A drill collar is a drill string component that is used to provide weight on the bit for drilling purposes. The weight of the collar helps stabilize the drill and keeps the drill in tension, thereby increasing the cutting efficiency of the bit. Drill collars are usually constructed from thick-walled carbon steel, but can also consist of nonmagnetic copper, steel or other metal alloys.
The external diameter of the collar can either be smooth or machined with helical-shaped grooves, depending on the properties of the formation being drilled into. The collars also typically have threaded connections that are male on one end and female at the other end. Smooth collars are generally used under normal circumstances, while spiral collars are used in situations where the collar may stick to the formation. As the drill string extends down the borehole, the diameter of the drill collar increases in a tapered manner to maintain overall stability.
The large mass of the drill collar applies a downward force on the drill bit which assists in cutting tough materials such as rock.