Drill 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:

• 18 mm diameter for wood
• 10 mm for wood and plastics
• 10 mm for concrete etc
• 10 mm for metals, wood or plastic

Other drill bits are essentially the reverse, shaped like Archimedes' screws, with the ability to remove material and work like augers.

Center drills

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 center, a center drill provides a good starting point as it is short and therefore will have a reduced tendency to wander when drilling is started.
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.

Morse taper shank drills

The morse taper twist drills pictured at left are used in metalworking, the sizes shown range from 13.5mm (with the No. 1 morse taper shank) through to a 30mm drill (No. 4 morse taper shank). The full range of tapers is from 0 to 7 and while the taper is often used on the shank of other tools, it is exclusively used on drills.

The morse taper allows the drill to be mounted directly into the spindle of a drill, lathe tailstock or (with the use of adapters) into the spindle of milling machines. It is a self locking (or self holding) taper of approx 5/8" per foot [1] that allows the torque to be transferred to the drill bit by the friction between the taper shank and socket only. The tang at the end of the taper is only for ejecting the drill bit from the spindle (with the aid of a drift).

The arbor of a drill chuck is often a morse taper and this allows the assembly to be removed (with the aid of a drift) and directly replaced with the shank of a morse taper drill. A range of sleeves may be used to bring the size of the smaller morse tapers up to the size of the drive spindle's larger taper. Sockets are also available, these extend the effective length of the drill as well as offering a variety of taper combinations.

Core drill

A core drill is used to enlarge an existing hole, the existing hole may be the result of a core from a casting or a stamped (punched) hole. The drills are similar in appearance to reamers as they have no cutting point or means of starting a hole, they also have 3 or 4 flutes which enhances the finish of the hole and ensures the drill cuts evenly.

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:

1. 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.
2. 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 Hughes’s market share is still 40% of the worlds drill bit market.

References

Morse taper, center and core drills