Comb, Saw, Wheel, Cog...
…with teeth.
As youāve probably noticed in our list, a lot of our body-related words are used simply because the particular feature of the object resembles the physical form ofāor serves a similar function asāthe body part used to describe it. In this case, the projections of a comb, saw, wheel, or cog loosely resemble human teeth (maybe?), but, to a greater degree, they mimic the actions of the teeth: separating, breaking down, cutting in, and grinding. The size of the teethāand, for saws, wheels, and cogs, the angles at which the teeth are cutāresults in different āseparatingā abilities and efficiencies.
When we say weāre going over something āwith a fine-tooth comb,ā weāre being meticulous in separating the component strands (literal or metaphorical) of whatever weāre looking at.