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Metric Threading with an Inch Lead Screw

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  • #16
    I have absolutely no problems with my lathe ! never have never will. Not that I am a clever clogs. It is just because I bought a metric lathe. To be serious ages ago I read on how to make imperial threads on a metric lathe but apart from needing to buy a few gears. I forget how to do it still when needed I will look it up again. How's that for being lazy couldn't care less attitude? L O L Getting old and forgetful Alistair
    Please excuse my typing as I have a form of parkinsons disease

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    • #17
      Same thick applies though Alistair when doing imperial on your metric lathe.

      Also doing metric on a metric lathe isn't as cut and dried as many think, certainly not as easy as imperial on imperial lathe.

      Doing metric on a metric lathe requires you to have the correct follower gear fitted depending on what pitch it is. Some have as many as 3 different gears and the dial in may cases has to be changes again to relect the pitch so using this trick doing metric on a metric lathe means you only need the one gear and dial.

      In fact if you use this method all the while is easy to cut any pitch on any lathe, period.
      .

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



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      • #18
        There is another way although it has little to recommend it.

        Stop the spindle with the half nuts still engaged then wind the carriage back exactly 15 inches and re-engage the half nuts before restarting the spindle.

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        • #19
          That's the hard bit though, stopping a big lathe at an exact point.
          .

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



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          • #20
            The stopping point is not so important as moving the carriage back 15 inches from where it stopped.

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            • #21
              You have a big lathe, 7 to 10 HP motor, large 12"+ chuck and you are running up to a shoulder with only a 1/8" runout groove.
              Stopping in time would be important to me
              .

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



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              • #22
                John, I am not pretending that the 15" method is worth anything except something to bear in mind as an aid to understanding just what is required.

                Now that big lathe of yours does it have a spindle brake?

                Of course there is yet another way (I think this is right) involving a lead screw revolution counter. The spindle does not stop and is not reversed. For a metric leadscrew we would need a counter that would count to 127 and repeat and for an imperial lead screw a counter to count to 120 and repeat. Use it like this....Set a carriage stop or marker at the starting position. Start the spindle and engage the half nuts at the same time resetting the counter. Cut one pass and retract the half nuts then wind the carriage back to the start point and wait to re-engage the half nuts when your counter recycles. Depending on the thread pitch and how long it is you may have to wait a while but on the other hand there will be plenty of time to advance the cut.

                I am not recommending any one system over another but show there are several ways to get the hide off this particular kitty cat!

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                • #23
                  Originally posted by John Stevenson View Post
                  You have a big lathe, 7 to 10 HP motor, large 12"+ chuck and you are running up to a shoulder with only a 1/8" runout groove.
                  Stopping in time would be important to me
                  Try this:

                  1 Set an indicator stop on the bed ways to register carriage travel at the thread relief,

                  2 Set it so zero is a bit less than a full rotation,

                  3 Make a few dry runs to zero to calibrate your neuro-muscular loop,

                  And you can thread to a narrow relief stopping almost right on the thousandth - and it works better at a couple hundred RPM than dead slow. Your reflexes came from your tree-dwelling ancestors when dinner was an important daily event easily frustrated by failure to match the swing of the club to the head of the speedy dinner item.

                  Try it, but practice it first (no tool) as a dry-run training exercise. Increase the spindle RPM by increments until you simply aren't quick enough. You will be surprised at your eye-hand coordination when it comes to analog lever flipping. Your eye tracks the sweep of the pointer as it crosses zero triggering the instant muscle twitch that works the half-nut making the spirit of your 2500th great grandpa proud.
                  Last edited by Forrest Addy; 08-24-2015, 05:23 AM.

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                  • #24
                    Can't fault what you are saying Forrest but Mr Artful was advising throwing the lathe into stop with half nuts still engaged

                    Stop the spindle with the half nuts still engaged then wind the carriage back exactly 15 inches and re-engage the half nuts before restarting the spindle.
                    The point I was making was whilst as you say if the haft nuts are linked to the sphincter mustle it's perfectly possible but just throwing a big lathe into revere or just a stop isn't easy given the mass of the rotating parts.

                    The method in the initial video is very interesting and once distilled down can work with any thread on any lathe / leadscrew combination.
                    In fact if you adopted this method from day 1 you would have no problems threading for life.
                    .

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



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                    • #25
                      Something to consider especially for the typical home shop machinist is if your threading process involves a lot of stopping and reversing our Chinese single phase motors we should have a supply of spare start and run capacitors on hand. Of course a 3 phase motor and VFD could be fitted but for me that would cost about half the price of a complete new lathe.

                      Just to recap, winding the carriage back 15" method requires the lathe to be brought to a halt with the half nuts engaged but the turns counter method does not require the spindle to be stopped and the half nuts can be disengaged at any time.

                      Every method has its downside and the only one I can see for the feed screw turns counter and the 15" methods is the waiting around time for the cycle to repeat. Maybe I should rig up a turns counter and report back!
                      Last edited by The Artful Bodger; 08-24-2015, 04:40 PM.

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                      • #26
                        Originally posted by The Artful Bodger View Post
                        Every method has its downside and the only one I can see for the feed screw turns counter and the 15" methods is the waiting around time for the cycle to repeat. Maybe I should rig up a turns counter and report back!
                        Why can't you move the carriage only 7.5"

                        7.5" = 190.5 mm = 127 * 1.5 mm

                        which is an integral multiple of the 1.5 mm screw pitch?
                        Regards, Marv

                        Home Shop Freeware - Tools for People Who Build Things
                        http://www.myvirtualnetwork.com/mklotz

                        Location: LA, CA, USA

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                        • #27
                          Thank you Marv I am pleased you stepped in. I had a suspicion there would be lesser intervals possible but my mathematical skills did not bring any to the fore.

                          7.5" for a 1.5mm lead screw but how would that work for a 3mm pitch? I am assuming that 15" will be valid for all pitches of lead screw and all pitches of thread being cut. Maybe I am wrong and 127 (or 120) turns of the lead screw is the real common factor?

                          I am tending to favour the turns count over the distance moved method as I can see the distance moved (15" or 7.5") will often bring conflicts with the tailstock. The turns count method also avoids having to stop the spindle. I really need to make a test rig to try this!

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                          • #28
                            There is a program on my page (STICK) which will work out the length needed for any combination of pitches and leadscrews. Basically, one wants to find the shortest length that is an integer multiple of both the pitch being turned and the pitch of the lathe's leadscrew.

                            The program will also make the calculation for turning Inferial pitches on a lathe with a metric leadscrew.
                            Regards, Marv

                            Home Shop Freeware - Tools for People Who Build Things
                            http://www.myvirtualnetwork.com/mklotz

                            Location: LA, CA, USA

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                            • #29
                              Thanks Marv, I will take a look at your site.

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                              • #30
                                And what if you bump into the tail-stock before you reach the 15" position?

                                You can do very well with a handle in the rear end of the head-stock spindle - the more so with smaller threads. Just disengage the motor drive/belts.

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