QUALITY FUNCTION DEPLOYMENT
A number of concepts and techniques have been developed to aid in the product design function. For example, several of the principles and methods of Taguchi (who is most recognized for his contributions in quality control), such as "robust design" and the "Taguchi.
loss function," can be applied 10 product design. These topics are covered in Part IV on Quality Control Systems (Section 20.3). The topics of concurrent engineering and design for manufacturing are also related closely with design. We discuss these subjects in the following chapter (Section 25.3) because they also relate to manufacturing engineering and process planning. In the present section, we discuss a technique that has gained acceptance in the product design community as a systematic method for organizing and managing any given design problem. The method is called quality function deployment.
Quality function deployment (QFD) sounds like a quality related technique, And the scope of QFD certainly includes quality. However, its principal focus is on product design. The objective of QFD is to design products that will satisfy or exceed customer requirements. Of course, any product design project has this objective, but the approach is often very informal and unsystematic. QFD, developed in Japan in the mid196Os, uses a formal and structured approach. Quality function deployment is a systematic procedure for defining customer desires and requirements and interpreting them in terms of product features and process characteristics. The technique is outlined in Figure 24.9. In a QFD analysis, a series of interconnected matrices are developed to establish the relationships between customer requirements and the technical features of a proposed new product. The matrices represent a progression of phases in the QFD analysis, in which customer requirements are first translated into product features, then into manufacturing process requirements, and finally into quality procedures for controlling the manufacturing operations.
Figure 24.9 Quality function deployment. shown here as a series of matrices that relate customer requirements to successive technical requirements. Shown here is a typical progression: (1,1customer requirements to technical requirements of the product, (2) technical requirements of the product to component characteristics, (3) component characteristics 10 process requirements. and
(4) process requirements to quality procedures.
It should be noted that QFD can be applied to analyze the delivery of a service as well as the design and manufacture of a product. It can be used to analyze an existing product or service. not just a proposed new one. The matrices may take on different meanings depending on the product or service being analyzed. And the number of matrices used in the analysis may also vary, from as few as one (although a single matrix does not fully exploit the potential of QFD) to as many as 30 . QFD is a general framework for analyzing product and process design problems, and it must be adapted to the given problem context.
Each matrix in QFD is similar in format and consists of six sections, as shown in Figure 24.10. On the left-hand side is section 1, consisting of a list of input requirements that
serve as drivers for the current matrix of the QFD analysis. In the first matrix. these inputs are the needs !Iud desires of the customer. The input requirements arc translated info Output technical requirements, listed in section 2 of the matrix. These technical requirements indicate how the input requirements arc to he satisfied in the new product or service. In the starting matrix, they represent the product's technical features or capabilities. The output requirements in the present matrix serve..as the iC1PUtrequirements for the next matrix, through to the final matrix in the QFD analysis.
At the top of the matrix is section 3, which depicts technical correlations among the output technical requirements. This section of the matrix uses a diagonal grid to allow each of the output requirements 10 be compared with all others. The shape of the grid is similar to the roof of a house, and for this reason the term house of quality is often used to describe the overall matrix. It should be mentioned that this term is applied only to the starting matrix in QFD by some authors, and the technical correlation section (the roof of the house) may be omitted in subsequent matrices in the analysis. Section 4 is called the relationship matrix; it indicates the relationships between inputs and outputs. Various sym bois have been used to define the relationships among pairs of factors in sections 3 and a. These symbols are subsequently reduced to numerical values.
On the right-hand side of the matrix is section 5, which is used for comparative evaluation of inputs. For example, in the starting matrix, this might be used to compare the proposed new product with competing products already on the market. Finally, at the bottom of the matrix is section 6, used for comparative evaluation of output requirements. The six sections may take on slightly different interpretations for the different matrices of QFD and for different products or services, but our descriptions are adequate as generalities
Let us illustrate the construction of the house of quality, that is, tile matrix used for the first phase of QFD. This is the beginning of the analysis, in which customer requirements and needs are translated into product technical requirements. The procedure can be outlined in the following steps:
Identify customer requirements, Often referred to as the "voice of the customer," this is the primary input in OFD (section 1 in Figure 24.10). Capturing the customer's needs, desires, and requirements is most critical in the analysis. It is accomplished using a variety of possible methods, several of which are listed in Table 24.2. Selecting the must appropriate data collection method depends on the product or service situation. In many cases, more than one approach is necessary to appreciate the full scope of the customer's needs.
Identify product features needed to meet customer requirements. These are the technical requirements of the product (section 2 in Figure 24.10) corresponding to the requirements and desires expressed by the customer. In effect, these product features are the means by which the voice of the customer is satisfied. Mapping customer requirements into product features often requires ingenuity, sometimes demanding the creation of new features not previously available on competing products.
3. Determine technical correlations among product features. This is section 3 in Figure 24.10. The various product features will likely be related to each uther in various ways. The purpose of this chart is to establish the strength of each of the relationships between pairs of product features. Instead of using symbols, as previously indicated, let us adopt the numerical ratings shown in Table 24.3 for our illustrations. These numerical scores indicate how significant (how strong) the relationship between respective pairs of requirements is.
Develop relationship matrix between customer requirements and product features The function of the relationship matrix in the ()FD analysis is to show how well the collection of product features is fulfilling individual customer requirements. Identified as section 4 in Figure 24.10, the matrix indicates the relationship between individual factors in the two lists. The numerical scores in Table 24.3 are used to depict relationship strength.
Comparative evaluation of input customer requirements. In section 5 of the house of quality, two comparisons are made. First, the relative importance of each customer requirement is evaluated using a numerical scoring scheme. High values indicate that the customer requirement is important. Low values indicate a low priority. This evaluation can be used to guide the design of the proposed new product. Second, existing competitive products are evaluated relative to customer requirements. This helps to identify possible weaknesses or strengths in competing products that might be emphasized in the new design. A numerical scoring scheme might be used as before (see Table 24.J)
Comparative evaluation of output technical requirements. This is section 6 in Figure 24.10. In this part of the analysis, each competing product is scored relative to the output technical requirements. Finally. target values can be established in each technical requirement for the proposed new product.
At this point in the analysis. the completed matrix contains much information about which customer requirements are most important, how they relate to proposed new product features, and how competitive products compare with respect to these input and output requirements. All of this information must be assimilated and assessed to advance to the next step in the QFD analysis. Those customer needs and product features that arc most important must be stressed as the analysis proceeds through identification of technical requirements for components, manufacturing processes, and quality control in the succeeding QFD matrices.
EXAMPLE 24.1 Quality Function Deployment: House of Quality
Given: We are engaged in a new product design project for the case of child's toy. The toy would be for children ages 39. It is a toy that could be used in a bathtub or on the floor. We want to construct the house of quality for such a toy (the initial matrix in OFD), first listing the customer requirements as might be obtained from one or more of the methods listed in Table 24.2. We then want to identify the corresponding technical features of the product and develop the various correlations.
Solution: The first phase of the OFD analysis (the house of quality) is developed in figure 24.11. Following the steps in our procedure, we have the list of customer requirements in step 1 of the figure. Step 2 lists the corresponding technical features of the product that might be derived from these customer inputs. Step 3 presents the correlations among product features, and step 4 fills in the relationship matrix between customer requirements and product features, Step 5 indicates a possible comparative evaluation of customer requirements, and step 6 provides a hypothetical evaluation of competing products for the technical requirements'