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  1. #97
    Benutzer
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    Nov 2008
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    50

    Smile

    The ball-nut doesn't have to be double to be zero-backlash. The best way to find out about one particular type of ball-screw, is to look up the speck-sheet. It usually says how the pre-load is achieved. Usually it seems to be achieved by oversized balls, or by grinding the ball-nut a special way (maybe you could say that there is two nuts in one nut?! (hehe). Thus no double nut is needed. A C2 ball-screw does not have to have less backlash than a c7, but yes I guess it's true, since they probably don't sell a c2 ball-screw without pre-load. I have not seen any though.
    I would bet on NSK's, but then again I have never used Hiwin. I don't think I will even buy a new c2 screw though.

  2. #98
    Gold Member
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    Apr 2009
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    5515
    I'm not an expert on this, but...

    As far as removing backlash, it can also be done by offsetting one of the ball races as well.

    The accuracy grade has more to do with the lead accuracy of the screw. I don't think swapping ballnuts or bbs out would change that, just the amount of preload.

    A C2 screw is an amazing piece of machined work, but it means nothing if the rest of your machined is not built to anywhere close to that tolerance!

  3. #99
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    Apr 2007
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    1876
    Thanks, of course a C2 grade is far beyond my needs, it just happens to be one of the options in the "new old stock" section, so the price is higher than C7 hiwins, but no so high that it is crazy.

    That being said, I can make one of the C7 rolled hiwins for 1/2 the price, but none of this stuff is really "cheap" so it is a question of doing it once vs twice.

  4. #100
    Benutzer
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    Nov 2008
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    50
    If you're not sure if you can take the lead accuracy of a c7 screw, and the price for the c2 one is not a deal-breaker, I would probably say that you got it right. It's better to pay a bit more and be sure, then to try and see if it works. It never seems to work to well with CNC. (At least not in my experience, as little as I have.)
    To even be able to have any use of c2 accuracy, you would need some fancy machinery though. But even if you don't It'll probably beat the c7 one, as long as you have the right support bearings and such.

  5. #101
    The C grade is only the lead accuracy and this pdf will show you what they refer to.
    http://www.zappautomation.co.uk/pdf/...l accuracy.pdf
    Backlash is down to the nut and the best way to get zero backlash is a double ballnut with pre load added.
    there are other ways to get zero backlash like oversized ball but this is not the best way.
    Zapp Automation Ltd
    www.slidesandballscrews.com

  6. #102
    Benutzer
    Join Date
    Nov 2008
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    50
    Yes, it's true the C grading only refers to lead-accuracy, but some people have mentioned it harder to get zero-backlash on c7 screws, because of the relatively large lead-variation compared to C5 or less. For some reason it also seems to hold more true with rolled screws!? This is not my opinion, but something I have read several times here on Cnczone. I have only used mostly C5, so I can't say if this is true or not.
    I'm not sure if Zappautomation refers to my post when he says that oversized balls is not the best way to get zero back-lash, but if that is the case; I was not talking about the DIY approach, but about factory made units from THK and NSK and such. Maybe it's not that common any more, but I think they still use this method in production. If someone know better, then please feel free to jump in.

    I have several new ball-screw of C5 and C3 quality without backlash, and only 2 of them have double nuts. I don't think double nut is better or worse than anything. Double nut tends to be quite a bit longer, but on the other hand they seem to have a bit better load rating. Just for this thread, and just for fun I compared two of mine:
    NSK W2509******* (10mm pitch) (Load rating) Dynamic 11600N Static: 19000N (skewed pitch pre-load)
    THK BNFN 2510A-2.5 (10mm pitch) (Load rating) Dynamic: 15800N Static 33000N (double nut, oversized balls pre-load)
    There is of course more to it than basic load ratings, but it could be fun and it at least gives an indication.
    I don't actually have the THK one, but I have a couple with smaller lead and 5mm pitch.

    Ball-screws are a very interesting subject, something could be talked about for hours. I know I could at least...

  7. #103
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    Aug 2013
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    4
    Hi All,

    I have some ball screw assemblies, out of a Medical Interments Mfg. cnc due to such tight tolerance work, and they are 0.001" or less backlash.

    They are still well coated in grease with no rust or scratches and well protected. These will be excellent for a mill or router project.

    The NSK part numbers are:
    W3206G-91P-C5Z 07H1-0020
    W3206G-91P-C5Z 08H2-0010
    W3204G-127P-C5Z 08H3-0031

    I have no use for them, but if anyone else does, message me and we'll talk.

  8. #104
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    Mar 2010
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    11

    Re: Ballscrew Basics

    Nice article.

    I had some issues with ball screws in the past and it was on bearing journal end and a lock nut. If you spend money on angular contact bearings for bearing support, you need to make sure you get the journal machine, usually ground to precise tolerance specified for the angular contact bearings. also, external thread for lock nut will need to be accurate. A lock nut is very critical also since the face of the lock nut will be applying preload to the angular contact bearing sets. Poorly made nut on poor thread will run crooked and will apply uneven pressure to the bearings. If you purchase ball screw support bearing blocks from ball screw manufactures, they will come with a good lock nut as well and all you have to worry about is journal thread being made properly. if you try to machine the screw end, and build your own bearing block and a lock nut, you better know what you are doing. Poor bearing block and lock nut will end up crooking the fixed end of ball screw and the rest of the screw will run crooked. You know when you have poor bearing block ends when you lock up the lock nut and screw runs hard and loosing the lock nut will help smooth out the screw movement. This is because linear bearing is fixing screw nut movement but screw is no longer parallel.


    you can tell rolled ball screw from ground ball screws by looking at the screw shaft. Rolled screw shaft has a channel that runs along the actual ball raceway. it looks like a mini raceway but are just a groove made when materials are pushed in roll process. ground ball screw does not need such channel so there are only one groove that is the ball race way.

    there is a 3rd type of ball screw called whirled ball screw. it does not use grinding so it is cost effective yet it can achieve better accuracy than rolled ball screw.

  9. #105
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    Jan 2013
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    32

    Re: OK I cant afford ground ballscrews....what's the next best thing?

    Anyone knows why in general 1605 ballscrews are cheaper than 1204?

  10. #106
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    Quote Originally Posted by JukeBox View Post
    Anyone knows why in general 1605 ballscrews are cheaper than 1204?
    ill hazard a guess that....

    the larger diameter is easier to produce accurately, especially in long lengths.

    the larger nut is easier to machine.

    theyre more common.

    just an idea.
    with so much stuff on hand, one spends more time locating it rather than using it.

  11. #107
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    Jan 2013
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    Re: Ballscrew Basics

    Quote Originally Posted by headsmess View Post
    ill hazard a guess that....

    the larger diameter is easier to produce accurately, especially in long lengths.

    the larger nut is easier to machine.

    theyre more common.

    just an idea.
    The second and third statements make sense.

    But why would be larger diameter easier to produce accurately?

    Of course I am referring to rolled ball screws.

  12. #108
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    Re: Ballscrew Basics

    Quote Originally Posted by JukeBox View Post
    The second and third statements make sense.

    But why would be larger diameter easier to produce accurately?

    Of course I am referring to ROLLED ball screws.
    i believe the part i put in capitals would explain my train of thought...

    rolling is a deformation process. and ball screws take a LOT of deformation compared to a standard iso threads. depending on the ball size of course.

    a 25mm bar is pretty solid.

    an 8mm bar bends simply by looking at it... there fore the smaller sizes would take more to setup, and make, so as to be up to standards required. more support, more guides, closer tolerances... which then requires a better operator and a better machine... or maybe they have to use less pressure but increase the number of passes? etc.

    think...on the smaller sizes, not only do the rollers produce valleys and crests...but they would also tend to stretch the material axially... i assume...

    now grinding or machining...the forces are far less...

    i could be completely wrong. as i said...its just what im guessing... there could very well be completely different factors at play...maybe the 12mm bar is simply less readily available than the 16mm stuff? or requires less work to straighten BEFORE any other operations are done.


    anyone feel free to correct me...i wouldnt mind knowing the real reasons myself!
    with so much stuff on hand, one spends more time locating it rather than using it.

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