Soooo, I ended up with 2 Bridgeport Series 1's one is actually a a clone called Exacto? Which is actually pretty nice, from the 70's I guess, good shape with an integrated oiler. That oiler made it the future doner for a CNC conversion. X/Y is easy, I already have a ball-screw servo kit for that.

But the Z axis is a little more of a question. I like to leave the free quill, so the knee it is. Looked at a few kits online and they never appear to swap the screw. The Acme screw that's with the machine is actually pretty nice, the handle and gear mesh feel nice, few thou of play, but overall pretty good.

I can't really find a reason NOT to do the knee, leave the existing screw, get a massive NEMA stepper, reduce it a little with a HTD belt and be done. Make sure the program understands the backlash and go for it.

Most parts are 2.5D, and some kooky thread milling thing I'm going to have to do with a 4th axis, but I think I can do that with plenty of space to get the knee lash out before the cutter engages. It will have to be done in a single pass, but fairly straightforward. (See my other post for that one)

I already own the mill, got a spare kit for x/y, just need to do the Z and get an Acorn and move on.

So is there any real reason this is a bad idea?

Thanks
Aaron