Use in Spotting Hole Centers
Traditional twist drill bits may tend to wander when started on an unprepared surface. Once a bit wanders off-course it is difficult to bring it back on center. A center drill bit frequently provides a reasonable starting point as it is short and therefore has a reduced tendency to wander when drilling is started.
While the above is a common use of center drill bits, it is a technically incorrect practice and should not be considered for production use. The correct tool to start a traditionally-drilled hole (a hole drilled by a high-speed steel (HSS) twist drill bit) is a spotting drill bit (or a spot drill bit, as they are referenced in the U.S.). The included angle of the spotting drill bit should be the same as, or greater than, the conventional drill bit so that the drill bit will then start without undue stress on the bit's corners, which would cause premature failure of the bit and a loss of hole quality.
Most modern solid-carbide bits should not be used in conjunction with a spot drill bit or a center drill bit, as the solid-carbide bits are specifically designed to start their own hole. Usually, spot drilling will cause premature failure of the solid-carbide bit and a certain loss of hole quality. If it is deemed necessary to chamfer a hole with a spot or center drill bit when a solid-carbide drill bit is used, it is best practice to do so after the hole is drilled.
Center drill bits wander as easily as anything else in hand-held power drills—so for such operations, a center punch is often used to spot the planned hole center prior to drilling a pilot hole. However, a center drill bit works nearly as well as a spotting drill bit for most rigidly-clamped drilling operations, especially in softer metals such as aluminum and its alloys.
The small starting tip has a tendency to break, so it is economical and practical to make the drill bit double-ended.
Read more about this topic: Drill Bit, Metal Drill Bits, Center and Spotting Drill Bits
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