Six Sigma Approach to Halve the Cycle Time of a Grinding Process on Carbonitrided Parts : Case analysis and solution executed at SKF-Bari Factory, Italy

Sammanfattning: The main purpose of this thesis work was to halve the cycle time of a grinding process of the groove of carbonitrided ball bearings rings. The study was conducted as a Six Sigma project at SKF in Bari, Italy. Reaching the goal was important for the company as it meant avoiding the purchase of another grinding machine. The groove grinding process on carbonitrided rings features double the cycle time than the same process on through hardened rings and halving it was required to fulfill the volumes demanded. The Six Sigma methodology was applied through the implementation of DMAIC. The analysis of the problems and identification of areas for improvement were carried out deploying different tools such as an Ishikawa diagram, Hypothesis Testing and Statistical Process Control. The outcome of this was the need to optimize the carbonitriding process minimizing the depth of the layer without enlarged carbides in order to render the design of the part easier to machine and subsequently find the process parameters for the groove grinding yielding a cycle time of six seconds. Many trials were conducted with the supplier of carbonitriding, until the carbon and ammonia potentials were set at levels resulting in a depth of the layer allowing the change of the design. An optimizing DOE was instead performed for the grinding process which highlighted the parameters settings yielding the target cycle time. Finally actions and controls to perform in order to maintain the gains were defined.