The whole knee z axis is a pretty dumb argument.
No, it isn't. I know I've learned a few things from reading about peoples experiences on this topic. I will bet I'm not the only one, Ed included.
i replaced the screw with a ballscrew and a custom pedestal. I replaced the original straight cut bevel gears with hardened spiral cut ones. BTW, i've got 500lbs worth of gas springs under the knee.
So basically you've made some significant modifications that will allow the knee to be used as a Z-axis dependably.
When I first read Ed's post, I agreed 100%. I have never seen a knee used in place of a quill feed. Of all the mills in the small shop I work, I couldn't imagine using the knee as a Z axis. Not in the worn out abused shape that they are in, anyhow. It just seems that using the quill would be the logical choice. I'm glad to learn that using the knee is indeed an option. With the right combination of mods, it CAN be done, and achieve results just as good or better than a quill. With the right combination of mods, pieces of metal can fly to the moon, and return to earth, too.
I have full confidence Ed was talking about a standard mill running on a cheap screw on a brass nut. (Ever seen a brass nut fail? It's not pretty!) If given the task of converting our J-Heads to CNC, would I choose the knee as the Z instead of the quill? No, I wouldn't. I suppose it's just my personal preference. I would feel more at ease driving the quill, instead of diving into unknown territory. To be done reliably, it seems there is quiet a bit of work involved, and most likely more costly. If given the task to convert our J-Heads to CNC and use the knee as a Z-axis....Could I do it? Oh heck yeah, I know that now for a fact! So case in point...It's not a dumb argument.
In no way should anybody think that Ed has a screw loose, or is inexperienced in any way from reading this topic. Those of us that have been here for a while know for a fact Ed is a very capable machinist, and could teach us all a thing or two. He does some super good work, and is a valuable part of the forums. He's a forum "regular" and helps when he can, so "Thank you, Ed"