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A pilot bit is a small drill bit that is used to drill a guide hole for a larger drill bit. By drilling a hole with a pilot bit, the actual drill bit is able to easily bore a hole in the proper dimension. Often when drilling larger diameter holes into a piece of wood or metal, starting the hole with a large bit creates a tough-to-drill scenario that often requires more time to drill than when using a pilot bit. When the hole is started with a pilot bit, the larger drill bit need only cut the outer edge of the hole instead of bore the entire surface of the hole.
Large drill bits also are prone to creating a rough hole when used to bore a hole unassisted. When the same size hole is bored using a pilot bit, the edge of the finished hole is typically much smoother. Using a smaller bit to drill the initial guide hole permits the use of a hand-held drill to bore the hole to size. Without a guide hole, the large bit will usually grab the edge of the material and violently twist the wrist of the hand drill operator. Drilling a large hole with only a single pass using a hand-operated drill motor is usually avoided, as drilling such holes is typically more successful using a drill press.
Drill bits intended to be used with hole saw assemblies and certain routers often do not emulate a typical drill bit. In this type of application, the pilot bit is often an altered version of a drill bit. Consisting of a short drill bit with very elongated twists, the pilot bit is not intended to be used as a final drill bit. Created only for punching the initial guide hole in the work piece, the finished hole produced by this type of bit is not typically smooth or perfectly round.
In the case of the hole saw, the pilot bit is intended to hold the saw bit square on the work piece. By having a center drill bit plunging through the work piece, the large round saw bit is not as likely to wander off of the mark when cutting a hole. The twists on the shank of the drill bit are also used to softly pull the saw cutter through the work. As the drill bit makes its way deeper and deeper into the work, the saw is coaxed into the work at a speed that will not force the saw and get it stuck.