There are numerous types and sizes of bits for drills. The average twist drill is useful for most household purposes since it can cut through both metal and wood. The power bore bits are made in 3/8-inch, 1/2-inch, 5/8-inch, 3/4-inch, 7/8-inch, and 1-inch diameters. The tip on each of these is a sharp point that establishes extreme accuracy in centering. A precision spur cuts the fibers and lends stability to the full-circle cuts. Slightly less popular are auger-type bits like those used using a bit and brace. Since they have a point at the tip, these can be centered more precisely at a point indicated with an awl mark. Auger bits for power drills are designed in larger diameters—usually 1 1/8, 1 1/4, and 1 1/2 inches. These bits have a spur to outline the hole by breaking up the fibers. The cutters then take out the stock without tearing.
The electrician’s bit is heat-treated to hold up to the severe use in boring both soft and hard woods. It is made with a 3/4-inch diameter to allow standard cables to be passed through obstructions, and it has a tapering head so that preliminary marking and cutting can be avoided.
The self-feed bit is for drilling in even bigger diameters— from 1 1/4 inches to 2 9/16 inches. This cuts large holes using minimum power in either hard or soft wood. It looks like a bit with a ripsaw edge but can cut on an overlap, angle, or cut on close centers.
Flat bits are useful for broadening the diameter of a hole larger than the capability of the standard bits. Even with a 1/4-inch shank the flat bit would make a rough boring up to 1 inch, whereas ordinary twist bits are not available for more than 3/4 inch. The flat bit should be used with great care to prevent overheating the motor or splintering the wood. Several sets of bits are generally available.
The twist drill set has several small sizes, 1/16 through ¼-inch. A ¼-inch drill will take on all sizes of twist drills up to and including ¼-inch, but not any larger, since the shank of a twist drill usually has the same diameter as the rest of the bit.
Power bit sets are available in sizes from 1/4 through 1 inch, typically in a vinyl plastic holder that keeps them from knocking against each other’s edges. These cut a clean hole efficiently through wood, having much less friction than a twist drill of the same bore, and therefore, less burning of the wood, binding of the bit, or stalling of the motor.