Dual Leadscrews with one Driver

[rweatherly]

My JGRO machine has a lot of flex around the long leadscrew. I want to do dual screws on the long axis. Can two stepper motors be driven by wiring them in parallel?

[Jojje Bergan]

Yes

[flyboypete]

Hi
You will need two seperate stepper motor drivers and wire the drive signals to the drivers together so that both steppers see the the same drive signal.
I have done this on my router and it works well and was cheaper than buying a larger driver and stepper. I also used a toothed belt to transfer drive to the leadscrew and so they are mechanically linked also.

best
Peter

[S Lovette]

Technically, both answers are correct.

If you drive steppers directly from a voltage source with a low voltage unipolar L/R driver, no chopping or current control, you can literally wire the motors in parallel. The current will flow through each motor at the same time and they will step synchronously.

However, an L/R drive arrangement is inefficient and therefore not likely to be used for CNC.

A high voltage, chopped drive is far more typical. As the second post states, in this arrangement you are better off providing a separate drive for each motor.

A chopped drive regulates the current to the motor by chopping the voltage on and off at a high frequency to yield an average current. It may not work properly if the current can split between two motors wired in parallel because the current may not divided evenly between the two motors. If you have well matched motors (same inductance and resistance in the windings) it is conceivable that they would have balanced current and operate acceptable in parallel. However, if the torque on the motors gets out of balance, for example, if one motor stalls (goes too fast and loses torque when it can't keep up with the magntic field) while the other keeps going, then one motor will carry most of the current, nearly twice what is intended. It will get very hot and ....

Well you will prove the old electrical engineer's adage: Electronics works on smoke, because when all the smoke escapes from your motor, its going to stop working.

regards,

Spencer

[dliley]

Hello, On my router I have two X motors driven from a single gecko drive. It works great. Both motors are the same type and size, I wired them in parrallel
and set motor current on driver to twice the motor current of each motor. But I am using racks and spur gears for the X axis. Should be the same with leadscrews I would think. Good Luck

[FrankG]

I'm glad you asked... as I'm thinking of going dual screw on amill that I've just started to gather parts for... But I was hoping to place a stepper on either end of the threaded rod...

I understand that I'd have to invert the direction term, and drive it off a seperate controller, but didn't want to get into 4 separate controllers per axis...

So if I understand the experience of others... I can parallel the 2 steppers on one end and the two on the other end But have to use current limiting resistors...

The only issue that arises is one missed step and it will really cause grief ie; skewed and binding...

I don't expect 400oz/in out of 4 individual 100oz/in steppers, but is say 300 reasonable to expect??? I have the steppers already (4 per Y, 2 per X and 2 per Z (maybe just one per Z))...


frank@theworkshop.ca

[ViperTX]

rweatherly....so, you're trying to eliminate the flex by having 2 lead screws....I think you're approaching this from the wrong end.....

[S Lovette]

Frank,

In order to invert the direction of a stepper motor you can simply reverse the leads on one of the windings. If the motor has 4 wires, reverse two from the same winding. If the motor has 6 or 8 wires, flip the wires so the current runs the other way through half of windings. You will have to see a motor wiring diagram to make sense of this. In this manner you can run two steppers off one drive in the opposite directions. Of course with your proposed arrangement it doesn't sound necessary.

If you have a currrent controlled drive you don't want to use current limiting resistors. That will just dissipate a portion of your power as heat instead of torque. The only option is to run the same motors in parallel, with some risk, as dliley has done successfully, or use separate drives.

Missed steps should not be a issue if your motors and drive are properly sized for the load. How often have you seen an ink jet printer print a straight line jagged due to a missed step?

If missed steps are a concern because you are near the motors torque capacity, you can avoid skewing and binding due to a missed step if you tie the two screws together with a toothed belt.

[Bruggles]

I agree with viper, if one shaft flexes by spanning the distance two would do the same, assuming they are both the same as the original. If you are trying to lessen whip, i would look at a larger diameter ballscrew or possibly spinning the nut instead of the screw.

[rweatherly]

It is not the shafts (pipes) that are flexing (I'm sure there is some, but that's not the main problem). The gantry, when you twist it, rotates around the central leadscrew -- one side moves in one direction and the other side moves in the opposite direction. The center of the gantry, where the leadscrew is, does not move.

[Bruggles]

I see, the dual screws would help in that circumstance, you could also consider making the gantry or bearings more rigid, which would help out for heavier cuts, but either way should help out.

[spalm]

Rweatherly,

Do you mean you can push the left side of the gantry away from you and pull the right side towards you? Or the left side can go up while the right side can go down? The first case calls out for dual screws, the second case calls out for better supported rails to reduce flex. (Actually professional rails will solve both, but we are talking JGRO here.)

I have been playing with dual leadscrews at the hobby level and it is a tough call between one motor with a belt verses twin motors. They both have their problems. Twin motors is the easiest, but needs constant fudging to keep them aligned, with either dual drivers or just parallel a single drive. A couple of missed steps on one motor (or a couple of power downs) and it will lock up.

One motor with a belt needs some way of aligning (squaring) the gantry, but should only need to be done once.

I still don’t know which I would recommend.

Steve