What is Process Capability ? Process Capability Indices-Cp and Cpk

Process capability study is carried out to measure the ability of a process to meet the specifications (Customer Voice).

SPC– Statistical Process Control is used to measure and control the Process Capability and controlling quality during the production process.

The frequency distribution diagram called Histogram and Control Charts is the basic 7 QC Tools that are used to measure, analyze, and control the Process Variations and Process Capability.

Table of Contents

Process

The process is a set of interrelated activities that convert the inputs such as equipment, materials, people, methods, and environment into desirable output.

The quality of the process is measured by the output of product quality characteristics. If product characteristics/specifications conform to engineering/customer requirements, then the process is considered as stable/capable and performing well as per set standards.

A process is considered stable if there are only Common Causes of Variation present in the process.

Process capability Meaning and Definition

Minimum spread of process variation present in a manufacturing process.

Process capability means how likely a product is going to meet the design specification.

It is the 6 Sigma range of process Inherent variation.

In SPC, Control limits are one of the most basic requirements of a process capability study, So it is very important to ensure the condition of the 4M input while calculating trial control limits.

Process Capability = +/- 3 Sigma = Total spread of 6 Sigma

Need for process capability (PC)

For meeting customer requirements/specifications.
To measure and control the spread of process.
To provide more realistic tolerances for product dimensions.
Improve the process performance capability

Factors influencing PC

Condition of machine and equipment.
Operation type and operational conditions.
Type of Raw materials.
The Skill of Operators and Inspectors.
Measurement method.
Gauge/Instruments condition.

Tools for PC estimation

Histogram.
Control charts.
Analysis of Variances.
Run chart.

Process Capability Indices: Cp and Cpk

Process capability – Cp: This is defined as the tolerance width divided by the total spread of process.

Cp indicates the spread of variation present in a process.

Process Capability Index – Cpk: This is the capability index which accounts for centering of process and is defined as the minimum of Cpk upper and Cpk lower

Cpk is a measure of the process performance capability.

Cpk indicates the shifting of the process from the target value or mean value.