Dynamo overhaul

[rowan.bradley]

I am about to start overhauling the 6v Lucas dynamo from my A10 Super Rocket. If the commutator looks a mess can I skim it on the lathe to create a nice flat surface? How much copper can be removed? Do I have to do anything with the gaps between the segments before doing this? E.g. scrape them out to be sure there isn't a buildup of carbon dust shorting the segments together? Fill them with some sort of insulation? What sort? What should it be glued in with? Or is it better to use emery cloth or similar instead of a cutting tool?

Thank you - Rowan

[Swarfcut]

 As a rule of thumb, aim to remove as little material from the segments as possible.

 If the dynamo works there's not much wrong and maybe just a polish with a nylon pan scourer or even fine sandpaper is all it needs. Contact cleaner or turps/WD can be used as a lubricant and wash off. Avoid steel wool, as fragments will conduct, emery paper is also considered a no-no.

 Deeper damage requires a skim on the lathe, take very fine cuts with a sharp tool, and only remove material until the surface just cleans up.

 Finally the gaps between the segments must be cleared out or cut back as necessary, a cut down hacksaw blade was the old way, these days a craft knife can do the business.

 With an unknown armature a first electrical test saves a lot of wasted effort...there should be continuity between adjacent segments, continuity between  directly opposite  segments and no connection between any segment and the centre shaft. Look for consistent resistance values in each of the two tests. Plenty on YouTube about testing brush type armatures.

 Swarfy.

Additional. Trev's right... See next post.

[trevinoz]

Swarfy,
As all of the coils on the armature are in series there should be continuity between any one segment and the eleven others.
Rowan,
If you machine the commutator, when you undercut the mica ensure that there is no copper bridging adjacent segments. Sometimes microscopic bits are there and can be extremely difficult to see. Check for loose segments, a common problem.

[Rex]

Skimming the comm really should be a last resort. Even if the comm is visibly worn leave it if it still works.
You need a really sharp lathe tool to skim it, and take off the minimum amount as the copper bars are so shallow.
Be prepared to be bored stiff undercutting the segments by hand, for which a ground-down hacksaw blade is ideal.

[rowan.bradley]

So the mica between the segments should be undercut but not filled with anything? Seems to me like a recipe for getting stuff, possibly conductive, possibly carbon dust stuck in the gaps.
Question 2: should there be no conductivity between any segments except neighbouring and opposite, or should they all show some conductivity because they are all in series?

Thank you - Rowan

[Rex]

Question1. No the gaps aren't filled, but I wouldn't worry too much as electric motors etc have been this way since they were invented, and that was a long time ago..

[JulianS]

Below from Lucas.

[trevinoz]

Rowan,
See my answer above, there should be conductivity between any segment and all of the others.
But, conductivity doesn't mean that the armature is good. If it has been working OK leave it alone. just clean it and maybe give it polish with an abrasive.