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Threading on backside of lathe

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  • Threading on backside of lathe

    I have a need to thread up to a shoulder. There have been several references to run the lathe in reverse and put the tool bit on the back side of the part. So for Right hand threads is it lathe in reverse, feed toward the tailstock or is there more to it?

    Any and all help will be appreciated.


  • #2
    It's been done providing your cross slide has enough travel and you have enough room to get the tool post and cutting tool set up behind the work.



    • #3
      You will need a runout groove in either case, so there is really nothing to be gained by the exercise. If a runout groove cannot be tolerated, and the spindle cannot be slowed enough for your comfort level, the spindle can be turned by hand. There have been several threads and writeups on making an expanding mandrel for the spindle to permit the use of a crank to turn by hand, I use an adjustable wrench on the chuck jaws.
      Jim H.


      • #4
        I don't know if it's possible to thread completely to a shoulder without using some sort of thread-milling technique. I used the upside-down tool and lathe in reverse to chase threads for my mini-vise project on a Clausing lathe at shop class:

        Notice that there is a groove on the chuck side of the piece where the threading cutter can traverse the entire surface of the part. The depth is advanced by 0.015" or so for each cut, and the half-nut is engaged at the same point so that the thread always starts at the same point. To get closer to the shoulder, the tool would have to be ground so that the left edge just touches the shoulder at full depth, and there will still be a groove at least one thread pitch wide.

        The following pictures show the groove and the threads:

        Paul , P S Technology, Inc. and MrTibbs
        USA Maryland 21030


        • #5
          Easiest with a groove to start in. In fact, if an end groove can't be used, I have no clue how you would run in the tool at all.

          Toolholder position can be awkward depending on how much travel your cross slide has. If the threads are tapered, don't forget to set the correct backside angle. Tool clearance should be the same from the back unless you are cutting tapered threads with a large taper. Watch the backlash and half-nut engagement, lest the carriage kick into the shoulder as the nut engages. The nut needs to be engaged smoothly and completely prior to start, as there is no dead time to prior to the tool starting the cut to weasel the nut full in... In the rare case I do this, I roll the spindle around to the same position before engaging the half nut each cut, then power on. There are people that power the spindle, the engage, but that takes a lathe with smooth engagement of the nut and a good hand. I have neither in my shop, and am missing the second in the shop at work.


          • #6
            Thanks to all that replied. As usual for me, I was overthinking the supposed problem. There is some room for a design change.


            • #7
              You do not have to put the tool on the back side of the work, just turn it upside down while still on the usual side.

              A relief groove is still needed, but can be much smaller and even "V " shape.

              Sent from my XT1053 using Tapatalk


              • #8
                I've done internal threading to a relief groove. The tool in the hole was blind so I relied on a mark on the outside to go by for throwing out the half nuts. I also ran it at an almost painfully slow back gear speed to give myself more chance to not crash the tool.

                I've yet to do the reverse rotation/back side cut. But it seems like a far more fool proof method. I intend to have a go at it for the next time I need to do an internal thread to a shoulder.

                Let's face it, not many of us do a LOT of single point threading. So each time we tackle even an outside thread we tend to slow down and double check. And for threading to a shoulder I'd wager that most of us run the lathe a lot slower and really focus. Especially with a coarser thread that will have the carriage moving more per rev. Now take that added care and attention and toss in an internal thread in a small hole to an internal shoulder? WELL! ! ! ! I'd say a little creative care invested in setting up to allow cutting the right hand thread using reverse is not a bad thing at all.
                Chilliwack BC, Canada


                • #9
                  "If a runout groove cannot be tolerated, and the spindle cannot be slowed enough for your comfort level, the spindle can be turned by hand."

                  Yep. If you're the type that can pat your head and rub your belly at the same time, you can fade out the tool while you turn the chuck.
                  Never tried it w/carbide. Don't believe I will.


                  • #10
                    I single point thread quite often and as a matter of course. Avoiding it out of fear will never produce proficiency.

                    Two advantages I have are a QCTP with threading tools set up and a VFD that allows slowing the spindle to a walk or even a crawl. I do turn by hand if it is critical, and backing out by hand produces a nice fade out when desired. I never use carbide, there is no need for it.

                    I had the good fortune to pick up a complete set of the Aloris thread cutters, V thread and Acme, at auction, but they are not that expensive when compared to carbide form tools. They are easily resharpened and will last a lifetime in normal use. I made my own QC toolholder for their use.
                    Jim H.


                    • #11
                      Just run it normal and stop it in the runout groove.

                      I always use quality carbide from reputable brands no faffing about getting full profiles correct and having to constantly sharpen them mid job.


                      • #12
                        I make a top hat shaped bushing on a fairly frequent basis that is 5/8-16 thread to a shoulder that is 1" diameter. I've ground a threading tool so that it is just over the width it needs to be on the LH side and utilize a dial indicator to dis-engage the threading op. I run at 100 rpm or 55 if I'm not feeling on top of my game. I cut an .080" relief groove at the shoulder, but really don't need to. The indicator is setup to operate on the last .100" inch of travel and when the dial hits .1", I drop the halfnuts and back out. Haven't nicked the shoulder yet, and this allows me to get the max thread engagement for this application.


                        • #13
                          As someone that has single pointed somewhere around 200 to 250 threaded items in my 50 some years of lathe use I'd say that I'm not anyone that has heaps of experience. But I consider myself as well beyond a beginner at it. And likely as not roughly equal to any other home/hobby machinist with similar time at all this.

                          And to that I would add a hearty "AMEN! ! ! !" to what JCHannum expressed a few posts back....

                          .... Avoiding it out of fear will never produce proficiency.
                          By all means don't wait for an actual critical piece to work on. TRY things out on scrap metal. Consider it as that machining course you could never find time to attend. But TRY this stuff. Learn from both the successes and failures. Each has something to offer. So don't be afraid of either.

                          This includes threading to a shoulder by some means. Either use the painfully slow speeds possible with back gear or a dial gauge or threading on the back side of a part and running the machine in reverse or some other means. Ideally you'll try all the methods. But what ever you do just DO IT! ! ! !
                          Chilliwack BC, Canada


                          • #14
                            I found some videos that show threading from the backside of the lathe, using an internal threading tool (like a boring bar):

                   (Heavy accent and baby crying, but shows decent technique and high speed)

                   (Large boring bar. The guy is smoking and wearing long sleeves - things NOT to do)

                   (Also uses boring bar, quick 1 minute video)

                            Finally, Tubalcain shows how he uses an upside-down threading tool:

                            Paul , P S Technology, Inc. and MrTibbs
                            USA Maryland 21030


                            • #15
                              While a run out groove looks nice and neat, it's not absolutely necessary. Disengage the half nuts at the end of the cut and let the workpiece turn a few turns before retracting the cutter, and it will cut the groove for you. Disengage just before the shoulder, then use the handwheel to travel to a stop or an indicator setting. The groove won't be as neat as one made with a square end cutter, but it will do the trick. External or internal threads, no difference.