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How to cut buttress threads?

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  • How to cut buttress threads?

    I have to cut an internal buttress about 2 or 2 1/2 dia.
    Never cut one before and could use any tips.
    First questiono I set the compound at the angle and work the tool in like v-threads ?Or do I bring the tool in at a 90 angle and cut the flat?
    Secondly: How do I know when to stop? The mating piece is over eight feet long,no way to bring it to the lathe to mate it to the part being cut .
    Maybe I should cut a male part first in order to sharpen my skills and to have a test part for the internal threads.

  • #2
    I will say your going to have to make a short male test thread the same dimensions as the long shaft.

    I have cut two buttress threads and I plunge cut both of them. I made a cutter that fit the thread I was copying.

    It was not an easy job, there had to be a better way.
    It's only ink and paper

    Comment


    • #3
      Hi,

      I've done external buttress threads for punch press balls screws. I always wanted to choke the machine operator for locking the press down and the maintenance guys for torching it off. They do suck to make.

      I had to grind a what was pretty much a form tool cut them. So, I had the compound feeding straight in. The tool cut on both sides simultaneously. Though it got to be quite a load on the tool as the depth of cut got deeper. I used a 1/2"x1" piece of M42 HSS to hand grind the tool.

      Oh, and don't forget to make sure you make the tool for threading the correct direction, buttress threads can be run both ways.

      Good Luck!

      dalee
      If you think you understand what is going on, you haven't been paying attention.

      Comment


      • #4
        These folks make inserts for American Buttress and API Buttress threading.
        http://www.toolflo.com/cats/On%20Edge.pdf they are on page 136 of their catalog.
        Byron Boucher
        Burnet, TX

        Comment


        • #5
          Just a couple of days ago cut a left handed buttress thread (.750-7 tpi) to save an old Starrett vise I got second hand. I found the two most difficult problems were grinding the tool to match the profile of the old vise screw. Secondly, the presentation of the cutting tool to the work is also critical, a fishtail doesn’t cut it. I set the compound to 90؛ and fed the tool straight in to the work the proper amount for a 7 tpi. (.071) I took .002-.003 per pass.

          I was fortunate enough to be able to mount the old screw in the lathe and adjust the tool post so the ground tool fit the old threads perfectly.
          Good Luck.
          Last edited by Ron of Va; 12-01-2010, 06:30 PM.

          Comment


          • #6
            I measured the thing today,it's 2 3/4 shaft threads look like they are 3/4" deep.
            This is going to be a tough job for my wore out 12" atlas.
            The form tool needs to be over an inch wide.
            I wonder if I can make two tools,cut the flat and half the angle to depth,then come back with the second tool and finish the angle.
            I can see it,just don't know if I explained it very well.
            I am repairing a Lincoln # 7015 four post car lift.One column has the buttress screw driven by a gearmotor and the other three post have cables tensioned to the screw so it all lifts evenly.
            The lower limit switch died and the screw ripped out the threads on the bronze lift nut.
            I can't get a reply from Lincoln so I have to do it the hard way.

            dalee: I promise I will measure and draw everything a dozen times before I turn on the lathe,thanks for the headsup,that could easily be overlooked
            Last edited by 1-800miner; 12-01-2010, 08:03 PM.

            Comment


            • #7
              I suggest you rough out all you can before you start using the form cutter to finish the thread. I also suggest you try it on a piece of scrap brass or aluminum before you do the real nut.

              That is going to be a very hard load on your lathe. Don't get in a hurry and take very light cuts.

              How are you going to hold a 1" square cutter in a boring bar? Are you going to weld the form tool to the end of a bar to make the threading tool?

              Man, that can get real intense real fast.
              It's only ink and paper

              Comment


              • #8
                That seems like a pretty wide chip to take on your lathe, even with taking multiple shallow passes.

                Consider making a tool which is only 1/4" wide with the requisite angle 1/8" wide and a 1/8" wide flat. Or go 3/8". Set your compound parallel to the axis of rotation so you can use it as a fine offset relative to the lead screw position. Then take lots of passes to rough it out:
                Code:
                      4321
                      765
                      98
                Advancing less than half the tool width for each one, longitudinally.

                You can also do this, reversing left to right, with a form tool.

                You haven't given the pitch and angle of the thread, that I can see.

                If your threads are 0.75" deep on a 2.75" shaft, then the bore of your nut would only be 1.25 which only gives you 1/2" shaft to support a 3/4" deep cutter, less a margin for clearance.

                Last time you were lucky in that the nut apparently stripped out while the lift was on the floor. If it strips out or has some other failure while someone is under a car ...
                Make sure you round off the inside corners so you don't create stress risers.

                Comment


                • #9
                  Originally posted by 1-800miner
                  I measured the thing today,it's 2 3/4 shaft threads look like they are 3/4" deep.
                  This is going to be a tough job for my wore out 12" atlas.
                  The form tool needs to be over an inch wide.
                  I wonder if I can make two tools,cut the flat and half the angle to depth,then come back with the second tool and finish the angle.
                  I can see it,just don't know if I explained it very well.
                  I am repairing a Lincoln # 7015 four post car lift.One column has the buttress screw driven by a gearmotor and the other three post have cables tensioned to the screw so it all lifts evenly.
                  The lower limit switch died and the screw ripped out the threads on the bronze lift nut.
                  I can't get a reply from Lincoln so I have to do it the hard way.

                  dalee: I promise I will measure and draw everything a dozen times before I turn on the lathe,thanks for the headsup,that could easily be overlooked
                  Hi,

                  Honestly, and with no disrespect meant, your 12" atlas isn't capable to do the job. It's not going to have enough power or rigidity to make those cuts. I was using an 18" Colchester with a 10hp motor and it had all it could do. A 15" LeBlonde with a 5hp wasn't up to it. Though I was making ball screws from 3 1/4" 4140.

                  Unlike vee type threads, the tool has to cut both sides at the same time. You maybe able to rough out a small part of the thread, but at some point you will need form the whole thread. And it will be at the bottom half of the thread where the load is the greatest anyway.

                  I would honestly pass on this job or find a lot bigger lathe to use.

                  dalee
                  If you think you understand what is going on, you haven't been paying attention.

                  Comment


                  • #10
                    While you are making the male test piece, you might want to make a tap or chaser. Then you could rough out the thread on the lathe and finish it with the tap.

                    It seems to me to be a real challenge for a 12" Atlas. I have one and mine is certainly not very rigid. I would probably try to cut it in several sections and would probably not even use a tool that cut the entire form. That would take some careful planning and measurement but I think it is feasible. Practice a lot on scrap before you start the real thing.

                    A good sharp tool with the correct angles will certainly make things easier.
                    Don Young

                    Comment


                    • #11
                      A 1" wide by 3/4" deep thread sounds like an application for thread milling to me.
                      Design to 0.0001", measure to 1/32", cut with an axe, grind to fit

                      Comment


                      • #12
                        Buttress Thread

                        Might be able to thread mill it with a dovetail cutter. Or get a couple of carbide inserts for that thread and make a thread mill. I've done that a few times for specials.
                        Kansas City area

                        Comment


                        • #13
                          How about using a buttress thread insert?

                          http://www.usshoptools.com/current_y...n_buttress.pdf

                          Comment


                          • #14
                            3/4" deep threads on a 12" atlas? Not going to happen. Your gearbox wont even be able to match the pitch of the thread. This is going to take a big lathe, or a cnc mill and thread milling.

                            And personally I would never feel good about a part of mine going on a car lift. eek..

                            Comment


                            • #15
                              Screwing a job (up?)

                              Good advise Macona.

                              Its usually difficult to cut a thread with a lead 2+ x the lead of the lead-screw - often considerably less.

                              Despite what some say, it is not all that difficult to cut that form and size thread on a small(er) lathe at all. I will cover that later if needs be. Just about everybody here will have all that is needed in terms of the tools required to cut the thread.

                              But before some start throwing thread milling - particularly internal thread milling - about, I suggest that they take a chill pill and read this extract from Machinery's Handbook.



                              I bought this machine from the UK - no NOT China but the UK - for such stuff amongst lots of others for high-helix (small lead) spiral gears as well as thread milling - also for small leads.

                              http://www.hemingwaykits.com/acatalo...ep_Mill__.html





                              My limitation is my 3mm lead (~ 8tpi) lead-screw.

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