Powering the knee on a knee mill for Z-axis travel

[davidpbest]

I found this video the other day and was rather intrigued by it. The guy in the video cut a hole in the side of his knee and installed a servo motor and cogged belt driving a ball screw. He had replaced the ACME screw with a ground ball screw and nut. Cutting into the knee would concern me only from an aesthetic standpoint and not a structural standpoint because I feel that there is enough cast iron that the hole would not matter.
He (Owen) utilizes gas struts to offset the weight of the knee. Utilizing the knee for the Z-axis movement eliminates all of the clutter associated with a quill Z-axis conversion and also maintains all of the manual controls.
The one downside I see to this is the possible wear on the knee ways, although as a pure hobby machinist, the mill will probably outlast me in that role.

You really have to admire this guys resourcefulness. I winced when I saw the image of the hole in the side of the knee, but I guess with the addition of the gas springs that Z-axis works pretty well. I'd love to see some repeatability tests on tolerances achieved. The air brush mist coolant system is inventive, but it's not MQL, and this the amount of alcohol he's spitting out has no chance to evaporate quickly enough to prevent a fire hazard. As an aside, finding denatured alcohol (my coolant choice as well for aluminum, but delivered in a Fogbuster) has become a real supply challenge - at least here in the PNW. IMO he made the right choice with the Acorn kit, but I think I would have gone with a new Nuc PC instead of that antique, if for no other reason than to eliminate the additional enclosure. But all in all, very impressive. If/when I do my CNC conversion, I'll try to hire this guy as a consultant. Thanks for posting.