Capacity Expansion and the Capability–Defect Gap After Automating a Screw-Fastening Station: A Lean Six Sigma DMAIC Case Study in an Automotive Safety-Components Production Line

Abstract

Background: Automotive safety-component suppliers often must add capacity at short notice, and automation is common. This study quantifies the capacity, equipment-effectiveness, process-capability and nonconformity effects of integrating an automated screw-fastening machine (OP8) into an assembly line. Methods: An Action-Research DMAIC study combined 2024 monthly capacity, OEE and nonconformity records with ordered 100-part series of fastening torque and nut height from the original equipment (OP6/OP7) and OP8 for two product variants, analyzed with I-MR charts, resolution-aware Anderson-Darling tests, normal and Weibull capability indices with confidence intervals, and Brown-Forsythe, Hodges-Lehmann, Cliff’s delta and Welch tests. Results: Rated capacity rose from 117 to 153 parts/h (+30.8%), of which 8 parts/h (22% of the gain) is attributable to OP8; OEE remained at 88.0–91.7%. Torque standard deviation on OP8 was 2.3–3.0 times that on OP6 (Brown-Forsythe p < 0.001), whereas nut-height spread was 11–23% lower. Poor screw-fastening reached 0.66% of OP8 output in June, July and September, about 21 times the OP6 rate, although no logged reading was out of specification. Conclusions: Capacity increased, but capability indices computed from machine-logged, apparently screened data neither explained nor predicted the attribute defect rate, and the defect reduction cannot yet be credited to the improvement actions.