Mixing steppers and servos on same machine?

[philofab]

Hi, I'm new here. I've decided to build my own 5x10 table after an almost 6 month long battle to get a machine from a local company that has less than favorable ethics. I have a lot of mechanical and electrical experience but am very much a novice to CNC control.

Anyway, I'd like to build a welded frame table in a 5x10 size with a waterbed that will allow me to plasma cut sheet, and experiment with routing and water jet cutting. The main planned use of the router or water jet would be HDPE, ABS sheet, and maybe Lexan with 1/8 to 3/16 thickness. Eventually I want to also add a rotary tube axis for plasma cutting round tube using the preexisting X & Z axis. I know it's asking a lot for a machine to do more than one thing but I only have a 20x30 garage for this so it limits my room for equipment.

So it seems my needs and budget call for LinuxCNC software and fairly heavy oz servo motors (leaning toward 850oz right now) setup for the X & Y axis, however is there any software or hardware limitation with my using LinuxCNC with a standard stepper on the Z axis? I can't find anything that says you can or can not mix servos and steppers on the same machine with correct power supplies and drivers for each. Is there any benefit to using a servo on the Z?

Thanks in advance.
- Philo

[NIC 77]

I don't see a problem with using a stepper for your Z.

The advantages of a servo are closed loop positioning. With a properly sized stepper, you shouldn't loose steps anyway.

The advantages of the stepper on the Z axis are the higher holding torques at lower RPM to support your Z weight that isn't counterbalanced. You could use a servo with gear reduction but it can be a more complicated design.

The advantages of a servo over a closed loop stepper is the rated torque remains constant (sometimes it drops off after 3000 RPM or so, it depends on the servo, it's a general statement) throughout the RPM range of the servo, while the torque from a stepper drops off quickly as RPM increases.

So if you're not trying to match the speed capabilities of your Z with your X and Y, then I don't see a problem with what you propose to do.

[asuratman]

Can you mix servo and stepper in one board in Linux ?

[NIC 77]

Can you mix servo and stepper in one board in Linux ?

Never used Linux. Why so difficult?!?!

Can't talk....running away....

[wendtmk]

Can you mix servo and stepper in one board in Linux ?

Not sure what you mean by one board. You're going to need separate drivers for the stepper(s) and servo(s), as well as the associated hardware to support them, but both can be handled at the same time by the linuxcnc controller.

Mark

[rodw]

Linuxcnc won't care if you mix servos with steppers. It depends what sort of servo drives you have. If they were step and direction, you could use a 7i76e but if they were 0-10 volt, you could go with a 7i76e and a 7i77 that will allow you to do what you want.

I'm still not convinced you need to use servos on a plasma or waterjet because there are no cutting forces so the motion environment is very predictable. The mill or router has quite different parameters to plasma so good luck with your multi process machine. I had that in mind and ended up going with a dedicated plasma machine.

[asuratman]

thanks for good info ...

[philofab]

I appreciate the input from everyone. I've now decided to use the same motor / controller for each axis to simplify having a spare. I ended up starting to design my own gantry and machine in order to use the same Nema 34 motors on all of the axis too.