Thanks for the responses, I didn't know about the duty cycle, the last machine I worked on that didn't use pneumatics came out of the ark and the solenoid on that machine was on most of the time. I guess you don't know unless you ask.

The problem I think I've got is that the quill seems to move quite a lot, not necessarily in the z plane, but locking the axis stops all movement.

I think I'll finish my conversion and see how much of a problem the movement is.