Spinners are defined in the rules:

11.2 Limits
Rotational weapons exceeding any TWO of the three limits below must be submitted for review and be preapproved
by the event organiser.
11.2.1 Weight
The spinning element is more than 20% of the robots total weight. (This includes any directly coupled
motor components rotating on the same axis).
11.2.2 Speed
The spinning element spins above 500 RPM.
11.2.3 Size
The spinning element is greater than 24 inches in diameter

I think a chainsaw would probably slip under the 20% and 500rpm rule (if set as such)