In short: an air gage (also called an air gauge) blows a small, regulated flow of air through jets in a plug, ring or snap gage. The closer the part surface is to a jet, the harder it is for air to escape, so the back-pressure (or flow) changes with size. A readout converts that change into a dimension, typically resolving to 0.00005″ or finer, without anything touching the part.
The principle: clearance changes pressure
Shop air passes through a filter-regulator so it arrives at a constant pressure. It then goes through a fixed restriction and on to the jets (nozzles) in the gaging tooling. When a bore is slightly larger, the gap between each jet and the wall grows, air escapes more easily and back-pressure drops. When the bore is smaller, the gap shrinks and pressure rises. Over a short working range this relationship is close to linear, which is what makes air gaging so precise.
The parts of an air gage system
| Component | What it does |
|---|---|
| Filter-regulator | Removes water, oil and dirt from shop air and holds a steady supply pressure. |
| Air plug (probe) | Measures bores. Opposed jets read diameter; more jets can check taper, ovality or several diameters at once. |
| Air ring | Measures shafts and outside diameters. |
| Air snap gage | Measures ODs, thickness or width from one side, often on the machine. |
| Setting masters | Certified rings, plugs or discs of known size used to set the gage. |
| Readout | Converts pressure into size. Older units use dials or columns; digital readouts such as the Air Gage Pro add tolerances, color zones and data output. |
Why shops choose air gaging
- Non-contact. Air, not a stylus, touches the part, so soft, polished or thin-walled parts aren't marked or deflected.
- Self-cleaning. Escaping air blows coolant, chips and oil out of the measuring point.
- Fast. Insert the plug and read. Operators need very little training, and results don't depend on feel.
- Durable. The tooling has no moving parts to wear or stick.
- More than diameter. Jet placement lets one gage check out-of-round, taper, straightness or several features at once.
Where air gaging has limits
An air gage is a comparator with a short working range, usually a few thousandths of an inch, so each tool is built for one nominal size. It reads the average of the surface under the jet, so very rough or porous surfaces need care. It also needs clean, dry, regulated air. For most production bores and shafts held to tenths, those trade-offs are worth it.
Setting the gage: single or dual master
Every air gage is set against masters of known size. Single master systems set at one point, usually nominal, and rely on factory-matched tooling and readout for the scale. Dual master systems set at a low and a high master, calibrating the span directly. See single vs. dual master air gaging for how to choose.
Questions
What is an air gauge?
An air gauge (or air gage) is a non-contact measuring instrument. Air flows through jets in a plug, ring or snap gage, and a readout converts the back-pressure into a size reading.
What is the function of an air gauge?
To measure bores and shafts quickly and without contact, typically to tolerances of .005″ or less, and often to check form such as out-of-round or taper at the same time.
What is the working principle of an air gauge?
Regulated air passes through a restriction to the jets. The gap between each jet and the part controls how much air escapes: a smaller gap raises back-pressure, a larger gap lowers it. The readout turns that change into size.
How accurate is an air gauge?
Air gauges repeat in the sub-micron range. Accuracy depends on the masters, clean regulated air and stable temperature. See air gage vs. dial bore gauge.
Is it air gage or air gauge?
Both. "Gage" is the common spelling in US industry and standards; "gauge" is the general English spelling. They mean the same instrument.