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Change gears for 9mm lead, three-start thread?

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  • #16
    I have produced 5tpi on a lathe which only normally manages 8tpi. I doubled the leadscrew gearing and set 10tpi. The extra strain on the mechanism caused me to replace the frangible link pin in the drive with a solid one for fear of the original breaking during the threading. I was doubly careful not to lose my concentration when doing that job.

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    • #17
      The specs for the thread are probably on this site.

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      • #18
        Old Mart: I'm thinking nothing more strenuous than delrin here. Strength really isn't the issue, it's ease of machining given that it may be difficult and also need tweaking to 'make' it fit.

        Originally posted by elf View Post
        The specs for the thread are probably on this site.
        Thanks for that. Seems to be mainly glass threads and imperial threads. Could be there but I'm not sure I'm going to find it. The name is rather unhelpful being "INE 30/25 Tall" as it doesn't come up with anything useful when searched for. For reference, this is the thread I'm looking at:



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        • #19
          Originally posted by BCRider wrote View Post

          Why not? Even if cutting in metal instead of plastic it's still just a thread with something similar to an acme form but rounded. It's simply that the advance is so large per turn.
          in response to:

          Originally posted by Peter. View Post

          "You might want to make a winding handle for the leadscrew. You won't be able to cut such a short, high-helix thread under power."

          A general rule of thumb is that if the thread being cut is coarser than that of the leadscrew, it is better to turn the leadscrew and use it to drive the spindle. This way, there is a lot less strain on the gear train. A handle is the simplest, but a DC motor on the end of the leadscrew can also be used.

          In the majority of cases the thread being cut is finer than the leadscrew so the spindle drives the leadscrew.

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          • #20
            The strain on the gear train at the coarse pitch more than the threading tool was what worried me. Those pictures of bottle threads clearly show a rounded crest buttress type, which is an excellent choice for a soft low strength plastic.

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            • #21
              Originally posted by djc View Post
              A general rule of thumb is that if the thread being cut is coarser than that of the leadscrew, it is better to turn the leadscrew and use it to drive the spindle. This way, there is a lot less strain on the gear train. A handle is the simplest, but a DC motor on the end of the leadscrew can also be used.

              In the majority of cases the thread being cut is finer than the leadscrew so the spindle drives the leadscrew.
              That's an interesting new angle. Hadn't thought of it from the point of view of the gearing not handling it. Certainly hadn't considered the crank being on the leadscrew rather than the spindle. I've ordered the 90 tooth gear - it's more expensive than a bottle connector but I'm viewing it as something useful for other things - so I'll proceed with extreme caution when starting the drivetrain and go from there. I don't doubt your wisdom but it could be a non-issue. If it isn't I'll have a think about how to work around it - it's hardly what you'd call a time-critical problem...more an interesting problem with an interesting learning curve. More so than I expected to be honest.

              Old Mart: I've experience with cutting buttress threads before....just not intentionally!
              Wasn't aware at first that Chinese lathes measure the angle of the compound from perpendicular to the ways rather than parallel....or is it the other way round?! I tend to just plunge cut lately. Working more on being able to thread away from the headstock for internal threads at the moment; think I have it sorted, just waiting to complete the carriage stop before I continue that project.

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              • #22
                As far as I know, lathe compounds set at zero are parallel to the spindle axis. If you make a threading tool that faces the rear of the bore and turn the spindle backwards, cutting away from the chuck (which is much safer) produces a righthand thread.

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                • #23
                  Originally posted by old mart View Post
                  As far as I know, lathe compounds set at zero are parallel to the spindle axis. If you make a threading tool that faces the rear of the bore and turn the spindle backwards, cutting away from the chuck (which is much safer) produces a righthand thread.
                  Then the Chinese standard is that zero is perpendicular to the spindle axis. It only caught me out once....but I'd did take me a while to work out why!

                  I have some left-handed internal threading bars that should do the job. There's some question of helix angle for the inserts but I suspect the reality is that it won't make a lot of difference for the (smaller) pitches that I'm likely to use. Only one way to find out though :-D
                  Also picked up a left-handed external tool (for peanuts) as I want to thread towards the chuck but away from a shoulder, before parting it off. To maintain concentricity of the external thread at the chuck end, the taper in the middle and the internal thread at the tailstock end.... but that's a different project.

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                  • #24
                    Originally posted by Cenedd View Post
                    I have some left-handed internal threading bars that should do the job. There's some question of helix angle for the inserts but I suspect the reality is that it won't make a lot of difference for the (smaller) pitches that I'm likely to use.
                    It does not work well out of the box....

                    You can use right hand inserts in a left hand tool and vice versa but you need what is known as a shim or anvil. I think the inserts are ground straight up and down when looking end-on. The seat on the tool is angled to give clearance/helix angle. So if you have two degrees helix on your right hand bar, you need a four degree shim to get two degrees on a left hand insert.

                    For external threads, where there are no clearance issues, maybe a wedge under the tool where it clamps in the toolpost would achieve the same thing. Shank of tool would not be parallel to lathe bed but that does not matter.

                    There are many free guides to insert threading. I have ones from Sandvik, Seco and Stellram, all of which explain this nicely.

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                    • #25
                      Thanks djc that's useful to know. It took me a long time to work out there isn't a difference between left and right inserts but the spiral direction (points spiral clockwise or anticlockwise off the triangular base) and that the difference was in the holder. I believe there may be some differences between internal and external inserts for some thread forms specifically with the nose radius. Can't find anything concrete though. Not many of my tool holders are designed with shims but I'll see what I can do now armed with a set of angle blocks.

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                      • #26
                        Afternoon all. Just wanted to say I'm still here and haven't forgotten this - don't want to seem like I'm asking for help and then either not listening or not going through with it.
                        90 tooth and 25 tooth gear turned up the other day. The latter purely because it was available, seemed like it could yield some useful ratios.... and because its aim was good when it jumped in my shopping trolley Naturally neither of them fitted the shafts on my lathe but a quick touch of my shiny new 16mm reamer* made of the finest Chinesium and a light file of the keyway sides and they slide on nicely.
                        Also reamed the second spacer, reduced the diameter by about half a mm and re-blued it. That now sits on the driven shaft (in front of the top left gear in the pic on page 1) so that I can mount a 60 tooth gear on the A shaft without it acting like a slitting saw on the spacer.

                        Finally, I set up the gear train, mounted my sharpest thread-cutting marker pen and checked the thread pitch. Seemed to come out at 1mm pitch....which was a little finer than what I was expecting. I've gone back to the drawing board and worked out what I did wrong - final gear in the wrong slot resulting in entirely bypassing half the gears to give 60-Idler-120. I shall sort it out and try again but currently my time is taken up by my son's illness and trying to fight the medical system to actually DO something.



                        *Bought it for another project and rather conveniently it is the correct-ish diameter for the change gears. I say "ish" as it seems to be a gnats oversize. More like 16.05 perhaps as much as 16.10mm - slightly harder to tell when you're reaming holes that already exist and aren't freshly drilled and perfectly aligned.

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                        • #27
                          [QUOTE=BCRider;n1834652

                          If that is your lathe in the pictures above and not something ripped off the web I'm thinking that you'll want to mark 3 equally spaced gear teeth on the last gear that drives the lead screw and one tooth gap on the gear that drives the lead screw gear. Leave the drive engaged the whole time and instead lift the gears out of engagement and rotate the lead screw to the next mark in each case. At least I think that will work to give you the three equally spaced entries. And if that worked it would certainly ensure equally spaced starts.[/QUOTE]

                          For the three space indexing could you not just jam a spacer of sorts from the ways to the jaws of the chuck.?A quick rough way of indexing three ways at 120 degrees.

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                          • #28
                            Originally posted by plunger View Post

                            For the three space indexing could you not just jam a spacer of sorts from the ways to the jaws of the chuck.?A quick rough way of indexing three ways at 120 degrees.
                            I was thinking to just set the compound parallel with the ways and use that to move the cutter by 3mm between each start (not each pass). I think the compound dials are accurate - I know the carriage isn't. A dial indicator for confirmation won't be a bad idea. Currently I have no thread dial (working on it, found some 28T and 30T 0.6 module gears for RC cars that seem to mesh nicely) so I normally have the half nuts engaged the whole time. I know it can be freely disengaged if the thread lead is evenly divisible by the leadscrew pitch but sods law dictates that this is one of those situations where my 2mm pitch leadscrew doesn't allow for that.

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                            • #29
                              Well, I had a few moments to slap some change gears on spindles. Used the Sharpie trick again and got full coverage - or the whole bar turned green. Not quite what I was intending! Quite the opposite in fact....which led me to realise that I'd got the calculations the wrong way round and had produced a 0.444 mm pitch thread. Sorted my calculations out and have found a number of combinations that would work....if they fit on the banjo.

                              My A spindle will take a 60 tooth max (had to turn the spacer down slightly to get that much) but most of the combinations fall apart when to get the B gear to mesh with the A, the end of the banjo then hits the gear that sits behind the A gear. Doesn't help if I spin the banjo round to mesh from the other side either. Tried these:
                              45-30/120-40
                              60-40/90-30
                              45-40/120-30
                              This one may be more promising - will try to simulate it but I've not got a second 40 tooth.
                              60-40/120-40

                              If anyone has any thoughts on combinations I've not come up with, I'm game for trying them but otherwise I think the potential ways of getting round this may be unreasonable for just being curious/stubborn/bloody-minded. That said, 9mm pitch would be ideal for winding a compression spring I 'need' to make. I'm curious though - partly as a thought experiment, partly to learn and partly should the need arise - what the options for getting round this would be. I can see a few options:

                              1. Cut down the end of the banjo. Not ideal as I'd probably be removing the ability to use more useful combinations and end up having to buy/make another banjo.

                              2. Add an idler gear into the train between A and B. Not sure if I can just hijack one of the SHCS that is in roughly the same place or if it needs to be somewhat more secure for a spindle - maybe a flange?

                              3. Same as 2 but slidable so different size idler gears can be used.

                              4. Make a 2nd banjo that pivots further down the D shaft and hence will clear the spindle gear (one on same shaft as A) and would allow either the gears to be meshed or an idler positioned if/as/when in addition to gears on the 1st banjo.

                              5. Some overly complicated replacement banjo that would mount more gears and probably need to be angled to fit them - I may just have "invented" a quadrant.

                              As I say, I'm not planning to do this currently but it's going round and round my head so would appreciate comments on how it would be done so I can mentally put it to bed for now
                              Last edited by Cenedd; 11-09-2019, 08:55 AM. Reason: removed autocorrect or fat-fingeredness.

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                              • #30
                                Do you really have two 120 as in your original list?
                                120/40, 120/80
                                120/60, 90/40
                                100/50, 90/40
                                100/80, 90/25
                                100/25, 90/80
                                60/30, 90/40
                                50/25, 90/40
                                Obviously above can be rearranged and 90/80 replaced by 45/40 and so on. I like the versions using both the 90 and 25 since you bought them specially.

                                Earlier comments were about driving the leadscrew rather than the spindle. You have a big bolt/nut holding the gear on the leadscrew end. A ratchet spanner, or drill with adaptor to socket, etc will fit this. When running back to avoid the risk of undoing this bolt take the strain off things by a) putting a bungee on the saddle to the tailstock to pull it back so no effort from the leadscrew and b) simultaneously twist the chuck with your other hand so no effort from the gears there either.

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