Quality 101: All About Air Gages
Air gages, or more precisely dimensional air gages, are measuring devices that use air nozzles to sense the surface to be measured. Using this technique, air is passed to one or more sensing nozzles and the resulting flow is measured by the air gage readout that is calibrated to display in linear dimensions. This technology was introduced in the 1940s, and since the 1950s has gained wide acceptance as a highly precise means of measuring internal and external diameters as well as many other features.
Readouts that measure the air flowing to the gaging nozzles have evolved from instruments that used tapered glass tubes with little pucks that float up and down within the tube, or alternatively, highly sensitive mechanical pressure gages coupled with needle valves or orifices serving as flow restrictors. Both of these designs have been largely replaced by readouts that use piezoresistive pressure transducers and electronic amplifiers coupled to digital or analog displays; typically these air/electronic instruments have serial data ports facilitating data logging and statistical process controls.
Regardless of the specific features offered by the manufacturer, all air gage readouts are pneumatic comparator instruments, and to display a precise measured value they must be referenced to a master artifact (or artifacts) of known value. For measurement of internal and external diameters, ASME/ANSI standard plain ring gages, plug gages or set disc are typically used for most applications.
Single Master vs. Dual Master Air Gage SystemsPrecision measurement requires controlling both the bias and the scale factor of the measurement system. In other words, the combination of the readout and the gaging member must be zeroed, and the sensitivity of both the readout and the gaging member must be set to a precise value. If the manufacturer carefully controls the pneumatic sensitivities of both the readout and the associated air gage member, one setting master will be sufficient to “zero the gage” for the application, otherwise a second master is required to set the sensitivity (scale factor). Both factory pre-scaled (single master) systems and user scaled (dual master) systems have tradeoffs in terms of accuracy, cost and user friendliness; and both are offered by several manufacturers. Regardless of the readout type or the calibration system used, it is the characteristics of the sensing nozzles that primarily determine the benefits and limitations of air gage technology.
The Gaging NozzlesFigure 1 shows the air gage sensing nozzle. It displays a jet hole where air exits the nozzle and an air gap at the nozzle face. For clarity, the illustration exaggerates the air gap: normally it is 1/10 the diameter of the jet hole or less. When limited to 1/10 the jet diameter, the area around the exit of the jet hole becomes the dominant restriction in the air passage, and a direct linear relationship between the air flow and the gap height exists. When connected to the appropriate air gage readout some very useful gaging tools result; the air probe shown in Figure 2 is one such tool.
Designed to measure internal diameters, the air probe incorporates two diametrically opposed air nozzles in a hardened steel body with air passages connecting the nozzles. (Note: Manufacturers’ nomenclature for internal diameter measuring air gage members varies; they may also be referred to as air spindles or air plug gages. The writer prefers the latter.)
As supplied to the user the air probe body will be sized to “slip fit” in the hole to be measured, typically with less than 0.001 inch clearance to the low product limit. Note also that the nozzle tips are recessed below the probe’s body; and that there is a vent groove leading to the area around the nozzle that allows air to escape the nozzle area and assures that the jet hole is the dominant flow restrictor in the air circuit. Now let’s look at some of the neat features this measuring tool has-as well as its limitations.
Rapid, High-Precision Measurement With a Minimum of Operator SkillThe probe is body piloted in the hole, so there is no need to centralize or rock the gage to find the true diameter. Opposed gaging nozzles make the measurement independent of how the gage is positioned in the hole. If the gage is displaced in the radial direction, the increase in air flow in one nozzle is offset by a corresponding decrease in the opposite nozzle; this “differential measurement” is not limited to two nozzle configurations, arrays of three or more nozzles can be used to inspect for lobed out-of-round conditions or to average an out-of-round condition. Coupled with air/electronic gage readouts, variable data is easily obtained for real-time SPC.
Noncontact MeasurementSince the nozzles are recessed below the body diameter, wear on the body does not directly influence the accuracy of the gage; furthermore, the air has a self cleaning effect, blowing debris away from the area being measured. These features coupled with the aforementioned differential measurement feature make dimensional air gages outperform other methods of gaging in many applications-particularly those involving high precision IDs and ODs.
Small Sensor SizeFor some applications this may be the most significant attribute: the air gage nozzle may be the only sensors small enough and rugged enough to fit into the work pieces.
What limitations does air gage technology have? We would be remiss in not commenting on these in this overview of air gage technology.
Limited Range, Relatively High Acquisition CostWithout a doubt, limited gaging ranges of individual gaging members are the most significant limitation to this technology. Unless the features to be measured are very closely grouped in size, an air gage will be required for each feature. Whether this is cost effective drills down to the cost of the air gage vs. the costs of passing out-of-tolerance parts forward in the production process and whether a gage with a lower acquisition cost can perform the function with the available level of operator skill.
Comparability to Other Means of MeasurementAir gages sense the average height of the surface areas directly opposite the jet holes in the gaging nozzles, while gages with hard contacts ride on the peaks of the surface profile. This can result in a variance between measurements made by these types of sensors on rough surfaces. These variances will be minimal or nonexistent on a part with a ground or honed surface, but may be significant on parts with rough surface finishes. In general the variance will not exceed twice the difference between the center line averages of the surface roughness of the workpiece and that of the setting master.
Air Gaging Deserves Another LookBy Andreas Blind
How do you wring the best performance from a manufacturing process if you don’t have quick, accurate, reliable and repeatable measurement systems? The obvious answer is that you can’t. Every manufacturing process produces variations, requiring manufacturers to employ a wide range of metrology systems to control part quality.
If your process produces rough parts, or parts with large tolerances, then any number of measurement systems could conceivably meet your measurement requirements. If your process produces semi-finished and finished parts, or parts with extremely small tolerances, then your options are quite a bit more limited.
Even though it is possible to measure reasonably small tolerances with contact and optical gaging, pneumatic measurement systems offer reliable, cost-effective possibilities for many manufacturers.
Pneumatic gaging is like the Rodney Dangerfield of the metrology world: even though it has quite a few stellar qualities that make it stand out from its competition, it gets a lot less respect than it deserves. Some manufacturers have limited their use of pneumatic gaging to small tolerance in-line gaging, mostly due to outdated perceptions of pneumatic gaging, costs associated with air consumption, noise level, and the supposed limited interoperability (flexibility) of these “dedicated” systems.
In fact, today’s pneumatic measuring systems offer many advantages: great accuracy, robust mechanical construction, operator friendly interface, low cost, and superior system flexibility.
In decades past, the air compressor, the filters necessary to clean the air, the piping to deliver air to the system, the constant air consumption required to measure parts, and the dedicated nature of the system itself added up to a meaningful expense. Today, all of those “costs” have been severely curtailed through better technology.
Modern high pressure pneumatic systems require only an inexpensive off-the-shelf air filter for submicron accuracy. Screw compressors, reservoir tanks, and the ability of high pressure measurement systems to shut air off between measurements have also sharply reduced the cost to operate an air gage.
Finally, the flexibility of today’s air gaging systems adds a great deal of value, avoiding the dedicated nature of previous systems. Interchangeable air tooling and other easily changeable features permit users to reconfigure an existing system for multiple part sizes, thereby easily extending the life cycle of the system for years.
Accuracy, repeatability and response time of air gage systems becomes a major advantage where tolerances are less than 50 microns, for example on semi-finished parts to finished parts.
The edge in rough finish part measurement generally goes to electronic contact systems mainly due to associated costs and the lower importance of accuracy, repeatability and response time. For semi-finished parts, the two types of measurement processes become more equal, although that really depends on where during the manufacturing process the measurement is actually taken (in-line, off-line, post-process, in-process).
But for finished parts, the edge tips quite a bit more in favor of pneumatic gaging as it has substantially better accuracy and repeatability, as well as a faster response time.
One final item that is worth mentioning, or maybe we should treat it like a question instead, is the robust nature of any measurement system. Using modern manufacturing methods, pneumatic systems by their very nature are quite a bit more robust than many other types of measurement systems. Since the only real moving part of a pneumatic system is the air, it can withstand quite a bit more punishment than other types of systems and is not affected by dirt, oil or swarf (metal shavings) on the part.
Although pneumatic measurement systems are not used for form or contour measurements, and do not offer the flexibility of an optical system for measuring a wide variety of part types, they can deliver high measuring accuracy, repeatability and speed, day in and day out over thousands of parts, without the need for constant maintenance. For high volume metrology, air gaging deserves another look.
Andreas Blind is vice president, Jenoptik Industrial Metrology