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As machine tools have become larger, faster, and more automated, the role of the machinist has changed from a primary operator to a system monitor, who is further removed from the machining process. Estimation of system reliability must still consider the human monitor as an integral component, but human failure estimates are harder to make as the operator is further removed from direct system control. To illustrate system reliability analysis for specific machining tasks, the reliahility of both manual and computer-numerical controlled (CNC) lathe operation tasks were evaluated. A task analysis was initially completed, detailing common failure modes in each suhtask.Using the Technique for Human Error Prediction (THERP), human operator reliability was estimated at 0.90 for a 30-element manual task, and 0.95 for an 18-element CNC task. These estimates were considered acceptable for nonproduction work. Further decreases in reliability in the CNC task are expected as the result of operator program changes during machining. THERP was most useful in developing initial estimates of human reliability for the tasks, hut additional studies are recommended in the areas of operator scanning strategies and human reliahility in performance of more complex machining tasks. 1

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