tool cut on both sides of the thread which increases the likelihood of introducing a taper from excess cutting force. And, as per the diagram in #6, what about crest-root interference?

                Do you have any means to accurately measure the pitch diameter of the thread?

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                • #9
                  Originally posted by 754 View Post
                  Try threading missionary style, and report what happens. .
                  LOL!

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                  • #10
                    Originally posted by txfireguy2003 View Post
                    Okay guys, I'm stumped here. I single point threaded a shaft to fit a "nut" with a known size and thread pitch (I made it, and used a drill and tap to create the internal threads). I first turned the shaft down to the specified OD, then set up the gears on my lathe for the correct thread pitch. I threaded in reverse using an internal threading tool on the back side. I got good looking threads, cleaned them up and tried to thread the two parts together.... only got a thread or two, so I took another pass of a few thou. After that, I could only get 3-4 threads. I continued to cut deeper and deeper unto I could get 10-12 threads, but then the first several threads were very loose, then it got tight and stopped before I reached the bottom of either of my threads. It's as if the threads were cut on a taper, like pipe threads, only a very small one. What am I doing wrong?

                    For reference, I can thread the tap back into my female part by hand, so I'm pretty sure toffee threads are right, it's just that my male threads start lose and get tight.

                    Sent from my SAMSUNG-SM-G891A using Tapatalk
                    Are you really really sure that the male thread you have cut is the same pitch as the nut you tapped? Have you held the tap against your cut thread, because it sounds like a pitch mismatch to me.
                    As a matter of interest, why did you use an internal threading tool and cut in reverse, why not just a plain ordinary threading tool and cut from the front? I don't think its the cause of the problem, I'm just curious.
                    I know I'm going to regret asking this, but what is a 'toffee thread'
                    'It may not always be the best policy to do what is best technically, but those responsible for policy can never form a right judgement without knowledge of what is right technically' - 'Dutch' Kindelberger

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                    • #11
                      Surely a taper produced as a result of too much 'stick out' would be the opposite to what the OP is getting. If the end bends away from the tool, there will be less cutting here, and so a wider thread. The OP has deeper threads at the sticking out end, with thread jamming nearer the chuck.

                      I agree he should check that his gearing really is producing the right pitch.

                      I'm not sure what a spring cut means, but I think it means that you take several extra passes with no more infeed, till you're 'cutting air'. I like to run a file over the crests - and on both sides of the crests - during these check passes, so the check passes can remove the burrs that result from the filing.

                      If it's a deep thread - say 10 TPI or coarser - I use the top slide to hit both the left and right flanks before centralising the tool in the thread and going for full depth. Feeding at 29 degrees is the other way to deal with deep cuts. You really can mess about with deep threads, until you're near the end - then you've got to gt it dead right.

                      With shallower threads, with practice you should be able to do it with a couple of heavy passes, a light filing, and a final pass. But don't expect the depth to come out just as the book says. I've got some heavy threads I'm proud of, but many times I've cut too deeply and had to start again.
                      Richard - SW London, UK, EU.

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                      • #12
                        Wow, thanks for all the replies gents, I'll try to respond to as many questions as I can.

                        I used the reverse technique just to try it out, as it seems easier and safer to me, but I'm still learning. Added benefit is that I can run higher RPM to get closer to the proper speed for the material (although I was running slow this time).

                        I did check the thread pitch against a couple of taps and a pitch gauge.

                        Toffee threads are the result of cell phone autocorrect.

                        I don't yet have a means to measure pitch diameter, but I'll be ordering a thread mic soon. I turned to the correct major diameter, then used the cross slide to cut treads down to the correct minor diameter (per the graduated dials, which have been very accurate to date), then tested the fit, not even close, so I made another pass and another and so on until I could get the two parts to thread together. I will try again with the compound instead, set to 29 degrees.

                        I did measure the turned major diameter prior to threading, but not in multiple spots to look for taper, I'm not sure where the taper would come from though. I'm only cutting maybe 3/4" of threads (it's just practice, so I didn't measure the length very closely). Shaft bending is possible, but like one poster said, it would tend to cause a larger diameter at the tip and I have the opposite problem.

                        I used a boring bar type internal tool (3/4" bar) because I wanted to try the reverse technique and didn't have a left handed external threading tool (which would be flipped upside down).

                        If I missed any questions, I'll try to answer them in another post.

                        Sent from my SAMSUNG-SM-G891A using Tapatalk

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                        • #13
                          Really sounds like you have pitch error either in the nut OR the "bolt"
                          Location: Helsinki, Finland, Europe

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                          • #14
                            Are you starting in a relief area (run out area if you thread towards the headstock) or just plunging in and going? That may cause a taper if your set up is not stiff.

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                            • #15
                              If you have some music wire or any other firm steel wire that will fit down into the angled faces of the threads while sticking up out of the thread then you can at least do a comparative measurement along the thread to see if the pitch diameter is the same or has a taper. You don't need a thread mic for that at least. In my case I've got lots of different sizes of music wire laying around. But if you don't have that perhaps you can scrounge up three same size small number drills to use?

                              You also wrote that you wanted to get the speed up to where the cutter is quite happy. For a home machine where you're engaging the half nuts for each pass I'd suggest you avoid doing that. It's "crunchy" enough engaging the half nuts on a slower turning lead screw. Upping the speed to where the threading lead screw has to slam the whole carriage into motion is going to beat up the engagement faces of the half nut threads. SOMEWHAT faster is OK. But I wouldn't go crazy on that aspect. Of course with finer threads the lead screw is turning at a slower fraction of the work piece RPM. So you can scale up the speed in that case. But I would not go up over a speed that puts the lead screw at such an RPM that it becomes hard to smoothly engage the half nuts.

                              I'm not talking here about the spot on the threading dial where the nuts hang up and then drop in. I'm talking about the lead screw turning fast enough that the half nuts skip and shudder over the passing teeth or where you can feel yourself forcing the half nuts onto the threads during most of the engagements instead of a clean drop in. With coarse threads where the lead screw is spinning at a much faster rate for a given spindle speed I've had that happen and had to lower the gearing just to make it so I could drop in on the desired point without fighting the lever due to the carriage starting to move.
                              Chilliwack BC, Canada

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