Refinement of the Process Capability Index Сalculation

Abstract

The variability of product performance is the reason for the introduction of special methods to ensure product quality, particularly statistical methods. These include introducing statistical process control (SPC) in production and calculating the process capability index to determine the manufacturing ability to meet the product’s quality requirements. To a large extent, the ability of a process to meet the requirements was determined by the location of the process or the mathematical expectation of the controlled quality characteristic value. Process setup center variability within the boundaries of the Shewhart control chart of the average values was supposed to be the natural state for a statistically controlled process. However, the calculation of the process capability index did not consider the possibility of a shift in the actual value of the process setup center for a controlled characteristic from its mathematical expectation. It was proposed to adjust the process capability index for the setup center’s possible deviation. It demonstrated the possibility of critical errors in determining the ability of a production process to meet requirements without considering the process setup center. The effectiveness of the proposed solutions was also demonstrated by the example of determining the ability of the welding wire manufacturing process to meet the requirements for metal yield strength of the welded joint of metal bridge span constructions.

Keywords

• statistical process control
• shewhart control chart
• control limit
• specification limit