QUALITY MEASUREMENT: Why Air Gaging Still Matters
After measuring holes for 90 years, air gages remain a powerful metrology tool. Air gages use compressed air to measure changes in pressure or flow rates. Such tests can determine attributes and measure distances between precision orifices and the workpiece. Air gaging offers sufficient magnification and reliability to measure small tolerances. It can easily determine dimensions with tolerances smaller than 0.005 inch and when gaging tight tolerances, a resolution as small as 0.00002 inch can be achieved. Because air gaging is a noncontact form of gaging, it is ideal for measuring extremely thin-walled parts for soft materials. It is fast, accurate and readily used in production environments.
Air gages measure length. Closing off of an open-air jet causes back pressure to be built up in the channel from the air supply. This backpressure can be graphed on a pressure distance curve and as the open-air jet becomes more restricted, the backpressure builds up. The pressure distance curve created by this effect is repeatable and usable for displacement measurement.
"The pressure distance curve is what makes up the air gage and is what ensures its practical use on the shop floor," says George Schuetz, director of precision gages at Mahr Federal Inc. (Providence, RI). "We have introductory information from the 1940s about air gaging, and the tooling and applications have not changed dramatically from those times, it's simply built upon the laws of physics.
"The air tooling itself is nothing more then a precision cylinder that holds-usually-two open air jets which feed to a common channel. The clearances are important, but the restriction placed on the open-air jet causes pressure to be built up which can be measured. Back in the 1940s, people understood the importance of the two jets for differential measurement. They also knew that more jets could be added for finding geometric condition-or that taper, straightness and other relationship conditions could be measured with air tooling."
Hole measurement historyAir gages first started ensuring quality in 1919. One of the first air gage systems was regulated by "bubbling" air through a specific level of water within a cylinder in which the air was passed. The specific gravity of the water has a known constant value. However, "This process could not prevent the water from evaporating and thus changes to the readings by the air gage occurred over time," says Jack A. Gaughan, sales manager of custom gaging at Edmunds Gages (Farmington, CT). "Other early air gages used bourdon tube technology to expand and contract based upon back pressure."
Practical dimensional air gages began to appear in the late 1930s. "This was when flowmeter instruments with operating pressures of 10 psig were developed," says Don Moors, president of Western Gage Corp., (Camarillo, CA). "About the same time dial-type back-pressure instruments with gage pressures at the midpoint of the scale ranging from 20 to 25 psig were developed. Both of these designs proved practical for many gaging applications, with many proponents of both systems."
During the 1940s, there was increased demand for tighter tolerance from war-related needs. After this, aeronautical requirements forced tolerances to get even tighter. Air gaging and their displays took measurement to another level and for the first time operators were able to measure 0.00002 inch on the shop floor-well before electronic amplifiers and probes.
One thing that hasn't changed considerably in the past 90 years is air gage's tooling. While there have been upgrades in materials, coatings and processes, according to Schuetz, "You could take a piece of tooling used in 1945, put it on a gage built in 2006 and it would still function. Also mechanical-style, analog displays have not changed too much over this period. Air gage displays today still use some type of pressure-sensing mechanical device with a magnifier to move a needle to display a length change. These are very similar to ones made 60 years ago. What really has changed are the options for displays that can be used. In the past 50 years, the electronic readout is what brought air gaging to a new level by providing more information in an easier-to-understand format to the operator.
"The electronic pressure sensor is what brought the air gage display into the 21st century. Combined with the power of microprocessors and computers virtually anything can be done with the air pressure change," Schuetz says.
It's true that one innovation today not present 90 years ago is that air gage systems can convert pressure readings to an electronic signal easily converted to a display on an electronic column or a microprocessor-based gaging system. "These displays are more of a one-size-fits all, where the resolution can be easily changed and the displays increased or decreased without changes to the min/max masters or the gage itself," says Don Kumpula, sales manager at Air Gage Co., (Livonia, MI). "The gage results can now be stored and the data downloaded to a network system to be analyzed for statistical process control."
Pressure, flow and varietyThe different types of air gages are based upon variations of two basic principles of compressed air: pressure and flow. Pressure type gages monitor the back pressure within the gaging circuit as the product size changes. Flow type gages monitor the amount of air passed through the circuit as the product changes size. Each of the popular types is defined within the attached system schematic diagrams.
There are advantages and disadvantages to each type of system. "The back-pressure-bleed system is one of the most popular, due to its inherent accuracy from a dual-master calibration routine," Gaughan says. "The differential system is also very popular due to its extreme ease of use. So there is no real low-end vs. high-end variation, just choices based upon basic principles and beliefs of what works best for the individual."
Air gaging is manufactured per its application, so there can be a variety of uses and types. "Some would include simple, lower-end applications for measuring ID or OD using open jets or ball jets," says Rose Garvey, president at Blanchette Tool & Gage Manufacturing Corp., (Clifton, NJ). "The more complicated applications measure taper, flatness, concentricity, thickness, center distance using leaf jets, plunjets, taper air gages and multicircuit gages. A high-end application could include a special 6-inch diameter rifle-barrel twist air spindle with a 10-foot long connector to measure a cannon bore."
Often, the number of orifices and the complexity of the fixture that holds the parts often determine the value of an air gage. "The more readings and the more difficult it is to locate and hold the part, the more expensive it is," says David Birdsall, president of Birdsall Tool & Gage Co., (Farmington Hills, MI). "The introduction of computers as a replacement to columns has greatly reduced the cost of air gages. Computers run between $2,000 and $3,000 each and can replace up to eight air columns at a time. There is no reason to ever purchase a column again. They are outdated and too limited for practical use."
There are single-master and dual-master air gages. "Using a dual-master system, the operator calibrates the system by observing that the span displayed by the comparator instrument corresponds to the span between a set of minimum and maximum setting masters," Moors says. "This method sets the combined sensitivity of all the components of the gaging system at one time. Sensitivities like flow restrictors, amplifiers, pressure indicators and gaging nozzles, as well as the effect of pressure drops in air lines are included in one overall calibration. Because of this, stringent control of individual components is not necessary to obtain accurate overall results using a dual-master system.
"Single-setting master systems require accurate control of the sensitivities of both the gage readouts and the air gage members at the factory prior to shipment," Moors says. "The sensitivities of single-mastered readouts are verified using master orifices that simulate air flow to gage nozzles. The gaging member sensitivities are controlled during manufacture by precise control of the gaging nozzle geometry."
One major advantage air gages have over other types of gages is maintenance. "Air gages need less maintenance. They are clean, simple and require fewer moving parts. The measuring surfaces don't touch the piece part being measured," says David B. Hickel, engineering manager at Etamic Corp., (Plymouth, MI). "Also, air nozzles help clean debris from parts while they are being measured. Reduce mechanical apparatus and you reduce the risk of broken parts on your gage."
What to ask forBecause air gaging is manufactured per its application in a variety of uses and types, how are operators supposed to know which is the most appropriate air gage to ensure quality? According to Hickel, operators need to ask "What is the tolerance being controlled? What is the surface finish on the featured surface and what environment will the gage be used in? And what other geometric features will have a direct influence on the characteristic being measured?"
Birdsall suggests a list of questions that should be asked before purchasing and operating air gaging include, "What are the productions levels? Do they want to check the parts in-line or off-line. Are they checking every piece or random samples? What is the tolerance?"
Because air gaging has to be used for a variety of tolerances, its amplifier's flexibility needs to be questioned. Having one type of amplifier that can accommodate various sizes and tolerances makes implementation on the shop floor much easier for operators due to consistency, and also is less costly in the long run. "The popular column-type amplifiers usually have selectable ranges that can be used for the various tolerances," Gaughan says. "In contrast, most of the dial-type amplifiers have one fixed range like 0.001 inch, so if you need to measure a product with a 0.002-inch tolerance you need to buy a different amplifier. With a column product you simply select a different range."
An important question to ask is whether there a useable output that can be used for data collection, data recording or intranet purposes. With more and more emphasis on lot sampling for statistical purposes, the output from an air age becomes extremely important to its usefulness to ensure quality of parts being measured.
"Most dial-type air stands do not offer an output because of their mechanical limitation," Gaughan says. "However the column-type products, known as air-to-electronic units, usually provide an RS-232C type output signal of the dimensions measurement that can be tied to a printer, data gatherer, PC or intranet, for recording or statistical analysis."
Instead of companies asking what they need from air gaging, Thaddeus Hosford, senior mechanical designer at Verimation Technology Inc., (Novi, MI) says that they should tell suppliers what they need. "I am a believer of consultive sales, in which the sales force proposes what the end user will require," he says. "This type of sales requires that the salesmen have a thorough knowledge of the measurements required, dimension, location, tolerance, finish, material, temperature, ambient conditions and whether it is manually or automatically made. The buyer should have an idea of what they want and many times defining all of this information requires bi-directional information sharing."
Seek a smooth surfaceDespite its many capabilities, air gaging does have limitations. Noncontact air gaging requires a surface that has enough area to cause a consistent flow restriction on each air gage measurement orifice. "This may only be a 0.005- to 0.010-inch bigger diameter than the orifice diameter. It depends on the location repeatability of the gage position and other factors," Hosford says.
Noncontact air gaging requires better surface finishes than contact gaging. The smoother the surface, the more accurate the measurement results will be. "The reason noncontact air gaging is affected by surface finish is that the air gage measures the flow of air escaping past the air gage orifice," Hosford says. "This is the cylindrical area above the highest surface feature and the average cylindrical area from the lowest feature to the highest feature. Conversely, when using contact gaging, the gage contacts the highest surface features. Therefore if you compare the noncontact air gage measurement, it will be different by half the average surface finish height. However, this is impossible to test, because when the air gage is mastered it uses masters that are hardened and ground with a fine surface finish."
In addition to being sensitive to the surface finish, air gages are also affected by any impurity in the air or what is on the part. "They also require regular cleaning because if dirt builds up on the orifice or on the part, the air volume changes and affects the readings and the measurements," Birdsall says.
Another 90 years?Despite their many quality-ensuring benefits, some believe air gaging may not be around for another 90 years. "There is a pretty good chance that air gages will fade away over time," Birdsall says. "As more and more sensitive probes are designed, as computer systems are integrated into the design, the requirement for air gage applications will decrease. The one and only advantage of air gages is that they can be designed to noncontact on precision surfaces with only air touching the surface."
Electronic gages are considered to be a strong replacement. Laser gages are also becoming more popular as a replacement when the tolerances are not too tight. "There are several companies that provide precision laser noncontact physical measurement gages," Birdsall says. "These laser gages consist of fixed and separate throat, single- and dual-axis configurations. They are capable of performing noncontact measurement of ID and OD, gap, pitch, width, edge, thickness, position and movement. They are very accurate but like all noncontact gaging, they depend on a clean environment, smooth surface finishes on the parts and no obstructions."
However, while the quality industry keeps its eye on new technologies, most believe that air gages will be a mainstay for tight tolerances, dirty environments, and inexpensive and fast measurement. When applied correctly, air gaging provides operators a simple, robust and accurate measuring tool. "It is still regarded as one of the best shop floor measurement systems. I don't think much will change," Schuetz says. "Fixed mechanical plugs, CMMs, and vision systems have been nipping at its heels. But for tight tolerance and high-volume parts, air gaging may be the choice for many years to come. There are other gages or systems that can do the measurements of air gaging, but they have limitations that could include speed, ease of use, accuracy and cost. Air gaging may be the simplest high performance gaging on the shop floor system for years to come."
For more information on the companies mentioned in this article, visit their Web sites:Air Gage Co., www.airgage.com;
Birdsall Tool & Gage Co., www.birdsalltool.com;
Blanchette Tool & Gage Mfg. Co., www.blanchettetool.com;
Edmunds Gages, www.edmundsgages.com;
Etamic Corp., www.etamic.com;
Mahr Federal Inc., www.mahr.com;
Verimation Technology Inc., www.verimation.com; and
Western Gage Corp., www.westerngage.com
Sidebar: Tech Tips
- The different types of air gages are based upon variations of two basic principles of compressed air: pressure and flow.
- Air gaging has increased in popularity as part tolerances have gotten tighter.
- In addition to being sensitive to the surface finish, air gages are also affected by any impurity in the air or what is on the part.