Process Capability: Process capability Study, index and Analysis

What is process capability? || What is process capability study? || What process capability study can || Why should I know the capability of processes? || Process variation || Process Capability Index || General Process Capability Procedure

What is process capability?

Process capability Measure of how capable the process is to meet customer requirements; compares process limits to tolerance limits.

It can be appearances at short term and long term performance of a process with regard to customer satisfactions. It is looks long term performance level of the progression after it had been conveyed under statistical control. In short, process capability is the range over which the natural variation of the process occurs as determined by the method of natural variation, is also the capable of the arrangement of manpower, machine, method, material and measurements to produce a product that will constantly meet the customer probability and design requirement. The purpose of process capability to carryout process potential / capability studies to determine whether the process, given its natural variation, is capable of meeting established customer specification and expectations.

It is measures how well manufacturing processes are currently performing with respect to the product specifications, it indicates process capability is characteristic reproducibility of process' outcomes. The capability is often considered as the part of the production which will be within the product specification tolerance, it can be measured in the frequency of the defect produced in:

- Percentage (%)
- Parts per million (ppm)
- Parts per billion (ppb)

What is process capability study?

Process capability study is a systematic procedure that uses graphic techniques to detect and reduce the unnatural causes of variation until an accurate of statistical control is reached. Once study is completed, you will identify the natural variability of the process. The study provides a model to realize how the process is operating relative to the specification. Process capability study provide metrics for quantifying that variability through display at how variability is affecting the processes, and study also provide data concerning what the process could do under best circumstances, and thus gives a performance target to eliminate.

What process capability study can:

Why should I know the capability of processes?

Process variation

Process variation is the unavoidable differences among specific measurements or things produced by processes. Sources of variation can be in different - different forms like:
(1) Unit - positional variation
(2) Unit to Unit variation
(3) Production lines to Production line variations
(4) Time - time Variation
(5) Measurement errors like repeatability & reproducibility etc.
There are two type of variation - In the processes, natural variation due to the cumulative effect of many minor unavoidable causes and a process operating with only chance cause of variation present is said to be in statistical control. The variations occurred due to may improperly adjustment of machine, defective raw materials or machine / equipment operator error, variation are occurred which is called assignable cause of variation. See example, Process variation in Processes:

Process variation template | Process variation Format | SPC pdf | PDF | Excel | Example | Sample

Process relation with Specifications

Process Capability Index

Cp & Cpk are statistical process indexes, these indexes display if the process is capable of producing features within the acceptable tolerances. The indexes i.e. Statistical process control(SPC) is the tools of statistical methods or techniques such as control chart to explore a process results and to take appropriate actions to attain and sustain a state of statistical control and to improve the process capability. To manage process capability index needs measures of process capability, there are four most common and timed tested measures of process capability are:

Process Capability Index Template | Process capability index format | PDF | Excel | Example | Sample

Index Usage Equation
Cp The Cp index assesses whether the natural tolerance (6 σ). Process Capability for two-sided specification limit, irrespective of process center. C = UCL - LCL / 6 σ or
C = Engineering Tolerance/Natural Tolerance
Cpu Process Capability relative to upper specification limit. (USL - X-Bar) / 3 σ
cpu | PDF | Excel | Example | Sample
Cpl Process Capability relative to lower specification limit. (X-Bar - LSL) / 3 σ
cpl | PDF | Excel | Example | Sample
Cpk Process Capability for two - sided specification limit accounting for process centering. CPK | PDF | Excel | Example | Sample

General Process capability procedure


A process capability study is the end results of the analysis done on process outcomes results, a process capability study reflects what our customer sees, and its indicates what our short terms and long terms situation. Generally, process capability is accompanied when the prior to taking delivery of new process equipment. Before approving newly installed process machinery and equipment for real production use, as production begins, to establish capability of equipment, tooling, material, operator combination. For the process capability study, needs to train engineers and supervisor measurements, study guidelines, variability of the processes so that the performance potential can be detected under normal in control condition.

First Published On Date: Friday,April 14, 2017 03:30 PM

Author: ARUN RAVAL | Business & Systems Analyst