This page contains a Flash digital edition of a book.
The Improvement Process Using contribution analysis of


the FTA provides a prioritized direction for improvement and a useful metric for quantifying effectiveness. This activity is intended to be cyclical and con- tinual in a closed PDCA loop, as shown in Figure 7. Improve- ments may come from several perspectives, but the follow- ing may prove most useful in a warranty evaluation where error probabilities are relatively low. • Error proofing: Human error rates will tend to be the highest probability events on the FTA.


Fig. 7. The diagram illustrates a PDCA loop as ap- plied to the use of FTA.


parts in the field should be explicit (and constant over the revisions of the FTA).


• Adding redundancy: If two sys- tems must fail for a fault to occur, the probability of their occurring simultaneously is much smaller than any one check system.


• Evaluating the factors that lead to basic events to further break down (and expose to correction) underlying causes.


• Evaluating and improving gage error through MSA (Measure- ment System Analysis) methods.


• Refining probability evaluations via specific capability studies or maintenance record mining.


Reporting and Communication


Interaction with the warranty engineer or quality engineer and the customer regarding this type of analysis may require some education regarding the meaning of the results and the approach used. Te following is recommended: • Te cross functional team in- volved in the analysis should be identified.


• Te “top event” should be clearly communicated and decisions about its scope should be ex- plained.


• Te overall structure and logic of the FTA should be described, branch by branch, so it makes sense to the customer.


• Te meaning of the probability numbers should be explained, including the units chosen.


• Te translation from probability of occurrence to actual expected


• Te provisional and conservative nature of the “top event” prob- ability should be described. Te probability is more useful as a measurement of improvement than as an absolute prediction of rate of occurrence of a defect.


• A comparison of actual warranty to predicted warranty within the FTA should be made, especially over a period of years.


• Where suppliers have impact on the results, they should be in- volved in reviewing or improving on the FTA logic and estimated probabilities.


• Reports of FTA results, espe- cially at the initial use, should be made in person where the appropriate explanations and de- scriptions can be accomplished efficiently.


A Case Study In one metalcasting facility, ma-


chined aluminum castings were being provided to the automotive market and were required not to leak air. Te aluminum wall thickness with this requirement was often at 0.11 in. (3mm) and process capability for this requirement was not 100%. Various methods were used over the years until the metalcaster settled on a helium leak detection system. Considering the high level of dis-


satisfaction with even one product that leaked to the end user, an FTA was constructed with a team of production, maintenance and quality staff. Tis was initially presented to the customer


after the team received four hours of FTA training over several months. In a series of six team meetings, an FTA was constructed with 64 basic events, and a “top event” probabil- ity of 1.25 x 10-4 events/hour of production was calculated. Te cus- tomer was impressed with the causal analysis and commitment to further reduce the incidence rate by way of objective contribution analysis. Revision B of the FTA suc- ceeded in reducing the probability to 6.23 x 10-5 events/hour, about


five months later. A third revision was undertaken after a year of further ex- perience and data collection. Revision C has a projected incidence rate of 2.82 x 10-5 events/hour, a 77% reduc- tion in roughly two years of work. An added benefit of the communication with the customer through FTA is the understanding that the defect rate will never be zero, and it is unneces- sary and unhelpful to initiate standard corrective action documents at each occurrence. Te foundry is committed only to annual, serious review of the FTA with a Pareto contribution analy- sis. Individual claim parts are received as they occur and simply reviewed to ensure no obvious assignable causes acted to create the defect. The metalcaster reports internal


benefits from a clear understanding of what matters in the control of this defect and a much better team environment in problem solving between maintenance, quality and production staff. Maintenance also reported a better objective basis for spare parts inventory levels as a re- sult of the close scrutiny the records received through the failure prob- ability determination effort. Te use of this method was devel- oped as a part of an overall improve- ment cycle for warranty. Casting suppliers and purchasers can consider this method a means of combating not only warranty claims, but ineffective, superficial responses to such claims. Te increase in customer understand- ing and appreciation for the investiga- tion is a welcome side benefit. 


This article was adapted from a paper presented at the 2014 AFS Metalcast- ing Congress.


Jul/Aug 2014 | METAL CASTING DESIGN & PURCHASING | 47


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60