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  • Wrinkled threads???

    Hi guys,

    I'm down to cutting some threads and they look strange while forming and for certain look wrinkled or wavy near the point. Is this normal?

    I am turning 18 TPI, ID, using 4130. I am using a new carbide insert. As groves are cut, flats are temporarily left. As groves get deeper the flats take on an appearance of "split threads". On close inspection using loupes, edges along the flats appear raised giving the impression that threads have been split. Additional cuts eventually remove the raised areas but when the thread is "pointed", the very edge of the point is obviously wrinkled or wavy.

    Is there a cause for this or is the normal?

    Harold
    For those having fought for it, Freedom has a flavor the protected will never know.
    Freedom is only one generation away from extinction.

  • #2
    Wrinklies

    Harold,

    as one older "wrinklie" to a younger one, I would guess that your lead-screw half-nuts are not engaging properly such that the "stop" (that limits travel of either or both the half-nuts or the actuating lever) has not "hit bottom" and so the carriage will lag the correct position as the nuts engage too far up the lead-screw thread (probably the "trailing" or back) flank/s.

    Can you post some pics please? Those excellent high-contrast black & white pics will do fine as well if needs be.

    You can get an indication of the correct meshing/engagement of the half-nuts if you leave the power off your head-stock spindle, and fully engage the half-nuts at any mark on your thread-chasing dial. Turn the head-stock spindle a couple of turns. Check that the indicator/arrow/mark on the chaser is exactly on a mark/line on the chaser dial. Adjust as required.

    You will now be able to see if the half-nuts are fully engaged as if they are the line and the pointer/indicator will be co-incident.If not, the lever needs to be pressed further down until it hits that stop.

    I suggest that you engage the half-nuts at least 1" out from the start of the screw-thread as this will give you time to either push the lever right down or to disengage the half-nuts or stop the lathe spindle.

    Just take your time. This is a classic "Hare and Tortoise" situation.

    I would not be concerned about seeming to ask a lot of basic questions as my guess is that many others will "latch onto" them as something they either want to know or comment on or were not confident to ask for themselves - or just didn't think of them until you ask them.

    I've had quite a few "head-scratchers" from some of your questions and comments I can tell you!!

    Keep 'em coming.

    Comment


    • #3
      Hi Tiffie,

      I will be happy to post a few B+W images. It's nearly 2300 HRS in Alaska so if you will just hang in there like a hair on a biscuit, tomorrow morning I will either supply images of that which I turned tonight or better yet I will turn a new piece and photographically record progressive steps through completion. Does that sound ok?

      In passing, the original chasing dial's pointer was quite a bit off when installed witnessed by that shown in the following image.



      Not being the "sharpest knife in the drawer", coupled with being chronologically challenged (I'm so old that I get my hunting & fishing license free in Alaska ), I was easily confused by which side of the line I engaged the half-nut so I have removed the factory installed arrow and scribed a line on the chaser's body so that it will line up with lines and numbers. I have also learned tonight that I can fully engage my half-nut half way between each line as shown by the blue lines in the following image. What is the significance, if any, of being able to engage on the blue lines?




      Harold
      For those having fought for it, Freedom has a flavor the protected will never know.
      Freedom is only one generation away from extinction.

      Comment


      • #4
        Originally posted by oldtiffie
        Harold,

        as one older "wrinklie" to a younger one, I would guess that your lead-screw half-nuts are not engaging properly such that the "stop" (that limits travel of either or both the half-nuts or the actuating lever) has not "hit bottom" and so the carriage will lag the correct position as the nuts engage too far up the lead-screw thread (probably the "trailing" or back) flank/s.
        I've had similar problems. One way of figuring this out is to set up to thread about ~20 tpi, and just take that very light first pass - and then keep repeating that w/o advancing the compound. Any error in half-nut engagement will be immediately apparent. You can then figure out a scheme for avoiding it or at least being able to feel when it happens. On my lathe I can run into this when I engage late, so becoming more deliberate helped prevent this from happening.
        Bart Smaalders
        http://smaalders.net/barts

        Comment


        • #5
          Originally posted by hwingo

          I am turning 18 TPI, ID, using 4130. I am using a new carbide insert. As groves are cut, flats are temporarily left. As groves get deeper the flats take on an appearance of "split threads". On close inspection using loupes, edges along the flats appear raised giving the impression that threads have been split. Additional cuts eventually remove the raised areas but when the thread is "pointed", the very edge of the point is obviously wrinkled or wavy.

          Is there a cause for this or is the normal?

          Harold
          It seems from the description that you are doing the thread dial perfectly, and what you are seeing is displaced metal, that the threading insert pushes out of the way instead of cutting.

          This is pretty normal with all threading, but may be worse with carbide and low speeds because carbide is inherently "dull" compared to a honed HSS cutter. Harder, but dull do to being made of little hard bits in a metallic binder.

          Some materials also do that, mild steel like 1018 being a known offender. The 4130 I don't know about, 4140 seemed OK to me when I last threaded it, but I was threading it at IIRC 9 tpi, so the turned-over edges would be less noticeable.

          Using a sharp HSS cutter will improve it, but any unsupported edge of material will have some tendency to push away instead of being cut, at the low speeds inherent to threading.

          A fine wire brush can get rid of the bit that is left when the thread is done.
          Last edited by J Tiers; 04-11-2009, 08:27 AM.
          1601

          Keep eye on ball.
          Hashim Khan

          Comment


          • #6
            (Edit: I gotta learn to type faster, sorry for the repeat info)
            Harold, To me it sounds like you're describing a burr being formed that rises above the original OD of the work. I see this on every thread I cut in steel. Some metal gets pushed up and out of the thread rather than being cut off. I deal with it by threading until I reach my target minor dia, then I run a file lightly over the top of the threads, then test the thread with a nut. Then make another cut, file, repeat as necessay. I'm no expert by anyone's standards, but I've asked some "real" machinists who tell me that this is normal, and that's why they make the full form threading inserts... to remove the burr and give the correct shape to the top of the threads. Perhaps someone can tell both of us how to keep it from happening!

            Hope that helps,
            Kerry
            Last edited by kmccubbin; 04-11-2009, 08:37 AM.

            Comment


            • #7
              i will second the use of HSS cutters. i have not used an insert style carbide, but with the brazed carbide cutters i have tried the point always seems to crumble and then the tread goes to [email protected]#%. try grinding a 60* point on some HSS steel, give it some top rake if using a flat tool holder, of course some side rake too, then use a fine hone to touch up all the edges and give it a try.

              Comment


              • #8
                hwingo, you said new insert. Are you using a threading insert and holder or a standard triangle insert and holder to cut the thread?
                It's only ink and paper

                Comment


                • #9
                  Good Morning Guys,

                  Just having my first cup of coffee while replying to your overnight post.

                  Starting with Carld’s question, “Are you using a threading insert and holder or a standard triangle insert and holder to cut the thread?”, I am using a right-hand internal thread insert in conjunction with a right-hand internal threading bar. I see you are from KY. I was born in Dawson Springs, KY.

                  J Tiers, I wondered about that as I read your reply (to include Kerry and Quadrod). Various things are coming back to me from years past. I seem to recall a similar explanation by a local machinist (from years ago) as he was threading using carbide. Over 20 years ago I had a “hobby shop”. My son was killed and I lost interest in just about everything thus I sold my equipment. Nineteen years later I find myself becoming re-acquainted with new equipment, principles of machining, and a wealth of forgotten information. On reflection, I always used cobalt or HSS threading tools. My threads were clean and well formed throughout the process (unlike what I am seeing today). I cut at very low speeds (using back gears on my SB lathe), hence, I never remember seeing threads take shape like those I described (which were cut at low speed and using carbide). I used the conventional “V Cutter” that was hand-ground and honed when threading steel and aluminum. That likely explains differences I now realize during thread formation when using carbide as opposed to HHS.

                  Bart, I really like your “testing idea”. I previously promised Tiffie that I would capture some images during thread formation. As soon as I follow through with my promise (which will be shortly), I will definitely do as you have suggested. I have a feeling this will prove valuable for several reasons:

                  1. It will prove or disprove inaccuracies in my chasing dial
                  2. Through familiarity I will learn of any “quirks” or irregularities inherent to my machine’s chasing dial and how to circumvent consistent inherent quirks. All machines seem to have their own “personality” and a relationship must be forged between the operator and machine.
                  3. This test will instill trust in my equipment or at the very least aid me in determining which regions might be "off kilter" on the chasing dial
                  4. It will serve to build confidence in my abilities to overcome inherent irregularities or recognize problematic areas that should be avoided

                  So Bart, I really like this idea!

                  Now, if you guys will excuse me, I must go into the shop and whisper “sweet nothings” into the ear of my beloved lathe with hopes of wooingly seducing her into performing. I shall return with some images.

                  Harold
                  For those having fought for it, Freedom has a flavor the protected will never know.
                  Freedom is only one generation away from extinction.

                  Comment


                  • #10
                    hwingo,

                    I may have missed it but I don't believe you stated what the ID of the bore you are threading is. Is it possible that the insert is "heeling" or rubbing due to lack of relief?

                    Internal threading is subjected to relative relief issues as the bore size decreases.The same tool bit in a 4" bore has far more relative relief that it would in a 1" bore.

                    Comment


                    • #11
                      That's what I am thinking too. That's why I asked what he is cutting with. When I get wrinkled threads internal or external it always seems to be side clearance of the cutter. If the nuts are picking the thread up wrong it will be cross threaded. I always try to use the same number I started on to do the whole thread.
                      It's only ink and paper

                      Comment


                      • #12
                        I have to disagree with Quadrod, if you decide to grind a hss tool do not put any rake on it, the top needs to be flat. You will need some clearance or relief on the sides and front of the tool, especially the side that is advancing into the cut.
                        James

                        Comment


                        • #13
                          J.Randall, that is fine i am a noob too. the SB9A i cut my teeth on ( heck still cuttin them teeth ) has a rocker tool post so top rake was built into the tool holder. now i have a SB13 and a QCTP and the tool bits are flat. flat works and i also tried some top rake too, it worked also but that does not mean it is the correct thing to do.

                          Comment


                          • #14
                            Quadrod, you might cut a usable thread that way, but the geometry won't be correct. If you have to use a rocker with built in rake grind the top of the tool back flat to eliminate the rake and get a correct thread form.
                            James

                            Comment


                            • #15
                              Well, well, WELL! This took much longer than I thought but I most definitely learned something while doing this exercise.

                              Let me first address Glenn before I get started. The ID is 1.01 inches. Glenn, you made a good point which I did not take into consideration when cutting the previous threads, that being, sufficient heel clearance. Before reading your post I decided to wear my surgical loupes so I could really see what I was doing. When it came time to thread, I removed one QCTP and installed my QC threading holder (with tool in place from previous threading). With magnification I could easily see that interference was about to occur so I rotated the bar just enough that the heel would not drag. You can bet your bottom dollar that I was dragging the heel with my previous attempt. So you have made a good point.

                              On to images. After setting the threading tool where the heel wouldn't drag I applied layout fluid so I could see lines form on the work.



                              After making a couple of very light passes I stopped to capture the following image which clearly demonstrates broad flats (still covered with layout fluid) and beginning groves (raw metal).



                              I made several more deeper cuts and begin to see, for the lack of a better word, extrusion of metal on each side of the grove. When seeing this three dimensionally, one can easily see metal rising above each "flat".

                              Closer magnification demonstrates more clearly raised edges.



                              When inspecting finished threads, I could clearly see a thin black line on each crest. This can be seen in the following image. It appears that extruded edges have folded (collapsed) on themselves thus forming a pseudo-crest and this concerns me. If this is in fact a false crest then threading is incomplete and flawed. What are your thoughts on this? See following image.



                              Harold
                              For those having fought for it, Freedom has a flavor the protected will never know.
                              Freedom is only one generation away from extinction.

                              Comment

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