TB6600 connections

robertjenkins wrote:

The AC output of the mains transformer goes to a (what is I presume a) rectifier unit which has a 24V DC output. The negative side of this is connected to the 0V of the BoB and all the other 0V terminals on the original drive board. So I don't see how I can avoid having the 0V of the BOB the same as the 0V of the 24V input.

In that case, all is good, but you might as well yank the BOB and throw it as it is doing nothing there.
Drives have separate inputs for the rest of the drive stuff, BOB should be also there to isolate stuff.

And those drives use a TBxxN109 or similar, it is weaker than TB6600 but it is much better in every way.