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machining a tailstock feedscrew.

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  • #16
    Originally posted by plunger View Post
    I would think 12mm trapezoidal would be a good choice .I say this because I can get silver steel or drill rod of 12 mm diameter off the shelf.

    Back to cold weather . It got as cold as six degrees c. I cant handle it. Its so weird. We are not used to wearing anything else but t shirts, slip slops and shorts in the dead of winter.I cant imagine living in a place below zero.
    I think you are on the right track with the 12mm drill rod and trapezoidal thread. You could thread say 300mm all in one go, and then cut it in half to use one for the tap and the other for the screw.

    The cold weather is like any other weather, long clothes, in layers. Say 2 or 3 shirts, etc. You do get used to it, believe it or not. Mechanic's coveralls are great in cold weather. I know, I work at a truck shop... whatever the weather is that day, that is what I have. And in the summers, 40C.
    25 miles north of Buffalo NY, USA

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    • #17
      Originally posted by old mart View Post
      Making the leadscrew is easy, the left hand thread is produced by threading away from the chuck. The nut will be the problem, it is too small diameter to singlepoint and taps could be hard to get hold of especially in South Africa.
      Difficult but not what I'd call impossible by any means. And while small in size the tool needed to cut the internal threads on just a short nut won't need to be overly long. Maybe 3/4", or 2cm or so? So even with the smaller size needed to clear it would still be well within the less than 4D stickout.

      I think I'd make my own single point tool from some 10mm drill rod then harden and temper it. To keep some additional height while still achieving the front and back fitting needed I think I'd do a bit of off center machining along the lines in the sketch.

      For getting the nice sharp and correctly relieved cutting edges from this, which comes out tangent, marker the metal and then with a good head mounted magnifier and a small rotary grinder sneak up on the cutting reliefs until there's the smallest still visible line of marking dye. At that point a couple of strokes with a fine sharpening stone will remove the last of the dye and give a nice edge.

      The angle on the inside of the relief will need to be the most aggressive. When we fold out a 1/2-10 angle the trig tells us that the angle of the helix at the smaller diameter root size is pretty small. Like 0.36" diameter and the 10 TPI.

      So the minor diameter is 0.36 which times pi = 1.13 And we have 10 TPI. So that's a triangle with 1" on one side and 11.3" on the other which gives us an helix angle of just over 5°. So to give yourself the cutting relief you need you need to make the side of the tooth in the relief angled at around 10°. The other side and the end can get by with something like 4 to 5°.

      That's better... I was thinking it was something like 45 and had to stop and pause for a second.

      Click image for larger version  Name:	Small Acme internal single point cutter.JPG Views:	0 Size:	36.6 KB ID:	1959207
      Last edited by BCRider; 08-28-2021, 05:20 PM.
      Chilliwack BC, Canada

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      • #18
        Originally posted by BCRider View Post

        Difficult but not what I'd call impossible by any means. And while small in size the tool needed to cut the internal threads on just a short nut won't need to be overly long. Maybe 3/4", or 2cm or so? So even with the smaller size needed to clear it would still be well within the less than 4D stickout.

        I think I'd make my own single point tool from some 10mm drill rod then harden and temper it. To keep some additional height while still achieving the front and back fitting needed I think I'd do a bit of off center machining along the lines in the sketch.

        For getting the nice sharp and correctly relieved cutting edges from this, which comes out tangent, marker the metal and then with a good head mounted magnifier and a small rotary grinder sneak up on the cutting reliefs until there's the smallest still visible line of marking dye. At that point a couple of strokes with a fine sharpening stone will remove the last of the dye and give a nice edge.

        The angle on the inside of the relief will need to be the most aggressive. When we fold out a 1/2-10 angle the trig tells us that the angle of the helix at the smaller diameter root size is pretty small. Like 0.36" diameter and the 10 TPI.

        So the minor diameter is 0.36 which times pi = 1.13 And we have 10 TPI. So that's a triangle with 1" on one side and 11.3" on the other which gives us an helix angle of just over 5°. So to give yourself the cutting relief you need you need to make the side of the tooth in the relief angled at around 10°. The other side and the end can get by with something like 4 to 5°.

        That's better... I was thinking it was something like 45 and had to stop and pause for a second.

        Click image for larger version Name:	Small Acme internal single point cutter.JPG Views:	0 Size:	36.6 KB ID:	1959207
        This looks like a good idea. Its half an hour past midnight so the maths gets to me . I must try absorb this tomorrow morning.Hopefully i can copy this without any maths involved.

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        • #19
          Bc rider I need to try understand this. If I machined this out of a piece of silversteel and just ground relief on a bench grinder as you said but did not cater for the helix ,would it not work or possibly drag and rub in the thread it is cutting. Am I sopposed to try grind it off its axis so to speak at an angle of what the slant of the thread is in layman's terms. So If so could I just try eyeball grinding it because I have no real means of measuring that as I dont have a tool and cutter grinder setup with angles or the ability to measure it. I have a simple porotractor .Thanks for the drawing

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          • #20
            Yes, you can eyeball grind it, thats how I do it because I've only got a double ended grinder, no T&C or other fancy equipment. The single pointed thread doesn't need to be super accurate, because you are going to finish it with a tap anyway.
            Don't forget that Acme and square threads have clearance at the top and bottom, so the tap needs to be slightly larger in diameter than the spindle, if possible.
            'It may not always be the best policy to do what is best technically, but those responsible for policy can never form a right judgement without knowledge of what is right technically' - 'Dutch' Kindelberger

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            • #21
              That's right, if you don't give the leading side of the trapezoid enough angle it won't cut because of the high feed rate on the very small diameter. You've got the minor diameter at around 0.36" which is a circumference of 1.13 inches. And for a single turn at 10TPI you will advance 0.1 inch. Doing the trig that's a touch over 5° worth of helix angle. So you need that same slightly more than 5° just to match the thread. But to actually cut you need the angle to be even steeper. Thus the suggested 10'ish degrees.

              Here's another sketch looking at the cutting tooth. This is just looking at the flat end face of the cutting tooth. So some fairly careful small point grinding on the angled flanks of the Acme pattern tooth that will cut the threads. I'm thinking that this is not a job for a bench grinder. I'm thinking more Dremel or similar. Or a 4 inch angle grinder with a nice thin cutting disc and hold the grinder firmly and delicately touch the tooth flanks to the edge of the disc. A head mounted magnifier is a BIG help for stuff like this. In fact I'm sort of thinking that it's mandatory for something this dainty. Hopefully you have one.

              Oh, silly me.... It's annealled. So you can file much of the rake angles and just leave the last whisker of dyed edge until after hardening and tempering and do the final honing for the keen edge. That's how I'd do it. Or a medium diameter grinding point on a Dremel to get the small hollow ground spots as suggested in the sketch.

              Again I'm assuming you would run this boring bar style cutter so it makes the cutting pass on the way out. So the side with the greater angle is the leading side.

              Click image for larger version  Name:	Small Acme internal single point cutter 2.JPG Views:	0 Size:	43.0 KB ID:	1959255
              Last edited by BCRider; 08-29-2021, 01:01 AM.
              Chilliwack BC, Canada

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              • #22
                Not all South Bend tailstock feed screws were Acme. My 10K had square threads.

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                • #23
                  If Plunger is making both the screw and the nut it doesn't really matter if its square or acme. Personally, I find square easier to cut.
                  'It may not always be the best policy to do what is best technically, but those responsible for policy can never form a right judgement without knowledge of what is right technically' - 'Dutch' Kindelberger

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                  • #24
                    I have a SB-9 and can provide photos and dimensions.

                    As for making the nut, you will probably have to make an insert in any case because the present screw is a right hand and it should be left hand. I would suggest that you consider the process that Evan Williams posted about 12 years ago. Here is a link to that post.

                    https://bbs.homeshopmachinist.net/fo...s-the-easy-way

                    Unfortunately, it was mostly based in photos and Evan had hosted them himself. At some time in the past years, he took that server down and those photos are no longer available. However, I had downloaded that thread, with all the photos. Since Evan took them down without any explanation and he owns them, I will not post them again. But if you want the complete post with the photos, PM me and I will e-mail it to you with your promise to also honor his apparent wishes that they not be made public.
                    Paul A.
                    SE Texas

                    And if you look REAL close at an analog signal,
                    You will find that it has discrete steps.

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                    • #25
                      Originally posted by Paul Alciatore View Post
                      I have a SB-9 and can provide photos and dimensions.

                      As for making the nut, you will probably have to make an insert in any case because the present screw is a right hand and it should be left hand. I would suggest that you consider the process that Evan Williams posted about 12 years ago. Here is a link to that post.

                      https://bbs.homeshopmachinist.net/fo...s-the-easy-way

                      Unfortunately, it was mostly based in photos and Evan had hosted them himself. At some time in the past years, he took that server down and those photos are no longer available. However, I had downloaded that thread, with all the photos. Since Evan took them down without any explanation and he owns them, I will not post them again. But if you want the complete post with the photos, PM me and I will e-mail it to you with your promise to also honor his apparent wishes that they not be made public.
                      Thankyou for the offer . I think i remember the thread. There was a bit of controversy as to who invented it. I hope Evan is okay. Now that you mention it i havent seen him post for a long time. I have an idea as to how to do the nut but i think i will go the route of either machining a nut or machining a tap.

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                      • #26
                        Just remember.... When making the screw,
                        if they got too long, you need to use a follower
                        rest because the stock will begin to bow from
                        the tool pressure. Long and skinny means
                        more deflection.

                        --Doozer
                        DZER

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                        • #27
                          Originally posted by Doozer View Post
                          Just remember.... When making the screw,
                          if they got too long, you need to use a follower
                          rest because the stock will begin to bow from
                          the tool pressure. Long and skinny means
                          more deflection.

                          --Doozer
                          Yes . Luckily its short. Ive never used a follower rest before so its a new experience for me. I may try making a follower rest as mine is pretty bulky. Another project involving hours of googling.
                          My first experience using a steady rest(not follower rest) on this lathe was a disaster. I was holding a long piece of 40mm steel with an ugly outer hard skin. (hot rolled) I never took into consideration that the heat build up on the shoes of the steady rest would cause the rest to bind. Before I knew it the bar of steel was spinning in my chuck ,sending sparks all over the place. It chewed up my beautiful Emco (rohm made chuck) three jaw chuck jaws .
                          No one made a comment about the 3d printed crosslide nut in # 1 that I supplied a link too. Could it work.?
                          Last edited by plunger; 08-30-2021, 09:56 AM.

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                          • #28
                            Originally posted by plunger View Post

                            No one made a comment about the 3d printed crosslide nut in # 1 that I supplied a link too. Could it work.?
                            I completely missed that -- it's perfect !!
                            25 miles north of Buffalo NY, USA

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                            • #29
                              What's the thread minor diameter? Looking at the tool sketch above, the thread minor diameter must be more than 10mm for the tool to clear the threaded hole. Even if you back up the lathe to withdraw the tool, you still need enough clearance to make the first pass.

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                              • #30
                                Originally posted by strokersix View Post
                                What's the thread minor diameter? Looking at the tool sketch above, the thread minor diameter must be more than 10mm for the tool to clear the threaded hole. Even if you back up the lathe to withdraw the tool, you still need enough clearance to make the first pass.
                                This is where my knowledge is lacking. But if I made it metric and used a two mm pitch on a 12mm thread. Theoretically it would be ten mm on a 12 mm x2mm pitch (You have have to love the simplicity of metric) But I dont think this applies to trapezoidal threads ,I think it may be less , I will have to try google that.
                                I may as well do the southbend extension while I am at it .
                                Last edited by plunger; 08-31-2021, 10:19 AM.

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