Quality Scan: Measuring Manufacturing Reality
Everyone in manufacturing knows that the ultimate size of a part is influenced by many different aspects of the production process. The tolerance is simply a convenient way to specify the maximum acceptable combined impact of all the variables. Each step in the process consumes some portion of the tolerance, and leaves less for the next step.
What is often overlooked, however, is the fact that gaging the product is part of the manufacturing process just as certainly as turning or grinding it. The portion of the tolerance consumed by gaging is not available to the other processes, and that means they have to be more precise than might otherwise be necessary.
That’s why, in the old days, before making a measurement, an operator would polish the anvils of the micrometer on his or her lapel before attempting the measurement. This self-imposed ritual ensured that the gage contact surfaces were clean so the operator could rely on the measurement. If the contact surfaces weren’t cleaned every time, then some portion of the total tolerance allowed would be consumed by the “dirt variation,” leaving less of the tolerance available for the machining operation.
Not many parts are measured with micrometers anymore, but the principle is certainly applicable to today’s sophisticated gages and systems. As manufacturing operations face ever-tighter tolerances in the global quest for zero defects, the impact of gage variability grows ever more significant.
Gages, after all, are manufactured products, and even the very best come with built-in variability that will consume some portion of the available tolerance. This variability is typically quantified with statistical techniques like Gauge Repeatability & Reproducibility (GR&R) studies such as the “10-Part, 3-Operator” protocol, in which three different operators measure the same 10 parts using the same gage.
Aside from the obvious impact of factors like surface finish, geometry, and temperature, the act of measuring a part will alter its dimensions at the submicron level being measured by today’s gages. For example, even if there is no variation in part position from measurement to measurement (and how likely is that?), continuous physical contact between the gage and the part surface will cause some change. The surface of most parts is not smooth. Consequently, the gage contacts will push the peaks into the valleys and, to some extent, change the part characteristic(s) in a minuscule way.
As the total available tolerance shrinks, the impact of such phenomena grows, and engineers are faced with some tough choices. The obvious responses, better machine tools and tighter process controls, tend to be very expensive. The less obvious response, better gages, is almost certainly less expensive, and offers several extra benefits.
Considering a 10% GR&R for a 0.001" (25 µm) part tolerance, a gage user might assume that no measurement reading can exceed 0.0001" (2.5 µm). In reality, however, due to the criteria applied in the approved test protocol, no measurement reading may exceed 0.000033" (0.84 µm). In other words, the act of gaging cannot consume more than about one-third of the desired GR&R results. Modern, top-of-the-line gages can cut that to 0.000020" (0.5 µm), or one-fifth of the desired GR&R results. The remainder is available for consumption by other error-causing variations such as vibration, temperature, surface finish, shape, etc.
The cost difference between investing in a new gage and investing in a new machine tool is reason enough to justify a very hard look at this option. But cost saving is not the only advantage offered by today’s gaging systems. Most of them have capabilities that simply aren’t available in older units.
Today virtually any top-of-the-line gage will be compatible with an electronic data management system that will not only provide gaging results, but also analyze the measurement data using built-in statistical programs. Gaging electronics also will probably interface with existing plant data systems using common protocols, to simplify closing the process-control loop.
New gages generally deliver their improved accuracy over much greater measurement ranges, which simplifies the task of assembling them into sophisticated cells and networks. They also tend to be tougher and easier to maintain than their predecessors. Many of them offer modular designs that reduce repair time and spare-parts inventories.
Perhaps the solution to your tolerance problems isn’t a more expensive machine tool, or a new process. It may be as simple as a more precise, repeatable gage that gives back some of the tolerance your old gages are eating.
It’s worth checking out.
This article was first published in the May 2006 edition of Manufacturing Engineering magazine.