Improved Buttress Thread Machining for the Excalibur and Extended Range Guided Munitions

Raytheon Missile Systems Raytheon of Tucson, Arizona, manufacturer of the Excalibur and Extended Range Guided Munitions ERGM have consistently received critical components containing Class III, 7 degree45 degree buttress threads out-of-tolerance from their suppliers. These out-of-tolerance conditions have caused failures of the components at the buttress thread joints. The Rockwell hardness of the 4340 steel Rc53 being used for the components is a major contributor to the out-of-tolerance condition. The National Center for Defense Manufacturing and Machining NCDMM was requested to evaluate and optimize the machining of these Class III buttress threads to eliminate the failures. After reviewing the supplied material and current processes, the NCDMM developed a plan to optimize cutting of both the internal and external threads using advanced technologies and methodologies. After completing numerous detailed designs of experiments DOE, the NCDMM was successful in consistently machining the internal and external buttress threads in the 4340 material. These new machining parameters ensure that the threads meet the required tolerances specified in ANSI B1.9 - 1992.