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

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

Leslie Watts
L M Watts Furniture
Tiger, Georgia USA
http://www.rabun.net/~leswatts/wattsfurniturewp.html

----- Original Message -----
From: "carlcnc" <carlcnc@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Sunday, February 10, 2002 12:32 PM
Subject: [CAD_CAM_EDM_DRO] timing belts vs ballscrews/rack-n-pinion


> after having built a few wood routers with ballscrews I am
> wondering if timing belts wouldn't be a better [easier] solution
> than rack and pinion when you need more speed over longer travels.
> Looking for pros and cons.
>
> thanks
> Carl
>
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