A drilling bit tipped with tungsten carbide. A 9% cobalt carbide generally gives the best results, and comparisons are usually referred to bits of this standard. Tests with tungsten carbide bits indicate that efficient drilling is possible only up to a hardness of about 55 Shore; beyond this, wear increases rapidly until, at 62 Shore, the cost becomes prohibitive. Several factors affect the cutting life of the bits, including the grade of carbide used, the rake angle of the cutters, the length of cutting edges, and support of cutters.