The last time I did it (MANY moons ago!!) I simply took a hack saw blade and pulled it between the sections.
I never measured the depth, 1/16" is probably a good number.
The function of the groove is to eliminate the conductive path from one section to the next, so not much depth is needed.
I think most of what I was doing with the hacksaw blade was cleaning the existing groove, rather than actual cutting.
Dirt and carbon from the brushes seems to pack into the grooves.
If you are going to turn the commutator, look at it at each end where it was not worn by the brushes.
That "virgin" area will show how deep the undercut was previously.
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