You Won't Err With Air

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.

Columns From: 9/3/2001 Modern Machine Shop,

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 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 thin-walled, made of soft materials or otherwise delicate. Continuous processes 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 wide range of shapes that would be difficult with mechanical tools.

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 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.

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