Other Definitions
drill bit (dict)
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Drill BitDrill bits are the working end of drill tools. Bits are interchangeable, meaning that they can be removed from the end of the drill, either to replace a worn part or to change the size of the part. Household and machining drill bits The most familiar type of drill bit is that used in handheld and press-style drills. These twist drills usually look like cylinders with a helical path of material missing from them and a sharp edge to cut material. Drill bits have different shaped tips for different materials. The picture on the right shows some drill bits: Other drill bits are essentially the reverse, shaped like Archimedes' screws, with the ability to remove material and work like augers.
Center drill bits Center drills are used in metalworking to provide a starting hole for a larger sized drill, or a suitable inverted cone in the end of a workpiece. Jobber or taper shank twist drills, depending on size, may tend to wander when started on an unprepared surface. Once a drill wanders off-course it is difficult or at the very least time consuming to bring it back on course. A center drill is short and therefore will have a reduced tendency to wander when it initially starts drilling.
The small starting tip has a tendency to break and with the larger body of the drill it is economical and practical to make the drill double ended. A center drill is also employed to make the mating receptacle for a machine center. These centers are used when turning or grinding workpieces. A workpiece machined between centers can be safely removed from one process (eg;- turning) and setup in a later process (perhaps a grinding operation) without losing any concentricity.
Oil drilling bits There are two types of drill bits used in oil or natural gas drilling rigs, a drag bit, and a rock bit: - a drag bit is used for soft rocks, like sand and clay. The drill stem is rotated, and teeth on the bit tear up the rock.
- a rock bit (also called a roller bit) consists of teeth on wheels which turn as the drill stem is rotated. These teeth apply a shearing pressure to the rock, breaking it up into small pieces.
The original patent for the rotary rock bit was issued to Howard Hughes Sr. in 1909. It consisted of two interlocking wheels. The success of this bit lead to the founding of the Sharp-Hughes Tool Company. In 1933 two Hughes engineers invented the tricone bit. This bit has three wheels and is still the dominant bit in the market today. The Hughes patent for the tricone bit lasted until 1951, after which time other companies started making similar bits. However, the Hughess market share is still 40% of the worlds drill bit market.
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