Dual motor idea!

[Ross77]

Hi All.

Looking for some advice on an idea I had to use 2 steppers on each axis to get over the high resolution and low rapids vs the low resolution and high rapids problem.

To start with this is for a small mill that will be used for stainless steel and Ideally Alu, the spindle is limited to 4000/5000 rpm so cant get near optimal surface speed for Ali so don't need crazy fast cutting speed but would like decent rapid speeds.

Initially I was looking at belt drive and picking a set of pulleys to give 1:1,1:2, 2:1 that had the same centre spacing so it would just be a case of changing the pulleys to change the machine performance depending on what I was cutting.

Then I thought that it could be possible to have dual motors, one geared at 1:2 for high resolution cutting and then the other geared at 2:1 for fast rapids. both motors would be driven for one driver and have the motor wires switched by relays on a rapid command from the controller.

So good idea? all ready done or pie in the sky?

Cheers

[C_Bubba]

IF I am reading you correctly, I think it is a bad idea.
1. It is extremely bad to have switches (the relay) between the driver and the stepper. Inductive kickback can let the magic smoke out:{(
2. Even disconnected, I would think there would be wasted power turning the disconnected stepper.
3. Two different "steps/move" to contend with, how to handle that?

Just my thoughts

[Jonathan]

I tend to agree with the previous post. You would have to switch off the stepper driver when changing, or add a circuit to better absorb the back-emf from disconnecting the motor. It seems impractical to me since how do you switch between motors accurately? When one motor is switched off and the second one switched on, the second one is likely to have to start on a micro-step position, so the driver would have to somehow start the motor without the rotor moving, even when the rotor isn't aligned with a pole tooth.

Are you sure you get the machine fast enough with the right motor and 1:2 for high resolution and do you really need a 1:2 ratio? A servo motor would make this easy.

[Ross77]

ok stupid idea then

Cheers, got a bit carried away there. Think I will just go for multiple or changeable pulleys then.

Jonathan
1:2 was more for the cutting speed as avg. stainless is 75mm/min so with a 5mm pitch ball screw that would only be 15 rpm. 1:2 brings this up to 30rpm so a bit smoother and as I'm after 0.1mm accuracy that will give me 8 steps head room.

Downside means that rapids are only 2500mm/min which seems a bit slow.

Servos would be best and I have quite a few motors but no drivers. hopefully this will be building the pcb mill to make servo drivers........

[JAZZCNC]

Downside means that rapids are only 2500mm/min which seems a bit slow.

You say this is for small mill so probably 300mm max travel so at 2500mm/min thats 41.6mm in 1sec so 7.2s to cover full travel.!! Seems like you'll be going to lot of trouble and expensive with using servos when machine will probably spend 90% of time cutting at 75mm/min.?

Personally I'd go for 2.5mm or 3mm pitch 1:1 for the torque and resolution and use Lead shine closed loop steppers.! This will make a nice and very accurate machine plenty fast enough for SS.

[m.marino]

You say this is for small mill so probably 300mm max travel so at 2500mm/min thats 41.6mm in 1sec so 7.2s to cover full travel.!! Seems like you'll be going to lot of trouble and expensive with using servos when machine will probably spend 90% of time cutting at 75mm/min.?

Personally I'd go for 2.5mm or 3mm pitch 1:1 for the torque and resolution and use Lead shine closed loop steppers.! This will make a nice and very accurate machine plenty fast enough for SS.

+1

Michael