Boyan,
The spring in the upper right is the one under the micrometer and is the key to pressure adjustment for engraving uneven surfaces.
The friend who sold me the mill has seen this arrangement and plans to use something similar to run a spindle at slower speeds with reduction driven by his chinese spindle.

I'll make a video once I get the equipment and post it here to show what can be achieved with a setup like this,

- Nick