[Cherryville Chuck]

It may be what Nick said but a little die grinder of mine quit recently after doing something similar and it looked like a chunk had broken off one brush and caused the other to get chewed up in the aftermath. The speed control may also be faulty but I’ll have to replace the brushes first to find out.

 

[Cherryville Chuck]

If that's true Bj then up cut spiral bits would probably be your best choice. If I can I like to make a small groove with my table saw and then finish with the router. The saw groove makes a big difference as it gives some of the chips an escape path and it lets air get to the bit which keeps it much cooler.