Laser micrometers and vision systems get most of the attention at trade shows. They're effective technologies, but they have limits that keep air gaging an important option. Here's how a technology from before World War II measures up.
A proven principle
The first air gages were developed in France before World War II by a carburetor company that needed a reliable way to measure its carburetor jets. The principle is simple: pressure and flow change with clearance. When clearance decreases, flow decreases and pressure rises; when clearance increases, flow increases and pressure drops. The gage is set with a setting master of known size, and the readout places each part's reading on the scale between your tolerance limits.
The benefits still hold
- Easy to use, with no special training, and repeatable between operators
- Economical: tooling costs little compared with other measuring methods
- Non-contact, so it doesn't mark parts or deflect thin walls, and it measures plastics and other non-metallic parts
- Very small diameters, under 1 mm
- Fast, even on difficult geometries, which saves CMM time
- Built-in cleaning of coolant and cutting fluid, and no moving parts
Air gaging measures inside and outside diameters, taper, perpendicularity, parallelism, out-of-round, lobing, straightness, flatness, matching and angles.
How it compares with lasers and vision
Laser micrometers and vision systems can measure a wide range of parts and sizes, but lighting, shiny surfaces, and coolant or oil on the part all affect results, and getting good results takes some expertise. An air gage is built for one application, so it lacks that range, but for the task it's built for it is the fastest and most reliable method there is.
Some features can't be measured with lasers or vision at all. A small, tight-tolerance bore is a good example: a laser can't reach it, and a vision system is limited by bore depth and by pixel size. Air gaging repeats at the sub-micron level, around 0.0002 mm (0.000008″), in seconds.
Where air gaging has advanced
The plugs and rings haven't changed much, other than getting smaller. The readouts have. Today's air gage readouts track serial and lot numbers, send offsets back to the machine to reduce scrap, and connect to robots and PLCs through RS232, 24 V digital I/O, USB, Ethernet and fieldbus. That lets air gaging slot into a modern cell with little human involvement. See automation and air gaging.