The tool attached to the lower end of the drilipipe; a heavy steel "head" c quipped with various types of cutting or grinding teeth, some are fixed, some turn on bearings. A hole in the bottom of the drill permits the flow of drilling mud being pumped down through the drillpipe to wash the cuttings to the surface and also cool and lubricate the bit.connector between drillpipe and bit. Originally, the drill collar was a means of attaching the drill bit to the drillpipe and to strengthen the lower end of the drill column which is subject to extreme compression. torsion, and bending stresses. Now the drill collar is used to concentrate a heavy mass of metal near the lower end of the drill column.
