QUALITY PROGRAMS IN MANUFACTURING
QUALITY PROGRAMS IN MANUFACTURING - Statistical process control is widely used for monitoring the quality of manufactured parts and products.
Several additional quality programs are also used in industry, and in this section we briefly describe four of them: (1) total quality management, (2) Six Sigma, (3) Taguchi methods, and (4) ISO 9000.
These programs are not alternatives to statistical process control; in fact, the tools used in SPC are included within the methodologies of total quality management and Six Sigma.
A. TOTAL QUALITY MANAGEMENT
Total quality management (TQM) is a management approach to quality that pursues three main goals: (1) achieving customer satisfaction, (2) encouraging the involvement of the entire workforce, and (3) continuous improvement.
The customer and customer satisfaction are a central focus of TQM, and products are designed and manufactured with this focus in mind. The product must be designed with the features that customers want, and it must be manufactured free of deficiencies.
Within the scope of customer satisfaction is the recognition that there are two categories of customers: (1) external customers and (2) internal customers. External customers are those who purchase the company’s products and services.
Internal customers are inside the company, such as the company’s final assembly department which is the customer of the parts production departments. For the total organization to be effective and efficient, satisfaction must be achieved in both categories of customers.
InTQM, workerinvolvementin the quality efforts of the organization extends from the top executives through all levels beneath.
There is recognition of the important influence that product design has on product quality and how decisionsmade during design affect the quality that can be achieved in manufacturing. In addition, production workers are made responsible for the quality of their own output, rather than rely on inspectors to uncover defects after the parts are already produced.
TQM training, including use of the tools of statistical process control,is provided to all workers.The pursuit of high qualityis embraced by everymember of the organization.
The third goal of TQM is continuous improvement; that is, adopting the attitude that it is always possible to make something better, whether it is a product or a process.
Continuous improvement in an organization is generally implemented using worker teams that have been organized to solve specific problems that are identified in production. The problems are not limited to qualityissues.
They may include productivity, cost, safety, or any other area of interest to the organization. Team members are selected on the basis of their knowledge and expertise in the problem area.
They are drawn from various departments and serve part-time on the team, meeting several times per month until they are able to make recommendations and/or solve the problem. Then the team is disbanded.
B. SIX SIGMA
The Six Sigma quality program originated and was first used at Motorola Corporation in the 1980s. It has been adopted by many other companies in the United States and was briefly discussed in Section 1.5 as one of the trends in manufacturing.
Six Sigma is quite similar to total quality management in its emphasis on management involvement, worker teams to solve specific problems, and the use of SPC tools such as control charts.
The major difference between Six Sigma and TQM is that Six Sigma establishes measurable targets for quality based on the number of standard deviations (sigma s) away from the mean in the Normal distribution. Six sigma implies near perfection in the process in the normal distribution.
A process operating at the 6s level in a Six Sigma program produces no more than 3.4 defects per million, where a defect is anything that might result in lack of customer satisfaction. As in TQM, worker teams participate in problem-solving projects.
A project requires the Six Sigma team to (1) define the problem, (2) measure the process and assess current performance, (3) analyze the process, (4) recommend improvements, and (5) develop a control plan to implement and sustain the improvements.
The responsibility of management in Six Sigma is to identify important problems in their operations and sponsor the teams to address those problems.
1. Statistical Basis of Six Sigma
An underlying assumption in Six Sigma is that the defects in any process can be measured and quantified. Once quantified, the causes of the defects can be identified, and improvements can be made to eliminate or reduce the defects.
The effects of any improvements can be assessed using the same measurements in a before-andafter comparison. The comparison is often summarized as a sigma level; for example, the process is now operating at the 4.8-sigma level whereas before it was only operating at the 2.6-sigma level.
The relationship between sigma level and defects per million (DPM) is listed in Table 42.3 for a Six Sigma program. We see that the DPM was previously at 135,666 defects per 1,000,000 in our example, whereas it has now been reduced to 108 DPM.
A traditional measure for good process quality is 3s (three sigma level). It implies that the process is stable and in statistical control, and the variable representing the output of the process is normally distributed.
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Under these conditions, 99.73% of the output will be within the 3s range, and 0.27% or 2700 parts per million will lie outside these limits (0.135% or 1350 parts per million beyond the upper limit and the same number beyond the lower limit).
But wait a minute, if we look up 3.0 sigma in Table 42.3, we find that there are 66,807 defects per million. Why is there a difference between the standard normal distribution value (2700 DPM and the value given in Table 42.3 (66,807 DPM)? There are two reasons for this discrepancy.
First, the values in Table 42.3 refer to only one tail of the distribution, so that an appropriate comparison with the standard normal tables would only use one tail of the distribution (1350 DPM).
Second, and much more significant, is that when Motorola devised the Six Sigma program, they considered the operation of processes over long periods of time, and processes over long periods tend to experience shifts from their original process means.
To compensate for these shifts, Motorola decided to adjust the standard normal values by 1.5s. To summarize, Table 42.3 includes only one tail of the normal distribution, and it shifts the distribution by 1.5 sigma relative to the standard normal distribution.
2. Measuring the Sigma Level
In a Six Sigma project, the performance level of the process of interest is reduced to a sigma level. This is done at two points during the project: (1) after measurements have been taken of the process as it is currently operating and (2) after process improvements have been made to assess the effect of the improvements.
This provides a before-and-after comparison. High sigma values represent good performance; low sigma values mean poor performance. To find the sigma level, the number of defects per million must first be determined.
There are three measures of defects per million used in Six Sigma. The first and most important is the defects per million opportunities (DPMO), which considers that there may be more than one type of defect that can occur in each unit (product or service).
More complex products are likely to have more opportunities for defects, while simple products have fewer opportunities. Thus, DPMO accounts for the complexity of the product and allows entirely different kinds of products or services to be compared.
where Nd = total number of defects found, Nu = number of units in the population of interest, and No = number of opportunities for a defect per unit. The constant 1,000,000 converts the ratio into defects per million.
Other measures besides DPMO are defects per million (DPM), which measures all of the defects found in the population, and defective units per million (DUPM), which counts the number of defective units in the population and recognizes that there may be more than one type of defect in any defective unit.
C. TAGUCHI METHODS
Genichi Taguchi has had an important influence on the development of quality engineering, especially in the design area—both product design and process design. In this section we review two of the Taguchi methods: (1) the loss function and (2) robust design.
More complete coverage can be found among our references , . The Loss Function Taguchi defines quality as ‘‘the loss a product costs society from the time the product is released for shipment’’ .
Loss includes costs to operate, failure to function, maintenance and repair costs, customer dissatisfaction, injuries caused by poor design, and similar costs. Some of these losses are difficult to quantify in monetary terms, but they are nevertheless real.
Defective products (or their components) that are exposed before shipment are not considered part of this loss. Instead, any expense to the company resulting from scrap or rework of defective product is a manufacturing cost rather than a quality loss.
Loss occurs when a product’s functional characteristic differs from its nominal or target value. Although functional characteristics do not translate directly into dimensional features, the loss relationship is most readily understood in terms of dimensions.
When the dimension of a component deviates from its nominal value, the component’s function is adversely affected.
No matter how small the deviation, there is some loss in function. The loss increases at an accelerating rate as the deviation grows, according to Taguchi. If we let x ¼ the quality characteristic of interest and N ¼ its nominal value, then the loss function will be a U-shaped.
D. ISO 9000
ISO 9000 is a set of international standards that relate to the quality of the products (and services, if applicable) delivered by a given facility. The standards were developed by the International Organization for Standardization (ISO), which is based in Geneva, Switzerland.
ISO 9000 establishes standards for the systems and procedures used by the facility that determine the quality of its products.
ISO 9000 is not a standard for the products themselves. Its focus is on systems and procedures, which include the facility’s organizational structure, responsibilities, methods, and resources needed to manage quality.
ISO 9000 is concerned with the activities used by the facility to ensure thatits products achieve customer satisfaction. ISO 9000 can be implemented in two ways, formally and informally.
Formal implementation means that the facility becomes registered, which certifies that the facility meets the requirements of the standard. Registration is obtained through a third-party agency that conducts on-site inspections and reviews the facility’s quality systems and procedures.
A benefit of registration is that it qualifies the facility to do business with companies that require ISO 9000 registration, which is common in the European Economic Community where certain products are regulated and ISO 9000 registration is required for companies making these products.
Informal implementation of ISO 9000 means that the facility practices the standards or portions thereof simply to improve its quality systems. Such improvements are worthwhile, even without formal certification, for companies desiring to deliver high quality products.