Air gaging is a fast, highly repeatable way to measure a simple diameter on tight tolerances. But it can do much more. This article covers using air gaging for form requirements: roundness, flatness, perpendicularity (squareness), taper, straightness and matching.
Most form checks rely on tracking the maximum and minimum readings and the difference between them (MAX−MIN). Modern readouts track this for you, which makes these checks simple; an operator can also watch the screen and do the math.
Roundness
Roundness needs no special equipment beyond the air plug or ring you already use for diameter. Rotate the part or the gage 180° and track MAX−MIN. This works best with a standard 2-jet gage: a 2-jet gage reads in one plane, so after 180° you are measuring the same plane again. With more jets, the gage averages the out-of-round and under-reports the actual deviation.
Flatness
Flatness uses a single air jet set into a granite or steel surface plate. Slide the part across the jet from one end to the other and track MAX−MIN.
Perpendicularity (squareness)
Use two air jets at different heights, 180° apart, with a square stop for the part to rest on. One jet sits near the top of the part and the other near the bottom. Rotate the part or the gage, whichever is easier, and track MAX−MIN.
Taper
A taper gage has at least two levels of jets: one set near the top of the taper and one near the bottom. The readout reports both diameters, and with the distance between the levels entered it calculates the taper angle in degrees, minutes and seconds. Higher-level readouts can also calculate the diameter at the face, or find where along the taper a given diameter falls.
Straightness (bow)
For bow, a special gage is built with four jets on one circuit: two on each side, with one pair closer together than the other. Track MAX−MIN as you move the gage along and around the part. On a tapered part, a third measurement shows whether the taper is concave or convex, and some readouts can graph the shape.
Matching mating parts
Air gaging is widely used to match mating components, such as sleeves and spools in hydraulics and parts in fuel injectors. Measure the sleeve bore with an air plug and the spool with an air ring at the same time, and the readout shows the clearance between them so you can verify the match.
Why it matters
Air gaging is extremely fast, so checking form at the machine saves the time and cost of sending parts to the CMM. If your print has form callouts you're checking elsewhere today, it's worth asking whether an air gage can do it on the shop floor. See custom air gaging.