Undercutting a commutator on an starter armature

[Afordman31]

I have read that you do under cut the starter commutator after turning in a lathe. My friend said that you don't under cut a starter commutator but you do a generator. Maybe someone can help me out. Afordman31

[van Dyck]

Stockel Auto Service and Repair c1969 says to undercut the mica on the generator armature. On the starter armature they say it is not necessary, but if you do, take very little off (1/32", full width).

[tbirdtbird]

you do not need to undercut a starter armature; but yes for a generator. The reason is that a starter does not turn fast enough or long enough to require this. I worked for a carb/starter/gen shop many many yrs ago, and have never undercut any of theirs or my own since. Be careful of turning these down, they require very little machining. Tom Wesenberg doesn't like to turn them at all, and I am sure he will chime in here

[Purdy Swoft]

You don't under cut the mica between the segments on the commutator of a starter. The only reason that a commutator would need to be turned would be if the commutator is out of round. If the commutator is out of round it will cause the brushes to bounce and if it was a generator armature it would cause the ammeter needle to jump. I hear that turning the starter commutator down weakens the starter. I've got an armature lathe in one of my buildings, I never use it.

[walt jones]

As mentioned above since the starter is in operation much less than a generater undercutting the mica in order to prevent brush wear is not as nnecessary. Another factor to consider is the fact that the current in the starter armature during operation causes a high voltage back EMF that can arc between the commutator bars if the insulating mica is removed from in between them.

[Tom Wesenberg]

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I would never undercut a starter armature. If fact I had to fix a starter that someone had undercut and some copper bridged the gap. All I could do was clean the cuts out and smooth the segments. If I was to turn it until there was no undercut left then the segments would have been way too thin. The high spring pressure and copper brushes keep the insulating material worn away as needed.

True, that I don't turn the commutators on starter or generators unless they really need it, which isn't very often. The starter turns slow enough and for only a few seconds at a time, so even if it was a few thousandths off center, it would be no problem. I'll spin the armatures in my lathe and run a smooth file over them if they need it, and then polish them with very fine crocus cloth.

I undercut the generator armatures by hand with an ATLAS hobby saw. These are only a couple dollars at the train store or hobby shop, and do an excellent job. I've seen where the motorized undercut will be too wide or go crooked at times. It's important that generator armatures are concentric so the brushes don't bounce or wear unneccessarily. Also if the segments are polished to a high shine the brushes will last a long time and the generator will make less noise.

[Willie Krash]

I agree, no to starters.
I'm always happy to defer to Tom. Just wanted to mention that after undercutting a generator armature and polishing it, it is ready for service.
If you want to take it a step further get one of the small brake lathe bits that are triangular with a hole in the center. You can mount it to a rod or use it by hand, Run one of the pointed sides down the comm where it has been undercut, do all the bars. It will chamfer the edges of the bars and will maximize brush life.

[Kurt in NJ]

It depends on the type of brushes used on the starter, with hard copper brushes the mica is not undercut (like the A has) some starters have soft brushes that need the armature undercut(mostly odd European)
I turn most all armatures, even if it looks smooth, the first cut of.002 will show how seemingly smooth is not, that a couple of bars are lower, and small low spots next to the mica
Many times I also resolder the wires to the commutator-- many years of abuse and heat cycles can weaken the solder connection, it does take a high wattage iron to do this, I use a 600 watt iron for this, it will bring the heavy copper wires and the bars to soldering temp in seconds, a small iron will take a long time to get the joint to the proper temp increasing the chances of insulation damage