How do I rest z axis

[grantx]

Ok so in my case the drives don't have fault signal outputs. I am going to have to think outside the box then. Im not willing to just leave this.

There must be another system that is separate to the drives and controllers that monitors the position of each side of the gantry relative to a fixed position. If they go out of alignment then the entire system disables.

If the left side of the gantry has a distance from the edge of Xmm then the right side must have the same distance. If at any point either position moves out of tolerance (lets say tolerance of 2mm each way) then emergency shut down.

What could I use to monitor this? Any ideas?

[Doddy]

What could I use to monitor this? Any ideas?

Not what you want to hear - but almost certainly better to replace steppers with closed-loop.

You *could* attach rotary encoders onto each screw and - not sure of any COTS solution - knock up an encoder comparator to generate a fault signal. Rotary encoders are in the £10-£20 range for cheap, and an Arduino is the cheapest way to implement the comparator. But that's all a little bit of a bodge.