This research examines the impact of task complexity, experience level, hazard perception, and simulated tool breakage on an overall mental workload rating. In this study, 100 participants were tested after using a desktop virtual reality training system that simulated a Computer Numerical Control (CNC) milling operation. Differences based on hazard perception and retention interval were also considered. Training conditions were varied for the participants so that the contribution of visual cues, auditory cues, and written instructions could be tested. The participants were asked to determine the best machining outcome during the training period. Results were consistent with previous literature and indicated that the overall mental workload was significantly influenced by task complexity and experience level. Results also suggested that hazard perception as new contributor could be added to the mental workload model for an industrial task of this type. Moreover, the impact of simulated tool breakage on performance was investigated, and it was found that performance was significantly affected by the presence of simulated tool breakage. (c) 2010 Wiley Periodicals, Inc.