Měřicí technika
Měřicí technika
Advances in Air Gaging

George Schuetz,  Mahr Federal Inc.

Over the years, the tooling for air gaging has remained basically the same: steel tubes or rings with precision orifices that set up a pressure/distance curve when in use.  As the orifice is restricted, flow is reduced and pressure builds up in the system.  This principle can be used to monitor the distance between the air jet and the part surface.  Orifices can be arranged in complex configurations to indicate form or relationship errors to the user.  Typical applications include average diameter, taper, squareness, center-distance, and straightness.  These fixed tooling components provide fast results for the user.

Unlike the tooling, however, air gaging displays have changed dramatically over the past 50 years.  Initially, the air meter consisted of a water-filled tube.  Changes in air pressure from closing the tooling orifice caused the water in the tube to rise or lower, and part of the gage setup process involved filling the tubes.  Ironically, one of the biggest sources of error in an air gage today is the very thing that used to be needed to make the old gages work—water in the air lines.

The next generation of air gage displays used a float in a tapered tube to monitor the flow of air to the orifice.  Some air gages of this style can still be obtained.  It's a simple design that works, but it's difficult to keep clean and resolution may be lacking for some applications.

Mechanical amplifiers were next employed to monitor the expansion and contraction of a bellows assembly as the pressure in the air system increased or decreased.  Differential air gage circuits came into play and provided an opportunity to manufacture tooling and air displays to a high level of performance.  These systems held such tight standards that a single master would place the air system into the "sweet" spot on the air pressure/distance curve.

Electronic air amplifiers took amplification one step further.  The first air-to-electronic transducers were LVDT's used to monitor the position of the same type of bellows used in mechanical gages.  Other systems use silicon based transducers to monitor the pressure change. Both methods provide high resolution and fast response.  Combined with the electronics in a gaging amplifier, they can make the gage a lot smarter and provide dynamic functions that can average readings and combine air signals for even more complex measurements.

One area that has not seen much in the way of advances until recently is the portability of the air gage.  Hand tools are used all the time in the quality/process control function to bring the measurement to the part.  With air gaging, the need for the piping and mechanical or electronic amplifiers always meant that the displays were rather big.  For small parts, this was no problem: the parts could easily be brought to the gage.  For larger parts, the tooling could be attached to a hose and brought to the part, but there was a potential problem in that the operator had to watch the tooling as he placed the plug in the part, then turn around to search for the air display in order to interpret the value.

In recent years, electronics have allowed even air displays to become smaller.  There are now air gages on the market that make the display portable, as part of the air tooling itself.  With this configuration, the air display is right in front of the operator, ready to be read, and there is no searching for the old air meter.  Like the amplifier version, these portable air gages combine electronic transducers with today's digital indicators, to provide selectable resolutions, tolerances and data collection capabilities.

Air gaging was thought to have reached its peak many years ago.  But in reality, there is an increasing demand for its application.  As tolerances have gotten tighter on the shop floor, air gaging is often the only way to make the check fast and easy for the operator.  With portable air gaging, that check can now be made right at the part.