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Cutting Threads

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  • Cutting Threads

    I'm new to machining. I am trying to learn how to cut threads in mild steel. When I cut the threads they appear to be galled, very rough. I am not using any lubrication or coolant. Also using a carbide insert type bit.

    My procedure is to turn the shaft down to the maximum diameter, then start laying threads about .005 per pass. I am turning down to the minimum diameter using the cross slide.

    Should I use the compound slide instead?

    My guess is that I should be using a HSS bit with a radius that matches my thread root and use a coolant. I am hoping somebody can point me in the right direction.


  • #2
    Hi Jb, what grade of steel are you using? Softer steels like rebar or 1018 are very hard to get a good finish on. Also use some thread cutting oil. Most prefer the dark cutting fluid like you find at Home depot or Lowes. I just use the Lps gold that i have. Try cutting threads in some 12L14 or some 1144 stress prof and you will probably have better results. Also feeding with the compound set at 29 degrees tens to help in that the cutter does most of the cutting with the leading edge instead of both edges.


    • #3
      I assume it is 1018 CRS. I picked up some cutoffs from work to practice on.


      • #4
        Its very difficult to get smooth threads on cold rolled, they will be quite gnarly looking. A good sharp HSS bit carefully set as precisely as you can get it and at least some type cutting oil with light cuts will give you the best results, but will probably still not be 'pretty threads'.
        If everything seems to be going well, you have obviously overlooked something........


        • #5
          Originally posted by jb-mck
          .... I am not using any lubrication....
          I'd say that is the major source of your problem! Get some dark thread cutting oil at a True Value Hardware store, Lowe's, or Home Depot and use it liberally!

          Last edited by Mike Burdick; 05-11-2009, 12:21 AM.


          • #6
            Originally posted by jb-mck
            I'm new to machining.
            ...then start laying threads about .005 per pass. I am turning down to the minimum diameter using the cross slide.

            Should I use the compound slide instead?


            I am also pretty new at this but a couple of things I do.

            first .005 (or more) is fine to start with but towards the end it may be too much. might want to back down to .0025 or .001 at the end depending on the pitch that might be a pretty deep cut.

            using the cross slide, the tool is cutting on both front sides of the cutter.
            this might be your main problem. j

            if you can use the compound and just cut with the leading front edge of the cutter it should be easier on the lathe.
            (it is doing half the work the two edges had to)

            my process is turn to diameter zero both cross slide and compound dial
            (which I don't always do since mine are not adjustable)


            advance compound (less and less as thread gets deeper)
            engage carriage
            disengage carriage
            back cross slide off so tool beyond major diameter
            crank carriage back before start of thread
            advance cross slide to zero

            you might want to wait and see if I get blasted for promoting
            some threading blasphemy but if I do we will both learn something
            Tom C
            ... nice weather eh?


            • #7
              Adding to that blasphemy..

              Compound at 29.5 (or so) degrees, last couple of light passes use the cross side only to clean up both edges of the thread.

              I've found it easier to thread from the back and run the lathe in reverse. That way I can thread at higher speeds away from the chuck. Less stress on me and better finish.


              • #8
                If you are not threading into a groove you definately want to back off cross slide BEFORE the lead screw is disingaged. Keeps the tool point from snapping off. Plenty of good cutting oil and light cuts as mentioned already.


                • #9
                  My sins were cutting tool not perfectly on center and too much unsupported stock sticking out of the chuck. I use a very small container with thread cutting oil and brush it on with a 1" wide paint brush. I also didn't have my lathe adjusted up tightor snug enough on the gibs.

                  (when I absolutly have to get it right, I have a handle I screw into the end of the quill and turn the lathe by hand. This is an idea I got from Evan)
                  - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
                  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.


                  • #10
                    Use oil and a hand ground HSS tool. Yeah I know. Tha pre-ground carbide once are so convenient and hand grinding a HSS a hassle but it does work better.

                    Any kind of oil will do but the best is either bacon grease for mild steel (honest!!) or Rigid heavy black pipe threading oil. Later on you'll accumulate a selection of cutting ouls and coolants like Tap Magi, Chesterton, mineral lard oil, etc but in general don't thread or use form tools dry.


                    • #11
                      And... at a scraping class I attended last year...Forrest also recommends Sheep fat for storing tooling.

                      Forrest was green before it became a buzz word.

                      But this is all good. I'm not "allowed" to eat bacon, but now I can buy it and say "it's for my threading". Ah.. the thought of sizzing bacon fat on my tool make my mouth water.


                      • #12
                        This is a pretty good starting place on threading.
                        "Those who hammer their guns into plows will plow for those who do not."~ Thomas Jefferson


                        • #13
                          When I started cutting threads 8 years ago, I made threads on plastic pipe until I got a feel for it.

                          Then I made threads on steel.
                          I showed them to my brother who had been cutting threads for 25 years.
                          He said I was doing everything I could do wrong as wrong as I could do it.

                          I remember finding out that the .015" cut at the threshold of chattering was a problem. He said take off .001, .002" or .003" per cut.

                          Now my threads look good from 3 feet away, but under magnification, I still have some tearing.

                          My brother's threads look like jewelry under magnification.

                          Maybe some of us were meant to only make functional threads and not fancy threads.
                          There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. -Ernest Hemingway
                          The man who makes no mistakes does not usually make anything.-- Edward John Phelps


                          • #14

                            Too items that help - SHARP tool and plenty of the old reliable high sulfur, black, cutting oil. The old Tap-magic with the high chlorine base fluid will help the cutting. A rigid tool mounting and a good lathe can help teh quality.



                            • #15
                              Thanks "ALL" for the input! I'll give her another go with oil, compound rest, and HSS bit. I will add results here tomorrow.