Types of dangerous sparks inside drilling machine that require to stop work immediately
You know the ordinary low level sparks which dance across the commutator? And you know the ordinary sparks from metalworking?   Any sparks other than those are dangerous and a sign it's time to stop using the machine.  

The amount of sparks has everything to do with the health of that particular set of brushes and commutator, and little to do with the make, age, class, etc. of the machine.