Thanks both.

So, I really like the look of the CSMIO/IP-M but the lack of built in support for slaving makes me nervous. From the manual it looks like it could be done via Mach3 but it's not at all clear what the consequences of doing this would be e.g. would homing / squaring work correctly.

I would happily just ditch one of the x-axis steppers (less hassle, cheaper, etc) but I can't see a way to combine an open back to the machine and a belt drive system without ending up with belts all over the place or the set up being impossible to tension. Any ideas would be greatly appreciated.

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I should say I'm not totally fixated on having an open back to the machine, it's nice to have and may make it more versatile but it's not the end of the world if it goes.