belts vs screws?


View Poll Results: Belt or screw drive?

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  • Belt drive

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Thread: belts vs screws?

  1. #1
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    Cool belts vs screws?

    Hello to all:

    I starting to plan out my DIY five or six axis wood cnc router. My question for all you experts out there is: Belt drive or screw drive? I have heard that belt drives are faster, more accurate and don't strain motors as much. I'm looking for as much input as I can get, so please give your two cents worth!

    Similar Threads:
    Last edited by neeboy; 04-01-2008 at 02:19 PM. Reason: misspelling


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    Geez, I guess I shoula looked at the other posts first! Ooops!



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    I have run machines with both and I have to say I do not like belts at all. I found that if the belt got warm ie on a hot day that it would grow and throw your machine out of alignment or running it under high load it would skip a tooth on the belt and really screw you up. Now this was running a belt over 10' long with square teeth, I know there are better belts out there but I say ball screws all the way.

    I'm not lazy..., I'm efficient!


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    In my limited experience, I prefer belt drive due to speed and yes it is easier on the motors. with my screw drive, I could only get up to 30" per min(with a 276 oz-in). With the belt drive, I can get to about 1700" per minute(but I also increased the from 276 to 640oz-in.). I would like to play with a nice ball screw, but that is out of my price range.



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    Many are going to rack and pinion because of speed increase and much less costly than ball screw. Check the posts related to rack and pinion before making a decision. I means you will have to use 2 motors in the x axis but with the price of motors on ebay its a no brainer...I would still use ballscrew or Acme on Z axis

    Bob



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    Tell me about resolution accuracy with the belts. With a 200 step motor, and a 2 inch diameter pulley, the resolution is terrible. I assume that you are micro stepping the motor to get it into a reasonable accuracy range. Is that true?



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    You can get pretty much the same speed whether you use belts, or screws. Regardless of what you use, you always trade speed for accuracy. If you get 2" per motor revolution with a belt, you can get similar performance (and resolution) with a screw with a 2" lead. Very high lead screws are about 80% efficient, though, so belts will give you a slight edge there. But screws imo are easier to install. The only way to have high speeds and high resolution is to go to servos, where gearing and high encoder counts can give you extremely high speeds and still have high resolution.

    Gerry

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    Ger,
    How about length of axis in the DIY machines. Do you agree that 4 feet is pretty much the limit for a screw drive? Given a 72 inch screw minus the allowance for the gantry width. But with a belt drive, longer is possible?

    The attached image is my inspiration for joining this thread. The owner posted this machine in the Vectric forum. I hope it's ok with him that I repost it here.

    I can't image buying an acme screw long enough to span that distance pictured.
    Dave

    Attached Thumbnails Attached Thumbnails belts vs screws?-4x8-foot-router-table-jpg  


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    Community Moderator ger21's Avatar
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    Quote Originally Posted by Glidergider View Post
    Ger,
    How about length of axis in the DIY machines. Do you agree that 4 feet is pretty much the limit for a screw drive?
    No, but you'll need to increase screw size and motor size, and cost goes up, sometimes real fast.

    Gerry

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    Quote Originally Posted by Glidergider View Post
    Ger,
    How about length of axis in the DIY machines. Do you agree that 4 feet is pretty much the limit for a screw drive? Given a 72 inch screw minus the allowance for the gantry width. But with a belt drive, longer is possible?

    The attached image is my inspiration for joining this thread. The owner posted this machine in the Vectric forum. I hope it's ok with him that I repost it here.

    I can't image buying an acme screw long enough to span that distance pictured.
    Dave
    My ball screw has 5' of travel. Of course, it is also 1" in diameter. ;-) There are screws available that are over 2" in diameter. They are heavy...



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    You could always use a rotating nut design so the screw stays stationary. Then you would have no whip at all. My techno has a rotating nut on the 11' x axis ballscrew. The screw is about 2" in diameter.

    http://www.glenspeymillworks.com Techno LC4896 - 2.2Kw Water Cooled Spindle | Moving Table Mill from Omis 3 CMM, 500Lb granite base | Epilog Legend 32 Laser Engraver


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    Community Moderator ger21's Avatar
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    I happened to measure the ballscrew on our Morbidelli today. It's 30mm, with a rotating nut, and moves a 1000lb+ head at about 1500ipm. There's about 13ft of travel, as it's a 5x12 machine.

    Gerry

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    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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    Quote Originally Posted by ger21 View Post
    I happened to measure the ballscrew on our Morbidelli today. It's 30mm, with a rotating nut, and moves a 1000lb+ head at about 1500ipm. There's about 13ft of travel, as it's a 5x12 machine.
    Does that have pop-up supports to keep the screw from sagging when the head is at the far ends?



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    I have been looking a lot at class 3 ball screws lately vs rack and pinion. Roton (and others) have 1 inch dia x 1 inch lead (1 turn per inch) class 3 screws as a standard, off the shelf product.

    A direct drive (through a coupling) stepper to a screw like this, it is very similar in rpm, torque, speed, and resolution to the cncrouterparts pro rack and pinion setup. As far as I can tell, the class 3 ball screw will be at least as accurate as the rack and pinion seutp, and possibly even more so, because of fewer components between the motor and final drive.

    This puts the ball screw rpm into very reasonable speed - often only 100 - 200 rpm for a lot of hobby cutting projects, which means that quite long setups are possible. 6 ft was the longest I found on Roton, but Thompson will cut them longer.

    I am still looking at the "total" cost of these seutps, but at least for 3 and 6 ft, it looks similar. The main challenge for me, remains economical screw mounting, but I think this can be done (for hobby use) at a reasonable price. Pro level ball screw mounting is really good, but not cheap. The difference in price between a 1 inch lead screw and 1 inch class 3 ball screw was really not that different - $ 25 vs 35 per foot.

    Coming back to the belt drive question, the main challenge is the lenth:width ratio. If you use a belt for cnc wood routing, target a length to width ratio of around 10 - 15:1. ( example - 60 inch long belt, target a 4+ inch wide belt) A 60 inch long x 1 inch wide belt will give mediocre results at best. I like belts, but 4 and 6 inch wide belts start to be difficult to place in constrained locations.



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    Community Moderator ger21's Avatar
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    Quote Originally Posted by kevincnc View Post
    Does that have pop-up supports to keep the screw from sagging when the head is at the far ends?
    It's got a small single support that looks like a wheel in the center. It's mounted on a spring loaded bracket to pivot out of the way when the head moves by. I've never actually watched it while running, though.

    The Masterwood we had at my last job had a similar screw with no support at all.

    Gerry

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    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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    Default Re: belts vs screws?

    Before I read anymore of the posts here I have to say something there is no perfect choice. However I will say it is probably easier to get away with sloppy mechanical engineering with a lead screw drive.

    Beyond that consider this, I work on machinery that uses both a high helix lead screw and belt drive on the same axis. These happen to be extractors for mold machines but the point is that sometimes the technologies are combined to yield a result. I've actually seen machines wear out zero backlash gear boxes before the belts failed, sometimes the machines go through multiple gear boxes on a set of belts.

    As for belt drives they can be extremely robust and have the advantage of reduced maintenance needs. However you won't get good results if the solution isn't properly engineered. The other aspects of this is YOUR expectations from the machine. A CNC machine that is cutting panels for boat build is a different machine than one trying to route out a nearly finished violin. I wouldn't have any concerns at all about a machine built with a proper belt drive to machine wood based sheet goods.

    To a lesser extent the same thing applies to leadscrew drives. Design in the worn sized lead screw and you will have issues with its performance. The difference here is that a lot of experience floats about these forums with respect to leadscrews, guys can tell about feasibility from experience and not have to walk though the engineering. That doesn't make leadscrews "better" in every situation just that it is easy to get feedback on a suitable implementation.



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    Default Re: belts vs screws?

    Quote Originally Posted by Glidergider View Post
    Ger,
    How about length of axis in the DIY machines. Do you agree that 4 feet is pretty much the limit for a screw drive? Given a 72 inch screw minus the allowance for the gantry width. But with a belt drive, longer is possible?
    4 feet is really nothing for a leadscrew. You do need to consider larger diameter leadscrews for long stretches. In that regard the best thing you can do for a design is to go to the experts, that is the lead screw manufactures and refer to their engineering data which many freely distribute. Within that information you will likely find tables for span capabilities, max speeds for both supported and unsupported ends, load capabilities and the like. As the span increases your have to increase the screw diameter which adds to the expense quickly. As things get larger you need to watch out for the rapid increase in expense of other parts of the machine. Bearings, mounting blocks, couplings, servos and the like rapidly increase in cost.
    The attached image is my inspiration for joining this thread. The owner posted this machine in the Vectric forum. I hope it's ok with him that I repost it here.

    I can't image buying an acme screw long enough to span that distance pictured.
    Dave
    The span isn't a problem. As noted you would need to make sure the proper lead screw is selected. If your goal is a machine to handle wood products I'd seriously consider a belt drive.



  18. #18

    Default Re: belts vs screws?

    My DIY machine has nearly 4' of travel in the Y and I have two 1/2-10 5 start Acme screws with no whip support. I'm cutting at speeds less than 100ipm and there's not a whiff of lead screw whip. Those lead screws are perfect for my semi hobby machine. My first machine was a belt driven Shapeoko. I looked seriously at using belts on my DIY machine. The Acme screws just seemed like a better-easier solution to me. I think they wound up being cheaper to implement as well.



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    Default Re: belts vs screws?

    Quote Originally Posted by fretman_2 View Post
    My DIY machine has nearly 4' of travel in the Y and I have two 1/2-10 5 start Acme screws with no whip support. I'm cutting at speeds less than 100ipm and there's not a whiff of lead screw whip. Those lead screws are perfect for my semi hobby machine. My first machine was a belt driven Shapeoko. I looked seriously at using belts on my DIY machine. The Acme screws just seemed like a better-easier solution to me. I think they wound up being cheaper to implement as well.
    your critical speed is on the order of 330 inches per minute if your acme screw is 50 inches long, about 500 inches per minute if you truly have a rigid end connection at one end of the screw.
    Ball and Acme Screw Critical Speed | Lintech



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    Default Re: belts vs screws?

    Planning to make all chain drive router :P

    http://free3dscans.blogspot.com/ http://my-woodcarving.blogspot.com/
    http://my-diysolarwind.blogspot.com/


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