AXYZ parts book and availability of parts

[JAZZCNC]

If the motion control system fails, roughly what does it cost to replace the original with the UCCNC system you mentioned?

Depends on which controller you use but the controller and I/O boards, plus software license will be around £200-£250. However, these are not direct replacements in terms of plug & play so you will need some experience in terms of retrofitting or will need to pay someone who does. How much this costs will depend on the machine and what's on it ie: ATC, pops-up's etc

On the mechanicals, are there any quick checks/tests one can do with a used machine to gauge how worn the racks and pinions and the rails and bearings are?

The rails should be free of dents and nicks, the bearings should be smooth running not growling, and without any play when grabbed firmly and moved side to side,

Racks should have relatively sharp teeth not rounded over, similar with the pinion, but more importantly, there should be minimal play(backlash) between rack and pinion.