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Threading - Knowing when to stop?

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  • Threading - Knowing when to stop?

    Hi folks,

    Fairly new to this but I've successfully threaded externally on my lathe a handful of times.

    I made a new leadscrew for my compound slide succesfully but it has a bit more free play than the original.

    I'm about to make one for the cross slide and I want to get it spot on.

    Every time I have threaded on the lathe I've struggled with knowing when to 'stop' cutting.

    It will be difficult to trial fit this particular one.

    Any tips on knowing when to stop? Is this something you can count off on the compound indicator ring?

    Lastly, despite a very sharp HSS threading tool the finish I get tends to look like it's been sandblasted. I polish the thread on my bench polisher to smooth it out, but this 'round off' the peaks of my thread. Any ideas how I can help this?

    Cheers,
    Rich

  • #2
    Have a shaft longer than it has to be ..turn the extended bit to the root depth of the threads..when you scratch this bit ..you are at the right depth...if nut still wont fit, chamfer end of thread and wire brush over threads...watch out for burrs on threads from badly made tool or blunt one .use good neat cutting oil ..like Rocal RTD

    ps ..always support end of shaft with tailstock

    and make sure the tailstock is in alinement with headstock

    pps ...you're brave making a lead screw ..like trying to run before you can walk

    all the best...mark
    Last edited by aboard_epsilon; 08-02-2011, 04:44 PM.

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    • #3
      I will assume the thread your cutting is an acme because most lathe screws are acme. Acme threads are not easy to cut clean and only with experience can that be achieved. Small depth of cuts and clean up passes when near size can produce a clean thread most the time. You have to take light cuts making acme threads or the metal will rip out leaving a rough finish.

      You won't or shouldn't have that trouble with a V thread.

      When your cutting a thread it is best to machine a relief area at the end of the threading pass. On the compound screw, cross slide screw and lead screw you should see an area where there is no thread that is the minor diameter of the thread and that is what you should do too. That allows you time to release the halfnuts at the end of the thread pass. After you have done a lot of threading you will get real good at releasing the halfnuts even without a relief area.
      Last edited by Carld; 08-02-2011, 04:44 PM.
      It's only ink and paper

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      • #4
        Are you asking when to stop the thread depth? There are ways to measure the thread. You might check the thread against the mating nut.
        Gary

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        • #5
          There is a Brilliant South Bend lathe video on how to cut an acme thread ..out there somewhere ..
          its on youtube ..but is reduced in size..so not that clear ...but look for the original .

          its black and white and made in the 40's

          all the best.markj

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          • #6
            For precise threading in the lathe I have made up a spindle crank. When doing it by hand you can stop at a very precise spot. The main problem becomes withdrawing the tool at the right rate to make a nice looking end to the groove - a bit of coordination between the left, cranking hand and the right hand on the cross feed.



            I was making a collet holder here and needed a good thread that ran almost up to a shoulder. No problems with overshooting.
            Paul A.

            Make it fit.
            You can't win and there is a penalty for trying!

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            • #7
              Do you know how to use threading wires ? On a standard screw thread, position two wires on one side of the thread and one on the other and mic across the wires.

              A leadscrew thread gets rather large for a mic, so you can try with two wires, and the mic at an angle. You wires should jam in the thread so they stay proud of the major diameter. Just bend a short piece of the correct wire so it springs into place before using the mic.

              Find part of the old worn screw that is still a good fit in the nut - usually the end nearest the handle. Put the wires on that and mic up. If your thread is correctly cut - tool geometry correct - then thread till the mic on the wires of the thread you're cutting reads the same. If your geometry is a bit out, you'll probably need to go a touch deeper.
              Richard

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              • #8
                This is a mini lathe and the screws aren't acme, they are 1mm pitch 10mm things. The main leadscrew is an acme thread, but the cross and compound slides aren't.

                The screws aren't actually worn, I have been re-engineering the thing to incorporate bearings and different travels etc. The factory design left quite a bit to be desired, I've improved the compound beyond recognition and I'm on the case with the cross slide at the moment.

                aboard_epsilon, that sounds like a very pragmatic solution and would work but for the fact that I don't know what the root depth is, I'm guessing though that it's a standard thing that I can look up (assuming the thread is M10, not just the nearest they felt like getting!). I will try that.

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                • #9
                  Just been trying to look up the root depth of this thread on the internet, it looks like 10mm thread with a 1mm pitch is not a standard thread? At least, I can't find it in any tables.

                  I don't have an M10 X 1mm right hand threaded nut to test fit it with, other than the one that's attached to the cross slide, which I'll be using to cut the thread

                  May have to resign myself to patiently taking the cross slide off for test fittings, then reassembling for another pass!

                  Comment


                  • #10
                    Originally posted by loply
                    This is a mini lathe and the screws aren't acme, they are 1mm pitch 10mm things. The main leadscrew is an acme thread, but the cross and compound slides aren't.

                    The screws aren't actually worn, I have been re-engineering the thing to incorporate bearings and different travels etc. The factory design left quite a bit to be desired, I've improved the compound beyond recognition and I'm on the case with the cross slide at the moment.

                    aboard_epsilon, that sounds like a very pragmatic solution and would work but for the fact that I don't know what the root depth is, I'm guessing though that it's a standard thing that I can look up (assuming the thread is M10, not just the nearest they felt like getting!). I will try that.
                    usually in metric you take the pitch away from the outer diameter and end up with the root ..or core if you like

                    so its 9mm for tapping

                    handy site

                    http://www.gewinde-normen.de/en/iso-fine-thread.html

                    all the best.markj
                    Last edited by aboard_epsilon; 08-02-2011, 07:53 PM.

                    Comment


                    • #11
                      Originally posted by loply
                      Just been trying to look up the root depth of this thread on the internet, it looks like 10mm thread with a 1mm pitch is not a standard thread? At least, I can't find it in any tables.
                      Here are all the thread data you will need, loply :

                      http://mdmetric.com/thddata.htm



                      .
                      Thomas

                      Problems worthy of attack prove their worth by hitting back
                      - Piet Hein

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                      • #12
                        8.773mm

                        minor diameter d3

                        according to Thomas' site

                        but you never know what the Chinese have done.

                        still may not thread into your nut

                        all the best.markj
                        Last edited by aboard_epsilon; 08-02-2011, 08:16 PM.

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                        • #13
                          One very important thing you need to realize is that neither the tops nor bottoms of the thread are supposed to touch the mating thread. If they do you will never get a good long lasting fit. Also, neither should be a sharp 'V' but should be slilghtly flat or rounded. That is especially important for the tops of the thread.

                          Don't depend too much on the existing thread to be 'correct', especially if the fit was loose to begin with.

                          If your crossslide has a removeable nut you may be able to remove it, lock the crosslide somehow, and cut your thread with only the compound.
                          Don Young

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                          • #14
                            Buy your 10x1.0 mm tap and die here http://www.amazon.com/10mm-High-Spee.../dp/B0006FI844 it will save you a lot of work.
                            It's only ink and paper

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                            • #15
                              One thing you can do is to make a dummy thread to test against the nut. I do this a lot when making o-ring grooves. Cut the thread till you think you should be done and then take it out and test it. If it is too loose try it again with a new piece and stop short of the depth you turned the last time. If it is too large put it back in and pick up the thread for more cutting. If you do this with short pieces of scrap stock you won't waste much material. When you finally get a good fit record the position of your dial and cut the real piece to the same depth.

                              EDIT: Borrowing a page from Don Young's post: You may have wear on the nut so if you calibrate your new thread based upon the size of the existing screw you will only re-introduce the same amount of play. By making a custom fit, as described above, you can make the fit much closer because you are calibrating to the nut rather than to the screw.
                              Last edited by DATo; 08-03-2011, 05:05 AM.

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