Process Capability

Measuring Your Process Capability
Planning, Design & Analysis

Process capability is the long-term performance level of the process after it has been brought under statistical control. It is the ability of the combination of your 5 M's to produce a product that will consistently meet the design requirements and the customer expectation. In this article, Dr. Nosh Kapadia explains: How tocalculate Cp, Cpk, Cpl and Cpu indices, Why should You know the Capability of Your and your supplier's Processes, The difference between Capability indices and Process performance indices, The relation between Process capability and defect rate and Some assumptions and precautions while calculating process capability indices.

Preamble: This article is devoted to the topic of process capability, withthe objective of making people aware of this subject and its significance to business success. The author believes that personal awareness is a prerequisite to personal action, and personal action is what we need for success. • • • It can be a source material for you to use in discussing this topic with your organization. It will address issues like what is process capability, how to measure it,and how to calculate the process capability indices (Cp, Cpk). It will also attempt to explain the differences between process capability and process performance; relationship between Cpk and non-conforming (defect) rate; and illustrate the four outcomes of comparing natural process variability with customer specifications. Lastly a commentary is provided on precautions we should take whileconducting process capability studies.

•

What is Process Capability? 1. Process capability is the long-term performance level of the process after it has been brought under statistical control. In other words, process capability is the range over which the natural variation of the process occurs as determined by the system of common causes. 2. Process capability is also the ability of thecombination of people, machine, methods, material, and measurements to produce a product that will consistently meet the design requirements or customer expectation. What is a Process Capability Study? Process capability study is a scientific and a systematic procedure that uses control charts to detect and eliminate the unnatural causes of variation until a state of statistical control is reached. Whenthe study is completed, you will identify the natural variability of the process.
http://www.symphonytech.com 1 © Symphony Technologies

Measuring Your Process Capability
Planning, Design & Analysis

Why Should I know the Capability of My Processes? • • Process capability measurements allow us to summarize process capability in terms of meaningful percentages and metrics. To predict theextent to which the process will be able to hold tolerance or customer requirements. Based on the law of probability, you can compute how often the process will meet the specification or the expectation of your customer. You may learn that bringing your process under statistical control requires fundamental changes - even redesigning and implementing a new process that eliminates the sources ofvariability now at work. It helps you choose from among competing processes, the most appropriate one for meeting customers' expectation. Knowing the capability of your processes, you can specify better the quality performance requirements for new machines, parts and processes.

•

• •

Why Should I know the Capability of My Supplier's Processes ? 1. To set realistic cost effective partspecifications based upon the customer's needs and the costs associated by the supplier at meeting those needs. 2. To understand hidden supplier costs. Suppliers may not know or hide their natural capability limits in an effort to keep business. This could mean that unnecessary costs could occur such as sorting to actually meet customer needs. 3. To be pro-active. For example, a Cpk estimation made using...