Counterboring is a machining operation that will enlarge an existing hole's diameter to a specified depth.
Traditionally, counterboring is done with either a special counterboring tool that has a pilot, or it is done with an end mill.
Eight more operations – including drilling, boring, counterboring, countersinking and tapping holes at various angles – remove an additional 60 pounds, resulting in a finished component that weighs half as much as the raw stock.
Mortises grooves and channels and counterbores and recesses cleanly.
This gets more complicated with different kinds of holes, such as countersunk, tapped, and counterbored.