Re: [CAD_CAM_EDM_DRO] timing belts vs ballscrews/rack-n-pinion

The only OEM machine I ever new of that used Some form of Belt drive is/was
Precix. Not sure what they used but almost bought one once. Specifications were
not any different than other machines but the Z axis clearance was a bit limited
on the one I considered.

Les, What about some of the Stainless Steel thin belting (all metal) ??

Do you have any experience with that ??

With the traditional belts, couldn't one find a way to drive the belt with more
than one pulley to minimize that "soft backlash" ?

I wanted to try a belt setup myself. I thought that it would be very quiet. One
would need to watch for accumulation of dirt on the belt, so location would be
critical.

Too bad IWF is so far away from me. I am stuck with IMTS in Chicago. There are
far less Routers than Metalworking machines.

Chris L

Les Watts wrote:

> It is a good question Carl.
>
> A year and a half ago I went to the Atlanta IWF, a woodworking show that had
> literally ACRES of cnc wood
> and plastic routers.
>
> I am sure I missed some but I did not see a single machine
> using timing belt drive.... even though timing belt linear motion
> drives are very commonly used for robotics. Many manufacturers make
> servo/stepper integrated linear motion
> units that use timing belts. Many will run at 10+ feet per second!
>
> So... why?
>
> They are light and fast as well as economical. I have used them for pick and
> place.
>
> I looked into this a long time ago and my answer came in the detailed
> engineering specs.
>
> The first is registration accuracy. For a typical 4x8' machine
> about the best I see is .008". Actually that is not too bad for some
> woodworking stuff. A lot of this is a kind of soft backlash from tooth
> deflection.
>
> I see the big one as stiffness though. Over 8 feet a 10 lb
> spindle force will result in as much as several hundreths of an
> inch belt stretch worst case. This is perhaps 10 to 100 times
> poorer than ballscrews and rack and pinion. An oscillation
> from the stretch and the carriage mass interaction can make this even worse.
> Kevlar and steel reinforced belts help a good bit but must be run at high
> tension.
>
> Some data is available from Gates at:
> http://www.gates.com/facts/documents/Gf000289.pdf
>
> It is in somewhat obtuse engineering nomenclature.
>
> So... the very best machines I saw at shows used linear
> motors... next best rack and pinion... then ballscrew.
>
> Most machines used ballscrews. I think the issue is economy.
> Ballscrews have a large reduction ratio built in as well as
> extremely good accuracy. Belt or rack and pinion often needs VERY expensive
> low backlash gear transmissions.
>
> There is no real reason a timing belt can't work ok...
> it is just that a case study and engineering data suggest other
> methods are better for wood routers. The best timing belts
> I have found are glass, steel, and kevlar modified curvilinear
> (Gates GT).
>
> I have used timing belt for light load pick and place as have
> many colleagues. I would think they might be good for laser and plasma as
> well.
>
> By the way the IWF is coming up this summer... It is about
> the biggest cnc router show in the world among other things.
> Sadly it is not open to the public but any small business can get in.
>
> Les