Mini-mill Modifications
The "Z-drop" problem -- excessive play in the Z-axis drive of the mini-mill -- is a real pain. It is caused by the factory drive method: a rack and pinion with a 3-spoke handle for course adjustment, a worm-wheel/screw gadget which drives the same the rack and pinion for fine adjustment, and a coil-spring to compensate for the weight of the spindle head. This slop in the Z motion can cause the spindle and tool to drop by as much as a few mils -- usually at the worst possible time.
Others have suggested replacing the coil-spring with a counter-weighted cable and pulley system to reduce the torque required to adjust the Z position. I tried this and it was an improvment, but it wasn't enough to do the trick. Eventually, my spindle dropped, the tool dug in, and I shattered one of the drive gears in the spindle head.

So I made two more modifications: I installed the LMS belt-drive kit, and I replaced the Z-axis drive with a lead screw. The photo shows the result.
There was no obvious, handy place to put a knob for adjusting the lead screw manually, so I added a power-feed in the form of a variable speed gear motor and remote control. For the motor, I used a modified electric screwdriver controlled by a basic PWM speed control box somewhat similar to the Minilathe unit although simpler to build. The remote controls consist of a switch to select UP or DOWN, and a momentary push button to turn the motor on.
In order to keep the drill press feature of the original mill/drill, I planned to use a”halfnut”, sort of like a lathe. In the end, I made a nut out of a delrin disk drilled and tapped for the lead screw and mounted so that it could be released easily. I machined a couple of slots on the edge of the delrin disk and made 2 clamps to attach the disk to the upper side of the head with machine screws. By loosening the screws a bit, I can turn the clamps enough to free the disk. Then I can spin the disk out of the way which allows the 3-spoke handle to operate as it did before the modification.
Richard 2006