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crunching, grinding and lousy threads

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  • crunching, grinding and lousy threads

    I can't cut a thread. I definetly have a 60* front , I ground back the sides too to about 10*. The top is ground down to the front at a guessed angle of about 30*. The front is ground back to about 5* and I tried this even greater. Have 29* combination set. When I attempt to thread it crunches and chunks. It sounds horrible and the threads are even worse. Jagged and like spikes all over. I am in dire need of help. I am a novice, and know nothing except I want to learn. I somehow feel it is my tool that is the problem.
    Thanks for any input.

  • #2
    Well two or three things occurr to me. :-)
    With a name like "ductwork" maybe you should
    be sticking to sheet metal. :-) (thats a joke
    son ) :-)
    Seriously what material are you trying to
    thread? Secondly why not try a "ready made"
    threading cutter. Even Harbor Freight, to get
    an idea of geometery.


    • #3
      ........Sometimes what you're trying to thread will be the cause of the problem. I don't rush threading and I like about zero backrake and maybe a bit more siderake then most. I mainly use HSS and I grind and then stone a GOOD sharp edge on it. I'm not in production so I use 2 bits. Depending on the thread I may use the first for 3 passes and then swap to a razor sharp fresh bit for the final 2 and zero infeed final pass.

      Son of the silver stream ..... Bullet caster.


      • #4
        Is your compound set to 29&1/2deg.and are you feeding the threading tool in 2 or3 thou. with the compound each pass?


        • #5
          The tool should be perfectly flat on top, no rake of any kind, and exactly on center. About 10 degrees of side clearance will work.


          • #6

            As Lew asked, what material are you trying to thread? I had very similar results trying to rethread old bolts - what should have been nice smooth flanks were covered in sharp, raggy scales.

            Try a more friendly material - either a free cutting mild steel of a known quality (ie. supplied from a metal merchant) or some aluminium or brass. If that solves it, there's probably nothing wrong with the tool you're using or your threading technique.


            All of the gear, no idea...


            • #7
              what spindle speed are you using? cutting oil?


              • #8
                is the cutting edge of the bit sharp enough to scrape a little finger nail when you try scraping with it. If not you cutter might not be sharp.

                I get the same results as you with dull bits and cutting dry. 3in1 oil works for me.
                - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
                Thank you to our families of soldiers, many of whom have given so much more then the rest of us for the Freedom we enjoy.

                It is true, there is nothing free about freedom, don't be so quick to give it away.


                • #9
                  too slow ? tough steel? rpm? depth of cut? the lathe? how long? diameter?

                  that is the question.

                  you can talk tool geometry all day.

                  If you dont make smoke you are going too slow IMHO.

                  you know, a night machine shop class at a community college kicks ass, I did a 11 credit day class and a 5 credit night class in 1991, I had so much fun that thats why I am not a civil engineer.(plus making $40-$80 an hour at a tattoo shop)

                  [This message has been edited by tattoomike68 (edited 04-17-2005).]


                  • #10
                    Are you set up with the cutter at dead center? Godbless, Ralph


                    • #11
                      From your description, it sounds like there are a couple kinds of rake on the tool.

                      While you may be able to grind rake on a threading tool, you would have to compensate angles. If you grind it to 60 deg, it has to be flat.

                      Any rake angle changes the effective angle of the cutter. Setting too low also does, setting too high won't cut worth anything.

                      I would start off by trying a straight-top cutter, ground to a 60 deg angle, and with relief on the sides below the edge.

                      As it is, you may have trouble with some parts of the tool having negative effective rake. Also, material may be something like drill rod that cuts badly, tool at wrong height, etc. You haven't described the setup in total.

                      I bet at least half of it is from wrong tool grind, from your description.

                      If you try to cut with straight-in advance with a light weight lathe that is a bit loose, you may get all sorts of trouble. it may work on a heavy machine, but not as well on light machines.

                      Depending on tool type, size, work diameter and thread pitch it may be required to grind more relief to avoid the tool bottom dragging on the cut thread and forcing the tool away from one side and towards the other.

                      [This message has been edited by J Tiers (edited 04-17-2005).]

                      Keep eye on ball.
                      Hashim Khan


                      • #12
                        There were some other posts in the last few days on threading. One mentioned running at 70RPM.

                        I use carbide lay-down inserts & run about 150SFM (as Mike said, close to smoking). That means 570RPM on 1" diameter, or 1100RPM on 1/2" diameter. (FWIW, big industrial machines typically run 250-300SFM).

                        Running too slow, even with proper tooling, forces the cutter into the material and allows the work to spring & jump as the toolpoint gouges & tears.

                        After following JT's suggestions (especially the one about "grind more relief to avoid the tool bottom dragging on the cut thread") try moving the speed higher.

                        Barry Milton
                        Barry Milton


                        • #13
                          THAT OLD GANG 'O MINE


                          • #14
                            Relief angles should be with regard to the helix angle of the thread, as well. Typically you won't need much or any relief grinding on the right side of the cutter, but will need it on the left side (towards the chuck). This is for normal threads. Left hand threads will require the reverse of this.

                            Another thing I don't think was mentioned is that there should be some flat on the point of the cutting tool, which will make the root of the thread. Even a small nose radius will be ok, but not a sharp point. Doesn't need to be much, but if it comes to a sharp point it won't give as good a result.
                            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


                            • #15
                              CLICK HERE

                              [This message has been edited by Carl (edited 04-18-2005).]
                              THAT OLD GANG 'O MINE