need help with screw drives

[cat427]

hi this is my firt machine i own a custom fab shop and thought it was time to build a cnc plasma table i have th table and gantry done it sits on a down draft unit but i think i made a mistake my x gantry drive is run on two 3/4 acme 6tpi screws with a 900 oz keling motor and two xl cog belts to time the screws the gantry is on linear slide rails and is constucted from alum. it moves freely and true but i cant seem to get more than 70ipm with out losing steps that is not going to work does any one have any suggestion also its run on a gecko 201 with a 40 12 amp power supply any help would be appreciated

[Pandinus]

What kind of gearing on the belts???
I'm not sure of the exact motor you have, but is it wired bi-polar parallel??
The 201 could go up to 80V, higher is better for high speed torque.

I'd assume @ 900 Oz its a nema 34 motor??? smaller is sometimes better for high speed torque... the higher inductance and higher rotational mass can slow you down...
6tpi at 71 ipm = 450 RPM * 200PPR * 10 microstep = 15000 pulses per second * your gearing???...

I run R&P on both sides of my X ... nema 23 - 387 Oz keling on a G540 ~ 36V and have achieved over 1800IPM gearing is only just over 2000 microsteps per inch.
That is basically one turn of the motor per inch.

With your big motor you could try to gear up your final.. lower motor speed should = more usable torque. But as soon as you spin your ACME to fast it will whip and cause you problems... how long is the axis??

Everything I read leaned towards rack and pinion/belt/chain drive for high speed - plasma and laser applications... and screws for lower speed higher torque - router and milling application... you can also get multi-start thread and high ration ballscrews, but I would think the plasma dust could gum them up real fast if not sealed... Rack and pinion with the rack facing down is decent design.

[cat427]

hey thanks for your reply i went and changed the drives to rack and pinion with a g 540 driver and it works great my pinion gear is a 20 t it gives me 4in per rev. should i slow that down the microsteping is set at 500 and all of my travels work great but will i get poor cut accuracy from the machine with it set to move so fast

[millman52]

Originally Posted by cat427 hey thanks for your reply i went and changed the drives to rack and pinion with a g 540 driver and it works great my pinion gear is a 20 t it gives me 4in per rev. should i slow that down the microsteping is set at 500 and all of my travels work great but will i get poor cut accuracy from the machine with it set to move so fast

Are you direct drive with the pinion attached directly to your motor?

If so you need to go through a belt reduction or gear reducer to achieve some resolution (accuracy).

My pinion gives me 2.36" per revolution. I have a 4:1 belt reduction ahead of that & still get over 400 IPM with terrific resolution, & loads of torque.

If your gantry weighs much at all your acceleration is going to be very poor.

[Pandinus]

Well the G540 is a 10 microstep unit @ 3.5 amps max.. I think your motors can take more amps than that.. 6 amps???? so your defectively "probably" getting about 500 OzIn ???

Mine setup is currently about 1 inch per rev... so as MillMan suggested a 4:1 reduction should be a nice drop for torque... How heavy is your gantry?? 3:1 might be good too... I think after tuning the motors and using it a bit you'll get an idea about changing the gearing.

my pinion is 8pitch 19 tooth so about 7.5 inches per rev .... Im running a 9 tooth primary and 70 tooth drive pulley for reduction... I'm considering going to a bigger drive for more resolution, but so fat things seem fine with the microstepping giving a theoretical 0.0005 resolution (Full step resolution of 0.005ish which should still be fine for a plasma)

The 4 inch pinion at 10 microsteps gives you a theoretical of 0.002" ... now microstepping is not very accurate for positioning but speeds up acceleration and smooths out the motion so your really looking at around 0.02" maybe 0.01"
So depending on your usage of the table that might be fine for you... Artsy signs and such... I'm guessing fine for personal use but if you want to do any repeatable production for someone it's their call on tolerances...
Then again what do I know... I still haven't dialed in a reasonably consistent kerf on mine yet which is more than 5-10 thou out at the moment :P

So congrats on getting a machine that moves at least any pics???

[cat427]

thanks for the input guys i have another question i have four outputs on my g540 for limit switches but when they trip in mach it just stops the machine and wont let the axis travel the other direction any suggestions

[Pandinus]

Limit switches are for a "Fault" as in the axis traveled too far... sounds like you need to set up "Home" switches.
Also in the settings page there is an "auto override" for faults... allows you to slowly jog off your switch... and you can set up a hotkey / input to overide the fault aswell