Timing Belts Math...Help!

[kanankeban]

Hi guys,
Is there someone out there that can expalin me in simple math, the principals of choosing the correct reduction ratio for a stepper motor. I'm trying to build a torchmate clone (www.torchmate.com) and I don't have a clue on the size of the time belt and pulleys used on that macine, they use a reduction I think of 3.6:1 and a rack and pinion setup, I will love to know why they choosed that setup .
It will be great if someone can write a short primer on how to choose the correct belt and pulleys and the math involved in such a task.

[Moondog]

Kanankeban..

I probably can't help much with the belt selection, but a good start might be a 10 or 16mm HTD belt with a 5mm pitch. That is a fairly universal pitch. For the calculations I recommend that you go to this site http://www.sdp-si.com/Cd/default.htm

This is a handy Centre distance finder and also allows you to play around with different pulley combinations. Very handy.

Hope this helps.

[kanankeban]

Originally Posted by Moondog Kanankeban.. I probably can't help much with the belt selection, but a good start might be a 10 or 16mm HTD belt with a 5mm pitch. That is a fairly universal pitch. For the calculations I recommend that you go to this site http://www.sdp-si.com/Cd/default.htm This is a handy Centre distance finder and also allows you to play around with different pulley combinations. Very handy. Hope this helps.

Thanks for the link...actually I have the SDP Manuals, but to be honest I don't understand quite bit, I just starting to learn The will help me get started is how to decide which ratio to step down, I know that the maximum speed I need to cut is about 380 IPM and the slowest speed will be 20IPM from here I need to find out what combination motor-belt & pulleys to use...another question I have going from this is what will be a good positioning speed before stariting to cut?
Regards...
Hector

[kevincnc]

Hector,
Since you didn’t mention a ballscrew I’m assuming that you’ll use either a belt or rack & pinion drive. Since you know you want to go 380 IPM we need to pick a hypothetical max speed for your motor to do some calculations. Let’s say your motor’s max speed is 1000 RPM before torque drops off significantly. You need to pick a timing pulley for the pinion that is relatively small but big enough to be bored to fit your motor shaft or whatever shaft the pulley is on. Let’s assume a 1.25” pitch diameter.

in/rev= Pi * pitch diameter= approx. 4 inches of travel per revolution.

(4 in/rev) * (1000 rev/min) = 4000 IPM

That’s obviously way to fast, and you wouldn’t have the power to accelerate fast enough to reach that speed anyway so you would need an intermediate reduction, like what ynneb did on his machine.

4000/380= about a 10:1 reduction would be needed to get the max speed, and therefore the most power possible from your motor.

380 IPM cut speed is pretty fast for a DIY and you’ll probably want to use servo motors to get the speed and accelerations that you want. I’m sure ynneb would share his ratios and performance with you to get you started.

Here’s link to a good timing belt site-

http://www.brecoflex.com/download_catalog.cfm

Catalogs B204 & 212 have some good info that would probably be helpful for you. You’ll need to pick a wide and strong enough belt that it can be tensioned without sagging too much, plus be able to take the accel & decel without breaking, and also not stretch too much under load.

Good luck!

Kevin