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.