New mill, which motors?

[Robin Hewitt]

Muzzer you jumped in while I was in mid-reply...
I shall start at 36V and if that's not enough then 48V and clamps it is.
When you say clamp is this the infamous crowbar circuit with a fat DIAC hard across the PSU to blow the fuse?
Cool!

[Muzzer]

No, I'm thinking of the "chopping / braking resistor" concept. Acts like a giant zener diode. Usually they have a switching transistor and a low value resistor. Once the voltage exceeds a threshold, the PWM duty cycles increases over a 5-10V band. That's the mechanism used within VFDs and servo drives but they are also available as standalone "braking units".
https://inverterdrive.com/group/Univ...raking-Module/

CNCdrive do one, although I had to repair the one I bought after it broke down across one of the insulators. A lot of the compts we use on our systems are little more than hobby grade - along with the low price you have to expect questionable quality or design and manufacture.
http://www.shop.cncdrive.com/index.php?productID=260

A big f*** off zener diode would do the trick but it's not very sophisticated and choice is rather limited.