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  1. #13
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    Default Ballscrew Technology

    Manufacturing technology of the ballscrew has advanced due to advances in tooling. We can "hard whirl" heat treated 60Rc spindles and the ballnut internals, this has reduced the cost of a precision C5 preloaded 16x5 flange style ballscrew from £240/meter ($440/36") to £140/meter ($252/36") obviously quantity reduces price. Similar rolled ballscrew £120/M.
    Visit www.marchantdice.com



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    Swede,

    Very nice article!! What type lube would you recommend for the angular bearings in your bearing block and would you put a lip seal on it or a just a shield?

    Regards,
    Pat



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    We use Kluber Isoflex NBU15. This is a high performance, high pressure synthetic lubricating grease, particularly suitable when extreme linear positional accuracy, precise repeatabality and low torque operation is required. It also offers exceptional stability in the presence of aqueous solutions, particularly water based cutting solutions.

    Our bearing blocks are fitted with rubber seals as standard, these are fitted integral of the housing and locate on the assembly spacers. The bearings (matched pairs) are fitted with metal shields. All Ballscrew support blocks housings are certified for accuracy!

    To view our bearing blocks http://www.marchantdice.com/ballscrews/bssalb.htm



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    Default Thankyou

    Thanks for this writeup... the time and effort taken is really appreciated.

    Line on the rolled leadscrews: I would guess this is created first to give the displaced metal somewhere to go as it is forced out of the grooves? Or it might form naturally during the displacement process? (Wild guesses both).



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    Quote Originally Posted by Calico

    couple questions:
    1. how you determine motor size if you have that certain ballscrew size ?
    any simple chart ? so if anyone build CNC, there is a minimum shaft size if they build CNC for certain size.
    2. I have a situation hoping you can guide me.
    I will build CNC about 1 meter by 1 .5 meter by 50cm Zdepth (all travel)
    I will cut Wood, fiberglass epoxy mostly for molding.
    this CNC for making a mold for bumper/spoiler/etc for cars, maybe dashboard.
    I will use CNC to make molding from RHino 3D and others apps.
    my questions:
    1. how to calculate bearing/linear bearing/shaft/ballscrew etc ? basicly ?
    2. how you calculate the motor size/torq. etc required ? how to choose stepper or servo for this purpose ?
    3. can you explain the ballscrew specifications ? say how strong a ballscrew can move an Axis ?
    4. can I ask more ?

    regards
    ruddy

    Look under the 'Resouces' tab at www.roton.com. There is information there related to many of your questions. You probably should have some idea of the feed speeds and forces on the cutter as well as the weight of any moving carriages. These would be your initial design parameters. The rest should follow from there. Then you can choose motors and drivers and power supply. The accuracy you are trying to achieve from the machine will also influence these choices.

    Chris



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    Excellent write-up about ball screws.

    I was considering precision ACME threaded rod for my next home built machine, but I think I will spend extra for the ball-screws even if they are rolled.

    Do you know the efficiency for the precision ACME threaded rod 1/2" -10 with multiple starts (1,2,5). I saw these rod at http://www.mcmaster.com/ or is it around 40% as well?

    Jason



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    Great write up! Any chance you can give an idea of the accuracy obtainable with just plain all thread rod? I'll probably have to use it until I build a few items with my CNC router. Would 3/8 all thread move a 36" X 24" gantry type router using 100 oz/in steppers? I plan on spending the majority of my money on the linear bearings. I don't care about speed and accuracy doesn't have to be that great either. I just need to be in the ball park. Half a mm would be better than I could hope for. Is that possible with all thread? Thanks again for the great article!
    Randall



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    Xterrian:

    Not sure if you're metric or imperial, so I'll follow your lead and use both

    3/8 sounds fine to me... threaded rod is less efficient than acme because of the different angles, but only roughly 25% difference I'd hazard.

    Repeatability will be good, accuracy (over longer ranges) less so. What's the pitch on 3/8 rod, around 1.5mm? Short range accuracy/repeatability should be at least a tenth of that, say around 0.15mm (wild guess), if your nuts are a good fit. Backlash and friction will be your bugbears.

    Can't speak for the size of stepper you need, depends a lot on the size of your gantry, friction, mass, etc. Get it built and test it by trying to turn the rods yourself, measuring the force you want and the torque you need to apply, then multiply the torque needed by at least 20 to get your stepper size, because steppers are specified for holding torque, not the torque it can apply when moving. Also it needs additional torque to accelerate your gantry, and a bit more power never does any harm. (Disclaimer: the former is largely guesswork)

    Jason: my guess is multiple starts will reduce efficiency further because the force is acting at a steeper angle to the resulting motion.



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    I'm Imperial
    My plan is to use 1" ID open linear bearings on a 1" supported hardened shaft for Y; Then .5" ID closed linear bearings on .5" oil hardened drill rod for X. My Z will be .75" ID closed linear bearings on .75" hardened shaft. (weird size choices I know, but those are what were available on E-Bay) I'll build the machine out of MDF and am guessing in the neighborhood of 20 pounds of weight to move around. I'll probably get it lower than that, but am trying to be conservative in my estimate. I plan on using thrust bearings on either end of a 1/2" thick delrin block for my all thread support at my driven end and a simple bearing at the far end. For my nut I plan to use the all thread to cut threads in another block of delrin. I just found the tutorial with formulas to estimate component sizes in the FAQ and am going to put my numbers in tonight and see what comes out. I plan on cutting mostly foam, MDF, Delrin, a little light wood now and then and one small part from 6061 Aluminum. I plan on using a small end mill and going extremly slow. I will probably end up making it smaller than my dream size, but want to make it big enough to upgrade later without buying new linear bearings and shafts. To think this all started because I wanted to turn an old printer into a stencil cutter. It just got bigger and bigger
    Thanks again for the quick reply. This place is great! If I could just figure out how to stop reading thread after thread until early morning.
    Randall



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    Thanks Mat-C, I was curious as I never heard of multiple starts before.

    Jason



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    Quote Originally Posted by Mat-C
    Jason: my guess is multiple starts will reduce efficiency further because the force is acting at a steeper angle to the resulting motion.
    Or it will increase the efficiency. (Actually it's the pitch that matters, the multiple starts just increase the load carrying capacity.) With a high pitch, the screw will rotate slower for a given speed. So less energy is wasted in accelerating and decelerating the ball screw. A high speed machine with a low pitch screw can waste more in accelerating the screw than the slide it is moving. Then you need a motor with higher torque, but slower speed.

    To get a "seat of the pants" feeling for the efficiency, try to backdrive it. That is, try to get the screw rotating by pushing the nut along it. The low efficiency of an Acme screw then becomes quite easy to understand. So does the better efficiency of a high pitch ball screw.



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    A word of warning, before everyone starts looking on eBay or firing off enquiries for multiple start screws; looking to increase load capacity and speed.
    Preloaded Assemblies
    1 Multi-start Rolled Ballscrews; I can guarantee that most multi-start rolled ballscrew assemblies are only loaded with balls in one track, or if more than one track is loaded most of the balls in the other tracks are doing nothing, just recirculating!!!
    Just think about it; Rolled screw, manufactured by the "plastic deformation" process, it can be hard enough to preload one track!
    To increase load capacity you need bigger balls, no pun intended!
    2. Multi-start Ground ballscrews; Again we have the same issues, even with precision grinding in a controlled environment I guarantee that the second, third, fourth and fifth track are not taking there designated load (the first track is doing the majority of work) Take one apart, the balls in one track will be different sizes to overcome this issue!
    One advantage of this is when the assembly requires a rebuild, just rotate the ballnut one track, fit new balls and your away!

    Regards / Kevin
    www.marchantdice.com/linear



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