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ways and lead to a casting defect. Te higher the volume and repetition of these circumstances, the greater the potential for variability and defects. Tese can be classified as common causes, because they are common to the process and not one of them can be assigned easily as the culprit for any specific casting defect. Once it is accepted that a process does not yield 100% conforming prod- uct, it is up to some form of inspec- tion to sieve out the defective articles. Yet none of the in-process inspec- tions—visual, functional or those using technology such as radiography or leak detection methods—is perfectly reliable; none of them catches every defective part 100% of the time. Tis leaves the typical part sup-


plier facing a warranty claim in a very troubling situation. It is tempting to explain the situation as related to an assignable process problem and correct something that may or may not have been the actual cause. Tis short-term





As improvement efforts are made, the change from the previous probability to the new one quantifies the change in risk, a very real measure of improvement in the frequency of the ‘top event.’”


solution may succeed in appeasing a customer but ultimately wastes time. It adds a burden to the process of questionable benefit and will not likely reduce the incidence of the defect itself or its escape to the end user.


A Better Way


Tere is a quality tool that can enable organizations to improve their customer warranty communications. Fault tree analysis (FTA) is used to estimate the probability of an event by establishing the potential cause-and- effect linkages and assigning a prob-


ability to these intermediate causes. FTA methods require the user to understand the methods of basic prob- ability and to estimate the likelihood of human error. Te power of FTA lies in both the visual representation of these chains of cause and effect as well as the quantification of risk. Tese advantages make it ideal for applica- tion to warranty communication. Te two basic logical relation- ships of AND and OR are combined to describe the potential chains of cause and effect leading to the “top event”—the undesirable occurrence that one is interested in avoiding or explaining. Figure 2 is an illustra- tion of the combination. Te diagram communicates a situation where the only way to elevate the temperature too high in the heat treatment solu- tion furnace zone 1 is to have a failure of both equipment and check systems. Equipment failure could arise from a burner failure or a circulating fan failure. Check systems could fail if either the alarm fails to alert staff or it is defeated by being mis-set. Te simple box describing an event was supplemented by an oval. Te oval indicates a basic event, one that is not going to be investigated any further for cause in the FTA. Te basic events are parallel to “root causes” in com- mon forms of WHY-WHY analyses. Events that arise from the basic events may be referred to as intermediate events. Te “top event” is the result of all intermediate and basic events.


Calculating Probability


Fig. 2. A combined illustration of an “AND” relationship of cause and an “OR” in a fault tree analysis is diagramed.


42 | METAL CASTING DESIGN & PURCHASING | Jul/Aug 2014


While the effort required to estab- lish a sequence of causes and effects in logical interconnection is instructive and valuable in itself, the great power of the tool lies in its ability to quan- tify the probability of the “top event”


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