Using Air to Measure Squareness

As far as reference surfaces go, after flatness, the most important is likely to be squareness. There are a number of different terms used to describe this relationship, including “perpendicularity,” “90 degrees” and “normalcy.” In engineering products, which may not have perpendicular surfaces, the right angle is used in the dimensioning of rectangular coordinates. The implementation and measurement of perpendicularity are never perfect and are assessed as having achieved a higher or lesser degree of perpendicularity.

The most common method to inspect for perpendicularity is to compare the part to a master square, which can be steel, granite or ceramic. An electronic height gage is a faster, more automated way of making this surface plate perpendicularity check. While the electronic height gage may not have the level of mechanical perpendicularity that is built into a precise master square, it is a very powerful and repeatable gage. And since it is repeatable, any inherent inaccuracy can be measured and corrected, thereby improving the total performance of the gage.

Master squares or height gages are great for machine tools or parts where the length being checked for perpendicularity is more than 4 inches (100 mm). However, for measuring perpendicularity where lengths are less — say, a hole to a face where the hole diameter is 1 inch (25.4 mm) and the face is about the same length — large height gages become too cumbersome to work with. They are not ideal at the point of manufacture where one should have repeatable, easy-to-use and accurate ways of measuring squareness. There are better ways.

In the manufacturing world, bore-to-face squareness is a very common relationship and one that often needs to be controlled. This is especially true in the manufacture of precision bearings that fit into assemblies, as the squareness of the bore to the face will influence the assembly’s performance.

One of the best ways to measure a bore-to-face relationship is with a form machine. Using the machine’s precision bearing and software, a reference cylinder can be created from the bore. The probe of the system is then moved to the face and the surface is profiled. With this, a very highly accurate squareness check is performed. This may be the most accurate method, but in a high-speed manufacturing environment, it may not be the most productive.

One way manufacturers might check the perpendicularity of the face of the surface to a bore at the point of production would be to create a fixture with a very slightly tapered cylinder mounted so that the cylinder is square to a surface. A part can be slid over the cylinder, holding the bore in place and perpendicular. Then, an indicating device (like a test indicator) is placed on the face of the part, and as the part is rotated, the indicating device monitors the perpendicularity of the face. This is a fast and easy check to use, but is subject to errors caused by repeatably holding the part (dirt, manual aligning and so on) and positioning the indicating device. However, as tolerances of squareness become tighter, this type of fixturing may not be capable of meeting the tighter tolerances.

A more accurate, repeatable and probably faster way to measure squareness is air gaging. An air gage is a versatile tool that lets you put a lot of measuring locations in small areas. Though most frequently used for diameter measurement, an air plug and platen can be readily configured to measure perpendicularity, although it kind of measures it from the inside out. Think about using a master cylindrical square; the normal way of using it is to place it on a granite surface plate to set up the reference square. The face of the cylinder is square to the surface and constitutes a comparative surface to check your part against.

However, with air tooling for squareness, the air plug is square to the platen that holds it and is manufactured to a very high tolerance. Therefore, the accuracy of the squareness plug is built into it, much like the cylindrical square on the granite plate. And rather than transfer the squareness reading from the gage to the part, the air tooling becomes the master and the part is compared to it using the built-in air jets. Using two jets at 180 degrees and different heights sets up the method for comparing out-of-squareness.

With air gaging, the squareness check is dynamic, much like other checks made with air gaging, straightness or diameter variation. With the squareness gage, the part is placed over the air plug, resting on the square platen. The circuit of the air gage is connected to a display that will monitor the dynamic change of the air signal.

The operator resets the dynamic reading and then rotates the part. As the part is rotated, it will naturally go through a square position to an out-of-square relationship to the jets in the air tool. The dynamic capability of the display will caption this difference and display the out-of-square condition.

There are a couple of things to consider here. First is how the perpendicularity is called out. The face may be called out to the bore or the bore may be called out as square to the face. Thus, it is important to know how the air gage is reading to get the correct relationship.

But the second and most critical thing to remember about this type of measurement is that the jets are only measuring over the distance between them. For example, the tolerance on the parts illustrated is 0.001 inch over the part's length of 1.00 inch. Looking at the gage, you can roughly say the pair of jets covers about 80% of the total length. Thus, the tolerance of the gage would be 0.0008 inch instead of 0.001 inch. However, with the power of the display units today, a multiplier can “project” what the display might look like if the total length of a 1-inch part is measured.

If the length of the part and the jets are very close, you can imagine how tight some of the tolerances can get. Close attention must be paid to the actual gaging tolerance to ensure the accuracy of the squareness measurement.

As shown in Figure 1, the tooling creates the reference square, and the single pair of jets will monitor the out-of-square condition of the face relative to the ID. This is not a static check, though; the part must be rotated 180 degrees. A dynamic total indicator runout (TIR) shows the relative squareness compared to the squareness built into the air gage fixture.

A second method (shown in Figure 2) is very similar but uses two pairs of opposing jets in a differential mode. The top and bottom jets on each side of the air plug are channeled to opposite sides of a special air display to provide a differential measurement. The dynamic TIR indicates a lack of squareness as the part is rotated on the reference platen. This method is used primarily when squareness readings should not be influenced by any taper condition.