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Single pointing left hand thread

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  • Single pointing left hand thread

    I'm cutting 3/4-10 threads on each end of a 1" bar for a buffer I am making. I can cut the right hand threads with a die if I have to but I need to single point the left handers. Does the compound need to be turned 29* pointing away from the chuck to do the left hand threads or does it stay the same as if I were cutting right hand threads (pointing towards the chuck)? Also any tips on getting a good surface finish? On a practice piece (used to be the real shaft) using a fresh point on an inserted tool the finish was really crappy (I had the compound pointing towards the chuck).
    Under construction - MGB roadster widened 11.5" with Corvette C-4 suspension front and rear, 440 hp LT1 V8 with a T-56 6 speed.

    Lots of pictures here
    Part 1 http://forum.britishv8.org/read.php?13,7581
    Part 2 http://forum.britishv8.org/read.php?13,22422

  • #2
    To cut a left hand thread cut from the chuck to the tailstock. Set the compound so the crank is on the left side toward the chuck. Set the threading tool on center line perpendicular to the work run the spindle in the forward direction. Set the levers in the correct position for the thread you want.
    It's only ink and paper

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    • #3
      Some are of the opinion that setting the compound over is not totally necessary:

      http://bbs.homeshopmachinist.net/sho...360#post501360



      Ian
      All of the gear, no idea...

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      • #4
        Originally posted by Jim Stabe
        Also any tips on getting a good surface finish? .
        make sure your side clearance is correct.....the side clearance needs to accommodate the helix angle which, for a left hand thread, is slanting in the opposite direction to what you'd have for a right hand thread
        in Toronto Ontario - where are you?

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        • #5
          I cut left hand threads for the well drillers in my area all the time. I made a left hand threading tool which I place in my tool post upside down. I then set my compound at 29 ½ deg. I rotate the spindle backwards and thread towards the head. Either way you do it, you will have to make a left hand tool. I feel that by setting up the way you would normally thread (except the rotation) your reactions will be normal in case of emergency. Neal

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          • #6
            Originally posted by Ian B
            Some are of the opinion that setting the compound over is not totally necessary:

            http://bbs.homeshopmachinist.net/sho...360#post501360



            Ian

            I would set the compound over. If the leadscrew is worn trying to cut the left side of the threads while moving to the right may cause the carriage to shimmy over and not cut right. To take up backlash and slop you should be cutting the side of the thread you are traveling toward.
            Andy

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            • #7
              Yeah, that was an interesting thread and mardtrp didn't understand what he thought he did. Plunge cutting is nothing new and for fine threads it works well but for coarse threads it has issues. As to a CNC lathe the program moves the threading tool at a 30 deg angle for each pass but I guess you can program it for plunge cut threading.

              I hope this thread doesn't to downhill from here. It seems like threads about how to thread always go astray. It's a real simple process that some like to make complicated.
              It's only ink and paper

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              • #8
                Thanks for the replies. I hadn't thought about the clearance for the helix angle nor had I thought about turning the tool over and reversing the rotation. I'll practice some more before tackling my next shaft.
                Under construction - MGB roadster widened 11.5" with Corvette C-4 suspension front and rear, 440 hp LT1 V8 with a T-56 6 speed.

                Lots of pictures here
                Part 1 http://forum.britishv8.org/read.php?13,7581
                Part 2 http://forum.britishv8.org/read.php?13,22422

                Comment


                • #9
                  It seems to me that swinging the compound around to cut only on the leading edge is all about not making plunge cuts, which feeding straight in is. Plunge cuts like to induce chatter. Rigidity can eliminate chatter. A locked down compound is a lot more rigid than one that is not(so you can feed it).

                  Most any thread that a home shop machinist will likely do will probably never go straight in very far and that will be done in very small increments.

                  My 10EE operators manual states that the last thousands or two should be done by feeding the cross slide straight in to ensure a properly formed thread(just at it's deepest point and most likely to chatter). Why not just do it all straight in?
                  It's worked for me for over 50 years, but then I have never run a 7" lathe either.

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                  • #10
                    Well, fine threads are mostly cut in one or two passes so they can be a plunge cut but most threads are best done with the compound to feed in for each pass. I know there are a lot of machinists that use plunge cut a lot but I get better thread finishes with the compound feed for most threading.

                    I too use a plunge cut of .001" sometimes to clean up a thread but most the time my threads come out just fine without a final plunge cut. Once you learn how to thread you usually find what works best for you and keep doing it that way. I use a little bit of every method as fits the situation except for using the cutter upside down. I see no good reason to do that but others love the hell out of that way. What ever blows your dress up I guess.
                    It's only ink and paper

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                    • #11
                      All good advice except you need to make an undercut with a parting tool if you are going to thread in forward rotation. Reversing the lathe with reverse rotation and upside down tool is ok if you have a camlock or LOO chuck mount. Threaded chucks can come loose with results you don't want in reverse. You should have no problems, why not single point both? You get a much nicer thread single pointing. Peter
                      The difficult done right away. the impossible takes a little time.

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                      • #12
                        Don't forget threading with tool upside down and in reverse will have cutting forces tending to lift the carraige off the ways. May or may not be a problem esp with heavier machines. Just a thought.

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                        • #13
                          I was taught to cut threads without setting over the topslide and thats how I do it I know both ways give results so what's the biggy.Grandpa Alistair
                          Please excuse my typing as I have a form of parkinsons disease

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                          • #14
                            No biggy at all you just do what blows your dress up or makes you whistle Dixie or blows your whistle or toots your horn so on and so on.
                            It's only ink and paper

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                            • #15
                              Cutting left is generally a pleasure (in my opinion) especially short run threads. I do jobs like what the OP is doing on a regular basis as I have a fleet (so to speak) of grinders, buffers and sanders to keep alive.

                              Undercut to max minor dia. Start tool in undercut and don't even bother with the compound, just straight feed it. Every 3rd / 4th pass, run a spring pass then move on. Sharp tool with no build up on edges and it goes like a dream. A light (brush applied) coat of yer favorite oil on the threads. My common thread is 5/8 11 Lh for the sanders. Only thing you need to do is note at each start what yer actual depth is. once the tool approaches the minor drop final depth to a few thou and finish it. As long as your cautious on hitting that max minor, then you know what you have for leeway for excess depth.

                              Personally I like to go the average between max and minimum minor dia. I've had excellent results this way, also do the same with acme threads. Hrm, I generally tend to straight feed internal threads. In fact the only thing I use the compound for is cutting standard threads, now that I think about it it's probably more from habit than anything =)

                              It's funny how threading kind of becomes something different for all of us. Teachings, habits (good and bad), etc...

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