1 Computer, 2 Machines & Mach 3

[Jonathan]

Wouldn't need to be too clever. Keep X, Y, Z for the mill and use A & B for the lathe as X & Z.

Yes, obviously. You would still have to interleave the two programs to make them both run at the same time. This isn't so easy since to get it to run smoothly you need to take into account how long each line takes. It does depend on how the G-code is generated.

So long as the settings at stepper driver level (usually microsteps and current) are the same for each machine. You don't want to be twiddling DIP switches every time you change over.

Yes I did assume that. Looking at the machines Mr Holmes has I think they probably could be the same. Even if it means getting slightly bigger motors than neccecary for one machine you may well still save overall.

Also, at some point or other, I guarantee you will try to unplug a motor while it is still powered up. This is a VERY BAD THING.

I don't see why you would be any more likely to do that with this setup.

What's the additional cost of duplication? One power supply and two drivers.

Yes, quite a lot. You're talking £60 each for good drivers, and a fair bit for the power supply too. Very easy to share the power supply - DPDT switch with center off. Or even a relay for added safety.

In Mach, you just have two profiles - one called lathe.xml the other mill.xml and choose the appropriate one in Mach loader or put shortcuts to each on the desktop.

That's exactly what I did.

If you were really smart, you could install a second parallel port and use LPT1 for the lathe and LPT2 for the mill.

Unless you actually run out of I/O there's not a lot of point. Since inputs for limit switches and Estop inputs on both machines can be shared, and only 5 (maybe 6) steppers between them then there will be enough inputs.

Yet another way of doing it, which becomes especially profitable when you need more than 6 drivers or want to run lots of things is to use one output of the parallel port as a 'select' input. When that's high it connects the outputs to one set of drivers (or whatever), and when it's low it connects to the others. Very simple logic circuit to do that.