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Didn't want to hijack the "Ball screw vs lead screw ... hype?" thread

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  • Didn't want to hijack the "Ball screw vs lead screw ... hype?" thread

    OK guys, time for another newbie question.

    Something mentioned in the other thread was backlash compensation when coding for CNC.

    I haven't had the opportunity to try my hand on a CNC mill, I have just begun working with my fleabay BP, manual with an x-axis servo drive.

    Please enlighten me. I was under the impression that a CNC mill was operated closed loop with the "true" position coming from an input from a DRO, eliminating backlash considerations.

    Any information is appreciated or please point me to some material that I can read up on this.

  • #2
    In a nutshell

    Closed loop is normally reserved for a servo motor application where as you say they read from a set of scales or a rotary encoder on the screw to allow feed back.

    Cheaper stepper drives don't have this and rely on the system being designed within limits that it doesn't miss steps, sounds crude but it's very effective.

    However any backlash even with compensation is bad because you get cutting forces grabbing the work and moving it as the control is trying to compensate.

    Imagine milling an internal circular pocket, at each quadrant point the machine changes direction on one axis, too much mechanical backlash and it leaves 4 indentations in the circle.

    The answer is to remove as much backlash as possible hence ball screws which are just very accurate screws.

    .
    .

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



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    • #3
      Backlash Compensation

      It all depends on what you are trying to accomplish. IF you are working on something that needs 0.5 micron tolerances you have to worry about how close everything is. If you are making your own parts for you own enjoyment then you have to consider how anal you want to be. I can make the parts i need with less than ideal machines and get the results I want. What do you really need, or are you trying to meet someone else's perceived needs? My little shop would drive some of the folks here crazy. It is what i can afford and I am able to work around the limitations of what my tools can produce.


      Are you planning on getting into high precision, high speed machining? You are probably going to want to look into zero backlash ballscrews. (they use 2 ballnuts and tension them with some preload and they are adjustable). Otherwise you can probably use a good quality set of ballscrews and never notice the .001, .002 backlash.

      Backlash compensation can help, but it also depends on the mass of the machine. Even a Bridgeport (a larger hobby machine...) table will move if you try a plunge cut with a 1/2" endmill without locking the table. Unless you have enough mass in the machine to help control wobble in those conditions, backlash compensation won't yield much results.

      Define the need based on what you want to accomplish, not what all the nattering nabobs say you need.

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      • #4
        Originally posted by kf2qd
        Are you planning on getting into high precision, high speed machining? You are probably going to want to look into zero backlash ballscrews. (they use 2 ballnuts and tension them with some preload and they are adjustable). Otherwise you can probably use a good quality set of ballscrews and never notice the .001, .002 backlash.

        Define the need based on what you want to accomplish, not what all the nattering nabobs say you need.
        True, but the big reason CNC guys are concerned with backlash is not precisely precision -- it's for cutting round curves -- if your leadscrews have backlash as your running back and forth, you end up with a weird-looking oval.

        You see a lot of pictures of that on CNCZone, using the shop-grade rolled acme screws.
        "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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        • #5
          Originally posted by kyfho
          I was under the impression that a CNC mill was operated closed loop with the "true" position coming from an input from a DRO, eliminating backlash considerations.
          I haven't seen a CNC that operates off a DRO. It could probably be done, and might work well, but would be expensive.

          The DRO itself may have backlash, too.

          Most hobby CNC's use open loop stepper motor control, not closed loop. They can be very accurate -- as long as you don't lose steps, as long as the computer doesn't crash, as long as there are no power failures, etc..

          A closed loop servo control is generally more robust, but costs more.

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          • #6
            Originally posted by MTNGUN
            I haven't seen a CNC that operates off a DRO. It could probably be done, and might work well, but would be expensive.
            They don't work off a DRO as such, but some machines do have linear glass scales as feedback. The Jenix scales for example are available in a differential output version (more noise resistant) suitable for CNC feedback. The motor shaft, coupler leadscrew, nut, nut housing etc. all have compliance that could drive a feedback loop crazy, so usually you have an encoder on the motor and the system uses the linear scales as a secondary loop. With good preloaded ballscrews and some careful tuning you might get them to work as the primary encoder.
            Paul Compton
            www.morini-mania.co.uk
            http://www.youtube.com/user/EVguru

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            • #7
              More backlash thoughts:

              The manual machinist knows how to compensate for backlash, and they instinctively operate their machines that way. The expectation is that a CNC machine could be operated in the same way.

              But in practice they are not for a variety of reasons:

              - CAM programs assume you have no backlash. So they do not create the right moves to avoid backlash.

              - There are many desirable things you can do to take advantage of a machine that has no backlash (note: no means very low, 0.001" or less. there is always some backlash if you look hard enough!). For example, circular interpolation where your endmill goes around and around to make a bigger hole.

              - Climb milling also depends on an absence of backlash and is very common for CNC because it gives better results.

              In the end, you really can't get around the shortcomings of backlash by adding scales. If nothing else, the scales don't prevent the table getting "sucked into the cutter" when climb milling.

              When doing that circular milling operation to cut a bigger hole with a smaller endmill, you can see the backlash show up as little "ears". Here is a simulated plot for 0.020" of backlash on a 1" circle:



              On the machine, you can see those little ears. Even with good backlash comp, you'll still get dwell marks there and usually a little bump, divot or other artifact.

              Here is the article that plot came from:

              http://www.cnccookbook.com/MTCNCStep...voBacklash.htm

              Eliminating backlash is not just a function of ballscrews. Backlash is "lost motion", and there are other sources. More articles:

              http://www.cnccookbook.com/CCBacklash1.htm

              http://www.cnccookbook.com/CCBacklash2.htm

              Cheers,

              BW
              ---------------------------------------------------

              http://www.cnccookbook.com/index.htm
              Try G-Wizard Machinist's Calculator for free:
              http://www.cnccookbook.com/CCGWizard.html

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              • #8
                Originally posted by MTNGUN
                I haven't seen a CNC that operates off a DRO.
                Heidenhain used to.
                .

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



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