Comparative Analysis of Stability and Capability of Two Drilling Processes in the Cast Product Using SPC Tools

Abstract This article presents the results of comparative studies of the machining processes of holes B and C, conducted under batch production conditions at a foundry enterprise. The analyzed qualitative characteristic was the hole diameter, which is a key dimensional parameter of the tested casting product. The aim of the study was to evaluate both drilling processes using Statistical Process Control (SPC) tools to identify the superior process in terms of stability over time and quality capability. The methodology included descriptive statistics, histogram and box plot analysis, Bland-Altman agreement testing, normality verification, and Johnson-transformed control charts. Capability indices (Cp, Cpk) and target-oriented indices (Cpm, Cpmk) were calculated using the Clements percentile method to ensure reliable interpretation for non-normal data. The results confirmed that both processes were statistically stable and demonstrated very high capability, with Cp and Cpk values exceeding 2.0, thus indicating a negligible risk of producing out-of-tolerance parts. Nonetheless, important differences were observed: process B showed lower short-term variability, nearly perfect centering within the tolerance field, and stronger capability toward the lower specification limit. In contrast, Process C, although characterized by slightly higher variability, achieved closer alignment with the nominal dimension and more balanced capability across tolerance limits, as reflected in its higher Cpm and Cpmk values. These findings highlight the need to combine stability, capability, and accuracy-to-target analyses to obtain a comprehensive picture of process quality performance, especially under conditions of asymmetric specification limits and non-normal data. From a practical perspective, both processes are stable and capable, suitable for further batch production. However, Process C can be considered generally superior due to its better alignment with the nominal value and greater actual process capability, which translates into a lower risk of producing out-of-spec products.