74 eldo starter motor

[Viator Trudeau]

When is it required to turn and under cut armature when rebuilding starter.

V. Trudeau

[The Tassie Devil(le)]

You mean the Commutator don't you?

The machining is required if the surface is not flat and true.

The replacement Brushes require a complete flat area to run on.

Bruce.

[TJ Hopland]

Usually a commutator is just 'dressed' when new brushes are installed.  This is done with a stone and or emery cloth.  Its kinda like putting a new set of rings in an engine.  A quick run with a hone will smooth out minor irregularities and then leave a slightly rough surface for the new rings or brushes in this case to seat on.   The tech will also clean out the gaps between the segments.  The bits of copper and carbon tend to short the segments together which hurts the efficiency of the motor.   

Undercutting is like having to machine a cylinder.  They will actually chuck it up in a lathe and machine down the commutator.  That is only done when there is a lot of wear and you have to know how thick the material is so you don't cut through.