Drill bit - Definition

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 bits 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.

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.