I've returned to the pipe cutter idea.
I've opened out the slot from 3.5mm to just over 5mm and it will now hold
neatly and freely rotating, a 16mm diameter x 5 mm ball race.
I know I can grind into the outer race with a Dremmel and thin disc
(freehand) and cutting 1mm spaced teeth is possible as proved by cutting
,freehand so rough, some 1mm spaced teeth around a steel washer. Running a
24 TPI hacksaw tightly around a pot shaft produced a ring of about 19 small
indents, so 1mm is about right for 16. Next a bit of trig to find a whole
number of rotations into 16 mm diameter to give about 1mm spacing. Set the
Dremmel on the compound/cross slide of a basic lathe (no dividing plate),
and ballrace tightened to some part threaded stud held in the chuck.
Then with lathe motor isolated find a set of gear ratios and pulley marks,
to give the required steps, by turning the first motor pulley by hand, and
maybe lightly clamping per step.

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Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/