We’ve written many times about how air gauging allows you to measure many jobs faster, more conveniently, and more accurately than other gauging methods.

For measuring all hole conditions, air gauging is unsurpassed for speed and accuracy. When checking any dimensional characteristic, air gauging offers sufficient magnification and reliability to measure tolerances well beyond the scope of mechanical gauges.

Air gauges also measure all common types of dimensions effectively and are particularly suited to checking dimensional relationships. Some of these are taper, parallelism, squareness, straightness, and center distance. Match gauging, which permits the selection of mating parts with a specified amount of clearance or interference, is easily accomplished with just one reading on one dial.

One of the great things about air gauging is that virtually no other gauging system can put the sensors (in this case, the air jets) so close together to allow for multiple diameters or geometric conditions in very small pieces of tooling. But the fact that you can get the sensors close together to make checks for taper or squareness may also impose limits, despite the measuring capability.

Let’s review some examples.

With the typical taper gauge, whether it’s a “jam” style that measures only taper, or a clearance style that measures two diameters and taper, the two pairs of jets are set to two different heights on the taper (see Figure 1). The gauge reflects a differential change between two diameters at a fixed distance along the part.

Figure 1: With the typical taper gauge, the two pairs of jets are set to two different heights on the taper. The gauge reflects a differential change between two diameters at a fixed distance along the part.

With a squareness (perpendicularity) gauge, jet spacing is similar, but the circuit created to combine the jets is different (see Figure 2). Top and bottom jets on each side of the air plug are channeled to opposite sides of a special air meter to provide a differential-type measurement. Lack of squareness is indicated by movement of the meter hand as the part is rotated on a reference platen. This method is used primarily when the squareness reading shouldn’t be influenced by any taper condition.

Figure 2: With a squareness (perpendicularity) gauge, jet spacing is similar, but the circuit created to combine the jets is different. Top and bottom jets on each side of the air plug are channeled to opposite sides of a special air meter to provide a differential-type measurement.

The limitations of these measurements stem from jet placement. With good machining, air jets can be put very close together. In fact, it’s not unusual to see jet spacing as close as 0.20 in. between the center lines of the jets. That’s a great feature of air gauge tooling.

But think about how this affects part tolerances. Let’s say the specified tolerance for taper is +0.001 in./–0.000 in. per foot. This 0.001 in./ft tolerance seems easy enough to achieve until you look at the complexity of the inspection process. First, most parts we see are much shorter than 1 ft, so most air gauges actually compare diameters that are just 3 in. or 4 in. apart. This is no problem. However, if you want to measure this taper tolerance over a very short distance, there can be issues.

Using the example above, where we have a 0.20 in. jet spacing, the part has to meet a gauged tolerance of 0.001 in. ÷ 60, or 0.000016 in. Most common air gauging resolves to 10 µin, and even high magnification air gauging only safely resolves to 5 µin. This doesn’t leave much room for any part, gauge, environment, operator, or master variation.

Now think about the master for the gauge. Masters like to be at least five to 10 times better than the system they are being used on. Or think about the gauges required to measure these masters: There are enough zeros in these numbers to make the national debt envious!

Beyond that, there’s the concept of trying to achieve a 10% GR&R. Because this is about 4% of part tolerance, or 0.64 µin., it’s just plain unattainable.

Perpendicularity gauging works in a similar way. It’s often specified over an entire length of the surface being measured and is subject to the same ratio reduction as the angular reading seen in the taper check.

So how does one achieve these measurements with certainty on the shop floor? With tighter tolerances, the old 10:1 rule for gauging is out the window. Achieving 5:1 might also be a stretch. At some point, some gauging just may not be capable of these tight tolerances over such a short area. Is it gauging at this point, or just an indication of what’s happening? In some cases, an indication of what’s happening is all that can be expected.