Can someone please explain why ball screws are more accurate than the standard 'v' profile machined screw thread? I'm assuming this is the case because the ball screws are used on CNC machines. Am I wrong and the ball screws are used for another reason (like less backlash)? If so, why? i.e. what makes the ball screw have improved qualities over the machined screw?
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Ball screw vs standard machined screw
- Thread starter Geswearf
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It's all about backlash, stiffness, wear resistance, and relatively low friction even when preloaded.
But they're still just ground or rolled. Which can also be done to an acme or V screw thread.
But with sliding friction of the regular screw zero backslash & high loading just isn't practical.
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But they're still just ground or rolled. Which can also be done to an acme or V screw thread.
But with sliding friction of the regular screw zero backslash & high loading just isn't practical.
Sent from my SM-N975U using Tapatalk
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Ball screws have about 80-90% less friction than a lead screw, which is not the same as a "machine screw," which would be even worse. Lead screws are trapezoidal in profile, and machine screws are "V" shaped, as you mentioned. Ball screws offer significantly longer life than either of the two aforementioned screw systems. Also, ball screws offer nearly zero backlash due to the pre-load and circulating balls within the system.
Contrast that to a lead screw or machine screw; as soon as you start turning the screw, you are introducing wear. It can, however, take years for that wear to show up if used in a hobbyist environment.
I have owned two mills. One had Acme (lead) screws, and now my Sharp mill with ball screws. The difference in feel and backlash is shall I say, astounding.
The one negative for the ballscrew system is that it can be back-driven. I.e., The head on the Z-axis can drive itself down to the table if power is removed from the servo.
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A ballscrew more closely approaches the qualities of an ideal screw and nut: Zero backlash and zero friction
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I think you set the preload in a ball screw with the size of the bearings. Also, modern CNC machining places demands on the linear positioning system that are almost unthinkable for a lead screw design. Velocities and accelerations that CAM software asks for are whole new world---your manual-machine intuition won't work here. Ball screws do have longer life but I think heat control might be the greatest advantage. I guess that's another way to talk about the friction reduction 7milesup mentioned.
Pictures of ball screws show them to be shiny and smooth. I would also assume that like bearings, they are made of hardened steel. Lead screws, or acme screws almost never look shiny and I'd guess are not hardened. Some years ago, I read of a guy who was polishing a long screw for some scientific apparatus that he was making (probably a Scientific American or Popular Science type article). IIRC, he cast a lead sleeve around his screw, removed it and added abrasive/polishing compound and ran the sleeve up and down the length for hours on end. I don't remember any more of this article, but I recognize this would not overcome the friction inherent in the thread profile vs a ball screw.
Is it simply easier and cheaper to make a ball screw than harden and polish an acme screw?
Is it simply easier and cheaper to make a ball screw than harden and polish an acme screw?
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For a CNC mill, a ball screw would be the best choice. The reason being that in order to reduce backlash to a reasonable amount, clearance has to be reduced to virtually nothing. The machining direction is constantly changing on a CNC mill and although there are software solutions to controlling backlash, they aren't ideal. Reducing backlash on a lead screw will increase friction and wear and increase required driving force.
OTOH, using a ball screw on a manual mill causes problems because of back drive. The only way to control it is either locking the axis or by manual control. Conceivably, use of a worm gear could eliminate the problem but the you would be adding the backlash in the gears which would be defeating the solution.
OTOH, using a ball screw on a manual mill causes problems because of back drive. The only way to control it is either locking the axis or by manual control. Conceivably, use of a worm gear could eliminate the problem but the you would be adding the backlash in the gears which would be defeating the solution.
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A ball screw uses rolling friction whereas a lead screw has sliding friction.
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This can play out in "interesting" way. For instance, if one were moving a mill with ballscrews, and was at the point where he was trying to get it over the lip into his garage shop. And that mill was facing with the table downhill. And the head was too tall to go in the door, so that he was stuck there a while trying to figure it out. The weight of the table would back-drive the ballscrews, and put more weight on the front of the mill. Making it front heavy. Which is on the downhill side. Which results in the whole thing falling over on its face.
Warning to the wise: When moving ballscrew equipped equipment, tied down each axis.
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