Re: roller chain instead of screws or rack?

Hello Mick,

Well, there IS a reason there are so many rack/pinion machines...
For a given low price, often a rack system will be better than a
screw system, if the format is large...

When the screw gets longer, lots of asdditional considerations come
into play (compression loading, whipping, rotational resonances, cost
of end mounts/screws, etc.). The rack pinion is a nice compromise
solution, when accuracy demands are not high.

A GOOD belt system will compare favorably to a rack pinion. However,
if you are not good at maintaining your equipment, the rack is
prob'ly more forgiving. ALL cnc machines need to be checked and
calibrated periodically. The belt will "fail" by stretching; the
rack/pinion will fail by "clogging"(may not be enough to notice
during use, but WILL affect part dimensions). So you calibrate belts,
and you clean rack/pinions... The best machines of either type will
use linear encoders to sense actual position, and send steps (or
current, in the case of true servo systems) to complete the closed
loop.

I know of several "belt" machines using the sstem described by Jon E.
SMS in the UK has/had? a WW router which uses the "stretchedbelt on
flat/around cogs" system, as does Precix in the USA(sold to the sign
and engraving industry). There are several variations used by
component mfrs for the mfg process control (conveyance and part
movement through the manufacturing process) which use custom aluminum
extrusions to serve the multiple functions of support, linear travel
and bearings, and motion.

Hope this helps.

Ballendo

--- In CAD_CAM_EDM_DRO@y..., Mick Jagger <got_milk_eh@y...> wrote:
> RE: large CNC tables and gantry setups
>
> As an added question, how does using a long screw 4-6
> ft compare. Is this the best solution for accuracy,
> but how about limits on how fast it would travel? Is
> a good belt system the better way to go or a rack?