The way it works is this, Insde there is a brass barrel with three holes drilled into the side of it, three 7mm ball bearings go into these holes
,the end of the brass barrel has a 2mm hole in it, that has a piece of 2mm drill as the "fine" spindle,it has a recessed area that the bearings
run on, the O/D of the bearings run in a hardened tapered steel barrel, The bearings are "loaded" by a brass nut that runs on 1.5mm ball bearings.
The coarse knob is fitted to the brass barrel, and the fine knob is fitted to the 2mm spindle.
This web page should give a better understanding. http://helixgate.net/tmb1.html
I found the most difficult piece to make was the recess in the 2mm spindle, as it has to be perfectly true and round, or it will feel lumpy
in use. Bigger bearings or smaller spindle would give a finer ratio.
While im on the subject, Ive found a great way to make the knurlled knobs, If anyone wants details, let me know!