This study examines a length deviation problem in steel sheet production on a mini leveller shear line, leading to repeated quality nonconformities and customer complaints. A systematic root-cause analysis was conducted using a fishbone diagram based on the 4M framework to identify the primary sources of process variation. The study indicated that excessive machine vibration, caused by a loose and worn coupling component, was the key factor contributing to length inconsistency. To address this issue, the defective coupling was replaced to improve machine stability. Process capability analysis was then used to assess process performance before and after the improvement. For each condition, 100 data points were collected and analysed. The results show a clear improvement in process capability, with the Cpk value increasing from 0.47 before the corrective action to 1.93 after. This confirms that the process became stable and capable of meeting customer specifications. Overall, the study demonstrates that addressing a specific mechanical root cause, supported by basic quality tools, can effectively improve dimensional accuracy and quality performance in steel sheet levelling processes.
Keywords
Mini leveller shear, Process capability index (Cpk), Fishbone diagram.