Conventional air gaging for measuring inside diameters is typically limited to a minimum size of about 0.060 inch/1.52 mm: below that, it becomes difficult to machine air passages in the plug tooling, and to accommodate the precision orifices or jets. But air gaging is among the most flexible of inspection methods, and with a simple change of approach, it can be used to measure very small through holes, below 0.040 inch/lmm in diameter.
Most air gages measure back-pressure that builds up in the system when the tooling is placed in close proximity to a workpiece. In the case of bore gaging, a smaller bore means closer proximity of the part surface to the jets: this results in higher air pressure, which the gage comparator converts into a dimensional value.
A few air gages measure the rate of flow through the system rather than backpressure: as tool-to-workpiece proximity decreases, flow also decreases. The flow principle can be effectively applied to measure very small through holes, even on air gages that were designed to operate on the back-pressure principle. Rather than installing tooling at the end of the air line, the workpiece itself is connected to the line. Smaller bores restrict the flow of air more than larger ones. Thus, the workpiece essentially becomes its own gage tooling. This approach works on all common types of back-pressure gages: single-leg gages requiring dual setting masters, as well as differential-type gages, which typically use just one master.
Air flow is proportional to the bore's cross-sectional area, but area varies with the square of diameter. Gage response in this setup, therefore, is non-linear. Nevertheless, this rarely causes problems, because the range of variation to be inspected is usually very small, and the gage is typically set to both upper and lower limits using dual masters or qualified parts. If numerical results are required, specially calibrated dials may be installed on analog comparators, while some digital comparators allow software correction.
Back-pressure air gages operating as flow gages for small holes have been used in a number of specialty applications, ranging from fuel injection components to hypodermic needles. Often, all that is required is a special holder that allows the part to be attached quickly and easily, with a good air seal. Air pressure and flow stabilize quickly, making this method efficient for high-volume inspection.
Like other forms of air gaging, flowtype measuring of small through holes is extremely adaptable. It has been used to measure IDs as small as 12 microinches/0.3 micrometers, and as large as 0.050 inch/1.27mm. Range of measurement can be as long as 0.006 inch/0.15mm, and discrimination as fine as 5 microinches/0.125 micrometers. In some cases, where the hole is so small that air flow is negligible, bleeds may be engineered into the system to boost total flow to a measurable level. On the other hand, excessive flow through large bores may be brought down into a measurable range by engineering restrictors into the system.
Some parts, including some fuel injection components, have two holes sharing a common air passage, and require that the holes be measured twice: once simultaneously, and once independently. To accommodate this requirement, special two-station air gages have been designed, where the first station connects the air flow through both holes, while the second station only connects the air circuit to one of the holes, and blocks the other.
Some applications are served well by microscopes and optical comparators, although neither is well suited to high volume production applications, and both are limited in the part configurations they can accept. Go/no-go gaging with precision wires is also practical only for very low volume tasks. the air gaging method described here often requires a modest level of application engineering, and occasionally a custom dial face or gaging fixture, but it lends itself well to high-throughput inspection of very tight tolerances.
In discussing air gaging in past columns, we've often emphasized its flexibility. With it, one can measure a wide range of dimensional characteristics, including inside and outside diameters, feature location, thickness, height, and clearance/interference. Air can also be used to measure geometry characteristics such as roundness, squareness, flatness, parallelism, twist, and concentricity. And we've seen how air gages can measure very deep bores, blind holes, and counterbores. The use of air gaging to inspect very small through holes is yet another example of the tremendous adaptability of this relatively simple, but very cost-effective technology.
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