Failure mode and effect analysis: a step toward total quality assurance.
The goal of FMEA is to provide a method of studying failure and its effects in product design or processes while establishing corrective action priorities before a design goes into production. FMEA will identify, document and evaluate all potential failures related to the design and development of a new product, process or service. Proper usage of FMEA has resulted in decreased workloads in the design and manufacturing areas by reducing product failures.
Foundries which will benefit most from FMEA techniques are those that have developed the supporting quality assurance programs, like statistical process control, which are necessary to successfully use this quality method. implementing FMEA
Implementing a FMEA program is dependent upon a successful team-oriented problem solving program (TOPS). The required information must be collected from many different departments and individuals to determine the high-risk areas for improved product quality and reliability. The team should include manufacturing, design and process engineers, quality and manufacturing personnel, including machine operators. Certain FMEAs may also benefit from customer input. A working SPC program must be in place simultaneously to provide this required information.
There are two types of FMEA techniques, design and process related investigations. Design FMEAs examine specific details that are inherent to the component. These would include tolerances, material specifications, improper assembly, contamination, stress calculations, etc. Process FMEAs on the other hand consider the entire manufacturing process. These applications might include; work stations, machines, gages, operator training, material handling, alloy or sand composition, heat treat, etc.
In the design of a new product or component, a casting for instance, the "design" FMEA will naturally be developed and utilized initially. But because most foundry operations rely on existing production processes, particularly molding and coremaking, the "process" FMEA takes precedence. In the foundry's case, a thorough investigation of the existing processes must be completed prior to putting a new part into production. The resulting information can then be used to develop a "design" FMEA for the new casting.
Developing the FMEAs
When developing a series of process FMEAs, a complete manufacturing flow chart must be generated. Key features of the flow chart might include: raw material receiving, material storage (raw materials, in-process, finished and rejected castings), production lines, melt areas, coreroom, cleaning room, etc (see Fig. 1). Once this chart is completed, a series of process FMEAs can be formulated. This phase requires the team oriented problem solving approach. Team members identify the areas within the process that can effectively utilize a FMEA. In doing so, the team must have the authority to add and delete areas as the analysis proceeds.
A FMEA summary form should be generated and completed during the team analysis. The information contained in the form should be intelligible enough for all operating personnel to understand, including quality assurance inspectors, auditors, engineers and supervisory personnel. Table 1 illustrates an actual FMEA summary form.
Another useful tool for generating information is the cause and effect diagram shown in Fig. 2. Some of the most commonly overlooked contributors to a potential failure are raw materials inspection, variations in process monitoring equipment, personnel, etc. A high degree of focus must be placed on people and maintenance during the evaluation in this step.
As the analysis progresses, the team will identify the potential causes and effects of failure, present controls, determine critical characteristics (Delta), recommend corrective action, assign a risk priority number (RPN) and identify key personnel. The risk priority number is the most beneficial portion of the entire analysis because it allows the information to be ranked in order of process influence. FMEA instruction manuals list guidelines for generating RPNs. The risk priority number is a product of the possibility of recurring failure multiplied by the severity of the failure and the ability to be detected. Applying SPC techniques will help to develop the necessary information during this stage of the FMEA.
The severity criteria should be discussed with all casting customers. The customer is not limited to the outside consumer but all who came in contact with the casting from the raw material phase through the final casting. These individuals have valuable information to contribute to the analysis. The occurrence criteria (see Table 1 ) can be generated using process monitored capability indexes. The detection criteria is generated by monitoring the scrap report versus the rejected scrap from the customer.
Once fully developed, process FMEAs can be used to supply useful information for developing "design" FMEAs. As new castings are produced the already developed "process" FMEA information can be utilized (i.e. severity, occurrence and detection criteria), since in most foundry operations there usually are few process changes. Even in the case of numerous process changes, much of the information will still be useful, because it can support the design engineering function. These benefits include lower scrap rate on test piece evaluations and a reduction in time to take a new product into production.
The completed design FMEA should be shared with the customer to maximize the benefit to the final product. Since the customer supplies valuable information while developing risk priority numbers they will also be affected by the casting performance.
It may be difficult to understand exactly how FMEAs apply initially. But it is important to analyze all process details that may affect the casting. The entire process is represented by a number of criteria. There are no limitations on applying FMEAs. Even in the case of a foundry meeting 100% of their customers specifications, but failing to maintain a timely delivery schedule, the result is a loss of customer satisfaction or business. The main point is: A process FMEA must include all potential causes of failure, even those the foundry has no direct control over. The results will continue to remain as product failures.
Failure Mode and Effect Analysis provides a method of studying failure and its effects while establishing corrective action priorities before a design goes into production. The application of FMEA is critical to the future success of manufacturers. Effectively this technique will add another useful weapon to the quality arsenal of any successful quality assurance program.
1. Ford Motor Company,"Potential Failure Mode and Effect Analysis," Form Number 1696, p 1 (revised 1988).
2. Chrysler Motors, Failure Mode and Effect Analysis Manual," Form Number 84-231-1200, p 2 (Revised 1986).
CAST METALS INSTITUTE OFFERS FMEA COURSE
On July 26,1990, the Cast Metals institute of the American Foundrymen's Society will hold a course on Failure Mode and Effect Analysis," a first-time program devoted to instructing foundry personnel in the application of FMEA. The program, to be held at the AFS/CMI technical center in Des Plaines, IL, will feature as instructor the author on the accompanying article on FMEA, Victor S. LaFay, technical director, The Hill and Griffith Co.
Instruction will involve development of a FMEA for an actual manufacturing process, including the layout and drawing of a manufacturing flowchart, and the application of a team oriented problem solving approach. Calculation of data, data interpretation and graphical representation will enable attendees to grasp the FMEA concept.
The foundry industry has recognized FMEA as an invaluable tool in understanding manufacturing processes and products. Recently, our industry has developed the supporting quality assurance programs that are necessary to implement a successful FMEA program. The application of this type of analysis is critical to the future success of manufacturers. A thorough understanding of this technique will add another useful weapon to the quality arsenal of any successful Quality Assurance Program.
For more information, or to register for this innovative CMI program call AFS/ CMI at 800/537-4237.
(Tables and other figures omitted)
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|Title Annotation:||also includes related article on a failure mode and effects|
|Author:||LaFay, Victor S.|
|Date:||May 1, 1990|
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