Integrating Measurement-System Analysis and Statistical Process Control to Assess Capability Improvement of Thread Pitch Diameter in a CNC-Turned Stainless-Steel Fitting

Process-capability claims are only as reliable as the measurement system behind them, yet small machining enterprises rarely evaluate their gauges before reporting capability indices. This case study integrates statistical process control (SPC), capability analysis, Gage R&R and metrological verification for the thread pitch diameter of a UNS S30300 stainless-steel fitting CNC-turned in a Mexican micro-enterprise and measured with a screw-thread micrometer with 60° cone and V-groove anvils. Pareto analysis attributed 57 % of 175 scrap parts to pitch-diameter deviation. After insert-grade reselection, a standardized finishing technique and instrument verification, the within-subgroup Cpk rose from 0.18 (95 % CI 0.02–0.35) to 1.40 (1.05–1.75) and the overall Ppk from 0.10 to 1.33; baseline indices are only indicative because the baseline chart showed out-of-control signals. Variability decreased significantly at both the individual and the subgroup level, whereas the mean shift was not significant at the subgroup level (p = 0.076). Crossed ANOVA Gage R&R studies (10 parts × three appraisers × two trials) showed that GRR fell from 89 % to 46 % of the tolerance and the part × appraiser interaction disappeared, but the system remained unacceptable (ndc < 2). Capability indices are reported as short-term, measurement-inclusive estimates; a measurement-corrected estimate is not yet reliably quantifiable with this gauge.