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  • Electronic Leadscrew

    Ok so i've heard these talked about a bit, never seen one in action before, and have read alot of the stuff i can find on it but...

    Has anyone here actually made/used one?

    With an electronic lead screw granted the gear train will be made redundant but what about the threading indicator?, and the keeping half nuts closed while doing a metric thread with an imperial lead screw issue?

  • #2
    Electronic lead-screw

    There was a thread or post from Germany or someone that was German - I forget - some months ago.

    It was all electronic and was based on pulses and synchronisation as I recall. The full process and diagrams were there.

    I though I had it book-marked but can't find it.

    Evan has a similar system for speed control of his lead-screw on his South-Bend lathe. Perhaps he might re-post the article or the link. It was quite clever.

    Perhaps this link might be of use:
    http://www.stellar-international.com/lathe.html

    Comment


    • #3
      I built one using a custom "ELS" software module for EMC^2 that I wrote.

      http://server.fricktion.net/~mfrick/...mcEls_v0_1.pdf
      (URL updated 1-28-2011)

      Works great - totally synchronized like it's got gears.
      I don't have a threading dial, but using a carriage stop, returning the leadscrew to home position, and ignoring spindle until it passes it's home position, I don't need a threading dial, nor do I need to keep the half-nut closed.
      Sure, I have to stop on each threading pass, but it works for any possible thread pitch within the limits of resolution. 4 is the lowest TPI I've tried.
      Last edited by Ryobiguy; 01-28-2011, 01:23 PM.

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      • #4
        Electronic feed/screw-cutting

        Thanks ryobiguy.

        That seems to be far and away better than the "German" item I referred to.

        Do you build the item for sale? I have a 230/240v 1ph 50Hz supply in Australia. If so please PM me.

        Electronics is pretty well a complete mystery to me as I am from the era/age of steam radio!!

        My lathe has no power feed or quick-change gear-box.

        Your solution would seem to fit in with me very well.

        I have a full set of changes gears but never use them unless screw-cutting.

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        • #5
          or join the Yahoo group

          http://tech.groups.yahoo.com/group/E-LeadScrew/

          Comment


          • #6
            You can have my software for free once I package it up and release it.
            It's tuned for my system, so it would take some tweaking to get it running on someone else's setup.

            Once you get a spindle encoder and stepper motor on the leadscrew and get those both wired up to a PC parallel port, you can at least try it out, and if you don't like it you haven't lost anything.

            Once it's hooked up to a PC, you could also hook up to EMC^2 and use a real CNC controller too. I haven't actually tried it out myself, I just wrote my own program that runs parasitically in EMC^2 'cuz I'm not yet ready for full CNC, and I don't quite have the hardware it needs. I don't have a quadrature encoder with index pulse (not totally necessary, could hack around that,) I don't have a second axis, nor do I even have the stepper's direction signal hooked up to the PC.


            Also, the Yahoo ELS group (mentioned above) is also developing a ~$150 standalone ELS controller based on the PIC microcontroller. It's almost in the production phase. It's got a LCD display (20x4 characters I think), a keypad, a stepper driver (somewhere around 2 amps?,) an MPG (or at least a hookup for one using inexpensive encoder,) can drive a second axis (with external driver) to do tapers and fully automated threading. I think I also heard something about a nice metal case for it too. Join the group for ask for details if you can't find them in the archive.

            -Matt

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            • #7
              Thanks

              Thanks bob-s and Ryobiguy.

              Very sound advice and very much appreciated.

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              • #8
                Matt - that was a great write up....i need more time to read it properly but wanted to say thanks for taking to time on it, great content
                .

                Comment


                • #9
                  I joined the e-leadscrew group when it first started and the idea's for what was needed were being thrashed out.
                  Most just wanted threading but not go to full CNC.

                  I advocated just using one axis of TurboCNC and a spindle encoder to do the threading but was told that was CNC.

                  So three years down the line they have come up with a single axis CNC that can be extended into a 2 axis CNC admittedly in a smaller case and no screen.

                  No sour grapes but once you start using a stepper motor, spindle encoder and driver you have re-invented CNC.

                  .
                  .

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



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                  • #10
                    I was disappointed by the discussion in that Yahoo group. I saw no consideration anywhere of the errors which all of classical control theory is devoted to dealing with. Steady-state offset error is, obviously, the big one in a leadscrew-driving system. Damping of oscillation is another biggie. (Fixing those are what the "integral" and "derivative" portions of a P-I-D feedback loop do.)

                    This is typical of the digital guys, who too often seem to think that simply counting pulses is all that's involved in control of moving parts. The phase-locked loop is a good way to generate a proportional error signal, albeit of limited resolution. But that's just the start of the control problem. Is a simple proportional feedback control system good enough for the application? What are the numbers?

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                    • #11
                      No sour grapes but once you start using a stepper motor, spindle encoder and driver you have re-invented CNC.
                      Not if there isn't a computer or some sort of numerical control involved . A simple switch controlled divider can do the job. For that matter it would be technically possible to simply use an encoder disk on the left end of the spindle that contained the right number of slots to produce step pulses directly. The point is that an electronic lead screw needn't have anything to do with a computer. A few logic chips don't make a computer as you well know from your work on the electronic gear cutter.

                      Feedback isn't necessary any more than it is on open loop CNC systems. All it takes is brute force to insure that things stay synchronized.
                      Last edited by Evan; 04-15-2008, 09:42 AM.
                      Free software for calculating bolt circles and similar: Click Here

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                      • #12
                        I have to agree with you Evan to a point.
                        As you say a computer isn't needed and pointed out the logic chips on the gear hobber but this is slightly different in that it's a continuous operation and has no way of knowing where to pick up if you stop and wind back.

                        It probably could be done with a few more chips thrown at it but the point I was making is that 5 years ago, plus, people were threading with TurboCNC. In fact it was developed as a lathe program in the first place, not mill.

                        Now 5 odd years on the ELS still hasn't seen public light of day but any half competent person in two days could get a lathe threading on T-CNC.

                        [Edit ] In this months MEW Tony Jeffree has started to convert his [ actually my old Myford ] lathe over to CNC but in stages.
                        His first exercise is the fit a stepper to the leadscrew and drive it with his Division master to mimic a power feed.
                        In doing this he still retains the acme screw, half nuts and manual use of the machine.
                        Later he's going to add the encoder and be able to do threading still with manual use of the lathe by the rack and pinion feed and still keeping the half nuts.


                        As you say feedback isn't necessary but if you are that paranoid about lost steps then fit a servo instead of a stepper.

                        My big CNC is still on an old DOS program called Ahha, quite dated now but it still works flawlessly so I can't be bothered changing.
                        I managed to get some old 12" Point of Sale monitors a while ago for peanuts, who wants 12" nowadays, monitors that is

                        These are ideal for old DOS applications as they hardly get in the way or appear overwhelming.

                        .
                        Last edited by John Stevenson; 04-15-2008, 11:26 AM.
                        .

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



                        Comment


                        • #13
                          Originally posted by sconisbee
                          With an electronic lead screw granted the gear train will be made redundant but what about the threading indicator?, and the keeping half nuts closed while doing a metric thread with an imperial lead screw issue?
                          Simon, no one answered your question, and folks here have asked for a synopsis of electronic leadscrews every once in awhile, so here it goes...

                          On a manual lathe, the outboard quadrant gears, or the quick change gear box gears, use the leadscrew to drive the carriage at some division of the spindle RPM. An electronic leadscrew does the same thing, by adding a shaft encoder to the lathe spindle, and driving the carriage directly with a servo or stepper motor, which is controlled by an electronic divider circuit. In that sense, it's very similar to the divider circuit that Brian Thompson made for Sir John's Electronic Gear Hobber.

                          So not only do you not need quadrant or quick-change gears, but you also don't need a leadscrew. That means you can cut Imperial, Metric, Transcendental/Diametral, Module pitches at will. For fun, the guy on the Yahoo group alternated Imperial and Metric threads on a piece of rod. You could also cut non-linear threads (increasing or decreasing TPI) if that turned you on

                          As you can probably tell, Electronic Leadscrew gets awfully close to full-blown CNC control of the Z-axis on a lathe, so ELS never showed up on manual Western lathes, since almost no new manual lathes have been designed since the 60's. The only manual lathe I've ever seen with an Electronic Leadscrew is the Eisen (Taiwan) copy of the Hardinge HLV-H. Notice that there's no gearbox, or leadscrew:



                          So now the complicated part

                          In theory, Electronic Leadscrew is as simple as synchronizing two motors (the spindle motor and the carriage motor) with a divider circuit. But in practice, it's a lot harder than that. The first issue hobbyists run into is generating enough pulses on the lathe spindle so that there are no steps cut into the threads. Brian Thompson/John had the same problem with the electronic gear hobber -- you need to generate a lot of pulses on the lathe spindle so that when you divide it out for the servo/stepper, you don't cut stair-steps.

                          In Matt's example in the EMC document he posted, he's got 4,000 pulses per revolution between the spindle and the carriage stepper. So to cut 4 TPI, the stepper needs 15.15151515 steps for every rotation of the lathe spindle. That means you really need to do floating-point math to minimize round-off errors in the division, or you'll get periodic errors on the thread.

                          The other complication is what Rantbot alluded to: the simplest Electronic Leadscrew setup is an open-loop system (without feedback) like Matt's, and you directly drive the carriage without worrying about whether the carriage is actually where you think it is. The problem is that as soon as you start cutting metal, the stepper is going to be facing very non-uniform torque, so either you use a large stepper that won't lose steps and get out of sync with the divider circuit, or you use a PID feedback loop with a servo motor. There's a big thread on the Yahoo group right now trying to figure out how much torque the carriage stepper needs, depending on the size of the machine and the cutting forces involved...

                          As far as I've seen, all the hobbyist Electronic Leadscrews (and Brian Thomson/Sir John's Electronic Hobber) have taken the open-loop, large stepper motor approach. But if you take that approach, it takes a lot of tweaking to get everything set up, and then you have to watch it carefully, or the divider circuit will lose sync with the spindle, and crash the carriage into the headstock. John Dammeyer (the guy driving the Yahoo ELS) had that problem a lot, so he's added the ELS to an old Gingery lathe, but he hasn't added it to his South Bend lathe, which should tell you something.

                          In answer to your last question: you still need to use the thread dial to re-start the threading pass, because, like a human, it needs to have the same starting point to engage the threads that have already been cut. The only way to avoid the thread dial is to completely automate (CNC) the Z-axis, so the electronic leadscrew circuit rewinds the carriage to a known point, and re-starts the thread pass by itself. But that's really full-blown CNC...

                          Cheers,

                          Robert
                          "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

                          Comment


                          • #14
                            Thank you Robert, very clear and concise.
                            I haven't read the EMC article but it's interesting that Matt uses 4,000 pulses per rev, exactly the same as Brian and myself found on the gear hobber.

                            Another point we have come across on the hobber is that as it stops it can loose steps due to the speed of the pulses diminishing, same on startup.

                            So if you stop the hob and restart you can't guarantee it will be in mesh as it was before. Possibly a servo may cure this but it is inherent with the system.

                            Ironically a lathe driving a single pulse 'encoder' with an adequate power spindle motor to keep the revs constant doesn't suffer from this and will do repeated passes whether stopped in between cuts or not.

                            .
                            .

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



                            Comment


                            • #15
                              Originally posted by lazlo
                              In Matt's example in the EMC document he posted, he's got 4,000 pulses per revolution between the spindle and the carriage stepper. So to cut 4 TPI, the stepper needs 15.15151515 steps for every rotation of the lathe spindle. That means you really need to do floating-point math to minimize round-off errors in the division, or you'll get periodic errors on the thread.
                              Not quite -- I have 4000 STEPS per inch. My spindle encoder is totally low resolution, I'm using the larger back-gear as an encoder with a zero-speed gear sensor. I only have 66 pulses per revolution on the spindle.

                              I do not have to resort to using floating point numbers either. My implementation is all integer math, has no cumulative rounding error, and does a great job of interpolating many steps between each spindle pulse.

                              As to whether this is a "full blown CNC" -- sure it uses a computer, it also uses numbers, and it also controls things. But I wouldn't really call it a CNC, at best perhaps a pseudo-CNC.

                              -Matt

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