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Gage repeatability and reliability important to foundry quality effort.

Gage Repeatability and Reliability Important to Foundry Quality Effort

Evaluating the accuracy of foundry gage equipment provides better understanding of a casting's true conformance to requirements.

As a casting is being manufactured, numbers generated by gages and testing equipment are used to determine if the product will meet the customer's requirements. Because such numbers are the basis of many decisions made in foundry operations, metalcasters need a complete understanding of them.

Failure to understand the inherent variability of generated numbers will result in inaccurate decisions at all levels of the decisionmaking process.

Inaccurate decisions can result in: * misdirected capital expenditures; * scrapping finished products unnecessarily; * costly rework; * poor utilization of manpower.

The reason why the variability of generated numbers is not recognized is that it is human nature to accept a number when: * it is generated in a logical manner; * approved procedures were used; * trained personnel performed the work; * equipment was calibrated; * the number "looked right."

Only by applying innovative quality assurance techniques will management and operating personnel at a foundry have "real" understanding of the numbers that are being generated. The two most effective techniques for improving understanding of generated numbers are gage repeatability and reliability (gage R&R), which evaluates the variation in testing equipment and operator performance, and team-oriented problem solving.

Both of these techniques effectively complement any foundry process. Experience has proven that the successful implementation of a gage R&R program can only be accomplished when there also is an effective team-oriented problem solving system within the manufacturing operation.

The team-oriented problem solving approach to decisionmaking is a key element to a successful quality assurance program. Many foundry operations (without realizing it) have been using this approach to decisionmaking for years. Some examples are: scrap meetings at the operation level; quarterly profit meetings at the management level; and quality circles. The bottom line is communication.

Implementing Gage R&R

A gage R&R program must be implemented with commitment by management. Every quality assurance program must have this commitment or it will not be successful. Therefore, management must commit the manpower, equipment, time and necessary resources to complete the gage R&R studies. This results in the generation of information that will enhance the decisionmaking process at every level of the manufacturing process and, thus, in significant dollar savings in the manufacturing operation.

After the necessary manpower and resources have been approved by management, a successful gage R&R program begins with the selection of the gage to be evaluated. The team-oriented problem solving approach must be used to select the most applicable gage to be evaluated. Because of time and manpower limitations, it is impossible to run gage R&R evaluations on all of the pieces of equipment in a manufacturing operation.

Thus, the designated "action team" will use traditional tools, such as a Pareto analysis, to determine the first gages that will be evaluated. The Pareto analysis, can be based upon either the "dollars" that are directly affected by a gage or the number of times a gate is used within the operation.

A key element to successful implementation is proper training of the operators and advance review of the test procedures. Many times, the operating procedures are outdated, poorly documented or in need of revision. Since management is expecting results from the allocation of the manpower and resources, the team cannot afford to repeat the gage R&R study because of testing errors.

Next, the team will determine which gage R&R technique will be employed. The most commonly used technique in the foundry industry is the "General Motors Method." There are two basic procedures that will be used to gather test data, the short method and the long method. The short method is completed with two operators testing a minimum of five samples twice. The long method is completed with a minimum of two operators testing ten samples at least twice. The preferred testing frequency is three times.

Computation

After the required testing is complete, the data are entered onto a form like the one shown in Fig. 1. Results are computed using the formulas given in Fig. 1. The computed data will provide the percent variation due to the equipment (equipment variation), the percent variation due to the operators (appraiser variation) and the percent gage repeatability and reliability. This information can be applied directly to the variation in the generated numbers from the process that is being monitored.

The calculations in Fig. 1 can be computed with a simple hand held calculator. However, computers have become a useful tool in the interpretation of the data. Many software companies have developed software that improves our understanding of the generated data by presenting it in graph form. Some of the graphical representations available are accuracy/deviation; accuracy/linearity; capability study; control chart; and variability. Graphs give the action team useful information that can be used in problem solving techniques.

Case Study

The accuracy/deviation graph is an excellent example of the improved understanding that results from graphical representation. Figure 2 is a comparison of two operators in a gage R&R study of the level of fixed carbon of seacoal.

In this comparison, it was determined that operator A had more variability (approximately [+ or -] 1.0% of the computed true value) than operator B (approximately [+ or -] 0.5%). The graphical representation further shows that the data generated by operator A averages slightly higher than the true value and that operator B averages slightly lower than the true value.

When the action team draws a conclusion based on an operator's data, they will have the most accurate information available. In this case, the conclusion reached is: when operator A is operating the gage, that individual will have a greater variability and a slightly higher reading than operator B.

Each of the graphical representations will supply useful information to the action team. From the information generated in the gage R&R studies, the team can then employ team-oriented problem solving techniques to improve the manufacturing process and testing procedures. If the team determines that training is required, for example, Fig. 2 shows that they will get maximum utilization of their time and money by training operator A.

Benefits of Gage R&R

The greatest benefit realized by companies that have implemented gage R&R programs has been an improved understanding of the products they produce. Better understanding leads to a reduction in the unnecessary scrapping of finished product.

As a product is being manufactured, numbers are being generated within the operation to determine if it meets the customer's requirements. If gage R&R studies were not completed, there would be no way to determine whether the product is out of tolerance or if the gage being utilized is in error.

If this product knowledge is missing, a manufacturing operator must ask, "How much product has been scrapped unnecessarily?" The management in that facility must ask, "How many dollars have been lost due to product that has been scrapped unnecessarily?" It is not difficult to get management and operation to commit to gage R&R studies when these questions are asked.

These benefits improve the relationship between the manufacturing operation and the customer. A reduction in scrap will result in a dollar savings for the operation and, ultimately, the customer. Since "quality" is customer-driven, any improvement in the manufacturing/customer relationship is essential.

Further improvement in this important relationship can be realized through application of gage R&R studies at the customer's operation. How many times has a customer rejected a product that met requirements based on testing at the manufacturing facility but failed at the customer's location?

The result might have been: * possible loss of business; * strained customer relationship; * costly rework or scrap of the product; * freight cost for product return; * lack of confidence in testing; * loss of supplier certification. Many of these situations would be avoided if gage R&R studies were utilized by manufacturers and their customers.

The application of gage R&R studies is not a solution to all of our quality assurance problems. However, it is another tool that will complement the continuous improvement effort that is being undertaken in today's manufacturing facilities. [Figure 1 to 2 Omitted]
COPYRIGHT 1989 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1989, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Title Annotation:also includes a related article on a Cast Metals Institute seminar
Author:Lafay, Victor S.
Publication:Modern Casting
Date:Oct 1, 1989
Words:1367
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