A heavy duty facing lathe failed when the tool post caught one of the jaws on the rotating chuck, causing the spline shaft that drives the main spindle to fracture. A detailed analysis of the fracture surfaces (including fractography, metallography, and analytical stress calculations) revealed areas of damage due to rubbing with evidence of cleavage fracture on the unaffected surfaces. The results of stress analysis indicated that repeated reversals of the spindle produced stresses exceeding the fatigue limit of the shaft material. These stresses led to the formation of microcracks in a retaining ring groove that were accelerated to sudden failure when the tool post and chuck collided.