A pilot drill is used to start drilling a hole before a larger drill is used to drill a hole of the desired size. A pilot drill is often used before the final hole is drilled in order to ensure the larger drill runs straight and the hole is definitely made in the right place. The presence of the pilot hole guides the larger drill, making it much easier to keep the drill straight. It also prevents the larger drill from slipping on the material to be drilled; this can be a safety hazard and can potentially ruin the project at hand.
In some cases, a pilot drill hole is made before a screw is put into a surface. The purpose of the pilot drill hole is essentially the same as when the hole is made to accommodate a larger drill. It keeps the screw straight and prevents it from slipping and damaging the surface being used. It also requires substantially less pressure and force to insert a screw into a pilot hole than into untouched wood. Depending on the durability of the material into which the screw is being placed, a pilot hole may also be necessary to prevent the material from splitting or cracking.
Unfortunately, many of the difficulties that apply to larger drills also apply to small drills. When making a new hole, for instance, drill bits of all sizes tend to slip or "wander" away from the desired location, particularly if insufficient force is used. Before using a pilot drill, then, many people use a center punch to make an indentation in the surface to be drilled. The center punch forms a small indentation that keeps the drill bit held in place, ensuring the hole is drilled in the desired location and the surface of the material is not damaged by the wandering drill bit.
A pilot drill may be used on a variety of different materials for several different reasons. It is, for example, important to drill a pilot drill hole into wood before inserting a screw. The screw could otherwise act as a wedge and cause the wood to crack and split apart. It is also very important to make a pilot drill hole in metal surfaces, particularly if they are smooth and polished. A drill bit could easily slip on such a surface, and a guide hole prevents that from happening, thereby reducing the risk to the material and to the individual using the drill.