Need Help!- Plama Cutter Gantry Drive Unit

[antsals]

Hi,
After a little of help / advise. I'm just currently building and designing as I go along a CNC plasma cutting bed. 1500mm x 1400mm bed size.

I'm just looking into the best way of driving the gantry unit. My plan was to drive the gantry from one side on a rack and pinion arrangement. My concern is should I be looking to drive it from both sides? The gantry is mounted two HIWIN linear slides at each side.

Does anyone have any experience with this and do you think I will get away with driving from one side of the gantry?

Thanks for your help,
Ant

[antsals]

Just a few pics of where I'm upto.

Ant

[bobsch]

Driving from only one side is going to allow racking. Drive it from the middle or both sides.

[antsals]

What do you think then:-
Rack and pinion both sides?
Lead Screw both sides?
Or Lead Screw underneath in the middle?

Thanks for reply
Ant

[multiplex]

definitely drive from both sides, using rack and pinion. are you planning on driving it direct with the stepper? you will probably need gear reduction to get any resolution out of the table

[antsals]

I was going to drive each side directly from the stepper motor on a 4mm pitch lead screw? Any reasons this wouldn't work?

Thanks
Anthony

[larryy1022]

drive both sides is far better than one side, i sugest gear rack. if u want better cutting quality, 3 pcs cheap 0.4kw servo is better than stepper motor.
but if you choose stepper, it will save much, even no need gearbox.

[antsals]

I already have the stepper motors they are old ones from my cnc mill conversion.
I was going to use 4mm pitch leadscrews. Why are rack and pinions better? When you say it's a better cut what's the reason?
Where do you get your rack?

[AndiKid]

Hey Antsals, in another thread Torchhead posted very interesting information about r&p.

Maybe it will help you and I hope it's ok for Torchhead that I post it here..



" If the final drive is rack and pinion you first have to realize the effect is a STEP UP in Speed (calculated as a ratio , of PI times the pinion Dia). Thus a 1" dia pinion results in approx a 3X speed increase and a 3X torque and resolution DECREASE. If you take that a stepper will spin (based on the DC applied voltage) of 600 to 800 RPM with a moderate load then you have a theoretical linear speed of 1800 to 2400 IPM with direct drive. Downside (con) is that you traded 2/3's of your torque and resolution for speed you may not be able to use.

So why do you need torque? because torque translates to lateral force and that yields acceleration (or cutting force on contact cutting operations). Why do you need acceleration? because with plasma to do rapid turns, starts and stops (to get sharp corners) you need good acceleration. Acceleration is the process of moving a Mass (weight of the gantry) from one speed up or down to another in a period of time. The lighter the gantry the faster you can accelerate the mass given a set torque. Conversely in order to accelerate a heavier gantry faster you have to have more torque (force).

So the Pros and Cons of a gear/belt reduction:

Pros

Better matches the RPM of the motor and the torque to a given load
Reduces mechanical vibration from motor to load.
Will accelerate a given load/weight faster.
Increases resolution (smallest distance you can move in one step)
Allows smaller motors to do more

Cons

Costs more.
Adds more complexity (more things to wear out and break)
Reduces speed to sub-sonic levels so you can't brag to your friends about how FAST your table is.
May not be needed if you can achieve acceptable acceleration numbers (min 15 IPS/sec) with a given load.


To gain back the lost torque and resolution you trade back some speed. A 3:1 belt reduction gets you back to base motor specs (with the 1" pinion) so you have 3 times more torque, 3 times more resolution and 1/3 the speed (about 600 to 800 IPM). More reduction gets you more resolution but because of the nature of steppers a diminishing amount of added torque. Generally numbers over about a 3:1 overall reduction (step down + step up of pinion) don't offer a lot more in return.

So.....some systems with direct drive will work fine depending on the gantry weight, motor torque ratings and the type cutting. You may have to compromise as the weight increases by lowering the acceleration targets and living with rounded corners.

It's all about matching the mechanics to the job at hand and motors you have to do it with."


Hope that helps..

Best - A

[larryy1022]

I didnt mean gear rack is better, I mean even a cheaper servo is better than stepper.
Mostly we use gear rack here, if you prefer leadscrew, that's ok too. attached you may check the new table we got from a factory. We plan to building a machine to test and demo our controller and THCs to customer.

for your cutter, I think a 200 USD stepper should be ok.
But later if you want to upgrade your machine, you may try servo. A good china 0.4kw servo is around 350 USD, Japan Mitsubishi E series 0.4kw is around 650 USD, pansonic is cheaper. With servo you need to spend another 250 USD for gearbox, a little more or less.

Anyway, if it's yr first machine, use old stepper is good option. A good machine matters with controller, motor, mechanical parts, plasma and thc.
It will take you quite a few time to work things out.

good luck.
larry