PCB milling machine

Mike wrote:

I'm building a PCB mill as my final year project. I currently have some 4-phase variable reluctance stepper motors
...
Are there any inexpensive boards that you would recommend?

Do you really mean "4 phase" or do you mean "4 wires, 2 phase"?

How inexpensive do you need? Rather than mess about with those steppers, which sound rather feeble, I would suggest you buy one of the inexpensive 3-axis kits from eBay. You can buy motors, controller and PSU for $150 or so.
www.ebay.co.uk/itm/CNC-Kit-3-Axis-Steppe...A-220V-/270824395338
As a random example, not as a recommendation.
Alternatively, and even more cheaply, you could consider driving thos motors direct from a ULN2003, though that is not a particularly efficient solution. It is extremely cheap though. (EMC2 can produce all 4 drive signals)
www.eleccircuit.com/uln2003-control-step...or-by-parallel-port/
This does require motor with 5, 6 or 8 wires, though.

Mike wrote:

(Mods: I *thought* I posted this yesterday - was it deleted, or did I mess up when posting?)
Mike

Something must have gone wrong when you tried to post...

John

Mike wrote:

It's 4-phase, 6 wires. I've attached the spec sheet.

Despite being described as a 4-phase motor, that wiring plan looks like a 2 phase unipolar motor to me.
en.wikipedia.org/wiki/Stepper_motor

However it might not be. You can normally wire a unipolar motor in a bipolar mode, and then the TB650 chips will drive it, but if you can't do that, then you probably can't.
Thinking some more about this, a variable reluctance motor can't be run bipolar, as the rotor poles can never be repelled by the coils.
This means that the ULN2003 approach might be the only one that is available to you with those motors.

With regard to the ULN2003, what do you mean by a 'less efficient' solution?

Normally you run steppers at 10x or 20x rated DC voltage. So, if you had a motor with a 1ohm winding and a 2A current limit, that would be a 2V motor. However, you would run it at 40V from a drive with a 2A current limit. This gives much better performance, as the inductance of the windings doesn't limit the current rise rate as much.
If you are using the winding resistance as the current-limiting element you can't do this, and have to run at the DC voltage limit.

Mike wrote:

Given the motor's length, it might be possible that it's a multi-stack type which would make it 4-phase.

Not necessarily, I have some 3-stack 2-phase motors. I have never actually figured out what they mean by "stack", actually.

In either case, do you think that the black and white leads in the spec sheet wiring diagram should be connected to the power supply?

If using a ULN2003, yes, as they effectively work as a switch to earth, so you would feed your +V to the black and white wires (through your current limiter) and switch the other ends to ground in sequence.

Mike wrote:

Took apart the stages today - with a mounting flange, I can use different stepper motors with the linear translational stages, which is handy. I've decided that I'm going to try and do this via USB instead of parallel port; this probably means I can't use EMC2, unless it's possible to use a USB to parallel port cable.

No, EMC2 is based on an assumption of realtime control. USB does not allow realtime control.
I think you can use Mach3 and the SmoothStepper. However that will cost you $150 for Mach3 and $150 for the Smoothstepper.
You can buy a lot of parallel cards for that money, or even a Mesa 5i25.