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  • Thread cutting help!!!!

    Hi all,

    New guy here so don't assume I know anything you can't insult me! `
    I bought an old Sheldon Lathe for the 50's and it has a srew cutting gear box and all the rimmings. my chuck is running around .0002 (3 jaw buck chuch) thats the best I can get it which i understand to be very good! I am following all the rules of screw cutting and gear setu and using the compound to move the cutter in just a few thous at a time. I also set the angle of the compound at 29, 29.5 and 30 degree to the right.. I cut threads preety rough and the nuts wont go on at all??? I tried 5/8-11, 1/2 x 20 etc nothing works. I am sure I am doing everything right starting on the correct number on the threading dial, Using plenty of oil, . My machine being old has a little play here and there but I learned to work around it by making sure I limit all the play possible before I start the job. I am also level to .0002 with apersision level. My Seldon as I read was build to a standard of .0005 so I think the .0002 is ok right? Any help at all is apprecitaed greatly!!!!!!

    Thx
    Joey V.

  • #2
    check to see if the pitch is the one you think you're cutting with a thread pitch gauge, the gear train could be wrong as Oldtiffie pointed out. You are using the threading leaver not the feed leaver, just in case. No insult intended, have seen this mistake made but I'm sure not by you. Shoot for .75 x threads per inch for compound depth and you should be pretty close. For steel I prefer soluble oil and water over oil for threading, go slow, 50 to 100 rpm. Try again and take your time, you will eventually get it right. Peter
    The difficult done right away. the impossible takes a little time.

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    • #3
      Are you measuring the 29 1/2 degrees from a perpendicular to the lathe axis? i.e. set the compound so it's feeding straight in. Then swing the base of it 29 1/2 degrees to the right (rotate it counter-clockwise 29 1/2 degrees.)
      ----------
      Try to make a living, not a killing. -- Utah Phillips
      Don't believe everything you know. -- Bumper sticker
      Everybody is ignorant, only on different subjects. -- Will Rogers
      There are lots of people who mistake their imagination for their memory. - Josh Billings
      Law of Logical Argument - Anything is possible if you don't know what you are talking about.
      Don't own anything you have to feed or paint. - Hood River Blackie

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      • #4
        SGW,
        I am moving the compound 29.5 so the rear of the toolpost is to my right cutter to the left toward the chuck. I use a fishtail guage to make sure it dead on. I then only advance the depth of the 60 degree cutter using the compound. The compound at that angle moves the tool like on a diaganal into the work. as for the gear box leavers I am 99% sure I have them right. You just choose a,b,c,d etc and then the other for howmany threads you want. I use the halfnut to turn the feed on and off going back to zero on the cross slide when I begin again. Just don't know????
        Thx!

        Comment


        • #5
          What criteria are you using to tell when to stop removing metal? Your problem might be as simple as your male thread pitch diameter being over sized by a tad.
          Last edited by Arcane; 11-08-2012, 06:12 PM.
          Location: Saskatoon, Saskatchewan, Canada

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          • #6
            I don't think you mentioned what material you're using. Some metals thread very rough on slow rpms. You can try varying the rpms, or try something guaranteed easy to thread, like free cutting brass. You can also try cleaning up the threads with a die, which will smooth them and also establish a uniform diameter.
            Gary


            Appearance is Everything...

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            • #7
              Threading can't be too difficult. I just finished having two or three high school
              kids do 3/8 - 16 threads (on aluminum) this afternoon. Part of a "hammer"
              project. So start with aluminum, and keep going in (on the compound) about
              .004 per pass, and when the crests flats almost disappear check with an appropiate
              factory nut. If it just barely starts take another .002 off and check again.
              Once you get the hang of it it's a "piece of cake". :-)
              OH! :-) be sure to start with the appropriate diameter, One of the kids had
              the shaft at about .420, couldn't figgure out why it was taking so many passes
              to get the nut to start. :-)
              ...lew...

              Comment


              • #8
                Maybe forget about the 29.5 degrees for now until you get something working. Just feed straight in, but feed a little more slowly.

                Before you start, you have a solid rod. When you have cut enough for the thread to look like a thread, your test nut starts a thread or two, but won't take properly. Another few thou off and the nut will start but bind. At this stage, take a very fine file and kiss the surface of the thread to remove some burrs. Feed in another thou or so, and the nut will go most of the way on, but bind.

                Take another thou or so off and you have a loose fitting nut. The key to threading is to take off less than you did that last time, for a tight but usable fit.

                Take some more off and you have a very sloppy fit.

                Where on that spectrum are you ? Because if you have the pitch right that's what will happen, even if the tool profile is wrong (so long as you don't use a parting off tool !).

                If you get a good looking thread but the test nut won't start, you started with rod of too large a diameter.
                Richard - SW London, UK, EU.

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                • #9
                  Nobody has mentioned it yet, but on a Sheldon (and a few others) you have to set the compound at 60° - sixty degrees on the scale in order to present the tool at 30° to the work....
                  If everything seems to be going well, you have obviously overlooked something........

                  Comment


                  • #10
                    In my experience this type of threading problem comes from not having the correct width of flat on the nose of the tool. You didn't mention what type of cutting tool you're using, but if you ground it yourself then this may be the problem. What happens is you've cut to the correct depth, but there isn't enough clearance between the threads for the threads on the nut. You can either grind the correct nose on the tool, or use the topslide set parallel to the work method so you can open up the clearance between threads after cutting to depth. Also you didn't mention where you got your info on cutting depth. There's usually a depth spec and a sharp vee spec. You also usually need to make a couple of spring passes after cutting to depth to make sure you're not still taking off metal. Remember that if you use the topslide parallel method you'll be feeding in with the cross-slide, which on a Sheldon is measured in diameter, not infeed.

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                    • #11
                      Pictures if possible would let us sort through this faster.
                      gvasale

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                      • #12
                        I think I got it!

                        Originally posted by gvasale View Post
                        Pictures if possible would let us sort through this faster.
                        I think I have the angle wrong for a sheldon!!! I think I need the cross feed at 60 not 30. I also started out with the bar stock to fat. Being new to all this never had a class at all where can I get a chart or something that tells me starting sizes and depths etc?
                        Thx fir all the help boys!!!!

                        Comment


                        • #13
                          Originally posted by Joey V. View Post
                          I think I have the angle wrong for a sheldon!!! I think I need the cross feed at 60 not 30. I also started out with the bar stock to fat. Being new to all this never had a class at all where can I get a chart or something that tells me starting sizes and depths etc?
                          Thx fir all the help boys!!!!
                          THAT is the reason I keep OBJECTING to the most common statement " set the compound to 29.5 (or 30) degrees". There are many lathes that the compound angle is calibrated the other way. With regard to diameter of work before trying to thread, I just went through this yesterday with half of a class at school. The kids jump in and start to thread without getting the diameter to the required size. :-) Two of them had the diameter at .400 to try and cut 3/8-16 threads. And couldn't understand why they were having trouble getting the test nut to fit.
                          ...lew...

                          Comment


                          • #14
                            Originally posted by Joey V. View Post
                            I think I have the angle wrong for a sheldon!!! I think I need the cross feed at 60 not 30. I also started out with the bar stock to fat. Being new to all this never had a class at all where can I get a chart or something that tells me starting sizes and depths etc?
                            Thx fir all the help boys!!!!
                            You're right. The Sheldon compound slide is marked relative to the work, so you want 60d relative to the work. When the compound is parallel it shows 0d. It makes sense to be marked that way when you're working from drawings. As far as specs go, if you plan on continuing your machining journey I would strongly suggest getting a copy of the "Machinery's Handbook". You can find older revisions pretty cheap on Ebay. Even the older versions will have all the thread info you'll ever need. Otherwise this chart is good but you need to really understand the dimensions to be successful.
                            http://www.engineershandbook.com/Tab...readlimits.htm
                            Last edited by Forestgnome; 11-17-2012, 01:30 PM.

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