The basic idea of a bit is to remove formation ahead of the drill string in order to advance the hole. The cuttings must be small enough to circulate out freely with- out balling or bridging the hole, but not so small as to force the bit to regrind them. Regrinding slows the progress of the bit and accelerates wear. The economy of bit also is a factor. A bit that lasts forever but doesn't make much hole is just as expensive as a bit that drills like a house-afire, but has to be changed before you reach total depth.