belts vs screws?

[neeboy]

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!

[neeboy]

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

[automizer]

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.

[benjamint76]

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.

[blackbeard52]

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

[Glidergider]

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?

[ger21]

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.

[Glidergider]

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

[ger21]

Originally Posted by Glidergider 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.

[Bear5k]

Originally Posted by Glidergider 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...

[Pplug]

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.

[ger21]

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.