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Backlash?

Discussion in 'Concepts and Ideas' started by Larry Fitch, Jul 1, 2015.

  1. Larry Fitch

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    I'm trying to come up with a design for a 4X4 router to use for making model airplane/glider parts in my shop. My current question is how to deal with backlash. I understand how anti-backlash nuts work on ACME lead screws, but how do you control these backlash issues with rack and pinion drives, or ball-screw systems? I imagine these other drives would be having these same types of issues. What am I missing, particularly with R&P drives. I'm trying to chose between ACME leadscrews and some R&P system.
     
  2. orangezero

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    I believe ball screws are preloaded in some way, but I've never personally seen one work. A large part of that may be because they are able to have tighter tolerances because they are essentially metal balls rolling against metal, vs a acme screw where it would be steel sliding over plastic or brass. In everyday function, wouldn't a screw need backlash to be workable for the average human? I just made some plastic nuts for my acme screws using a heated acme screw (propane torch). It saved me from buying $100-120 in nuts, assuming they work. I can spin them around, but boy are they tight right now. I assume running them with a motor will loosen them up a bit. I think I made them long enough I'm not even going to do the "spring between two nuts" method like I was originally planning.

    I don't hear about backlash with R&P systems, but perhaps I've always ignored the conversations. They are often used on big expensive machines, so they must work well. I would guess it would be minimized, again, with tight tolerances on the pinions and rack. They do typically use a spring to keep them together, perhaps that is all that is necessary.

    If you were trying to stay really cheap, the McMaster Carr 3/8in acme (1/2in per rotation) is only $16 something for 6ft sections. Really pushes the limits of whipping according to my reading, as 3/8 is typically not recommended for anything over 4ft. Just as an example, similar stuff in 1/2 is $40 and many other places had stuff that would really be slow or was 2-5x as much.
     
  3. Sonny Jeon

    Sonny Jeon Veteran
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    Belt-drive is commonly used on hobby machines, because the timing belts are designed to have no backlash. If your machine is lightly loaded (not cutting metal), belt-drive is perfectly fine. Although at 4' by 4', I would use 9mm belts, rather than 6mm. This should improve cutting.

    As for ball screws, these usually have a spring-loaded, anti-backlash mechanism integrated into the slide. These very stiff, precise, and very expensive. Ball-screws are common on industrial machines that require high precision, stiffness, and durability. Not the ideal choice for a light-duty hobby machine.

    If you are looking at a lead-screw and bushing type system, there are some spring-loaded bushings that can be used to help with this. Or anti-backlash nuts. These are difficult to get rid of the backlash completely, since the more you remove backlash, the more friction in the bushing there is. These are fairly cheap and you see this on hobby CNC machines that cut metal. You need them because they have a high mechanical advantage and stiffness to push through cuts. For your application, this would be overkill and lead to a lot of problems. Leadscrew design can be temperamental if you don't account for things like leadscrew whipping (undersized, going too fast, and not well supported).
     
  4. Hytech2k

    Hytech2k Master
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    Acme screw use spring loaded anti backlash nuts to pretty much eliminate any slop, my Z slides both use them with good results. Acme screws are limited to 4 ft generally due to issues with whipping at high speeds. R&P benefits from having spring loaded tensioner that keep the pinion fully engaged with the rack and aren't limited to length or speeds. My larger machine uses 9 ft racks currently and I run between 800-1000 IPM rapids.

    Check out this link for some more ideas and info on R&P

    http://www.cncrouterparts.com/pro-r...-47_49.html?osCsid=bqdkan8qos7jsk01dn6oi9o7i5

    or my build

    http://openbuilds.com/builds/area-51-cnc-parts-f-117-cnc-gear-rack-drive-system.1428/

    I like belt systems as well but like Sonny said I would definitely use a larger belt, I personally like a 3/8 to 1/2 belt for this application with a good urethene kevlar belt to reduce stretching to a minimum.
     

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