Six Sigma tolerance specification represents the acceptable range of performance values that a customer will accept. "Six Sigma" is a statistical term that indicates that in a batch of identically manufactured parts, 99.99966% of the items are within the acceptable tolerance specified by the customer, as defined by Professor Joel Cutcher-Gershenfeld in his course on MIT Open Courseware. This stands as an outstanding performance level in the industry.
Tolerance refers to the space of values that stand between the minimum and maximum values allowable for an item to work properly or meet customer expectations. When a factory produces large amounts of items and that it becomes unreasonable to check each item to ensure that they are meeting the requirements, statistical analysis estimates what the tolerance might be. Motorola University explains that analysts frequently express this statistical tolerance in terms of the expected percentage of parts that will fall within specifications and their confidence level that the number is accurate.
Cost vs. Tolerance Specification
In a Six Sigma environment, less than 3.4 articles out of one million will fall out of the range of acceptable values. Decreasing the tolerance keeps only the products that approach perfection, but creates a bigger pile of rejects. Increasing the tolerance lets more items be shipped, but takes the risk of disappointing the customer. Hence, the tolerance specification has a great impact on the cost of business in Six Sigma facilities, explains Thomas Pyzdek, author of the "Six Sigma Handbook."
Cumulative Properties of Tolerance
The variations observed in a manufactured product find roots in the slight differences introduced by each step of assembly. Deviations, even invisible ones, tend to partially if not fully add to subsequent variations in the product. Hence, to ensure that the product meets the requirements 99.99966% of the time, each individual step must adhere to even tighter tolerance specifications. Six Sigma tolerance at the end of a production line means much better than Six Sigma performance upstream, reports Motorola University.
From Design Tolerance to Process Tolerance
"This is not about the product but all about the process," would be a way to summarize the Six Sigma perspective. The variability observed in a product reflects the degrees of freedom that process steps introduce into an item. Hence, the effort of designing a product performing within tolerance specifications translates into the design of an assembly process that must operate within pre-defined process tolerances, stresses Pyzdek. This is the reason why the Six Sigma methodology achieves product quality by controlling and improving process variability.
The Six Sigma philosophy strongly promotes product of high quality, but will place the efforts to achieve perfect outcomes in the context of financial returns. Only process improvements that will yield increases in corporate profits should be pursued, explains Pyzdek. Hence, a Six Sigma improvement team will balance performance level with tolerance range and with increased profits. If recommendations to improve the process do not translate into significant upside financially, the team goes back to the customer to require them to relax the tolerance specifications.