Had a call from a friend earlier - "my CNC mill ain't working!" It's a Bridgeport-style vertical mill, built by Ajax, and by the looks of it, a decent machine in decent, little used, condition. Which is more than can be said for the control system - it's built-in PC still has a floppy disk drive and the whole lot is probably 25 years old. The current fault on it might be as simple as a dirty connector somewhere, or it might be something on a control board which would probably be uneconomic to replace.

We started kicking around ideas for a refresh of the machine. I believe that it uses DC brushed motors, no plate on them, and I have no idea what's on the back of them. Encoder, tacho feedback, or whatever. I'm trying to get some idea of what to use for the control system and motor sizing so that I can give him a rough budget and estimate of work involved. I have considered leaving the motors and their drivers in place (I believe 10V analogue) but not having any idea what the servo feedback is on these machines and given the reduced choice of control options for analogue, I'm not entirely happy with that. I haven't had the covers off the motor mounts (apparently belt-drive to the feedscrews) to look at mounting arrangements, etc, to get an idea of ease of replacement but that might be easier than grafting a new motion controller on to the existing drive electronics (assuming that that's not where the fault is!)

I'm happy to give him a few options for the control electronics (UC300+PC, dedicated Chinese controller (SZGH?) are likely options) but not sure about motor sizing for a machine like this, at least to be able to consider ease of replacement and costs. Any back-of-the-envelope guesses? I'm currently thinking something like 1.5KW/6Nm absolute servos with matching drivers (absolute as this machine has a mechanical limit switch and I'm not sure how repeatable homing would be). Table size is about 36x12" and the Z axis drives the quill rather than the table.

Spindle motor drives the spindle via a variable-speed gearbox so the control system only needs motor on/off control, not speed. He's happy to stick with that.

Any thoughts or suggestions?