Abstract

The authors used Six Sigma to improve the process of manufacturing gear boxes for mechanical power transmission at a foundry in India. The goal was to improve product performance by reducing variation in the casting of components, thereby reducing defects. In the define phase, the team created definitions for a defect, a defective unit, a defect opportunity, and the number of defects per million opportunities. The measure phase identified trapped gas in the casting molds, metal shrinkage, and the failure of two streams of metal to properly mix due to cooling as the causes of the majority of the total defects. The analyze phase used root cause analysis and failure mode and effect analysis to identify several process variables, including pattern design and maintenance, worker training, and the proportions of scrap and coal inputted into the molds, that were increasing the frequency of the major defects. The improve phase saw the implementation of an action plan that resulted in only two defects out of 525 opportunities. The control phase involved the implementation of standard operating procedures for all processes involved in the manufacture of the gear boxes, with feedback systems in place to control the processes.