A drill bit is a rotary-end cutting-tool having one or more cutting lips, and having one or more helical flutes for the passage of waste matter. Flutes also allow the admission of a cutting fluid when drilling harder materials.
Bits may have a countersink, producing a wider aperture at the very top of the perforation to allow for sinking screw or bolt heads into the material for flush mounting.
Like all cutting tools, better quality (usually more expensive is synonymous) drill bits are longer lasting and cleaner cutting. Choosing drill bits is mainly dependent on the material being drilled. Choice can be made easier if a few basic guidelines are observed:
- Drill point angle : Typically 118° for multi-purpose applications but a smaller angle is preferred for harder materials like steel, where 135° is ideal.
- Flutes : Single flutes are common, whereas dual-flutes allow for cleaner drilling and easier feeding of cutting fluids.
- Speed : As a rule of thumb, the harder the material, the slower the speed chosen. 300 rpm is ideal for most wood, while 30 might be the maximum speed for High-nickel Steel.
- Plastics : Great care must be taken when drilling plastics, taking melting-point into careful consideration.