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How to cut buttress threads?

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  • #16
    As others have said, this is really too big a job for the lathe that you have. However, if there's one thing HSM'ers excel at, it's stuffing a quart into a pint pot and doing jobs waay past their machine's normal capabilities.

    Cutting the whole sloping flank with a full width form tool will just cause chatter, especially as it'll be on the end of a boring bar. What you could do, however, is work along the sloping and vertical faces cut by cut.

    Rough the thread out, and then clean up the sloping and vertical faces separately. Feed down the vertical flank with the cross slide, and then set the compound slide to 45 degrees and work along it. Both flanks might take 5 or 6 passes each. This way, you can use a form tool that's only 1/5th of the sloping flank's total width.

    Good luck, post pics!

    Ian
    All of the gear, no idea...

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    • #17
      Eating an elephant.

      You've got it Ian.

      Eating an elephant can seem a daunting task - if you want to do it in one go - but its a lot easier with lots/more chewable and digestible bites.

      I'd have done the roughing using what are essentially "parting tools" and the cross-slide for the straight plunge in (as for square threads) and then the top-slide set at 45* for the 45* flank.

      The rest is just a clean-up - taken slowly.

      The cuts are making a spiral not a groove and with that size helix angle I'd be tilting the screwing tool/s.

      I don't know how the Law applies in the OP's state or country, but here in OZ car hoists (as in commercial garages etc.) come under the "Cranes, Hoists and Lifting Gear Code" and all components parts or assemblies must be designed, made to and Inspected in accordance with the Code.

      The Code also covers and requires testing for replacements if required.

      The hoist is required to be checked by an Inspector and load-tested at set intervals.

      If the hoist fails a test or is overdue it is regarded as being "out of test" and is not to be used until it is "passed".

      There is no way I'd make that nut here.

      The hoist has clearly had a substantial failure and must only be repaired and probably tested and Inspected in accordance with the Code.

      I don't think I'd like to be served with a writ or a warrant if anyone got hurt or worse.

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      • #18
        I would go with an insert for the thread you need to cut.
        Look at the link below and you can download thier program for selecting the right insert for the job. I use it all the time, it takes the guess work out of everything and they threads come out perfect.
        http://www.vargus.com/vardex/templat....aspx?pCatId=7

        JL..............

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        • #19
          Well I better tell "the rest of the story".
          It's my own car lift.I bought it at a military auction years ago.Bought four,sold three. In hind sight I should have sold two.
          They had three phase three horse motors.I have single phase.
          So I replaced the motor with a 3/4 horse with a hundred to one gear reducer in between(I know, but it is all I had)
          It was slow but so am I.It worked great for twenty years,until the limit switch quit. At x100 torque against the weak side of the threads it failed.
          If I can repair the lift I will change out the motor and gear reducer to somthing more modest.

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          • #20
            Ok, from the start I didn't think your lathe is up to the job. I suggested roughing most the metal out and then finishing with the form tool and that may work. Others have suggested using a smaller form tool and stepping it out to clean up the finished thread but that is difficult to do.

            I guess you will be using some grade of brass for the nut. First you need to make a male test stub of the thread probably in aluminum. Then try to make a nut using aluminum to see if you can even do it. Roughing out the internal thread will be tricky and I suggest you do the cutting on the back side so you can watch the progress. It will be very tricky to get a good finish using a smaller cutter because of all the overlapping passes required. You will probably have to use a full size cutter to get an acceptable finish and doing that with a boring bar is going to be even harder.

            In the end I think it may be better to have it done on a CNC lathe and they would not have to use a full size form tool to do it.
            It's only ink and paper

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            • #21
              Don't know if I can get it cut,but I sure started an interesting thread!
              I sure like the looks of that tool post milling machine,I can see lots of possibilities.
              Prolly buy a new car lift for the repair price ,but I gotta try, just because.

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              • #22
                Here is another option. Cast a nut. Maybe something like silicon bronze. Make a mold over the screw and then cast a core from that. I am betting you can get good enough surface finish from the cast for a car lift. Not talking breach block lugs here.

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                • #23
                  If you do try it by cutting the profile in steps, don't worry too much about getting the sloping flank perfect - it doesn't do anything, it's only the perpendicular flank that's taking the load. It needs to be somewhere close to give the loaded flank the required support, but it most likely won't even be touching the threaded shaft.

                  Ian
                  All of the gear, no idea...

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                  • #24
                    Never thought about casting! I have a lot of high density styrofoam. I can chuck that stuff up and cut threads until I get a good one. Then cast it.
                    I will have to play with cutter shapes so I cut rather than tear the foam.
                    Any thoughts on that?
                    Maybe I can fit a quart into a pint jar?

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                    • #25
                      What I would do is take the screw out and make a box around it about 2x the length of the nut. Then coat the screw in mold release and use silicone mold material to pour half around the screw, let it harden, put release on the new rubber and pour the top half. Now you have a mold you can make a core for. Use silica sand and sodium silicate to make a no bake core. Or a baked plaster typed core might give a better surface finish. Use that to cast around for the new nut.

                      I have never seen really great results with the lost styro methods.

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                      • #26
                        What a great idea. I should have titled this thread "how not to cut a buttress thread"

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                        • #27
                          Or "Saving your buttress one thread at a time."

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                          • #28
                            I want to know how this came out....anyone else?

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                            • #29
                              I had to cut an American butress thread inside a collar to match a screw shaft. I bought the full profile insert for my Valenite internal boring bar and it was a snap to cut. All I had to do was make sure the bar was parallel to the work and the height was set right and in about 20 passes I was done.

                              JL.....................

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                              • #30
                                Originally posted by ajfreeze
                                I want to know how this came out....anyone else?
                                Me too. A year should be enough to have a final answer. :-)
                                ...Lew...

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