Quote Originally Posted by Chaz View Post
Hi, thanks for the info. I'm in two minds about the sizing. One part of me wants to get the machine running quickly and without too much fuss, so near bolt on replacements is nice. In terms of limits, assuming they work, is there still a concern?

For E - Stop, depends how you wire it. If you use normal E Stop circuitry via the Acorn, you will likely just disable the drives (like what happens on Thor and CS Labs). You do not switch off anything else.

Not a replacement for Thor, I dont think. The table is not as big as I had hoped for but it will do. The idea was to have something that can deal with big flat steel / hard metal plates for things. With Thor, it was designed for high speed alu type machining. I might make new brakes for Thor (motor mounts) on the Bridgeport. It was too good a deal to pass on and I've always wanted one.

I could buy the replacement drives and run with the DC motors but Ive had DC motor issues before (Spindle on Triac) and not keen on that. I just want something that works reliably.

A lot of the drives being shown are all the same manufacturer, so perhaps just find the best priced ones?
When was the last time you checked all the limits were working on your machines?
With modern AC servos, limits aren't as high priority as they used to be, as if an encoder fails, the drive should realise it's outputting the AC to get the motor to move, but it's not seeing the encoder move, and fault out.
With DC servos, the drives have to rely on the controller realising there is an encoder problem, but often by that point they'll of run into a limit switch, as the drive will of been outputting an ever increasing voltage in the hope the encoder moves, not knowing the motor was still moving at an ever increasing speed.

My concern of using more powerful servos than what was originally fitted, would be potential damage should you do something stupid while using the machine. I know you could always lower peak current, but I'd personally put in the extra effort to mount smaller servos, than cripple bigger servos.

E-stop is always a bit contentious. Personally for CNC machines, I prefer that all drive power is ultimately cut in e-stop. It might not cut immediately (I'd guess the BST drives have a switch mode supply running from the mains, and not the main DC capacitor bank, so would need power to remain on to control a fast stop), but I'd want it to cut as soon as practical. The chances of a drive failing in use are pretty slim, but it's more for during setup, where a wrong setting could cause a runaway. Plus in future, you never know what/who may be changing things they shouldn't.


DC servos do work well, but you need to weigh up the benefits/costs of drives for them. They do need maintenance (clean out, and new brushes occasionally).
If you still had the original servo amps, then it would make economic sense to go with a suitable +/-10V controller, especially as they've already got quadrature encoders fitted, then swap them out for new AC servos as and when needed. However the cost of suitable drives, is likely to be not much less (possibly even more...) than new cheap AC servos and drives, so the economics don't really make much sense for spending money on drives for motors that might not be working correctly.