Harbor Freight CNC: Z axis

I'm a couple weeks late posting this - as they say, life comes at you fast.

My original projection for getting the CNC operational was "1 week for X, 2 weeks for Y and 3 weeks for Z, cut a meaningful part by Thanksgiving (week 9)". And I came in right on time. I had the Y axis operational week 2 but took another week to re-work it properly, and after a false start week 5, I got Z operational on week 6.

The setup is pretty simple. A stepper motor is cantilevered over column to drive a leadscrew that is between the machine head and the column. A thrust bearing on the bottom supports the load through the leadscrew, and an adapter holds the leadscrew nut to the head.

First things first, head needs modifying to provide a gap between the column and the head. If you flip the casting over there are three webs that extend across the casting (highlighted in red below). I had a buddy of mine from work who converted his mill to CNC take those down ~ 15mm. This allowed sufficient gap to run the leadscrew down the head once the rack and pinion were removed.

Without the spring lift the head wants to drop (gravity is a harsh mistress) and so I made a print that encompasses the bottom of the head casting and bolted the leadscrew nut to it. In the picture below it is the black plastic part which wraps around the bottom.

Although in practice gravity might not be enough, the head can still try to jump - may add an air spring pushing down to force the head down even against upwards pressure from a translating cut. For now you can see a clamp is holding it in place.

As mentioned the motor is mounted cantilevered off of the column. I printed an adapter 10mm thick with a 10mm extension going into the headstock. This does wiggle a bit here and there, it should be made of metal once I can. The printed part could be improved by having a 30-ish-mm extension going into the column and using several exposed bolt holes to secure in place.

Another view showing the coupler I made on the lathe. It was a 1" diameter bar stock turned down to ~18mm to clear the head when it slides up over it. Several tapped holes with grub screws secure the motor shaft to the leadscrew.

Thrust bearing. The metal part on top of the thrust bearing is made on the lathe, it fits inside the thrust bearing and has a hole to accomodate the leadscrew. It can then spin freely against the thrust bearing sitting atop the hard stop. I put a reasonable amount of force into driving the hard stop against the leadscrew when securing in an attempt to carry most the weight - I don't want to wear out the steppers' bearings. An alternative construction I am considering is putting the thrust bearing up top annd hanging it instead of supporting from the bottom.