(2) Undercutting  mica.    After  resurfacing  the commutator,  undercut  the  mica  to  a  depth  of  0.025  to 0.032 inch below the commutator surface using a power- driven  undercutting  tool  (fig.    4-10).    If  a  power-driven tool is not available, the mica may be undercut  by  hand as shown in figure 4-11. Note.    Use  care  in  undercutting.    Do not   widen   the   commutator   slots   by removing   metal   from   the   segments, and do not leave a thin  edge  of  mica next   to   the   segments.      Figure   4-11 illustrates   good    examples    of    good and bad undercutting. Figure 4-10.  Undercutting mica using a power-driven tool. Figure 4-11.  Undercutting mica using an alternate hand method. (3) Polishing  the  commutator.    After  the  mica has been undercut, remove all copper and mica particles with  compressed  air.    Polish  the  cornmutator  in  a  lathe with No.  2 / 0 sandpaper while the armature is rotating at a speed of 1500 rpm. b. Repair of the Remaining Parts. 4-7