Purpose – To investigate the potential of combining six sigma (SS) and theory of constraints (TOC) to improve production system performance.

Design/methodology/approach – Based on the literature, this paper proposes an integrated TOC/SS framework and applies this framework to an axle manufacturing company to improve its gear-cutting operation.

Findings – The results of the case study indicate that the company benefited tremendously from its emphasis on global improvement guided by the TOC concept. Managers were able to select a continuous improvement (CI) project that had greater impact on bottom line performance. On the other hand, SS provided various statistical tools and engineering techniques (such as value analysis, Pareto diagram and control charts) for defining the specific process to be improved, analyzing the root causes, and designing actions for making improvement.

Research limitations/implications – For the future effort of integrating TOC with SS and other CI techniques, we suggest the incorporation of a TOC methodology, thinking process (TP).

Practical implications – Overall, the integration of SS and TOC provides managers with an excellent platform for two CI techniques to work together and create great synergy.

Originality/value – This study adds to the CI literature and practice by demonstrating the value of a method of combining TOC and SS.