Double margin drill bits have an additional margin ground behind the leading margin, providing twice the number of contact points. This second margin performs various functions to improve hole quality. The trailing margin burnishes the cavity, making a true hole and smoother finish, helping to provide a more concentric drilled hole.

These Spotting Drill Bits are stub length and are designed to be extremely rigid so that they can precisely spot a hole for a twist drill. The goal is use the spot drill to make a little dimple in the workpiece that keeps the twist drill from walking so that the hole winds up in the right place.