This is Part 2 of a two-part article on process capability. Part 1 addressed the concept of process capability and how to calculate it, including what to do with data that is normal and non-normal. Part 2 looks at failure rates and material conditions for a surface finish example. Sometimes, a process has specifications that…
This is Part 1 of a two-part article on process capability. Part 1 addresses the concept of process capability and how to calculate it, including what to do with data that is normal and non-normal. Part 2 will look at failure rates and material conditions. What is process capability? From a conceptual view it is…
Capability analysis is an essential tool for statistical process control (SPC) and process improvement, applicable to a variety of environments from hospitals and labs to assembly and manufacturing. The formula for calculating a Cpk index is easily found: USL, upper specification limit; LSL, lower specification limit. *Estimated sigma = average…
Overall equipment efficiency (OEE) – the percentage of production time that is actually productive – has its roots in manufacturing where all production ceases when the equipment is offline. It is possible to gain a greater understanding of how effectively a system is operating, however, by expanding the concept to include all process steps in…
The capability of a process, statistically speaking, is the ability of that process – based on historical performance – to achieve measurable results that satisfy established specifications and statistical limits. In a nutshell, process capability indicates how well a process is able to perform its specified purpose. Why is process capability important? When the capability…
Have you tried to explain the concept of process capability to someone who then looks at you like you have two heads? Try using the following analogy to explain the subject more easily. Driving in a Construction Area Imagine you are driving on a highway that is undergoing construction. The width of your vehicle –…
Recently I visited several contract manufacturers (CM) to evaluate their ability to produce an electronic device that my company is developing for the automotive industry. I asked the project manager at each CM to provide me with an estimate of the typical rolled throughput yield (RTY) on their production lines. Only one of the project…
The process capability indices Pp and Cp describe how closely a process can operate within its specification limits. Many articles describe the difference between Pp and Cp simply: one is short term, one is long term. Moving beyond such a description, this article focuses on the untapped power of capability analysis and shows you how…
Z score is a measure of the distance in standard deviations of a sample from the mean. It is calculated as (X – X bar) / sigma. The table of z distribution is shown below. Table of the Standard Normal (z) Distribution z 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.0 0.0000…
In statistical process control, diligent technicians and auditors conduct detailed work to gather data for quality engineers, who then crunch the data into capability studies. Typically, the repeatability and reproducibility (R&R) rates for the data are first determined for the gauges used in the capability studies. The R&R studies are completed first to establish the…
“Speed is survival.” That could be called the mantra for all manufacturing and service organizations in today’s uncertain and continuously changing global economy. In this example, a leading manufacturer of color picture tubes (CPTs) was posed with a survival challenge: to improve manufacturing productivity. By adopting Lean Six Sigma methods, the manufacturer was able to…
In my process engineering experience I have found a good number of people confused about the important process measure indices of capability and performance. The capability index is useful for measuring: Continual improvement using trends over time, or For prioritizing the order in which process will be improved, and For determining whether or not a…
How to Calculate Process Sigma Consider a power company for illustration purposes: A power company measures their performance in uptime of available power to their grid. Here is the five-step process to calculate your process sigma. Step 1: Define Your Opportunities An opportunity is the lowest defect noticeable by a customer. This definition, of course, is…
Process capability (Cp) studies are usually performed whenever: Engineering tolerances are reviewed against the observed variability of the process and/or new equipment is evaluated, The capability of a process to meet customer specifications needs to be determined, or Process changes and/or improvements need to be evaluated. Our rapid pace and highly competitive industrial environments often require…
In the Six Sigma quality methodology, process performance is reported to the organization as a sigma level. The higher the sigma level, the better the process is performing. Another way to report process capability and process performance is through the statistical measurements of Cp, Cpk, Pp, and Ppk. This article will present definitions, interpretations and…
In manufacturing, key quality indices – performance capability index (Cpk), defects per million opportunities (DPMO) and first pass yield (FPY) – are prevalent criteria for gauging the performance of products and processes. These indices however, often are interpreted wrongly and used without taking into account the conditions of application. Moreover, alternative indices such as rolled…
In the beginning of a deployment, many companies set a goal of 3.4 defects per million opportunities (DPMO) using Six Sigma quality concepts in production, and later extend this concept to other operational areas. Fewer companies, however, have extended Six Sigma from a manufacturing application to manage customer satisfaction or customer complaints. But it is…
Measurements affect behavior. Wrong behavior results when metrics are confusing or do not represent what is truly happening. Leaders of many respected companies are paying the price for creating an environment in which measurements did not reflect accurately what was occurring in their organization. In contrast, the wise selection of metrics and their tracking within…
For many years industries have used Cp, Cpk, Pp and Ppk as statistical measures of process quality capability. Some segments in manufacturing have specified minimal requirements for these parameters, even for some of their key documents, such as advanced product quality planning and ISO/TS-16949. Six Sigma, however, suggests a different evaluation of process capability by…
During a Six Sigma project, the defect rate of the business process is calculated twice, first during the Define phase to show the impact of the problem and again during the Control phase to show how the process has improved. Although effective analysis of data that is not distributed normally is possible, completing one of…
The definition for Six Sigma was clear from the beginning – 3.4 defects per million opportunities (DPMO), allowing for a 1.5-sigma process shift. But the definition for zero defects is not so clear. Perhaps zero defects refers to the domain beyond 3.4 DPMO. Or perhaps it refers to designing defects out of the process or product,…
An organization is always successful in its journey towards business excellence if it has specific goals for each of its critical processes. Various quality management programs adopted and implemented by such an organization can streamline the activities towards the ultimate goal of business excellence. When the quality improvement program is Six Sigma, the goal for…
When a Six Sigma project is unsuccessful, a common cause of the failure is a poor project selection process. Compounding the problem of project failure, incorrect and inappropriate projects transmit a negative message to the organization about Six Sigma and can lead to the impression that the methodology is not applicable to all types of…
The principles of Lean manufacturing are applicable to any business process. This article reviews some of the common problems seen in transactional projects and outlines an example where simple graphical methods are used to interpret cycle time data. Identifying and characterizing the non-Lean processes facilitates the application of 5S, brainstorming and other improvement tools with…
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