Drill bit

Drill bits are the cutters 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.

Forstner drill

Forstner bits bore precise, flat-bottomed holes in wood, in any orientation with respect to the wood grain. They can cut on the edge of a block of wood, and can cut overlapping holes. Because of the flat bottom to the hole, they are useful for drilling through veneer already fixed, to add an inlay. They require great force to push them into the material, so are normally used in drill presses or lathes rather than in portable drills. They are impractical to use other than in power tools.

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The bit has a centre point which locates the drill for the start of the cut (and incidentally spoils the flat bottom of the bored hole). The cylindrical cutter around the perimeter shears the wood fibres at the edge of the bore, and also guides the bit into the wood precisely. The tool in the image has a total of two cutting edges in this cylinder. Sawtooth Forstner bits are available, with many more cutting edges in the cylinder. These cut faster but produce a less clean hole.

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Forstner bits have radial cutting edges to plane off the material at the bottom of the bored hole. The bit in the image has two radial edges. Other designs may have more.

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Forstner bits have no mechanism to eject chips from the bore, and must be pulled out periodically to clear them.

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Bits are commonly available in sizes from 8 mm to 50 mm diameter. Sawtooth bits are available up to 100 mm diameter.

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