View Full Version : Timing belts for motors

11-01-2010, 03:05 PM
Hi All
Been looking to build a CNC for a while now never got the time or money, so I've decided to go for it this year come hell or high water.
I will no doubt have lots of questions but here is the first.
I have seen many builds with the motors directly coupled to the ballscrews & many that are using timing belts with a 1:1 ratio,
Is one method better than the other or is it just so that the motors can be mounted in a better position.

11-01-2010, 03:46 PM
Coupling the motor directly to the ballscrew requires a certain accuracy with the motor mounting, plus some form of coupler. Using a timing belt means you can mount the motor seperately and allows for a certain amount of error in the motor mounts. Generally if you want 1:1 its better to drive direct as there are less opportunities for backlash to creep in. IMHO!

11-01-2010, 04:31 PM
Thanks Irving for your comments didn't think of the aligning help timming belts could offer, I will give it some thought as to direct drive or timing belts.

Robin Hewitt
12-01-2010, 12:39 AM
For milling I use a 36:45 XL belt reduction. Larger pulleys cut less slack and XL pulleys are cheap from RS.

This converts a common or garden 5mm pitch screw to the slightly less usual 4mm pitch at the motor, allows NEMA34 motors without overhanging the mill bed and makes the maths easy...

400 steps/rev on a 4mm (pretend) screw is 100 steps/mm, 10 microns

Switch it 800 steps/rev and you get 5 microns which is about the best you can hope for without spending a lorra lorra money on iron :whistling:

Switch it to 1600 steps/rev, if you can sustain the pulse train for a G0. Tell the hoy poloy that it "resolves" 2.5 microns, tell everyone else that it makes it quieter :heehee: