Air Gages

Air gaging is a technology that employs the use of air flow volumes and air pressure to determine the size of measured part dimensions. To achieve this measurement, it relies on the laws of physics, which state that flow and pressure are directly proportionate to clearance and react inversely to each other. As clearance increases, air flow also increases and air pressure decreases proportionately. As clearance decreases, air flow also decreases and air pressure increases accordingly. In all air gaging applications, regulated air flows through a restriction of some kind – a needle valve, jeweled orifice, etc. – prior to being expelled through the nozzles of a particular air tool. When the nozzle of an air tool is unobstructed and blowing to the atmosphere, maximum air flow occurs and the air pressure – called back pressure – between the restriction and the nozzle is at a minimum. As an obstruction is brought closer to the front of the nozzle, air flow from the nozzle diminishes and back pressure builds. When the nozzle is completely obstructed, air flow is zero, and back pressure reaches the pressure supplied by the regulated air supply. In this scenario, air flow moves from maximum flow to minimum flow, while back pressure moves proportionately in the opposite direction: minimum to maximum. These values of flow and pressure can each be plotted and graphed against the nozzle's clearance from the obstruction. The extremes of both back pressure and flow notwithstanding, these graphed values will always represent straight-line curves and establish the straight line or "linear" proportions that form the basis of all air gaging. A Brief History of Air Gaging The first air gages were developed in France before World War II by a carburetor company that sought a reliable method of measuring its carburetor jets. These early air gages employed a simple back pressure technology and, though crude by today's standards, they provided the basis for the development of virtually all air gaging styles currently in use. These air gages relied on a simple restriction-based air regulator system connected to a graduated indicator tube filled with water. The air regulator's first restriction served to reduce the air pressure from an incoming line while an open-ended tube from a "Tee" in the air line was submerged in a predetermined depth of water. Any air pressure in excess of the pressure at that particular depth caused a certain amount of air to be forced out the bottom of the tube and into the atmosphere. Simultaneously, the connected water column in the graduated indicator tube would fall proportionately. A second restriction was located just before a "Tee" in the air line, which was inserted between the top of the indicator tube and the air plug. This second restriction provided the zeroing control for the graduated tube. The air plug was inserted into the carburetor jets one at a time. The larger the carburetor jets, the more air would be allowed to displace the submerged air tube and, proportionately, water in the graduated tube would be displaced. As back pressure affected the level of water in the graduated tube, the distance between the air plug nozzles and work piece wall was indicated. In this way, the company was able to establish a standard back pressure reading and relate this reading to the size of the carburetor jets by selecting certain parts deemed "acceptable" and recording its particular back pressure reading. Once this standard was established, a comparative back pressure measurement was then performed on other manufactured parts to determine if they were within acceptable tolerance limits. The carburetor jets were then deemed acceptable or rejected according to a comparison to this initial back pressure standard. In 1943, a U.S. patent was granted for the simple back pressure system to exiled French engineers who had escaped Nazi control at the onset of World War II. The patent also incorporated the use of the newly developed air pressure regulator, which served to eliminate the evaporation problem of the first water based system, and used a more sophisticated dial readout. Air gaging – though in exile – had been born. Improvements in air gaging systems and styles continue to the present. Back pressure bleed, differential, and flow technologies comprise a modern air gaging acumen. Each style possesses particular characteristics that affect its ability to address issues such as diversity of application, accuracy, efficiency, and ability to compensate for tooling wear. In addition, modern air to electronic amplification has raised the resolution of air gaging to the level of millionths of an inch while allowing modern data collection and processing technologies to generate and download information for statistical process control. Master Gages: One vs. Two Master Systems Though more related to proper air system use rather than air system style, it's important to discuss the differences between a one and a two master system. The "master" philosophy, which an engineer or operator chooses, is pivotal and will affect the accuracy of the entire air gage system – regardless of air gage style. Simply put, a master gage is the physical standard by which an air gaging system, or any gaging system for that matter, is calibrated. To ensure their accuracy, masters usually have their actual size verified by mechanical comparison to certified gage blocks. A single master system, which uses only one fixed limit standard, calibrates an air gage amplifier or readout at only one scale location and, as such, indicates with assurance at only the nominal or zero point of a part tolerance. With a two master system, both the maximum and minimum sizes of the part tolerance are calibrated at the minimum and maximum part tolerance locations on the readout. With a single master system, the outer boundaries of the tolerance band are never calibrated by direct physical comparison to certified master gages. As such, the operator is forced to trust the linearity of the readout – the readout's ability to correctly translate actual size differences into correct scale readings at multiple scale locations throughout the part tolerance. Deviations in the linearity of the readout are more likely to yield inaccurate readings when a single master system is selected. This is especially true when parts are manufactured at the edges of their part tolerance. The best air gaging system in the world will be ineffective in correctly displaying part size if the readout can't translate back pressure into an accurate scaled reading. For this reason, a two master system is the typical recommended approach for selecting an air gaging system. Types of Air Gaging Systems • Back Pressure Bleed System It's the "bleed" feature in this configuration that accomplishes the back pressure bleed system's greatest benefit – its versatility. Tooling for different air gaging systems can be used with the back pressure bleed. And because of the high degree of flexibility and accuracy, many gaging companies have opted for the back pressure bleed method as the mainstay of its integrated air gaging systems. The back pressure system is configured with a fixed regulator to control incoming air pressure for maximum linearity. The key to this system's versatility involves the addition of a second adjustable restriction in the feed line opposite the output leg. It's this second restriction that allows users to adjust for different air gage tooling and readouts by varying incoming air pressure to suit the particular style of air tooling. The system's magnification is controlled by the typical adjustable restriction between regulator and air tool. The second adjustable restriction releases excess air to the atmosphere to adjust the zero position. Two setting masters – minimum and maximum – are used to calibrate the system, defining and displaying both ends of the particular tolerance range. Besides the versatility gained by the ability to use multiple air tool styles, back pressure bleed systems operate generally at higher air pressures than other systems, and, as such, permit greater nozzle drop – the distance of the air nozzles from the work piece. Farther away from the actual measuring surface, nozzles in back pressure bleed systems are more protected against wear and damage that can affect measurement accuracy. As such, typically these air tools last much longer than other conventional options. The higher air pressure also offers better self-cleaning properties, as there's less likelihood that a chip or debris will be mistaken as the actual wall of a part. In addition, this system is capable of the broadest magnification adjustment of any air gage system. It accommodates almost any size nozzles, as large as 0.093 in. or as small as 0.02 in. This is especially beneficial when small nozzles are required to check narrow lands. A modern variation on the back pressure bleed system also can include a special bias circuit. The bias eliminates the need for an expensive precision air regulator and accomplishes the same result, usually at lower cost. Incoming line pressure is split into two legs, one to the usual back pressure bleed configuration above, the other through a fixed bleed, to an air-electronic pressure differential chip. The output leg is also connected to the chip, which sends its differential signal to the amplifier column. As line pressure varies, the differences between the two legs cancel out each other, thus maintaining a relative zero regardless of changes in the line pressure. Applications where there are substantial fluctuations in line pressure can make this option attractive. • Back Pressure System If you remove the second adjustable restriction from the back pressure bleed system, you've got a back pressure system. This two-master system operates just as the back pressure bleed without the tooling versatility benefit. The back pressure system requires dedicated tooling and amplifiers with limited ranges. As such, this gaging option has found limited modern application. • Differential System In this system, sometimes referred to as a "balanced" system, the air stream is divided and flows through two fixed restrictions. One side of the system, the bleed leg, ends in a zero valve, which balances pressure to the fixed second leg of the system, terminating at the air plug. The difference between these two legs is measured by means of the differential pressure meter that bridges these legs. The system is set to zero using a single master for each tooling configuration, making setup somewhat faster. However, the differential system amplifier can only be set to zero. Damaged or worn tooling could result in inaccurate readings. As discussed previously, a single master system presents more inherent risks than a two-master system. Tooling for the back pressure differential system needs be ordered for each particular magnification. Because the single-master system has fixed magnification, worn, damaged, or fouled tooling must be returned to the manufacturer for service. Another drawback of this system is that each amplifier only accommodates one full-scale value. If an application requires the measurement of different tolerances, several amplifiers may be necessary. • Flow System As in the simple flow circuit discussed earlier, the air flow variation is measured and read in a flow meter tube, which supports a float of some kind. Flow systems require a two-master system and allow magnification and zero position to be set by two adjustable restrictions. As such, the flow systems provide good accuracy in reporting work piece deviations within tolerance, similar to the back pressure bleed system. The range of magnification is augmented by changing flow tubes and scales, rather than by a simple adjustment. Flow gages, by their nature, require a greater volume of air to generate movement of the float; this flow requirement can vary drastically from manufacturer to manufacturer. Flow systems require tooling with larger nozzles, which must be kept closer to the part by designing them with a shallower nozzle drop. As discussed, shallow nozzle drops can shorten tool wear life. Also, when the measurement of smaller work pieces necessitates smaller air plugs and smaller nozzles – as when measuring small lands – it's often difficult to attain proper amplification. To its credit, the flow system can be used with long hoses without affecting the response time of the amplifier. This feature makes the system suitable for checking long holes, such as gun barrels or oil drill bushings. It also facilitates other high production applications where air gage actuation time – the time it takes for the air gage amplifier to attain the air pressure necessary to make an accurate reading – must be kept to a minimum. Why Use Air Gaging Today? Air gaging is an efficient and reliable method of measurement that's ideal for measuring dimensions with tolerances smaller than 0.005 in. When gaging tight tolerances, a resolution as small as 0.000002 in. can be achieved. Its non-contact characteristic makes air gaging particularly useful for checking soft, highly polished, thin walled, or other delicate materials. Chief among the benefits of air gaging is its ease of use, which produces accurate results even when used by unskilled operators. Operation is as simple as presenting a tool to a work piece and observing a reading. Air gaging operation is fast, as well. A row of multiple column amplifiers can be scanned in one glance, reducing time and operator fatigue. And relationships – squareness, for example – that cannot be checked by fixed limit gaging and are costly by other means, are easily measured and amplified with air gaging. Once the basic air gaging system is purchased, relatively inexpensive additional tooling can be used for a wide variety of applications. Air gages effectively measure all types of dimensions and are particularly suited to measuring dimensional relationships and match gaging. Most air gaging systems operate at air pressures that can purge work pieces of contaminants such as abrasive particles and coolant at the measurement point, eliminating the need for a separate cleaning in most operations. Also, since air gage tooling has no moving parts, it's virtually immune to fouling. For these reasons, air gaging has been and will continue to be an effective and efficient shop floor metrology solution for years to come. Part II By: Robert Edmunds III - Edmunds Gages As seen at: NEWS & ANALYSIS@metrology world.com  

"Air gaging is the greatest thing since sliced bread," a friend once told me.  And he was right — air gaging is good.  It's fast, high resolution, non-contact, self-cleaning and easy to use.  For use in a high-volume shop, it's hard to beat.  But that begs the question, "If air gaging is so good, why would you ever consider going back to contact type gaging?" The answer is that while air gaging does provide all of the benefits listed above, it and everything else that obeys the laws of physics, has some limitations.  There are, in short, some trade-offs and for every advantage you gain in the measuring process with air, you will have to pay the price and sacrifice something else.  The real question is, "What are those limitations and how can you best work with them?" Air gaging gives you a fast measurement device that provides superior reliability in the dirtiest shop environment.  But you give up things like measurement range and a clear delineation of surface.  Air gaging has about 10 - 20% of the range of a typical electronic transducer with similar resolution. The response of air to surface finish, however, is more complicated.  Think of an air jet.  The measurement 'point' is really the average area of the surface the jet is covering.  Now consider the finish, or roughness, of that surface.  The measurement point of the air jet is actually the average of the peaks and valleys the jet is exposed to.  This is not the same measured point you would have if a contact type probe is used.  This difference is a source of real gaging error, and one which is most often apparent when two different inspection processes are used. For example, let's say we have a surface finish of 100µ" on a part, and we're measuring with an air gage comparator and two-jet air plug that has a range typically used to measure a 0.003" tolerance.  The typical gaging rule says you should have no sources of error greater then 10% of the tolerance.  In this example, that's 0.0003".  If we used this plug on the 100µ" surface, the average measuring line is really 50µ" below the peak line.  Double this error with two jets and you get 0.0001" or 30% of the allowable error.  That's pretty significant and air would probably not be a good choice for this part.  As a general rule, the limit for surface finish with an air gage is about 60µ", but it really depends on the part tolerance. This source of error should also be considered when setting the plug and comparator to pneumatic zero.  If the master and the part have similar surface finishes, then there is little problem.  Most master rings are lapped to better then a 5µ" finish.  However, if the gage is now used on a 200µ" finish part, there would be significant error introduced.  For most applications, there should be no more then 50µ" difference between the master and the part the gage is measuring.  Even this can be significant if the tolerance of the part is as little as 0.001". In some applications air gaging can be the best thing since sliced bread.  In others, you can get in trouble with the butter.  When measuring porous surfaces, narrow lands, and areas extremely close to the edge of a hole, stick with a fixed size, mechanical plug with probe contacts. However – there is a twist as long as you can do some testing and make a few assumptions. The fact is that air gaging is a comparison measurement. The assumption is that the surface finish is a consistent and repeatable result of the manufacturing process. If this assumption is taken testing between the air gage and a fixed mechanical gage can determine an offset value. This offset value can now become part of the air gaging result to project the size as if it were measured by a mechanical gage. But – this is a topic for another article. By George Schuetz, Mahr Federal  

1. Calibrate the air gaging system using the max and min masters in a normal manner. 2. Orient the tool with a horizontal centerline so the jets are situated vertical and located midway within the width of the maximum setting master. This will produce a readout indication at the high limit end of the tolerance zone on the scale. Note the exact readout indication. 3. Reposition the air tool so that the nozzles are reversed 180 degrees. Observe any difference in the indicator reading from step 1 above. Should a difference greater than one division on the scale be encountered, the tool should be considered for repair to correct "jet balance".

1. Calibrate the air gaging system using the max and min masters in a normal manner. 2. Orient the tool with a horizontal centerline and with the jets located horizontally and located midway within the width of the maximum setting ring. Observe the exact readout indication. 3. Rotate the tool so that the jets are reversed 180 degrees. Observe any difference in the indicator reading from step 1 above. Should any differences greater than two divisions on the scale be encountered, the tool should be considered for overall repair.  

It is perfectly natural that machinists should have an affinity for mechanical gages. To a machinist, the working of a mechanical gage is both straightforward and pleasing. Air gages, on the other hand, rely on the action of a fluid material, the dynamics of which are hard to (shall we say?) grasp. But air gaging has many advantages over mechanical gages and should be seriously considered as an option for many applications. Air gages are capable of measuring to tighter tolerances than mechanical gages. The decision break-point generally falls around 0.0005 inch; if your tolerances are tighter than that, air gaging provides the higher resolution you will need. At their very best, mechanical gages are capable of measuring down to 50 millionths, but that requires extreme care. Air gages handle 50 millionths with ease, and some will measure to a resolution of 5 millionths. But let’s say your tolerances are around 0.0001 inch and mechanical gaging would suffice. Air still provides several advantages. The high-pressure jet of air automatically cleans the surface of the workpiece of most coolants, chips, and grit, aiding in accuracy and saving the operator the trouble of cleaning the part. The air jet also provides self-cleaning action for the gage plug itself. However, the mechanical plug-type gages can become clogged with cutting oil or coolant and may require occasional disassembly for cleaning. The contacts and the internal workings of mechanical plug gages are subject to wear. There’s nothing to wear on an air plug except the plug itself, and that has such a large surface area that wear occurs very, very slowly. Air gages consequently require less frequent mastering and, in abrasive applications, less frequent repair or replacement. On some highly polished or lapped workpieces, mechanical gage contacts can leave visible marks. Air gaging, as a non-contact operation, won’t mark fine surfaces. For the same reason, air gaging can be more appropriate for use on workpieces that are extremely thin-walled, made of soft materials, or otherwise delicate. Continuous processes, as in the production of any kind of sheet stock, rolled or extruded shapes, also benefit from non-contact gaging. Air equipment can save time in almost any gaging task that is not entirely straightforward. Air plugs with separate circuits can take several measurements simultaneously on a single workpiece,or example, to measure diameters at the top and bottom of a bore for absolute dimensions, or to check for taper. Jets can be placed very close together for measurements of closely spaced features. Air gages are capable of measuring to tighter tolerances than mechanical gages. The decision break-point generally falls around 0.0005 inch; if your tolerances are tighter than that, air gaging provides the higher resolution you will need. At their very best, mechanical gages are capable of measuring down to 50 millionths, but that requires extreme care. Air gages handle 50 millionths with ease, and some will measure to a resolution of 5 millionths. But let’s say your tolerances are around 0.0001 inch and mechanical gaging would suffice. Air still provides several advantages. The high-pressure jet of air automatically cleans the surface of the workpiece of most coolants, chips, and grit, aiding in accuracy and saving the operator the trouble of cleaning the part. The air jet also provides self-cleaning action for the gage plug itself. However, the mechanical plug-type gages can become clogged with cutting oil or coolant and may require occasional disassembly for cleaning. The contacts and the internal workings of mechanical plug gages are subject to wear. There’s nothing to wear on an air plug except the plug itself, and that has such a large surface area that wear occurs very, very slowly. Air gages consequently require less frequent mastering and, in abrasive applications, less frequent repair or replacement. On some highly polished or lapped workpieces, mechanical gage contacts can leave visible marks. Air gaging, as a non-contact operation, won’t mark fine surfaces. For the same reason, air gaging can be more appropriate for use on workpieces that are extremely thin-walled, made of soft materials, or otherwise delicate. Continuous processes, as in the production of any kind of sheet stock, rolled or extruded shapes, also benefit from non-contact gaging. Air equipment can save time in almost any gaging task that is not entirely straightforward. Air plugs with separate circuits can take several measurements simultaneously on a single workpiece, for example, to measure diameters at the top and bottom of a bore for absolute dimensions, or to check for taper. Jets can be placed very close together for measurements of closely spaced features. Air plugs are available (or can be readily engineered as “specials”) to measure a Section K 2 wide range of shapes that would be difficult with mechanical tools. Examples include: spherical surfaces, interrupted bores, tapered bores, and slots with rectangular or other profile shapes. It would be possible to design a fixture gage with a number of dial indicators to measure several dimensions in a single setup, such as diameters of all the bearing journals on a crankshaft. But a fixture gage using air gaging will almost nevitably be simpler in design and fabrication, easier to use, less expensive and more accurate. Because of the relative simplicity of fixture design, air gaging is especially suited to relational, as opposed to dimensional, measurements, such as squareness (see illustration), taper, twist, parallelism, and concentricity. Air gaging isn’t perfect, though. Its high level of resolution makes air gaging impractical for use on workpieces with surface finish rougher than 50 micro inches Ra because the readings would average the highs and lows of the rough surface. Most important, air gaging has relatively high initial cost, so it is usually reserved for large production runs. Clean, compressed air is also expensive to generate and must be figured into the equation. In general, however, air gaging is the fast, economic choice for measuring large production runs and/or tight tolerances.   Reference: Schuetz, George. "You Won't Err With Air." Modern Machine Shop. Modern Machine Shop, 02 Sept. 2001. Web. 01 May 2017.  

Previously, we touched on several applications where air gaging is particularly practical. These include relational, as opposed to dimensional, measurements, such as distance between centers, taper and concentricity. Along with high resolution and magnification, speed and repeatability, air gaging exhibits great flexibility. Air gages are often simpler and cheaper to engineer than mechanical gages. They don’t require linkages to transfer mechanical motion, so the “contacts” (jets) can be spaced very closely, and at virtually any angle. This allows air to handle tasks that would be difficult or extremely expensive with mechanical gaging. Gaging the straightness and/or taper of a bore is a basic application that benefits from close jet spacing (see Figure 1). All it takes is a single tool with jets at opposite sides of the gage’s diaphragm. The gage registers only the difference in pressure between the two sets of jets, directly indicating the amount of taper. The concept can be applied to a fixture gage, to measure several diameters and tapers in a single operation. This fixture would be much simpler than a comparable gage equipped with mechanical indicators, each one outfitted, perhaps, with a motion transfer linkage and retracting mechanism. And the air gage could have an electrical interface to single in- or outof-tolerance conditions with lights—much quicker to read than dial indicators. From one basic concept, we have just described at least four options (there are more). How is that for flexibility? Note the basic principle: air circuits operating on one side of a diaphragm measure dimensions; circuits on opposing sides measure relational measurements or differences between features. The beauty of the concept is that you can choose to ignore dimensions while seeing only relational measurements, and vice versa. You never have to add, subtract, or otherwise manipulate gage data. It can all be done with direct-reading. Section K 3 Consider, for example, the fixture gate which allows the shaft to be rotated. Two jets on opposing sides of the journal, acting on the same side of the diaphragm, will accurately measure the diameter of the journal, even if it is eccentric to the shaft. As the shaft is rotated, the air pressure increases at one jet, but decreases at the other one. Total pressure against that side of the diaphragm remains constant, so we obtain a diameter reading. If the two circuits operate on opposite sides of the diaphragm (see Figure 2), the gage reflects not the total pressure of the circuits, but the difference between them. Higher pressure on one side or the other, therefore, indicates the journal’s displacement from the shaft centerline. Naturally, one can position the jets and circuits to measure both features at once, and add another set to check for taper. Could you design a mechanical gate to accomplish all this at once? Perhaps, but at the cost of mechanical complexity. The same principles apply to gages designed to measure the squareness of a bore (see my July, 1992, column). The included angle of a tapered hole, or the parallelism (bend and twist) and distance between centers of two bores (such as, a connecting rod’s crank and pin bores). Bore gages with the proper arrangement of jets can turn checking barrel shape, bellmouth, ovality, taper and curvature into quick, one-step operations. Contact-type air probes, which use air to measure the movement of a precision spindle, provide the major benefits of air gaging (high resolution and magnification) where the use of open air jets is impracticable for use in measuring. Open jest can’t measure against a point or a very narrow edge. The narrowest jet orifices are 0.025 inch, and these require a somewhat broader workpiece surface to generate the necessary air “curtain” to read accurately. Contact-type air probes, however, can gage with point or edge contact. Rough surfaces (above 50 microinches Ra) will baffle an open jet, but present no problems to a contact probe. And where open jets are limited in range to 0.003 inch to 0.006 inch measuring range, contact probes can go out to 0.030 inch (at some loss in sensitivity, however). Contact probes are often mounted in surface plates to measure flatness. Depth measurement of blind holes is another common use for long-range probes. You needn’t be a gage engineer to appreciate the flexibility of air gaging, or to understand how it can simplify gaging tasks. Just remember that virtually all kinds of measurements -- both dimensional and relational--can be performed with air, and that the more complex the task, the more air recommends itself. It is perfectly natural that machinists should have an affinity for mechanical gages. To a machinist, the working of a mechanical gage is both straightforward and pleasing. Air gages, on the other hand, rely on the action of a fluid material, the dynamics of which are hard to (shall we say?) grasp. But air gaging has many advantages over mechanical gages and should be seriously considered as an option for many applications. Air gages are capable of measuring to tighter tolerances than mechanical gages. The decision break-point generally falls around Section K 4 0.0005 inch; if your tolerances are tighter than that, air gaging provides the higher resolution you will need. At their very best, mechanical gages are capable of measuring down to 50 millionths, but that requires extreme care. Air gages handle 50 millionths with ease, and some will measure to a resolution of 5 millionths. But let’s say your tolerances are around 0.0001 inch and mechanical gaging would suffice. Air still provides several advantages.The high-pressure jet of air automatically cleans the surface of the workpiece of most coolants, chips, and grit, aiding in accuracy and saving the operator the trouble of cleaning the part. The air jet also provides self-cleaning action for the gage plug itself. However, the mechanical plug-type gages can become clogged with cutting oil or coolant and may require occasional disassembly for cleaning. The contacts and the internal workings of mechanical plug gages are subject to wear. There’s nothing to wear on an air plug except the plug itself, and that has such a large surface area that wear occurs very, very slowly. Air gages consequently require less frequent mastering and, in abrasive applications, less frequent repair or replacement. Written by George Schuetz, Director of Precision Gages, Mahr Federal Inc.  

We concluded that air gaging represents the method of choice for most high-resolution measurements on large production runs. While quite durable and reliable compared to mechanical gages, air gaging is not care-free. Accurate air gaging requires proper maintenance of the tooling, and vigilance over the air supply. Although the factory air supply may not be under the gage user’s control-- compressors and air lines may be shared by dozens of other users--the gage user must ensure that the air reaching his gage is clean, dry, and fairly stable in pressure. Tooling, on the other hand, is directly under the gage user’s control, and he is responsible for its maintenance. Proper tool maintenance simply means keeping it clean and dry inside and out. Contaminants such as chips, dirt, coolant, and cutting fluid may be picked up from workpieces, while water and oil are likely to come from the air source itself. Although the air flow tends to clear out most air passages on its own, some contamination may occur in the master jet or measuring jets. Even accumulations of only a few millionths of an inch can throw off a measurement. The gage must be inspected and cleaned when necessary. Repeated mastering that produces varying readings is a good indication of dirty jets. Shop air is difficult to keep clean and dry. Air dryers are not entirely adequate. The very act of compressing air produces moisture, and a compressor’s need for lubrication inevitably generates some oil mist in the line. Oil and water mist can actually act as an abrasive and cause part wear over long periods (for example, the Grand Canyon), so don’t leave the gage on overnight. Our near-term goal, however, is simply to prevent mist from entering the gage and fouling the jets. To do this, we must employ proper air-line design to intercept it before it enters the meter. Air main lines should be pitched down from the source, with a proper trap installed on the end. Feed lines should also be equipped with traps. Take air from the top rather than Section K 5 the bottom of the mains, so that moisture doesn’t drain into the fee. Long, gentle bends on feeds are preferable to hard angles and close ells. Bleed air lines before connecting gages to them. Gages must always have a filter in place when operating, and this should be changed when it becomes saturated. So, enough about moisture and that oil. Let’s talk about air. Air leaks are another common cause of air gage inaccuracy. To test, cover the measuring jets tightly with your fingers and observe the indicator needle. If it’s not stationary, check all fittings, tubes and connectors for leaks. Simple. Most factory air lines run at about 100 psi, but depending upon the demands of other air users, this can fluctuate widely. Properly designed air gages operate reliably over a wide range--some as wide as 40-150 psi--so a certain amount of fluctuation is acceptable. Other gages are more sensitive and must be isolated from fluctuations by using a dedicated or semidedicated air line. To check the sensitivity, simply leave a master in place on the tool and observe the indicator for movement as other air-line users perform their normal tasks. If large pneumatic equipment is being used on the same air line, surges over 400 psi might be generated that could blow out the built-in regulator and damage the gage itself. Again, isolation of the line is the solution. Tight, clean and dry: the requirements of air gaging aren’t very different from mechanical gaging after all. On some highly polished or lapped workpieces, mechanical gage contacts can leave visible marks. Air gaging, as a non-contact operation, won’t mark fine surfaces. For the same reason, air gaging can be more appropriate for use on workpieces that are extremely thin-walled, made of soft materials, or otherwise delicate. Continuous processes, as in the production of any kind of sheet stock, rolled or extruded shapes, also benefit from non-contact gaging. Air equipment can save time in almost any gaging task that is not entirely straightforward. Air plugs with separate circuits can take several measurements simultaneously on a Section K 6 single workpiece, for example, to measure diameters at the top and bottom of a bore for absolute dimensions, or to check for taper. Jets can be placed very close together for measurements of closely spaced features. Air plugs are available (or can be readily engineered as “specials”) to measure a wide range of shapes that would be difficult with mechanical tools. Examples include: spherical surfaces, interrupted bores, tapered bores, and slots with rectangular or other profile shapes. t would be possible to design a fixture gage with a number of dial indicators to measure several dimensions in a single setup, such as diameters of all the bearing journals on a crankshaft. But a fixture gage using air gaging will almost inevitably be simpler in design and fabrication, easier to use, less expensive and more accurate. Because of the relative simplicity of fixture design, air gaging is especially suited to relational, as opposed to dimensional, measurements, such as squareness (see illustration), taper, twist, parallelism, and concentricity. Air gaging isn’t perfect, though. Its high level of resolution makes air gaging impractical for use on workpieces with surface finish rougher than 50 microinches Ra because the readings would average the highs and lows of the rough surface. Most important, air gaging has relatively high initial cost, so it is usually reserved for large production runs. Clean, compressed air is also expensive to generate and must be figured into the equation. In general, however, air gaging is the fast, economic choice for measuring large production runs and/or tight tolerances. Written by George Schuetz, Director of Precision Gages, Mahr Federal Inc.  

Conventional air gaging for measuring inside diameters is typically limited to a minimum size of about 0.060"/1.52mm: below that, it becomes difficult to machine air passages in the plug tooling, and to accommodate the precision orifices or jets. But air gaging is among the most flexible of inspection methods, and with a simple change of approach, it can be used to measure very small through holes, below 0.040"/1mm in diameter. Most air gages measure back-pressure that builds up in the system when the tooling is placed in close proximity to a workpiece. In the case of bore gaging, a smaller bore means closer proximity of the part surface to the jets: this results in higher air pressure, which the gage comparator converts into a dimensional value. A few air gages measure the rate of flow through the system rather than back-pressure: as tool-to-workpiece proximity decreases, flow also decreases. The flow principle can be effectively applied to measure very small through holes, even on air gages that were designed to operate on the back-pressure principle. Rather than installing tooling at the end of the air line, the workpiece itself is connected to the line. Smaller bores restrict the flow of air more than larger ones. Thus, the workpiece essentially becomes its own gage tooling. This approach works on all common types of back-pressure gages: single-leg gages requiring dual setting masters, as well as differential-type gages, which typically use just one master. Air flow is proportional to the bore's cross-sectional area, but area varies with the square of diameter. Gage response in this setup, therefore, is non-linear. Nevertheless, this rarely causes problems, because the range of variation to be inspected is usually very small, and the gage is typically set to both upper and lower limits using dual masters or qualified parts. If numerical results are required, specially calibrated dials may be Section K 7 installed on analog comparators, while some digital comparators allow software correction. Back-pressure air gages operating as flow gages for small holes have been used in a number of specialty applications, ranging from fuel injection components to hypodermic needles. Often, all that is required is a special holder that allows the part to be attached quickly and easily, with a good air seal. Air pressure and flow stabilize quickly, making this method efficient for high-volume inspection. Like other forms of air gaging, flowtype measuring of small through holes is extremely adaptable. It has been used to measure IDs as small as 12 microinches/0.3 micrometers, and as large as 0.050"/1.27mm. Range of measurement can be as long as 0.006"/0.15mm, and discrimination as fine as 5 microinches/0.125 micrometers. In some cases, where the hole is so small that air flow is negligible, bleeds may be engineered into the system to boost total flow to a measurable level. On the other hand, excessive flow through large bores may be brought down into a measurable range by engineering restrictors into the system. Some parts, including some fuel injection components, have two holes sharing a common air passage, and require that the holes be measured twice: once simultaneously, and once independently. To accommodate this requirement, special two-station air gages have been designed, where the first station connects the air flow through both holes, while the second station only connects the air circuit to one of the holes, and blocks the other. Many other methods exist to inspect small holes. Some applications are served well by microscopes and optical comparators, although neither is well suited to high-volume production applications, and both are limited in the part configurations they can accept. Go/nogo gaging with precision wires is also practical only for very low volume tasks. The air gaging method described here often requires a modest level of application engineering, and occasionally a custom dial face or gaging fixture, but it lends itself well to high throughput inspection of very tight tolerances. Some users have experienced up to 300 percent increases in efficiency compared to other methods. In discussing air gaging in past columns, we've often emphasized its flexibility. With it, one can measure a wide range of dimensional characteristics, including inside and outside diameters, feature location, thickness, height, and clearance/interference. Air can also be used to measure geometry characteristics such as roundness, squareness, flatness, parallelism, twist, and concentricity. And we've seen how air gages can measure very deep bores, blind holes, and counterbores. The use of air gaging to inspect very small through holes is yet another example of the tremendous adaptability of this relatively simple, but very cost-effective technology. Schuetz, George. "Air Gaging For Itty-Bitty Holes." Modern Machine Shop. Modern Machine Shop, 02 Jan. 1998. Web. 23 May 2017.  

Air gaging has many advantages as an inspection method. It is quick and easy to use, requiring little skill on the part of the operator. It is highly adaptable to measuring special features for both dimensional and geometric tolerances, ranging from simple IDs and ODs to taper, flatness, and runout. With different tooling readily installed on the gage display unit, it can be highly economical. And as a non-contact form of measurement (in the sense that there are no hard contacts), air gaging is useful for measuring delicate or flexible surfaces, and for monitoring the stability of continuous processes such as drawing and extruding. Once the decision has been made to use air, the user can choose between three basic types of gages, each operating on a different principle. These are: the flow system; the differential pressure or balanced system; and the back-pressure system. Section K 8 In older flow-type gages, air flows upward through a graduated glass column containing a float. Exiting the column, it flows through a tube to the tooling, where it exits through precision orifices or jets. Flow increases with clearance between the jets and the workpiece. When clearance is large, air flows freely through the column and the float rises. When clearance is small, air flow decreases and the float descends. Flow systems are not very popular in production environments, because they do not readily provide high magnification, and tend to be sensitive to clogging. Differential systems offer linear response over a relatively long range: a single master is therefore sufficient to establish the zero point and still assure excellent accuracy on both the plus and minus sides. Both differential and back-pressure systems are very well suited to production gaging applications, for different reasons. Differential systems are capable of higher magnification and discrimination; are easier to use because of greater tool-to-part clearance and the requirement for only one master; and are more stable. Back-pressure systems offer lower cost, adjustable magnification, and greater interchangeability of tooling between manufacturers. See the table for a summary of benefits associated with these gages. Written by George Schuetz, Director of Precision Gages, Mahr Federal Inc.  

We have touched on different applications of air gaging: size, match gaging and form applications such as taper. In this column, we'll discuss air straightness plugs. The typical out-of-straightness condition is seen as a "bow" form within the bore that was introduced as part of the manufacturing process. The air straightness plug attempts to measure the depth of its curve. Usually this out-of-straightness condition is all within one orientation along the axis of the hole. Design: A typical air plug has four measuring jets in two opposing sets—two near the middle and two near the ends, as seen in the diagram below. This allows the plug to look at both extremes of the bow condition. There are no rules for the exact positioning of the jets relative to each other, as is sometimes the case with taper or squareness checks. Nor are ratios involved. The air jets at the extreme of the plug are positioned to inspect for the out-of-straightness condition, usually specified over the total length of the bore. In order to understand how a straightness plug works, we have to look at the combinations of jets typical in air tooling. Differential Measurement: A two-jet plug is a differential measuring system. Imagine a two-jet air plug inside a master ring with the indicator reading zero. Now move the plug so one jet is pressed against the side of the ring. This increases the back pressure on one jet and decreases it on the other. The indicator reading does not change because the combined pressure remains the same. When you insert the plug into a smaller or larger test part, however, the pressure changes, and the gage reads the differential. An extension of the two-jet air plug is a four-jet system. Four jets are added together, and if the plug is moved in any direction, an average (differential) reading is made. The four jets see four pressure changes and add them all together. If there is a change in any of the measured dimensions, the total—and the reading on the indicator dial—changes. The four jets are normally at the same level or plane on the plug. In theory, the four jets could be moved anywhere along the length of the plug independently, and if they are positioned at 90 degree angles to each other, they will measure an average diameter of the bore. If we move the jets so that two are on the same side at the extremes of the plug, and the other two are moved to the center on the other side of the plug, we have the straightness plug described above. If the part being measured is perfectly straight and the plug is moved up and down, it acts like a two-jet piece of tooling. The two jets on the top are offset by the two jets on the bottom, and the result is no change on the display. But if the bore is not perfectly straight, then the combined pressure changes and the differential is shown on the instrument. Dynamic Measurement: But don't think you've got it down so fast. If the straightness plug is simply inserted into the bore, the display shows a number. What does that number mean? Straightness needs to be made as a dynamic measurement, similar to a squareness check. With both of these form errors, the out-of-form condition is at its maximum along the axis at two positions, 180 degrees opposite each other. Looking at the diagram, we can see what the inner and outer jets see. When the jets are in line with the bow (up and down), they are seeing either their maximum or minimum reading, depending on the orientation. When moved 180 degrees, the inner and outer jets reverse roles. The same value is seen, and the plug is working in its differential mode. As the plug is rotated through 180 degrees exploring the bore, the jets experience maximum clearance, then find minimum clearance, usually at right angles to each other. The difference between the two is the out-of-straightness condition as seen over the total length of the plug measurement length. Think of it this way: If you looked at the bore from the end and drew a line around the extremes of its path, you would end up with an ellipse. If you had an ellipse in a hole, a two-jet air plug could measure the variation in size by rotating it in the part. Think of the straightness plug as a stretched-out air plug with four jets doing the same thing. The air straightness plug, though more complicated, maintains the advantages of a standard air plug—easy setup, easy use and high precision results. Schuetz, George. "The Three Ds Of Straightness Plugs." Modern Machine Shop. Modern Machine Shop, 04 Sept. 2002. Web. 06 June 2017.

Adjustable magnification; tooling flexibility. Less costly tooling. Higher air pressure: cleans part surface more effectively. Two masters provide greater traceability. More manufacturers; wide compatibility. Differential (Single Master) Gages Higher magnification, discrimination; longer range. Greater tool-to-part clearance reduces wear, speeds usage.. Better stability, dependability; no drift. Better for automatic control applications, and data collection for SPC. TSingle master makes gage easier, quicker to set.

Air gaging is often referred to as a non-contact form of measurement. This is accurate, to the extent that there's no metal-to-metal contact between a sensitive gage component and the workpiece. Nevertheless, air gage tooling—including air plugs for inside diameter measurements—does generally come in contact with the workpiece, and may show wear after several thousand measurements or years of use. (The comments here are equally applicable to electronic plug gages.) When, due to wear, the clearance between the gage and the workpiece exceeds Section K 9 the design clearance, centralization error results. The air jets then measure a chord rather than the true diameter of the part. As the distance between the chord and the bore centerline increases, we begin to see measurement inaccuracy. Another form of error occurs when the jet centerline is not on the plug centerline. In this case, the plug will always measure a chord of the part. How much centralization error is allowable depends upon both the diameter of the workpiece and the dimensional tolerance specification. Obviously, looser tolerances can "tolerate" more measurement error. But equal amounts of misalignment will cause greater centralization error in a small bore than in a large one. (For details on calculating the misalignment tolerance based on allowable centralization error, refer to this department in MMS 7/98.) For a gage to function properly, all the jets in an air plug (or ring) must have a common recess depth and orifice diameter. But recesses and orifices may become clogged with contaminants, damaged through accident, or worn unevenly through very heavy usage. This causes another form of measurement error, called "balance error." It is easy to inspect for these various forms of error. Certain types of gage usage will demonstrate characteristic wear patterns. In hand-held gaging applications, wear usually occurs fairly evenly around the circumference of the plug. If, however, the plug is horizontally mounted on the front of an air gage display, then the top surface will probably experience the most wear. If wear is expected to be fairly regular around the circumference, start by securing the gage horizontally, with the jets also horizontal. Place a master on the plug, release it, and note the reading (Figure 1A). Then carefully raise the master until it contacts the lower surface of the plug. If the plug is worn, the readout will change as the measurement moves from a chord, through the maximum diameter (Figure 1B), to another chord. Wear may be considered excessive if the reading changes by an amount equaling 10% or more of the part tolerance. If the gage is stationary, and the jets are normally oriented horizontally, wear might be expected on the uppermost surface of the plug between the jets. Again, place a master on the plug, release it, and note the reading (Figure 2A) . Then rotate the plug 180° and note the reading again (Figure 2B). If the surface in question is worn, the second reading will be higher. If the jets are normally oriented vertically, then the top jet, or its recess, are of the greatest concern. The test is performed as above, by placing the master, noting the reading, rotating 180°, and reading again (Figures 3A, 3B). Alternately, the test can be performed by lifting the master ring, as in the first procedure described. Normally, two-jet air plugs automatically balance themselves when one of the jets is closer to the workpiece than the other—as is the case here, where the master is allowed to rest on the upper jet. But if one jet or orifice is damaged or worn, this test will demonstrate the gage's inability to maintain that balance. Plug gages tend to be highly durable, because they contact the workpiece across a broad surface area. But that doesn't mean you can ignore the possibility of poor centralization or balance. Include these tests in your annual gage calibration program. Section K 10. Schuetz, George. "Checking For Centralization And Balance Errors." Modern Machine Shop. Modern Machine Shop, 31 July 1999. Web. 13 June 2017.

Contact gaging measures the dimension from surface roughness peaks of the part being measured (i.e. at plane A - A). Open nozzle air gaging measures the mean surface of the part, which is approximately the average of surface finish peaks and valleys (i.e. at plane B - B). Technically, the mean surface would be an imaginary plane established by using the material from the peaks to fill the valleys until a level or zero line is formed. The result is that there is a difference in measurement between air gaging and contact gaging. The amount of this difference is a function of surface roughness. The inside diameter of a hole will read larger with an air gage than it will if measure with a contact gage. Conversely, the outside diameter of a shaft will read slightly smaller with an air gage. The following chart shows the diametral difference between air gaging and contact gaging: RMS DIFFERENCE RMS VALUE DIFFERENCE 2 .000005 in. 50 .000140 in. 5 .000013 in. 60 .000165 in. 10 .000025 in. 70 .000200 in. 20 .000040 in. 80 .000225 in. 30 .000080 in. 90 .000255 in. 40 .000110 in. 100 .000280 in. Edmunds- air gaging vs. contact gaging. N.p., n.d. Web. 20 June 2017

In this article, we examined the history and fundamental principles of air gaging as well as the various styles of air gaging systems in use today. This discussion will focus on the components that comprise a typical air gaging system and how they work together. We also will address the most common air gaging applications used in industrial environments. Air gages, past to present, either measure flow or back pressure. Integrated air gaging systems are comprised of these basic components: air regulator, amplifiers, tooling, setting masters, connectors, and accessories. Let's take a look at each type of component and then examine various applications. Components of an Air Gage System All air gages employ a precision air regulator, which provides consistent air pressure to the amplifier. Depending on the system, this can be as little as 10 psi or as much as 44 psi. In addition to the air regulator, air gages use various types of tooling that deliver a specific air flow or pressure to the surfaces being measured. The tooling, which can be plugs, rings, or other shapes, is configured and sized specifically for the work piece it's designed to measure. Air tooling is designed with its nozzles recessed, to achieve the appropriate clearance for the air pressure of the system being used and to gain protection against wear or damage to the nozzles. The tooling also features vents that let air escape from the work piece without creating spurious back pressure or restriction of flow. Moreover, air gage tooling is designed with properly positioned nozzles. For example, two nozzles are needed to measure a diameter. The nozzles are balanced to ensure accurate and repeatable readings, regardless of the skill level of the worker using them. For instance, if a tool should be applied to the work piece radially off-center, the decrease in air flow from the closer nozzle is offset by increased flow through the further one. Hence, the flow and back pressure for the tool as a whole remains constant. Another common component of the typical air gaging system is an amplifier. Available in several styles, including an air-electronic column, dial-type meters, or flow meter tube, the amplifier provides visual representation of the size being measured, enabling the user to take readings quickly and accurately. Back pressure systems either use columns or dials to display readings; flow systems use flow meter tubes. If an application requires the operator to make multiple measurements, more than one amplifier must be viewed at a time. However, checking multiple measurement results from several dial readouts can be difficult. To make it easier to compare results, it's recommended that the air-electronic columns or flow meter tubes are parallel stacked, where all readouts line up vertically. Air-electronic columns also offer a more sophisticated system for multiple-function processing, as well as output of data for printing and for SPC and other data processing uses. Finally, setting masters are used to calibrate air gaging systems. Depending on the system, one or two masters – usually in the form of discs or rings – are employed. Usually, two masters are recommended for absolute accuracy. (See "Back Pressure Bleed System" in Part I for further explanation.) Typically fabricated from steel, chrome, or tungsten carbide, masters are furnished to tolerances ranging from class X to XXX. Make sure you understand the lab's relationship with NIST so you'll know whether the lab's masters are directly or indirectly traceable to NIST. Edmunds, Robert, III. "Putting Air Gages to Work." Edmunds Gages - Metrology World Article - Putting Air Gages to Work. N.p., n.d. Web. 27 June 2017.

Inside and Outside Diameters Air gages are most commonly used for measuring the size and form of inside diameters (IDs) and outside diameters (ODs). Two-nozzle air plugs, with nozzles diametrically opposed, are often used for internal measuring, and two-nozzle air rings are used for external dimensions.   Averaging Multiple nozzles are equally located about the circumference of the air tool to allow for average size measurement. Commonly used for thin-walled or out-of-round parts – four, six, or more nozzles are used, depending on the tool size.   Out-of-Round Air tools can gage a part for roundness. For two-point out-of-round conditions, a standard two-nozzle air tool can be used. If lobing exists in the part, an odd number of nozzles must be used, depending on the number of lobes.   Straightness A common application of air gaging is to dynamically measure the straightness or "bow" of an ID. In this case, a custom-designed air plug makes verifying a part's straightness simple and fast. (A straightness air plug cannot measure diameter).   Squareness To determine squareness of a part, for example a bore to face, air nozzles configured as a "z" are used with dynamic measurement to change the back pressure from square to out-of-square conditions.   Taper Angle variation of tapered surfaces is commonly checked with air gaging as the difference of two diameters.   Flatness To measure flatness, an air nozzle is mounted within a stationary platen. The part is then moved across the nozzle. This process provides a convenient, quick method to accurately gage flatness.   Groove Width The measurement of grooves is conveniently achieved with flat, blade-type air tools. Air gaging not only determines groove size, but with exploration around the workpiece, parallelism of the groove faces can also be determined.   Matching A specified clearance between two mating parts is often required to assure proper part operation. An amplifier allows for the individual display of the bore size, the shaft size, and the clearance between the two parts. Operators need only observe the clearance display to determine if the two components have the required match dimension.