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What's wrong with my threading technique?

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  • #61
    Get some ink or something on there and try the nut again. You should be able to see where it starts binding.
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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    • #62
      You say you tapped a blind hole in the "nut". All taps have a tapered section at the tip and only cut full threads at some distance back from there. How about a close-up photo of the tap that you used?

      You may need to use a bottoming tap to cut full internal threads deep enough. And your male part that you are threading may need to have a tapered section at the end to allow it to bottom out in your tapped hole. Or does it have a shoulder on it to seat on the "nut"?
      Paul A.
      SE Texas

      And if you look REAL close at an analog signal,
      You will find that it has discrete steps.

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      • #63
        I had asked earlier (#8) if the compound was being used, and if so at what angle. The setup being used is an insert mounted on a boring bar cutting on the back side of the part with the spindle in reverse. That's a good way to avoid crashing the tool that I've employed on many an occasion when working near a shoulder. Good on the OP for his creativity. What I'm wondering is, with the tool on the opposite side of the spindle, how are the cross slide and compound lead screws positioned as far as backlash goes. Could a compound floating off its lead screw allow the cutting tool to wander off its intended setting enough to cause the problems experienced?

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        • #64
          Originally posted by tom_d View Post
          I had asked earlier (#8) if the compound was being used, and if so at what angle. The setup being used is an insert mounted on a boring bar cutting on the back side of the part with the spindle in reverse. That's a good way to avoid crashing the tool that I've employed on many an occasion when working near a shoulder. Good on the OP for his creativity. What I'm wondering is, with the tool on the opposite side of the spindle, how are the cross slide and compound lead screws positioned as far as backlash goes. Could a compound floating off its lead screw allow the cutting tool to wander off its intended setting enough to cause the problems experienced?
          That I was also wondering. Everything so far sounds like its right but its not working so there is something overlooked..
          "drunken thread" pitch error or only partially cut female thread are the best guesses at the moment.

          Might help if OP would post more pics of the setup and "everything".
          Location: Helsinki, Finland, Europe

          Comment


          • #65
            It should be easy enough to make a "nut" using a piece of flat material and tap it through for 5/16"-24. If it threads onto the shaft without binding, and stays at the same amount of looseness over the length, then the problem must be the female threads of the knob. It might actually be good to have a taper so that the knob can be tightened like a pipe thread. It's also pretty easy to make a bottoming tap by grinding the end off of a taper or plug tap.

            Also helpful would be pictures of the actual setup on the lathe. Here is the setup I used on the Clausing lathe at the machine shop class, for 3/8"-16:



            And my first attempt at chasing threads on my HF 9x20 lathe:


            Left-hand square threads 3/4"-8 were challenging:


            http://pauleschoen.com/pix/PM08_P76_P54.png
            Paul , P S Technology, Inc. and MrTibbs
            USA Maryland 21030

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            • #66
              My two cents says the problem is the insert and the way it's being used. To cut internal threads in the usual setup the point of the insert faces the operator and is fed to the left to cut a right hand thread, and will have relief to clear the helix angle of the thread.

              If I understand what you are doing, the insert is put behind the work, right side up, point facing the operator, lathe ran in reverse, the tool feeding to the right to cut a right hand thread. If this is the case then the helical relief on the insert is opposite what it should be, it's cutting on the blunt side, and probably not cutting as much as you are expecting.
              Jim

              Comment


              • #67
                Originally posted by J Harp View Post
                My two cents says the problem is the insert and the way it's being used. To cut internal threads in the usual setup the point of the insert faces the operator and is fed to the left to cut a right hand thread, and will have relief to clear the helix angle of the thread.

                If I understand what you are doing, the insert is put behind the work, right side up, point facing the operator, lathe ran in reverse, the tool feeding to the right to cut a right hand thread. If this is the case then the helical relief on the insert is opposite what it should be, it's cutting on the blunt side, and probably not cutting as much as you are expecting.
                That would account for the bar turning the toolpost and failing to cut the huge amount (a full 0.120") undersize that you cranked in, but really CANNOT have actually happened.
                CNC machines only go through the motions.

                Ideas expressed may be mine, or from anyone else in the universe.
                Not responsible for clerical errors. Or those made by lay people either.
                Number formats and units may be chosen at random depending on what day it is.
                I reserve the right to use a number system with any integer base without prior notice.
                Generalizations are understood to be "often" true, but not true in every case.

                Comment


                • #68
                  Originally posted by J Harp View Post
                  My two cents says the problem is the insert and the way it's being used. To cut internal threads in the usual setup the point of the insert faces the operator and is fed to the left to cut a right hand thread, and will have relief to clear the helix angle of the thread.

                  If I understand what you are doing, the insert is put behind the work, right side up, point facing the operator, lathe ran in reverse, the tool feeding to the right to cut a right hand thread. If this is the case then the helical relief on the insert is opposite what it should be, it's cutting on the blunt side, and probably not cutting as much as you are expecting.
                  I'm not sure I agree with that. Tool behind the work, lathe in reverse, feed to the right when standing in front of the machine. Go round the back what you see is the same as what you see when cutting a thread normally - Work revolves towards you, feed to the left, so tool should be cutting as usual.
                  Correction!!! I'd forgotten that he was using a boring bar with, presumably a right hand internal insert, so JT is right, its the wrong helix angle on the insert.

                  Still don't understand why the OP chose to do it cack handed, why not single point tool from the front? Theres a lot I don't understand about this thread, particularly the bit about feeding in an extra 60 thou, and the thread still not fitting. Something must be moving because its not clamped up tight?

                  My intuition is that its all down to the tapped thread being taper because it isn't tapped all the way through. OP seems reluctant to do what we've been advising and try a standard nut and a standard bolt in the 2 threads.
                  Last edited by Richard P Wilson; 03-29-2018, 02:17 PM.
                  '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

                  Comment


                  • #69
                    These following statements can NOT all be true. Danlb alluded to it as well.

                    First:

                    Originally posted by txfireguy2003 View Post

                    After those pics, I continued cutting, using the compound to feed into the material, several spring passes etc and cut until I got to my expected final depth of cut. Not even close to fitting, so I kept cutting deeper and deeper (60 thou deeper on the compound) before finally giving up when I crashed the tool into the end of my threads. Even still, it matches my thread gauge perfectly, matches three different taps perfectly, but won't even start to accept the female threads. Then I crashed and gave up. It's time for dinner anyway.
                    And then:

                    Originally posted by danlb View Post
                    ,,,,,
                    5) Your theoretical thread height is 0.0361 inch ( pitch * .86603 for UN and UNR)

                    Dan
                    See the problem here?

                    The added cut is way more than the thread depth.... and it still won't enter the nut....

                    WE KNOW Dan's statement is true, that IS the thread depth. So that leaves the idea that txfireguy THINKS he cut the part that far, bit actually it did not cut. If he cranked in 0.060 on the radius, or anything reasonable close, the part should have pretty much fallen right through the nut,

                    Since he says it would not go in, then CLEARLY the cuts that were cranked-in DID NOT HAPPEN. Something moved, and the cutter was NOT cutting.

                    That one statement about crashing the cutter into the end of the threads is part of this.... if the thing WAS cutting, it should not have been able to crash in to the end of the threads, the diameter should have been smaller. But clearly it was not, unless txfireguy engaged the halfnuts in a way wrong place, or goofed when returnig the crosslide to the same place, which I will provisionally assume he did not do.

                    Even then, it should simply have tried to take too big a cut, it should not have crashed "into the end".....

                    So I am saying (and Dan seems to agree) that this is really suspicious. that 0.312 should have been down around 0.190 after all that was cranked-in (0.060 movement, double depth of cut 0.120"), but obviously it was not.... So the cutting did not happen.

                    Either the bar sprung, or the toolpost rotated, something allowed the compound to run that far in, without the cutter actually taking the cut
                    Last edited by J Tiers; 03-29-2018, 11:30 AM.
                    CNC machines only go through the motions.

                    Ideas expressed may be mine, or from anyone else in the universe.
                    Not responsible for clerical errors. Or those made by lay people either.
                    Number formats and units may be chosen at random depending on what day it is.
                    I reserve the right to use a number system with any integer base without prior notice.
                    Generalizations are understood to be "often" true, but not true in every case.

                    Comment


                    • #70
                      My thinking may be fuzzy, but here's how I see it. The tool orientation stays the same, pointing toward you. You stay in the same place, in front of the lathe. For both OD and ID right hand threads the side of the thread nearest you leans to the left, on the side farthest from you the thread leans to the right. You can check this by holding a suitably sized wire in the valley between threads and see which way it leans.
                      Jim

                      Comment


                      • #71
                        Originally posted by Richard P Wilson View Post
                        I' not sure I agree with that. Tool behind the work, lathe in reverse, feed to the right when standing in front of the machine. Go round the back what you see is the same as what you see when cutting a thread normally - Work revolves towards you, feed to the right, so tool should be cutting as usual.

                        Still don't understand why the OP chose to do it cack handed, why not single point tool from the front? Theres a lot I don't understand about this thread, particularly the bit about feeding in an extra 60 thou, and the thread still not fitting. Something must be moving because its not clamped up tight?

                        My intuition is that its all down to the tapped thread being taper because it isn't tapped all the way through. OP seems reluctant to do what we've been advising and try a standard nut and a standard bolt in the 2 threads.
                        Let me address this first. I chose to use the reverse technique as something new to try, and others have proven that it works, and it's safer that feeding toward the chuck, literally zero chance of a crash. Also, this particular operation is a simulation of threading a bolt handle from a rifle, which requires the bolt to be held in a fixture, then the fixture in the chuck. When feeding toward the chuck, I've had the part slip in the fixture, but if puking away from the chuck, there's mechanically no chance of movement due to some angles.

                        I'm not reluctant to try my parts against commercial versions, I just don't have any commercial versions available at the moment, I don't live in a hardware store. As far as the tapped hole being tapered, it may be, but I used a tap with very little taper on the end, maybe 2 or 3 threads at most, I ran the tap in until I ran out of length on my tap. Looking from the bank side of the knob, even though the threads don't go all the way through, the hole does, and I can see that I had at least 8 or 10 threads left.

                        I agree, it's possible something is moving, but I can't imagine what, I will check tightness of all parts tonight after work. I did notice that it seemed as though I want cutting anything for a while, and agree, something had to have moved, but I couldn't find anything that had changed.

                        Sent from my SAMSUNG-SM-G891A using Tapatalk
                        Last edited by txfireguy2003; 03-29-2018, 11:50 AM.

                        Comment


                        • #72
                          I took a couple more photos this morning before leaving for work, and measured the part I cut last night. Now, you all saw my micrometer measurements last night, 0.312, no taper at all. After threading, it measures 0.325!

                          On my next attempt, I will start by turning the shaft down smaller, say to .302 to account for the flats on the crests, as that may be the problem, but I don't know how to account for the major diameter growing.

                          Here's a pic of the setup.

                          Sent from my SAMSUNG-SM-G891A using Tapatalk

                          Comment


                          • #73
                            Now you tell us it got bigger..
                            It us not cutting it is smearing.
                            You don't have tool clearance or dull tool. It must have visibly been bending.

                            Just start off doing it the conventional way, missionary. Straight in,no fancy angle, nothing variable for us to guess at.
                            You tried it because you thought EASIER.... and it wasn't.
                            Start over, and no fancy tricks

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                            • #74
                              That boring bar clearly did not bend. So something had to have moved.

                              Originally posted by txfireguy2003 View Post
                              I took a couple more photos this morning before leaving for work, and measured the part I cut last night. Now, you all saw my micrometer measurements last night, 0.312, no taper at all. After threading, it measures 0.325!

                              ...
                              Burrs.

                              That is not a "topping" insert, which would clean off the thread to correct size and shape, but that is specific to thread size.

                              The cutter you have will throw up a burr on each side.
                              Last edited by J Tiers; 03-29-2018, 12:29 PM.
                              CNC machines only go through the motions.

                              Ideas expressed may be mine, or from anyone else in the universe.
                              Not responsible for clerical errors. Or those made by lay people either.
                              Number formats and units may be chosen at random depending on what day it is.
                              I reserve the right to use a number system with any integer base without prior notice.
                              Generalizations are understood to be "often" true, but not true in every case.

                              Comment


                              • #75
                                Originally posted by danlb View Post

                                The OP specified that he was using a 3/4 inch boring bar. He also said that he was taking cuts of .010 or smaller. There should insignificant deflection making a cut right next to the chuck with a bar that's 75 times bigger than the cut that's being made.

                                Dan
                                Yes, insignificant deflection. Of the workpiece. But if that boring bar is sticking out six inches or so.... And .010" is a pretty decent cut.

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