Quality Scan: Air Anyone?
Have you ever wondered why a measurement technology invented before you were born continues to be used in modern manufacturing processes? The answer lies in the processes used in today's manufacturing plants, specifically on the shop floor, right at the point of manufacture. Due to heat, coolant, swarf, and other byproducts of manufacturing, optical and contact gaging just don't cut the mustard, unless cleaning stations are utilized before measurement occurs. Even offline postprocess contact gaging requires the part to be somewhat clean before a measurement can be taken. Welcome to the new era of the air gage!
Typical system pressures range from as low as 22 psi or 152 kPa (low pressure) all the way to 42 psi or 290 kPa (high pressure). Using a pressurized air circuit means parts can be introduced in a nonclean state, while still allowing accurate, repeatable measurement results. This is possible because, as the part is being introduced into the air tooling, air bleeding from the measurement jets "blows" all the contaminants away from the measurement surface. If the part is exceptionally dirty, complementary "cleaning" jets can be manufactured into the air tooling for additional premeasurement blow-off.
An additional factor to take into consideration is the inherent difference between low-pressure systems (22 psi) and high-pressure systems (42 psi). Low-pressure systems require constant air flow (high consumption), are incapable of being turned off between measurements without losing accuracy, have slower response times, and have smaller measurement ranges. Using high pressure allows much greater measurement ranges (300 µm compared to 200 µm), more effective parts cleaning, 4x faster measurement acquisition, and permits the system to be shut off between measurements without loss of accuracy, thereby reducing air consumption while still delivering accurate results.
Air gages in general are much more application specific than, say, a CMM. Once an air gage has been manufactured, it typically can only be used on a specific part size. That being said, one of its core strengths is the interchangeability, or adjustability, of different components, allowing a greater range of flexibility. A good example of this would be connecting rod systems. Using a simple length-adjustment block, the center-to-center distance between the pin and crank diameters can be modified, allowing measurement of a much greater range of sizes. Combine this with interchangeable air tooling, and you get a highly flexible machine capable of measuring a much greater range of part sizes.
Air tooling delivers distinct advantages over other measurement systems. Capable of measuring IDs as small as 0.8 mm (difficult using any other technique), air tooling can be custom-manufactured using single, multiple, or annular air jets to cover the full range of dimensional and geometric measurement characteristics such as: ID, OD, average, match, height, edge location, taper, length, width, max, min, class, cylindricity, out-of-round, symmetry, barrel shape, hourglass shape, and perpendicularity. Also, unlike contact gaging, which requires fragile "flex fingers" to measure small bore diams, air tooling has no moving parts, thereby circumventing this limitation. With no moving parts, and using simplified designs with wear-resistant materials (carbide, TCIN coating, etc.), air gaging brings new meaning to the word "robust" to help you protect your investment by lengthening the life of your gage tooling and lowering maintenance costs.
Some general facets of metrology that need to be discussed are resolution and repeatability. Both of these stats depend on the style and quality of the manufactured tooling, the type of air jets (round or rectangular), the style and quality of the air-toelectronic converters, and the measured value. Resolution is the simpler of the two, considering 0.01 µm resolutions are achievable using basic off-the-shelf equipment. While resolution is important, repeatability is by far the Holy Grail of air gaging. Capable of repeatability of ≤0.1 µm (which is very difficult to achieve using contact-style gaging), air gaging consistently proves itself an accurate and robust form of metrology.
Taking all the above into consideration, if air gaging were still being implemented using the same technology and techniques created during its infancy, it would have disappeared as a practical form of metrology years ago. However, air gaging has not only managed to keep pace with modern manufacturing techniques, but actually managed to embrace the future through technological innovation, helping propel it into the foreseeable future as a viable form of metrology on the shop floor and in the quality lab.
This article was first published in the December 2009 edition of Manufacturing Engineering magazine.