Metrology
Metrology
Air Rings and Air Forks
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Air Rings and Air Forks
George Schuetz, Mahr Federal Inc.

We have talked many times in this column about the advantages air gaging brings to the manufacturing process, especially on the shop floor. In addition to their speed, ease of use and ability to automatically clean part surfaces before measuring, air gages are also capable of measuring to tighter tolerances than mechanical gages. At their very best, mechanical gages can measure 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.

When most people think of air gaging, they think of plugs. But a number of air ring and air fork styles provide the same benefits and some additional ones as well.

An air ring is the opposite of an air plug, and is primarily used to measure ODs. Air rings look quite simple—a steel ring with a pair of jets at a particular location—but there is a lot of engineering built into them. One trick is in calculating the size of the ring in order to achieve a good balance of clearance between the part and the opening in the ring. Too much clearance and your readout will start showing centralizing error. This happens when the part is shifted slightly in the tool and the air ring measures a chord rather than the diameter. Too little clearance and the indicator's range gets limited—or even worse—a geometry error can prevent the operator from inserting the part into the air tool.

There is also a good deal of engineering involved in building the proper recess in the jets to precisely position them a little lower than the body of the ring—to allow for ring wear—while at the same time making sure the jets are equally balanced to ensure the differential characteristics the air system needs to function.

Typically, two-jet air rings are the most common for measuring basic diameters. But as with an air plug, the position of the jets is determined by the part being measured. The ring can act like a through-hole plug, where the jets are in the center of the ring and the part is passed through. This configuration allows for exploring the whole part from end to end. But sometimes there are parts with multiple ODs, such as pullies or gears, where it is critical to measure the diameter right up to the face it's attached to. This is where a shoulder, or even a snout type ring comes into play. These are basically blind hole rings that position the jets as close as possible to the face to allow for measuring the OD right to its end. A snout style goes one step further and is used when there are other clearances or obstacles to get around.

Also like an air plug, air rings may be designed with multple jets—3, 4, or 6—to help find lobing or average diameters.

Another design decision is whether to bring the gage to the part or the part to the gage. Air rings can be portable and held in the hand while the part is inserted. Or the ring can be mounted horizontally on a bench fixture—where it is often called a "pot gage"—and the part is placed into it. For smaller parts the ring can be mounted vertically, with vee’s or chutes that are used to help align the part to the gage.

Sometimes, however, it's not possible to just take a part and place it in an air ring. On crankshafts, for example, there are many journals—some with tolerances too tight for a mechanical snap gage—and there is no way to place an air ring over the part. This is where an air fork comes in.

Making an air fork involves more than slicing an air ring in half. There are the same critical dimensions to worry about, but there are some added tricks needed to make it perform as well. One trick is to position the air jets on the expected diameter in a very repeatable fashion. With an air ring, the whole inner surface acts as a part reference. With an air fork, we don’t have that luxury, so precision-ground locating stops are manufactured into the back of the fork. This creates the reference, based on the expected part diameter, and positions the jets precisely.

By customizing the position of the air jets, air forks can really begin to shine. Jets can be placed near the end of the fork so that diameter can be measured right up to a face. Or multiple jets can be added to check two or three diameters simultaneously. With a little computing power, the three diameters can be read as one, and taper or shape can also be calculated.

Thus, when faced with measuring those tough ODs, you may want to put away the bench stand or mechanical snap and consider the advantage of air.