How bad are these Chinese ballscrews gonna be?


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Thread: How bad are these Chinese ballscrews gonna be?

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    Member QuinnSjoblom's Avatar
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    Default How bad are these Chinese ballscrews gonna be?

    So I bought a package on ebay for 500 bucks that included 4 1605 ballscrews (2x 1000, 700, 400) and 6 20mm linear rails with 12 carriages (hiwin clones). The linear motion stuff is great after I had them exchange a couple carriages. Good preload, no movememt. That stuff alone was well worth the 500 bucks. At first I thought the ballscrews were also good with zero backlash but after building my machine and taking some readings with a dial, they aren't so perfect. At first the measurements weren't making sense to me. Without putting much load on the axis, they are indeed zero backlash but when a decent amount of force is applied, they give about a thou in each direction, then spring back to zero. I have confirmed it's in the ballnut, not the bk12 end supports. I then realized why there's only backlash with excessive load. The ball nuts have plastic inserts in either end that are formed to the ballscrew and are pinched by a set screw in the nut housing. These basically take up the backlash but give with some force. I wanted to know how much actual backlash there was so i loosened the set screws pushing on the inserts and there is about 1.5 to 2 thou when the inserts aren't doing anything. When tightening inserts, backlash is relative to force applied. They can be tightened to the point where there's a bit of resistance in the ballnut and then it really takes a lot of force to get some backlash but I really just dont know how much force there's gonna be while machining (aluminum). One problem I see with backlash being relative to force is that backlash compensation in the software probably isn't gonna work the way it should. If I set it to compensate and it's doing a finish pass with minimal force, it will probably be over compensating since there won't really be backlash in that scenario. Might be better off leaving it at zero comp and just letting it be off a bit during high load roughing and when it does light finish passes, should be pretty accurate.

    Are these types of ballscrews with plastic inserts to take up backlash a common thing that actually works? Or is it just a way for the manufacturer to take cheap ballscrews and call them zero backlash? Should they perform decent? I'll run them for now and see how it does but I may end up upgrading eventually. Even if the ballscrews end up in the trash, I'm still happy with what I got. The hiwin clone linear motion stuff is really nice and would have been about 1400 bucks on its own for genuine stuff.

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    Default Re: How bad are these Chinese ballscrews gonna be?

    The ball nuts have plastic inserts in either end that are formed to the ballscrew and are pinched by a set screw in the nut housing.
    I believe that those are just seals to keep the dust out.

    You're not going to get zero backlash for the price you pay. .001-.002 is not bad, really.

    Gerry

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    Default Re: How bad are these Chinese ballscrews gonna be?

    Unless you bought double nut ballscrew assemblies you should expect some backlash. 0.001-0.002 is actually very very good for single nut screws, and even for double nut isn't bad. On the double nut screws there is a spacer between the 2 nuts to take out the lash, but you don't want them too tight or they can bind.



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    Quote Originally Posted by ger21 View Post
    I believe that those are just seals to keep the dust out.

    You're not going to get zero backlash for the price you pay. .001-.002 is not bad, really.
    I was assuming they were just seals as well, but they are definitely doing more than sealing whether they were designed to or not. If they were checked for backlash in the standard way, just running the axis back and forth on a dial, it would always return to exactly zero and seem like zero backlash since these inserts are actually pinching on the ballscrew. Only when pushing on the axis with some force does it give a bit and read some backlash. I guess they could just be meant as seals, but seem to be more than that. They are listed as "anti backlash ballnuts" so maybe that's what they intended them to do. But yeah, even with the inserts loosened completely, 1 or 2 thou doesn't seem too bad for the money. I'll see how they perform once i start machining and report back.



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    Quote Originally Posted by 109jb View Post
    Unless you bought double nut ballscrew assemblies you should expect some backlash. 0.001-0.002 is actually very very good for single nut screws, and even for double nut isn't bad. On the double nut screws there is a spacer between the 2 nuts to take out the lash, but you don't want them too tight or they can bind.
    Ok, that's reassuring that 1 or 2 thou is considered good. I think I'll get some pretty good performance out of this machine for the money I have into it



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    Default Re: How bad are these Chinese ballscrews gonna be?

    I have two cheap Chinese eBay ballscrews (which I am not using) and some NSK and THK ground ballscrews (which I am). I can't speak for others but from what I bought, there is a noticable difference in quality. I was sufficiently unimpressed with the Chinese stuff that I decided not to use them at all.

    One of my Chinese ballscrew assemblies actually feels ok but the other grinds and clicks when you turn it and backlash is detectble with the naked eye. I concluded that you can probably get lucky with them..., or just as easily, unlucky. Their QC is not exactly consistent and I don't believe their self awarded "C7" accuracy gradings for a minute.

    I originally intended to use my Chinese screws for a second smaller machine but when I saw them, I chose to use my cheap lead screws for the second machine instead.

    The downside to my double-nut anti backlash NSK ballscrews is that you get far less travel for the length of screw. I can barely achieve 30" of travel with my 42" X-axis screw because the double nut alone is 5"....



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How bad are these Chinese ballscrews gonna be?

How bad are these Chinese ballscrews gonna be?