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Quality Scan: Why Do We Measure?

  Art Whistler




Somewhere between a company's mission statement and the shipping dock, products get measured. Although for some there is pure pleasure in making measurements, this does not make the practice rational.We measure products for a reason: to control. (At trade shows in Germany, the exhibition for quality and measurement is called "Control." No accident.) We like to say that if you can't measure it, you can't make it.Well you can't control it either.

Measuring what we make tells us either that we are sticking to the plan, or that something has gone awry. Monitoring dimensional quality is like the temperature gage on a car dashboard: nice to have if it's always normal (in tolerance), but critical if it's not.

Many in the metrology profession lose sight of the larger context of measurement and its place in manufacturing. Our focus is often on a device or instrument, its particular specifications for accuracy, its calibration cycle, and the specific software that wraps around it and what that software can do. There is also the pressure of deadlines and record keeping. But controlling the size and form of things that are made is the point of all our efforts.

The quality function exists to serve manufacturing: to measure, monitor, report, and control. It's not surprising that so many customers of the metrology vendor are manufacturing engineers. They need measurement data to control their processes with immediacy, and they relate quickly to what a gage or device can do for their concerns. Properly implemented, the measuring device is at once predictive, defensive, and corrective. In this sense, one might think of dimensional measurement in manufacturing as akin to the immune system, sensitive to variables in the process stream, generating symptoms of trouble, and reacting to aberrations that infect healthy dimensions.

Yet the metrologist should be more than an accessory to manufacturing. On a part print, GD&T is code for the purpose or function of that part, and the metrologist should be able to help manufacturing decode that intent and express it optimally in the production process. In practice, however, the manufacturing engineer is often left to assume the responsibility for metrology. When quality personnel are steeped in an understanding of manufacturing processes, they can become equal partners in the team, not adversaries as often happens.

In many cases, measuring devices are integrated into the process itself, and provide feedback directly to the process controller.This works well for discrete features, or in simpler applications, but there is much disdain on the floor for closed-loop gage feedback directly to the controller, because when something goes wrong it can do so repeatedly. The preference for human intervention can be strong.

Some recent software developments have focused on fitting the measurement function (hand tools, gages, CMMs) into the manufacturing process more seamlessly by linking the inspection devices, measured data, SPC analysis, and resultant corrective action plans.The concept strives for improved management of manufacturing process efficiency by providing line operators with specific action plans for every eventuality. "If this dimension is wrong, here is what you do."

Because measuring then becomes inseparable from the continuum that is the manufacturing process, in a way that is consistent with systems logic and real-time process control, the concept is admirable. It is "in-process control," but with a human hand. It requires development of a knowledge base, but that can be a dynamic practice, and a reservoir of expertise that evolves with the process.

Manufacturing engineers are, of course, quick to grasp the value of this approach. If the ultimate purpose of measuring is to control the manufacturing process, integrating measurement into the process DNA should be a necessity. Too often the act of measuring remains "the way we always do it," postprocess, and control is diminished.

From my point of view, the purchasing cycle also changes dramatically. The old way involves a department or end-user purchasing equipment based on competitive merits, features, and benefits. Now, the vendor promotes this concept of measurement management in a closed-loop, yet people-directed process-control scheme made simple, to the company's top echelon. Even something as sophisticated as a CMM is promoted less under its own spotlight, and more for its role in the flow of measure-monitor-report-control. Suppliers of measurement equipment enter the door not at the purchasing or supervisor level, but with top management.


This article was first published in the September 2009 edition of Manufacturing Engineering magazine. 

Published Date : 9/1/2009

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