[this log entry copied from #Minamil where it was created 9 Aug 2021]
When HaD editors gave #CDCNC , the one-off precursor to this project, its 15 minutes of fame, they commended the Z-horizontal layout which avoided lifting the heavy tool with a tiny motor & gave chip clearance for free. So why abandon those advantages?
It wasn’t the plan. The new axes were going to be about the same size as in CDCNC, so I was just going to swap new parts into the old frame. What could go wrong?
This design uses 0.5mm pitch screws, instead of 3mm or longer pitch used in optical sleds, to get more linear force from the same little 15mm motors that just barely make CDCNC go. The finer pitch screws are also self-locking, not backdrivable, so the machine will keep position without constant power to the motors -- another benefit, right?
The ‘what’ that could go wrong is that self-locking leadscrews with little rotational inertia will chatter hard when loaded with force in the direction of motion. So while these screws can drive a fair load upward, opposed by gravity, they’re helpless to let even an empty slide come back down, assisted by gravity, with any useful speed. Counterintuitive, if you didn’t already know this. I don’t recall any mention of it in the tiny slice of CNC lore I’ve read.
So axes of this design won't be moving anything vertically without adding stuff to neutralize gravity. In which case it makes sense to turn the heavy, slow, and more often stationary axis back up to vertical.
That adds complexity and brings back the chip-clearing problem. But in trade we get a much smaller footprint.
 from Dr. Orang Vahid Araghi's thesis, page 187 first paragraph:
The first condition states that the lead screw must be self-locking. The second condition requires that the force applied to the nut be in same direction as the nut translation. The third condition defines a [...] limiting value for the mass of the translating part (depending on the lead screw inertia, coefficient of friction, and geometry of lead screw), below which instability does not occur.