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AMI4900: Engineering Design

Unit 2: Structured approaches to problem solving and process improvement

Section 2: Six Sigma

Six Sigma

Contents

• 2.2.1 Six Sigma - What is it?
• 2.2.2 Six Sigma - How does it work?
• 2.2.3 Six Sigma - DMAIC
• 2.2.4 Six Sigma - DMADV
• 2.2.5 Six Sigma - DFSS
• 2.2.6 Six Sigma - Who carries out Six Sigma Improvement Projects?
• 2.2.7 Six Sigma - Can it work with other techniques?

2.2.1 Six Sigma - What is it?

Six Sigma is a statistical problem solving technique. It operates in a disciplined manner to eliminate defects with the use of data. It is not a method which can achieve fast results but it does deliver sustained, achievable results when implemented correctly.

The objective of Six Sigma Quality is to reduce process output variation so that on a long term basis. 3.4 defect Parts Per Million (PPM) opportunities (or 3.4 Defects Per Million Opportunities – DPMO) is the target at which Six Sigma aims.

One of the first organisations to successfully implement and operate Six Sigma was Motorola who pioneered the technique in the 1980’s to consistently produce reliable radio handsets. This has proved to extremely cost effective and beneficial for Motorola who are now one of the world’s biggest producers of radio and telephone handsets.

To be truly successful and achieve Six Sigma a defect rate of 3.4 per million is demanded. A defect under Six Sigma is classified as “anything outside of customer specifications”.

A defect under Six Sigma is measured against an average. To really succeed in Six Sigma the drive is to achieve six standard deviations between the mean and the nearest specification limit.

Six Sigma is successful because it focuses on implementing a measurement based strategy. This measurement based strategy aims to improve processes by reducing variation and thus increasing consistency and eliminating defects.

Six Sigma is successful by focusing on improvement projects. These projects do not attempt to improve an entire process but concentrate on specific issues. For example, if a factory producing kettles was producing too many defects the Six Sigma approach would be to examine one particular aspect of the process, such as, flex attachment, and improve that until Six Sigma is achieved. Once Six Sigma has been achieved at one stage of the process a second is then tackled until that also achieves Six Sigma. By attacking and then achieving Six Sigma at different stages of the production process the entire process slowly improves.

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2.2.2 Six Sigma - How does it work?

To calculate Six Sigma it is necessary to establish a process to measure. Once a process has been identified it needs to be measured. The measurement of a process will establish a number of results. When the results have been gained a range of results can be calculated.

Figure 2.2.1

Establishing arange of values

 

The graph above shows how a number of measurements can establish a range of results. The two horizontal lines are the upper and lower limits of acceptability. The space between the upper and lower limits is the acceptable area. The aim of Six Sigma is to reduce the area of acceptability. It does this by employing a number of techniques to reduce the variability of the results.

Statistical analysis is then used to identify the average results and the standard deviations that occur as a result of this.

In statistical terms, sigma is the standard deviation of a set of data. Six Sigma is, therefore, six standard deviations. If a normal distribution pattern is followed then this is the same as two quality failures per billion. However, as already mentioned Six Sigma takes the rate of 3.4 failures per million; this is to reflect practical considerations.

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2.2.3 Six Sigma - DMAIC

There are three Six Sigma techniques. The first approach DMAIC is a tool for improving existing processes which are operating below specified levels. The DMAIC technique provides incremental improvements. There are five stages in the DMAIC technique. They form a logical path to improve quality and remove defects. The Six Sigma DMAIC process:

• Define
• Measure
• Analyze
• Improve
• Control

These five measures are key to the success. They demonstrate where and how work needs to be carried out.

DMAIC is a methodical quality improvement tool.

• D efine the out of tolerance range so that the key factors can be targeted.
• M easure key internal processes. These processes should be those which are critical. To measure non-critical processes would divert attention and resources away from the issue which really needs tackling.
• A nalyse why defects occur. This is similar to non-statistical methods. It can lead to improvements in quality but as part of the DMAIC process it is one tool which, in concert with the others can be a key technique.
• I mprove the process. Following the analysis of why defects occur improvements to the process can be made. It is vital that any improvements are within the tolerances defined in the first step.
• C ontrol the process. After all improvements have been made and the manufacturing process is back on track it is essential that continued measuring takes place. This is so that targets that have been set are maintained.

To be truly successful under six sigma the DMAIC process should be considered as a cyclical process with a continued drive to narrow the tolerance range. By doing this the variability of results will become ever smaller thereby producing a more consistent product.

The diagram below shows how DMAIC can be viewed as a circular cycle.

Figure 2.1.2

DMAIC Circular View

 

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2.2.4 Six Sigma - DMADV

The second Six Sigma technique is the DMADV approach. This technique is an improvement technique which is used during the development of new processes or products. Its aim is to produce Six Sigma results from the beginning. The DMADV technique can also be used to improve processes by more than just incremental values. The Six Sigma DMADV process:

• Define
• Measure
• Analyse
• Design
• Verify

The DMADV process uses design to make profound improvements to yield. Its five steps need to be carried out by personnel who are able to understand the issues and able to correctly evaluate the results presented.

Although DMADV is a process which uses design as a tool it is also a logical quality improvement technique. The five steps have the following requirements:

• D efine the process which is in need of improvement. The improvement is needed so that customer requirements are achieved. It is important to be as exact and precise as possible so that the correct process is improved.
• M easure the process. Once a process for improvement has been defined it must be measured to ascertain its variability.
• A nalyse the variability. Once this has been done it needs to be determined whether or not the variability is acceptable to customers. If the variability is within an acceptable range improvement by design may not be needed. If, however, the range is out of the acceptable range then further action is needed. The analysis of the variability is crucial as it influences the way in which improvement is needed. For example, if a car is found to be dated and in need of updating should it be improved, e.g., VW Golf, or should it be replaced, e.g., Renault 19 to Renault Scenic.
• D esign the changes in the process to reflect customer requirements. The process will need re-designing so that changes provide not only customer happiness but commercial viability. This is an important factor as an improvement needs to be about commercial success as well as meeting customer requirements. This means that over-engineering a process may remove all variability and therefore produce a product that meets all customer requirements but is too expensive to be feasible. It is important to remember there must be a balance between cost and customer requirements.
• V erify changes. After all the changes have taken place and the process has been re-designed it is essential that customers are consulted so that the changes are effective.

DMADV is also a cyclical process. This is an important consideration. It is vital that the process never ends. If this is allowed then any advantages gained by re-design will be lost as customer requirements change.

The diagram below shows the cyclical nature of DMADV.

Figure 2.1.3

Cyclical nature of DMADV

 

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2.2.5 Six Sigma - DFSS

The third Six Sigma technique is DFSS. This is Design For Six Sigma.

DFSS is the ultimate six sigma tool. It aims develop and design new products that can operate at six sigma levels from day one, rather than having to be improved during the manufacturing process.

The key to DFSS is understanding customer requirements. This is done by using various design tools, such as, the "house of quality". This enables products to be designed taking all customer, design and manufacturing issues into account with the aim of producing a product that will meet all requirements from all users.

2.2.6 Six Sigma - Who carries out Six Sigma Improvement Projects?

Six Sigma improvement projects are carried out by Six Sigma Yellow Belts, Six Sigma Green Belts and Six Sigma Black Belts , and are overseen by Six Sigma Master Black Belts .

The award of belts was pioneered at Motorola and reflects the influence of Japanese quality initiatives. The belt obtained is reflective of the experience gained in improvement projects and can also be verified by institutes, such as, the Motorola University .

The grading system illustrates a seniority in an organisation and also an understanding and ability to be successful in Six Sigma.

According to the Six Sigma Academy , Black Belts save companies approximately $230,000 per project and can complete four to six projects per year. General Electric, one of the most successful companies implementing Six Sigma, has estimated benefits on the order of $10 billion during the first five years of implementation. GE first began Six Sigma in 1995 after Motorola and Allied Signal blazed the Six Sigma trail. Since then, thousands of companies around the world have discovered the far reaching benefits of Six Sigma.

2.2.7 Six Sigma - Can it work with other techniques?

Six Sigma is not a stand alone quality improvement tool. It works by taking tools and methodologies from a wide range of disciplines. For example, DFSS uses the 'House of Quality' from QFD and has a strong reliance upon statistics. It is above all a logical technique that is capable of using any appropriate tool to enable a successful outcome.

To get true success from Six Sigma the root problems need identifying and tackling with the most suitable tool available. It is worthwhile spending time looking at Six Sigma and researching what tools from other problem solving techniques can be used with it.

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