generator / starter rebuilding - commutator??

[eschneider]

I haven't got a straight answer on this one from any source....

On lucas starters and generators, I thought when you turned the commutator with a lathe, one was supposed to back-cut the insulation between the commutator bars. I've recently seen a couple of posts (elsewhere) emphatically claiming that this will damage the commutator.

Either way, I'd like to know "WHY"? Intuitively, it seems like any crevice between the bars would collect conductive particles and short out the commutator bars.

No specific application - I was just doing some reading.....

 

[martx-5]

Undercutting an armature commutator was always part of the job after truing the comm on a lathe. The reason it was done, was because <span style="font-weight: bold">mica </span>was used between the comm segments, and very often it was harder then the copper used in the rest of the comm. There would then be mica high spots which could cause brush bounce. The problem was more prevelant in generators then starters with their hard carbon brushes.

All of that is pretty much a thing of the past, as mica is not used in automotive applications much anymore. The rebuilder I work for doesn't undercut any armatures anymore, unless it's from a very old application and the the man doing the job deems it neccessary...but that rarely happens.

BTW, if done properly, it will not damage the commutator. There are special undercutting machines that will do the job and we have a couple of them at work, but like I said, they may be used once or twice a month...and we pump out 3,000 units a day!

 

[eschneider]

Thanks! Great explanation!