SECTION 1.8
Identification of Significant Characteristics

Significant characteristics can be defined as the product, process, and gaging requirements which are necessary to insure customer satisfaction (and which the quality manager must have summarized on the Control Plan). The engineering department has the responsibility to identify significant characteristics as determined by the customer for the tools which the customer will use.

Most tool characteristics are non-functional. However, some of them carry tolerances which may involve high costs. The design review is one means of looking for these, through comparing the tolerances with the known process capabilities. The identification may also come from past experience with similar designs and tolerances, i.e., such tolerances have generated high internal rejection rates, high inspection costs, high supplier costs, etc. It is helpful if the design review can identify the potential recurrence of such difficulties. In some cases the designer will be able to provide relief. In other cases it may be possible to modify the process in ways which make it easier to meet the tolerances economically.

SECTION 1.8.1
Defining significance

Quality characteristics are decidedly unequal in their effect on fitness for use. A relative few are "significant", i.e., of strict importance. Clearly, the more important the characteristics, the greater should be the attention they receive in such matters as extent of quality planning; precision of processes, tooling, and checking equipment; strictness of criteria for conformance; etc. However, to make such a discrimination requires that the relative importance of the characteristics be made known to the various decision makers involved: the customer, engineers, quality, inspection, etc. To this end, the cutting tool manufacturer should utilize a formal system of seriousness classification. The resulting classification finds use not only in inspection and quality planning, but also in specification writing, vendor relations, product audits, quality reports, etc. These uses of seriousness classification dictate that the system must:

1. Decide how many classes or strata of seriousness to create (usually three or four).
2. Define each class.
3. Classify each characteristic into proper class of seriousness.

SECTION 1.8.2
Classifying characteristics

The system of classification was pioneered in the Bell System during the 1920s. Most subsequent sets of definitions and classifications have evolved from this venture. Classifying characteristics yields some welcome results by discovering misconceptions and confusion among departments, thereby opening the way to clear up vagueness and misunderstandings.

A problem often encountered in the practice is the reluctance of the designers to become involved in seriousness classification of characteristics. The designers may offer plausible reasons, such as: all characteristics are significant or, the tightness of the tolerance is an index of significance, etc. Yet, the real reasons may be the unawareness of the benefits; other matters have higher departmental priority, etc. In such cases it may be worthwhile to demonstrate the benefits of classification by working out small-scale examples, such as the one shown.

Figure 1-3.
Seriousness classification of broach defects