Belt driven screw ?

[tomcat47]

Hello everyone,
My name is Tom and this is my first post, I've been reading and
looking for a couple months now, this site is awsome I've learned
so much here and been inspired to build my own machine!
The z axis and gantry are nearing completion, being built of steel
and aluminum with many cast parts quite heavy, I'm guessing 125lbs.
Using 640oz motors from campbell designs all axis will be belt driven,
the drive screws are 1/2 8 double start acmes. HERE'S my question>
Should I use a 1 to 1 pulley config or 1 to 2 small pulley on motor
larger on screw or other I'm STUCK here! ( gecko 202s & mach 3)
Any help here would be greatly apreciated !!!!!
Sincerely Tom

[ger21]

I'd start at 1:1. If you need more speed or power, then adjust accordingly.

[Konstantin]

If precision of any importance then calculate the best gear ration for your table.

Konstantin.

[Zumba]

Like Ger said, start with 1:1.

What you end up with will depend on the friction in your linear motion guides, rigidity and accurate setup of all the machine components including linear motion guides, screw end bearings, motors, and pulleys (to prevent binding), and the voltage of your power supply.

125lbs is not that much weight. I've managed to get a tiny 250-oz NEMA23 stepper to move that much weight in a horizontal orientation at 550 IPM with a 5/8"x0.200 ballscrew, THK SR20 linear rails, 48v power supply, and Gecko G201 (1:2 pulley ratio). I probably could've gone faster but I was already peaking the parallel port output at 45kHz, and the G201 is fixed at 10 microsteps. Will resume tests later when I get a G100.

[millman52]

Do you really need that much speed???? If 1:1 will work ok at lower speeds (if you need low speed) Great. The less moving parts the better for adjustment & maintenance. At 8 TPI on your lead screw 1:1 is going to make for a near light speed movement at top speeds.

Assuming you are using steppers They will need to ramp up & down slowly for rapids or possibly loose steps getting there.

It all boils down to what you want/need from your system.

[Zumba]

Light speed? Not quite. 1:1 with an 8tpi screw, Gecko G202 10 microstep drive, and Mach 3 -->

60 seconds * 45000hz / (8tpi * 10 usteps * 200 steps/rev) = 168.75ipm

The power supply is important. A 24v power supply may result in stalling at high speed, whereas a 48v power supply will allow the gantry to muscle through anything and everything in its path.

[ger21]

It's a 2 start screw, so double that speed.

[Zumba]

Ah yes, you're right.

Still slow though.

[marting]

HI Guys, I've gut a cnc, the same - made of steel and aluminium having a gantry weighing 30kg (I don't know how that adds up in lbs sorry) and each axis driven with a 400oz/in servo motor and pulley ratio 1:5. In my opinion thing that it can be set to 1:3 the least, naturally depending on the motor rpm.

[tomcat47]

Thanks for the input so far its nice to be able to share my
hobby with interested parties!

By my calculations it will take roughly 32 turns of the screw
to move the gantry 130 inches, leadscrew whip should never be an
issue, at 1to1 1 motor rev would move things 1/4 inch 1/2 a
motor rev would move things 1/8 and so on.
So for any detail would i be asking to much of the motors at
1to1 no accel or decell room, thats why i'm wondering if it would
be better to have the motors spin faster than the screw, smaller
pulley on the motor larger on the screw @ 1to2 the motor would
spin 8 times to move an inch and one motor rev to move 1/8 ?
I don't have experience with these 640oz motors nor any other
steppers, don't know what to expect from them or how they'll
perform best.
Just for clarification, many agonizing hrs spent so far setting up
and machining these parts to assure paralellism, so far it's paid off
everything moves freely without any deviations ...whew!
Don't mind the lathe but changing setups on the mill is like pulling
teeth!!! thanks again Tom...

[ger21]

By spinning the motor twice as fast, it'll probably have about half as much power at the higher rpm. With a 1:1 ratio, you'll get a resolution of .00125 per step, which should be plenty fine for a router.