Waste Reduction of Press Load Testing in Steering Shaft Spline Production Process

Abstract

The automotive parts industry has continued to grow steadily over the past several years. Quality control of each component is of utmost importance, as it directly affects driver safety. The case study company, a manufacturer of steering column components, has recently faced quality-related defects during product testing. To address this issue, the research team applied various 7 QC Tools to identify root causes and develop corrective measures. The investigation began with the use of a Pareto chart to compare monthly production defect data, which revealed that the most defect was press load testing, where values frequently exceeded the customer-specified limit of 30 kN. A fishbone diagram (Ishikawa) based on the 4M principle was then used to analyze potential causes. The findings indicated that the main problems were the lack of machine adjustments and the wearing of the jig. As an initial corrective measure, the team installed shim plates (support fixtures) on both sides of the machine to stabilize the setup. Subsequent process monitoring using scatter diagrams and control charts showed that the press load values improved and remained within the customer’s specified range. Additionally, calculations of the process performance indices (P_p and P_pk) after the improvement showed better values, indicating reduced variation in press load measurements. Overall, the improvement reduced the defect rate related to excessive press load from 0.67% to an average of just 0.03% per month, representing a 95.52% reduction. The associated defect cost dropped significantly, from 7,635 THB to just 123 THB per month. Going forward, it is recommended to establish a systematic jig maintenance and machine calibration plan to prevent the recurrence of similar issues and to address other high-cost defects in subsequent phases.