Air Gages

Sliced Bread And the Limits of Air Gaging

"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

Checking Nozzle 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 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".

Checking Air Gages Centrality

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.

Air Gaging for Itty-Bitty Holes

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.

Choosing the Right Air Gage

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.

Back-Pressure VS. Differential Air Gaging Back Pressure (Dual Master) Gages

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 vs. Contact Gaging

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

Putting Air Gages to Work

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.

Air Gaging Applications

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.