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  • Why ballscrews?

    This is probably more of a "cnczone.com" question, but I think the signal/noise ratio is better here so...

    Why are all these CNC conversions of small mills switching to ballscrews? I understand that they have less backlash than the leadscrews the mills come with but is that the only reason, or is the reduced friction a big advantage?

    The reason I ask is because it costs less to use servos or add encoders to the steppers than it would to buy good ballscrews. The encoders can be used to remove backlash every time direction changes and save quite a bit of money. In fact, if the backlash is constant or can be characterized over the length of the leadscrew, you can probably do away with the encoders and just remember that "between positions 1000 and 1500 steps, take up 4 steps of backlash" or something like that. This makes the software slightly more complex but it's certainly not difficult to code.

    In my day job I write control software for an instrument where pumps must be controlled to within microliters of dispense volume. We do this by simply removing the accumulated mechanical backlash every time the piston changes direction of motion. It should work for at least a 2.5D CNC system which seems to be what most hobbyists are doing; "real" 3D will probably require the ballscrews.

    Part of the reason I'm asking this is that I want to add a power feed to my mini-mill's X axis and I'm thinking that I may as well use a stepper motor for the feed so if I eventually CNC the thing, some of the work is already done.

  • #2
    I am not a CNC person but the ball screws can have constant lube and low wear and minimal slop. I would think a CNC using standard Acme thread screw would have many problems keeping the accuracy that a ball screw would have. I imagine the programing problems would be great with a screw thread.
    It's only ink and paper

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    • #3
      No hijack intended here, but in a slightly different version of LWalker's question, can (or should) ballscrews be fitted to a Atlas/Craftsman 12x36 lathe that is only used manually, not CNC driven? I'm refurbishing such a machine now and will probably replace the lead and other screws. As long as I'm doing this it might be nice to gain the zero-backlash and smooth action of ballscrews, if it can be done. I'm a novice, so any advice is appreciated. - Bob

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      • #4
        Bob, it depends on how much time and money you want to put into the lathe. Yes, it will have less or no backlash but is it worth it on a manual lathe. Only you can answer that question.

        Just from talking to CNC people where I worked, having slop in the screws plays havic with programs. Mostly it seems to be the fact the cutter may not always end up at the same place or if the servo's back off and do not feed forward the tool will push away from the work. At least that is what I was told. I guess a program can be writen to feed in, back out and feed in to cut but I imagine slop in a CNC would be real trouble.
        It's only ink and paper

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        • #5
          Yes the servos can compensate for backlash when changing directions but the biggest reason you don't want any slop is so you can climb mill with the machine which will leave you a better finish in most situations. If your machine has .010 slop and you climb mill with it then the the cutter can pull the slop out of the screw and over cut the part. Which would lower accuracy and can also break the cutter if there is enough slop in the screw and nut.
          Jonathan P.

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          • #6
            Any slop in the screws will lead to surface finish issues as well as dimensional accuracy issues. Lead screws do not wear evenly. Mostly near the center where they are most often used.

            But the thing is is once you have installed ball screws in a machine that was meant for manual use with leadscrews you have effectively ruined it as a manual machine. Ballscrews should not be used on a manual machine. They do no stay where set like a lead screw. the cutter will back drive the screw and all sorts of bad things will happen.

            Thats why its better to buy an old CNC with a dead or dying control and put a new control on and reuse all the iron. You might replace a couple bearings but you now have a machine intended for CNC for cheaper than a new set of ball screws.

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            • #7
              Another thing to remember is: Not all operations are done with the
              table moving (as in compenstaing for backlash) but drilling a hole
              is done stationary and the table can move about due to the cutter
              action on the work. BAD SCENE!
              ...lew...

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              • #8
                ball screws reduce backlash, but don;t eliminate it without deploying antibacklash nuts. Also, the specs on a rolled screw might not be that much better than acme - ground screws give minimal backlash but are a expensive

                one of the guys on cnc zone who i think is knowledgeable swears by ballscrews with an antibacklash set up. on a manual mill complete with high end matched angular contact bearings. his handle is NC Cams if you want to search there for his detailed descriptions. can't remember how backdrive is addressed, maybe just through the bearing preload and low lead screw? never used one myself, but he's adamant there's benefit to it.
                .

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                • #9
                  Ballscrews, in big machines, are universally backlash-free. They're also low-wear, because there's no sliding surface. It's all rolling. To put my oh so humble opinion lightly, using ballscrews without preloaded nuts to eliminate backlash is retarded. That's the whole damn point of ballscrews. They aren't, in any fundamental way, MORE accurate than leadscrews. In fact, you can't buy a ballscrew in the same accuracy class as the most accurate leadscrews available. So that argument is a bunch of bullocks if you hear it from someone.

                  I've used Bridgeports with ballscrews, and I much prefer them. The issue that arises is table kick - the screws have very low friction, and the cutter can drive the table around. You need to always have your gibs partially snugged on the axis you are driving, and the other axes must be locked down. If you have servos on the axes, the servos will hold the screw still.

                  Having positional compensation for backlash is not the same as being backlash free. You will have a difficult time doing contouring when your screw has backlash in it, just like trying to continually switch from climb milling to contouring on a milling machine.
                  Last edited by toastydeath; 11-30-2007, 02:52 PM.

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                  • #10
                    lwalker,
                    In addition to Lew and jacpas's comments, I would add that one of the reasons you usually want to use CNC is to do circular interpolation and similar non-linear paths. When doing this, usually one or both axes have to reverse direction while in the cut. Any backlash is going to allow the cutter to chatter or move off the intended path briefly, whether you try to compensate extra steps or not. Your method might work fine if you don't require this reversal while in the cut.

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                    • #11
                      I wrote the "Ballscrew Basics" article on the CNC Zone:

                      http://www.cnczone.com/forums/showthread.php?t=8813

                      What everyone has said here is pretty accurate, but I don't know if anyone has mentioned efficiency yet. Ballscrews are 80-95% efficient at converting torque to linear motion, whereas acme screws are somewhere around 40%

                      Backlash comp in the software has never really worked well, and it is easily demonstrated by commanding a circular cut. Unless the system is tight and has zero backlash, there is a good chance the circle is going to be pretty ugly.

                      If you can find a ground ballscrew, I'd say there is a 95% chance that the nut mounted on it is zero-backlash already. With all of the factors put together, it adds up to ballscrews being vastly superior in CNC applications to other threadforms. They present the best overall solution when dealing with backlash, speed, efficiency, etc. HTH

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                      • #12
                        Originally posted by Bob Farr
                        can (or should) ballscrews be fitted to a Atlas/Craftsman 12x36 lathe that is only used manually, not CNC driven?
                        How are you planning to implement split-nut operation with a ballscrew?

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                        • #13
                          Originally posted by rantbot
                          How are you planning to implement split-nut operation with a ballscrew?
                          I'm not. In fact, I don't think I can and the comments here have convinced me refurb the lathe as Atlas designed it.

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                          • #14
                            Hi Swede,

                            Originally posted by 5Bears
                            If you can find a ground ballscrew, I'd say there is a 95% chance that the nut mounted on it is zero-backlash already.
                            I agree with the efficiency comment, but even a C3 ground ballscrew with a single nut is going to have backlash -- it has to, or the nut wouldn't turn.

                            The backlash is essentially the clearance between the balls and the spiral raceway in the nut. That's why preloaded, opposing ballscrews are almost always used in commercial CNC machines.

                            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."

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                            • #15
                              Originally posted by Bob Farr
                              I'm not. In fact, I don't think I can and the comments here have convinced me refurb the lathe as Atlas designed it.
                              That's another reason ballscrews haven't been used on lathes -- you can't make a half-nut for them The Schaublins use a ballscrew for both longitudinal feeds and screw-cutting, but it's locked in place and both longitudinal and cross feed is done by rotating the ball-nut -- there's no rack as on most conventional lathes.
                              "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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