Re: [CAD_CAM_EDM_DRO] Timing Belts and Pulleys

Frank wrote:

>1) With a 50 and 25 tooth pulley, will this give me an exact 2:1
>ratio or will there be an error factor? Will I need to calibrate my
>mill by counting steps and measuring the distance?
>
>
>

the pulleys will give an absolute 2:1 ratio over the long term.
Depending on
how accurately the hubs are bored, there may be some small cyclical error
every turn. That's why I always buy the pulleys with undersize bores and
bore them up to whatever size I need. I turn rings out of aluminum scrap
that have an exact ID to match the pulley's OD, then split the rings.
The OD
of the ring is not important, but I turn it on the same mounting so that the
ID and OD are perfectly concentric. I put these around the pulley when
mounting in the lathe, so the pulley is held very accurately concentric
to bore
the ID.

>2) Can I place the pulleys at the specified distance, 60.21mm? Or do
>I need to add some to allow for belt tensioning? In this case would
>I be better off making one side adjustable? I will probably use two
>stages to give me 4:1, so do I need to make the middle pulleys
>adjustable?
>
>

You can't get the belt on if you can't release the tension. I usually
make the adjusting
slots at approximately right angles to the belt tension, so the motor
can be shifted
sideways to tighten the belt. You can't always arrange to do it this
way. You need to
have at least one pulley flanges, or the belt will walk off the pulleys.

If you use MXL belts, you should have no trouble getting 4:1 in a single
stage.

>3) What is backlash like with timing belt pulleys? I've read some
>other posts, and it seems the elliptical tooth profile is a good
>choice.
>
>
>

Backlash with new belts and pulleys is nil. Only on VERY worn systems will
you find any backlash. You also should run the belts VERY tight. This
keeps
the belt from creeping on the pulley, which is what wears out the belt
teeth.

>4) How will timing belts effect the required servo stiffness? I
>currently have the gain quite high to get a rigid feel (still
>haven't milled anything though). Do the timing belts allow for a
>softer (and hence quieter) gain ie will the stiffness be multiplied
>like the torgue? Or, do the timing belts add a springiness to the
>drive?
>
>

As I said, you need to run the belts VERY tight. I'm running something
close
to the recommended 80 Lbs of strand tension on my Bridgeport setup with
3/8" XL belts. I'd suggest at least 25 Lbs strand tension on a 3/8"
MXL. That
would put a 50-Lb radial load on the pulley's axles. Make sure your motor
can withstand that much radial load before doing it.

The belts won't be very springy at this tension. And, you don't want
anything
springy, as this can lead to undesirable resonances.


Jon