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How small a thread do you cut?

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  • How small a thread do you cut?

    Ace Ventura posted pictures of a lathe thread chart for JoeFin which went down to 224 tpi. My old lathe had screw cutting to 192tpi. I never even thought about using that - the lathe tool grind would be hard enough to get so the thread was meaningful. I'm guessing that these horribly fine threads are there because the gears can do them rather than they are used - some of the UN threads get down to 80tpi I think and I've seen threads for microscopes at 36tpi.
    As a matter of interest, what are the smallest that members have cut on lathes (single point or chased)? (of course we want pictures...)


  • #2
    Well I suppose even if you use taps or dies they are made with machines. Alistair
    Please excuse my typing as I have a form of parkinsons disease


    • #3
      I've never had need to cut more than 40 threads per inch but I have seen 180 tpi. When they built the Synchrotron here in Saskatoon a few years ago, the leveling blocks for the light beam were built by a local machine shop. One of the shop owners was a brother to a friend I worked with who had been given one as a souvenir and he brought it to work. AFAIR, the threaded block was roughly 1 1/4" thick by 2 1/4" square, threaded completely through, and the "weight jack" part of it was about an inch, maybe 1 1/8" in diameter. I do remember that the pitch was definitely 180 tpi and it turned as smooth as anything I have every seen.
      Location: Saskatoon, Saskatchewan, Canada


      • #4
        Years ago when making a small working model of a flintlock pistol,I made a 224 thread tap of about a .020" diameter. 2 flute. Still have it somewhere.


        • #5
          I have a job waiting that is 5mm diameter by 0.35 or 0.4 pitch, can't get in with the pitch gauge at the moment and it's LH to boot.

          It's a very neat bit of kit, it's a set of three knives / scraper that close under centrifugal force as they spin and you place a piece of enamelled wire in, start up and it closes and spins and scrapes the enamel off so you can solder onto the wire. I have to make a new driving shaft so it can be put on another motor.

          Previous to this I had to do some 8mm threads at 0.3 pitch both internal and external. 0.3 is about 85 tpi.

          Actually they had 8mm x 0.3 on one part and 8mm x 0.35 on the mating part. I rang up about it and was told just make it to the drawing. when I pointed out they wouldn't fit I was told not my problem if made to the drawing ?

          Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.


          • #6
            In my history I never went higher than 80. Did a 48 yesterday.

            But on the other end of the extreme, I just noticed about a month ago that the large lathe at work will do 1/4 TPI (one thread every 4 inches).
            That's just built into the quick-change gear box - no changing gears required. Just a standard pitch on the machine, as well as a progressive mix between that and more sane pitches. That's kinda cool.

            No doubt others have use machines that are set up to do that and maybe even utilized it, but that's unique in my experience.

            Funny that I told the other two guys there about it and they never knew the lathe would do that and they've both been there over 4 years.


            • #7
              The thread I had to replicate when I made another tip for my Verdict DTI appeared to be 0.35mm pitch on a 1.6mm diameter, a sort of narrow 10BA - works out at about 72 tpi.

              My biggest (attempts to divert original topic) pitch was 4mm on a 39mm diameter, giving a pitch factor of 11. Hmm... that's not as impressive as I thought it was going to be.

              I once cut a 1/4" pitch in wood by filing freehand, as the retaining nut for a ceramic handle for an old kitchen table drawer - before I was tool-wise !

              The smallest thread I used a tap and die for was No 1 UNF, 72 tpi, for a screw to retain the spring on the top/ring octave key on a saxophone.
              Richard - SW London, UK, EU.


              • #8
                Finest I've ever single pointed was 6-48, but have used taps and dies down to 0-80.

                David Kaiser
                “You can have peace. Or you can have freedom. Don't ever count on having both at once.”
                ― Robert A. Heinlein


                • #9
                  The big lathe we had at work would actually do 1 thread in 10" !!!! It was a Promaster I bought from MSC. I liked the threading range,because we used it,among other things,to cut very large wooden threads with a router in the toolpost. I have no idea who would want to cut a 10" wide thread,though!


                  • #10
                    Smallest I have ever single pointed in lathe was 2-56 . Made 26 little hex head bolts for a model engine one time. Made them out of a 5-32 Allen wrench. Annealed the wrench with torch. Chucked it in 3 jaw chuck Turned it to size for 1/4 long and cut threads. After that I said never again I would buy them.
                    Every Mans Work Is A Portrait of Him Self


                    • #11
                      The smallest I have ever made that had to actually fit something is 2-56. As an experiment a while back when this subject came up before I turned some outside threads in stainless steel at 160 TPI just to see what they looked like. They look pretty good. Making a part to match would be a real challenge because the thread depth is less than most tolerances for most parts.

                      Free software for calculating bolt circles and similar: Click Here


                      • #12
                        Jerry Kieffer makes threads to at least 250 TPI and .020" diameter, I think even smaller. He wrote the article in HSM some time back on using optics for micro machining. He told me that the typical Chinese lathe won't do that fine work because the bearings, etc. simply aren't good enough. He uses Sherline equipment.


                        • #13
                          My Electronic Threading attachment for my Sherline lathe it theoretically capable of 249.5 tpi. Realistically, it would be limited to the sharpness of the tool bit. So far, I've only done down to 40 tpi.

                          Here is my most recent threading job.

                          It's a 1.140 - 32 tpi test plug for a lock repair I'm doing, a duplicate of the lock cylinder. I needed to have the plug on hand because the cylinder and existing lock housing needs to remain in service while I build the replacement housing. The plug will be used to gage internal threads when I thread the hole in the lock housing.

                          The plug was simple - the threaded hole in the lock case will be more interesting. I plan to thread it on my lathe with Electronic Threading the same way as one would line bore a hole. I will be making an adjustable between-centers boring tool to hold the threading cutter, and mount the lock housing on the cross slide. Stay tuned.
                          Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
                          ~Practitioner of the Electromechanical Arts~


                          • #14
                            My Rivett has a threading chart into the 600+ tpi range with change gears. NO IDEA what that would be fore. I don't think I could get a cutter sharp enough to cut that.
                            James Kilroy


                            • #15
                              The gear box on my SB goes from 4 to 224 but someone on the PM forum speculated that it was only because SB felt obligated to put a TPI on the chart for each corresponding feed. And maybe for those few maniacs that would actually find a use for threads that fine.

                              Tom - Spotsylvania, VA