Estudios r&r

MEASUREMENT SYSTEMS

Attribute Gage R&R
A SIMPLE BUT ROBUST TOOL SAVED ONE COMPANY $400,000 A YEAR.

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By Samuel E. Windsor, Delta Sigma Solutions LLC

easurement systems are routinely analyzed using traditional gage repeatability and reproducibility (R&R) studies. These studies are ANOVA (analysis of variance) methods used to quantify the variation attributable to gage repeatabilityand operator reproducibility. The gage R&R study as it applies to continuous data is widely used and written about. But another form of this tool—the attribute gage R&R—can improve process yields and reduce costs dramatically. Most processes require at least some form of subjective inspection or validation. It could be a check for blemishes on a painted or plated finish of a part or a judgmentconcerning the color, taste or smell of a product. In some cases, measuring equipment is available to access the acceptability of such characteristics. Many times, however, test equipment is not used due to cost or is simply not available. For example, although profilometers may be available during the inspection of a machined surface finish, the surface finish may be judged using a fingernail test.This fingernail type of inspection method has the potential for variability among inspectors and even variability by the same inspector over a period of time. Any variability in the measurement system will affect the measured process variability, thereby affecting the measure of process capability. Although the math is different, the effect of misreporting process capability is the same for bothcontinuous and attribute gage studies. One advantage of the attribute gage study is that, unlike the variable gage study, it can easily be applied to transactional processes. For example, a study could be performed on how customer service representatives interpret a customer complaint or the way a customer requirement is converted into an internal order. In the following case study during themeasurement phase of a Black Belt project, the simplest form (short method) of the attribute gage R&R is credited with saving a company more than $400,000 annually.

Background An electroplating company supplying silver plated machined parts for a telecommunications company was experiencing a rejection rate of just over 16,000 ppm at the customer facility. The parts were 100% visually inspected at thesilver plating facility for defects consisting of pits, blisters, voids and rough surfaces. When accepted, the parts were wrapped and shipped to the customer, where they were sampled and inspected by the customer. If parts were rejected, they were returned to the supplier for a process referred to as strip and replate. In this process the existing silver was removed, the part cleaned and new

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silver applied. The strip and replate process cost the plater two times the cost of the initial plating due to the cost of removing the existing silver and applying new silver. For example, a part with a surface area of 100 square inches would cost 10 cents per square inch to plate,at a total cost of $10 per part. The cost of the strip and replate process would be 20 cents per square inch. This means the rework cost $20 per unit. Investigation An investigation was conducted to determine what specifications were used by both the customer and supplier. In both cases the specification required the parts to be free of blisters, voids, scratches and roughness. Even withidentical specifications, nearly 2% of parts plated were rejected at the customer’s facility. Further investigation revealed no part was or could be expected to be completely free of blisters, voids, scratches and roughness. In addition, there was no real reason for the parts to be perfect. There was a need, however, for the blemishes and other defects to be minimal. The difficulty became defining...