Tool fracture probability for 6 weight percent cobalt tungsten carbide tools turning AISI 4340 steel (HB = 235 kg/mm2) at 200 fpm was found to be small at the beginning of a cut or during steady state cutting at feeds up to 0.100 ipr (2.54 mm/rev). Steady state fracture probability was found to be high only when cutting with severe chatter at feeds of 0.100 ipr (2.54 mm/rev) or higher. Tool fracture probabilities were determined by finite element analysis for elastic tool stresses, an intensified maximum tensile stress fracture criterion associated with circular defects [9] and Weibull plot giving percent failed versus fracture stress in uniaxial compression.