Standard seeks to curb inspection vagueness.

Position:B89.7.2 measuring standard - Brief Article

The standard establishes the ground rules for verifying conformance. This can be helpful in contract disputes, and may even be unnecessary.

According to Robert Campbell, a professor of engineering at Harper College in Palatine, IL, and Philip Stein, a measurements consultant in private practice, Pennington, NJ, in the past if inspection included gaging, it was assumed with some confidence that the gage was sufficiently accurate with respect to product tolerances. It was also assumed that any uncertainty in the measurement was negligible. In fact, it probably was negligible in the sense that gage accuracy did not have a significant influence on the overall acceptance rate.

The standard also defines a specific group of measurement and planning tasks, and establishes a resulting job classification, the dimensional measurement planner. According to Messrs Campbell and Stein, the intent is to eliminate the de facto measurement planning that takes place in final inspection or on the production floor and to replace it with a formal, professionally written plan. It also presumes that the planner has the requisite knowledge to do the job, implying that the job is not an entry-level position.

But times have changed since then, especially in the world of manufacturing.

While the B89.7.2 standard cannot cover all possible measurement procedures, committee chair Walt Beckwith, of Brown and Sharpe, North Kingstown, RI, says the committee captured elements common to all measurement processes and identified the underlying principles. "This will assure acceptable business decisions based on good metrology practice and on sound economics," he says.

"Rapid increases in machine tool capability have approached accuracies that challenge our ability to measure. Along with this increase in technological capability has come the obvious request for tighter tolerance zones," says Mr Stein. The result is that measurement uncertainty now becomes a major participant in accept-reject decisions and the contractual disputes that are likely to arise from those decisions.

To accomplish the business purpose, the cost of measurement is balanced against the risk associated with inadequate measurement quality. Expenditures...

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