After extended use, the comm can be grooved by the carbon & arcing. I'd
check it for continuity with battery & bulb between each segment & then
earth. You should get continuity between each segment. If in doubt, use a
meter - they should of course all read about the same. If one is lossy,
check where the wire is crimped or soldered into the ends of the comm
Heresy, I know, but I've always filed mine back to round with a decent,
chalk-loaded, safe edged file. Place the armature on the partially open vice
jaws & hold it with one hand if light. If massive it'll sit there obligingly
while you get on with the filing. The trick is to move the file in an arc to
trace the diameter of the commutator. Keep rotating the armature, taking ONE
cut at the comm at each turn.
Personally, I'd not put an armature in a lathe & turn the comm because of
the danger of flicking a segment out - and then you are knackered just like
the comm! It will take you a while, but careful hand work is the key to so
many difficult operations. Clean & rechalk the file frequently.
As to the armature windings, remember that if its old it is probably painted
with shellac (murdered beetle wing cases!) which is dissolved in alcohol, so
you won't want to clean it with meths, you see ...
If it was me, I'd dry brush & Hoover as much off as I could, but if its oily
crap, I'd wash it in petrol & at once pat it dry with a cloth. Immediately,
I'd place it in a warm place to drive off fumes. Leave it there for a couple
of days to ensure dryness, then paint the exposed insulated wire with a
decent oil paint - I like to use red or orange personally, but clear varnish
will do just fine.
Finally, you'll need to undercut the mica between the segments & this can be
achieved with a broken all-hard hacksaw blade, suitably ground to shape.
Larger comms may need a power hacksaw blade.
One tip - don't use emery paper on a comm, it gets stuck in the copper &
gnaws the brushes. 400 grit+ wet & dry used wet will be ok but make sure you
wipe it dry straightaway.
Repeat as necessary with the field coils.
J. Kim Siddorn,