Hardware and Tools - Printing in Plastic: Build Your Own 3D Printer - page 21

The second type of hole is a counterbore hole. A counterbore hole will use a larger bit (more on
what kinds of bits to use in a moment) to drill a hole into the plywood a certain depth without going all
the way through the plywood. You can see an example of a counterbore hole in Figure 2-13.
Figure 2-13. A counterbore hole does not go all the way through the plywood.
A counterbore hole is useful when you wish to bolt two pieces together without the head of the bolt
sticking up. Instead, the bolt head will dip below the surface of the work piece (also known as flush to
the surface ), providing a smooth flat surface, as seen in Figure 2-14.