The Voice of The Customer - Six Sigma

Quality Function Deployment

QFD was covered in some detail in chapter. Important as it is in improving an existing process, service or product, it is even more significant when we are considering a new design. In the Identify phase of PIDOV and the Deane phase of DMADV the voice of the customer is analysed using QFD to focus the design / development effort on important criteria.

Load / strength Relationships

A technical addition to QFD in the design process is the concept of load / strength relationships. Once we understand what the customer wants we need to assess the load model on the understanding that some customers will stress the product more than others and some products will be less robust than others due to manufacturing variation. The series of diagrams which follow show what we are attempting to achieve when designing a new product, and indicate ways in which our intentions are not met.

Ideal relationship

Figure shows the ideal situation in which a product has an undistributed strength that is greater than the highest of the loads expected from the load distribution. This situation would result in an inherently reliable product that did not fail in service.

A more realistic relationship

Figure is a more realistic version of Figure in which it is acknowledged that not only are the loads which a product experiences distributed but so it the strength of the population of product that have been manufactured. In fact, we know that variability exists in everything, and what we are attempting to do is to manufacture a product with as little variability in its characteristics as possible.

An inherently unreliable design

If Figure represents the situation at product launch, an inherently unreliable design is depicted. Failures will occur when the weakest products meet the highest loads, and this could occur as the first products enter the market place. On the other hand, if Figure represents the situation after a number of years of use, age - related degradation is represented. For example, a product depicted by Figure at launch could gradually weaken with age so that the distribution “slips” to the let. Dependent on at what stage in the product’s life this occurs, this may be acceptable to the user as failures will start when the “tails” of the two distributions overlap.

Effect of Incapable Processes

Figure shows an inherently reliable design (the mean strength is similar to Figure ) but the incapable manufacturing process has increased the variability in strength. The process may be assumed to be in control because the strength is singly distributed.

Efect of out-of-control processes

Figure shows an inherently reliable design that appears to be manufactured by capable processes because there is no increase in the spread of the strength distribution. However, the manufacturing process is out of control as evidenced by the small “freak” distribution of weaklings. This sub - population will fail early in service as the highest loads cause premature failure.

The efect of abuse

Figure shows the same situation as Figure and, in addition, recognises that although the design is inherently reliable, some early life failures may be caused not by weak products from an out of control process, but by use of the product beyond the expectations of the design and marketing team. This is shown by the small “freak” load distribution depicting abuse of the product by the customer.

Best achievable situation?

Finally, Figure perhaps represents the best situation that we can achieve. It would be exceedingly deficit to determine the degree of abuse and even if it were possible to do so with any confidence, to design for excessive abuse would result in an over - engineered product for the vast majority of users. The resulting cost increases may also make the product non - competitive. Before considering how best to achieve this situation, it is useful to understand a number of major factors that influence significantly the achievement of reliable, quality products.

In DFSS we need to understand both load and strength curves to assess the viability of the design.


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