One of the advantages of using air gages is that there is little contact between the tool and the workpiece. In fact, such tools are typically referred to as non-contact tools. But, strictly speaking, this is not entirely true. Air gage tools do come into contact with workpieces, and this may be reflected in the fact that they do suffer wear and tear over time. The progress of this degradation may be significantly slower than that of contact gages but eventually, it is bound to happen.
How Wear And Tear Leads To Centralization Issues
When your air gage tool is sufficiently worn out, the clearance between the workpiece and the gage will usually be greater than it was designed to be. This in turn leads to centralization problems where the air gage tools measure a chord of the workpiece in question rather than measure the diameter of the part. Centralization problems may also arise if the centerline of the jet is not aligned with the plug centerline. As the tool degrades and the space between the bore centerline and the chord increases, the centralization errors become bigger and bigger.
Obviously, machine operators will allow for some centralization error, but this depends on how much leeway their process allows them to. With looser tolerances, these kinds of errors don’t pose much of a problem. However, with tight and precise machining, this becomes a problem for the machine operator.
Understanding Balance Errors
Unlike centralization errors, balance errors happen when the orifices and cavities in the air gage jets become clogged or are damaged by misuse, or as we saw earlier, become worn-out unevenly. This is because for your air gage to work properly, it is important for all the jets to have the same orifice diameter and recess. Anything that changes these parameters throws your air gage tool off-track.
The next question then is how you can spot this wear and tear and what you can do about it. There are two main approaches that you can use to do this.
Though not always possible, you may be able to see contaminants that may be clogging up your jets. This will of course depend on the type of tool you are using, its size and so on. This is part of the reason it is essential to keep your gages as clean as possible, as well as the workspace that you use them in. However, visual inspection may not always be possible. Even in the circumstances that it is, it may not always be possible to understand just how badly the problem is affecting your measurements.
In order to get a more accurate picture of the problem, try the second approach.
Using A Master
This approach is based on the fact that for most air gage tools, wear and tear tend to follow a fairly predictable pattern. With gages that are hand-held, the wear is often around the plug’s circumference and tends to be relatively even. For these, secure the gage horizontally, then take a master reading. Having noted the reading, take the master and place it on the lower surface of the plug. Again, take another reading. If there is wear and tear of the plug, the readings will be different. Generally speaking, if the difference between the two is more than 10% above the acceptable tolerance, you may have to replace the plugs, or the tool all together.