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  • Ball screw vs lead screw ... hype?

    Well ...... I know it isn't *hype* per se, however is ball type always needed?

    I'm pretty new at this, but from what I can tell, ball screw is better for making 400 parts per day (less heat build up on the beds, hence better acuracy?) or if you're mounting heavy parts. I guess the drag of the lead screw type makes for less predictable slide actions and such that affect machining. Backlash is less as well. Got it.

    But if you're only making 5 parts per day, no larger than a CD jewelcase and just as light, and the sequence times are around 4 minutes, not to mention backlash compensation in the software .... is ballscrew really needed? Keep the ways/rails/gibs lubed and clean, especially if they are already in excellent and *tuned* condition bla bla and it should be fine, que no?

    It just seems like you could use a nicely made plain old ZX45 clone for much more than the *big guys* will tell you (Smithy et al). Of course, no *pro* would ever DARE use a leadscrew with a cnc rig. The shame of it all!

    Just curious.

    Farmdirt.
    I make messes.

  • #2
    Leadscrews properly done are fine,anti-backlash nuts are the key.It is easier and a lot cheaper to get accurate leadscrews than it is ballscrews.IIRC the best I could find for anywhere reasonable money were .009" per foot accuracy,while the mid-range acme leadscrews were .003"per foot.

    If you want to see what leadscrews on a CNC mill can do have a look at Evan's mill.Completely homebrew mill featuring Acme leadscrews running in plastic nuts if I recall.Does some very fine work.
    I just need one more tool,just one!

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    • #3
      Ball screws automate well, as in CNC. Either will work well with hand wheels.

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      • #4
        Apples and oranges. You don’t have a case.

        Friction, think about it the contact point of a ball verses a flat spiral flat. Enough said.

        If you were into mass production, balls. Axial or angular balls are king. The next step is magnetism.

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        • #5
          What exactly do you mean by "leadscrew"? That is not a type of screw, but rather a use for such a shaft.

          It could be normal 60 degree v threadas or acme... or whitworth? You need to be more specific.
          -paul

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          • #6
            Acme screws tend to wear, changing the accuracy and backlash over the length of the screw over time.
            Ballscrews will wear much less and general allow climb cutting without issue, where as a loose mill might have problems with climb cutting.
            Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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            • #7
              yes, u need ballscrew for CNC (in general)

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              • #8
                Lead screw is an accepted term for an acme threaded shaft.

                You're better off with ballscrews for a cnc application;

                * Even though you have compensation in the software, the ammount of comp needed will change more often (relative term there) with a leadscrew
                * a properly sized and lubricated ballscrew with preloaded nuts (they are also available without preload - mostly just for moving loads, not positioning though) will give almost infinite life and experience very little wear
                * the efficiency of a high lead ballscrew permits the use of smaller drive motors to move larger loads.

                For a good comparison between leadscrews and ballscrews try running through the calculations for each type on the Rockford Ballscrew website. That will give you a better feel for which suits your application. They manufacture both and probably don't care which you buy.

                Obligatory disclaimer: I'm not connected with RBS except as a satisfied customer.

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                • #9
                  Definitely ballscrews.
                  You can get reasonably priced ballscrews from China. These are C7 grade, rolled with less than .002" backlash.
                  Regards
                  Geoff
                  My place.

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                  • #10
                    A lot of points made, some valid and some not.

                    An acme leadscrew has poor efficiency that ranges from about 90% maximum to as low as around 20 to 30%. The finer the pitch the lower the efficiency because the greater the friction in the nut. The advantages of acme screws are simplicity, low cost, ruggedness, long life and the big one for manual machines: They are self locking if the helix angle is less than about 8 degrees.

                    The self locking property means that it can't be back driven by machining forces. This is very important when operating a manual machine as it means you won't be fighting varying cutting loads to try and maintain smooth operation of the hand wheels.

                    Ball screws are not suitable for a manual machine for the reason that they can be back driven and will not stay in place unless they are locked by some sort of friction device. They are much more efficient than acme screws and because of that they allow for smaller motors to drive larger loads in CNC applications. They usually are designed to eliminate backlash which is necessary in CNC machines. Backlash compensation is not an answer for CNC machines as it cannot be used in most operations that use interpolation.

                    Backlash compensation is relatively easy to implement in acme screw systems by using acetal nuts. The best are made using PTFE filled acetal which has very low friction and virtually no wear. They are designed to be a slight interference fit which also eliminates the requirement for wipers as there is no room for swarf to enter. I have been running an acetal nut on my Y axis on my mill for a couple of years now and it still shows nearly zero backlash.
                    Free software for calculating bolt circles and similar: Click Here

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                    • #11
                      Originally posted by Evan

                      The self locking property means that it can't be back driven by machining forces. This is very important when operating a manual machine as it means you won't be fighting varying cutting loads to try and maintain smooth operation of the hand wheels.

                      Ball screws are not suitable for a manual machine for the reason that they can be back driven and will not stay in place unless they are locked by some sort of friction device.
                      On my little mill there is zero chance of of depending on the lead screw providing much locking. I have the bent end mills as proof! I actually allow my gibs to drag some, so my lead screw efficiency is quite low, but it does allow me to make sharp corners. I always lock the unused axis. This mill is hopelessly incapable of even mild climb milling.

                      Again, a small mill, but I did have the opportunity to use ball screws manually on a machine and it was a pleasure to use. Can't say what it may be like on a larger system. So I'd have to say there's no hard fast rule.

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                      • #12
                        How well it locks depends on the helix angle of the screw.
                        Free software for calculating bolt circles and similar: Click Here

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                        • #13
                          Backlash compensation is relatively easy to implement in acme screw systems by using acetal nuts. The best are made using PTFE filled acetal which has very low friction and virtually no wear. They are designed to be a slight interference fit which also eliminates the requirement for wipers as there is no room for swarf to enter.
                          This is very interesting to me Evan because I have a lathe that is metric but converted from an imperial design, and the maker had the great idea of using a 1/2" diameter cross-slide screw with a metric pitch. The pitch is 5mm, which is VERY close to 5tpi. I've been thinking of making an acetal 1/2" x 5tpi nut for it, thinking it might work and serve effectively as anti-backlash too since the pitch is about .004" per turn different. Might try it now.
                          Peter - novice home machinist, modern motorcycle enthusiast.

                          Denford Viceroy 280 Synchro (11 x 24)
                          Herbert 0V adapted to R8 by 'Sir John'.
                          Monarch 10EE 1942

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                          • #14
                            Reading the OP original post and addding it to a previous one he's trying to compare the Microproto CNC mill which is a Taig mill underneath with a ballscrew machine or just ball screws.

                            The Tail uses standard 1/2" x 20 UNF thread as it's leadscrews, not threaded rod, nicely made screws but still vee thread at the end of the day.

                            They also uses a bronze split nut for backlash adjustment.

                            I have one of these machines but unfortunately it hasn't done enough work yet to become worn and give a good idea of how servicable it is for engraving over a long period of time.

                            My concern would be is that the OP wants to do a lot of fine engraving and when backlash does occur how badly will it affect the work.

                            As Evan has said you can double up with plastic nuts to get a better fit but the use of 1/2" UNF is questionable for an application like this.

                            .
                            .

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



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                            • #15
                              Once you put DROs on the axis the issue of acme/ball screw 'accuracy' is moot.

                              Set it up right and the DROs read position of the table, and really doesn't care how many turns it takes you to get there.

                              I never understood why servo's encoded off the shaft (well, I know why, but its a poor trade off). If you get your absolute position off the motor shaft, you still have ball screw play and way play. If you encode position of the table who cares about lash... the TABLE is where it should be.

                              My manual machine has DRO. Its a crutch, but the scales on a hand wheel suck.

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