An expansive bit works in a very similar way to a wood boring auger in that the outer edge of the bore hole is cut by a spur, and waste material is cleared by two sharp cutting lips.
Once the bit starts turning, the thread on the guide screw bites into the wood and starts to draw the tool downwards.
The primary spur (the one nearest to the guide screw) contacts the wood first, and the primary lip starts to bore a small hole. Next to engage is the outrigger cutter spur, followed by the cutter lip, which widens the hole to the diameter set by the user.
In essence, the expansive bit’s lips cut two holes. The first acts as a pilot and keeps the bit level. The second takes care of drilling out the actual hole
