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.