The 3 D's of Straightness Plugs

We have touched on different applications of air gaging: size, match gaging and form applications such as taper. In this column, we'll discuss air straightness plugs.

The typical out-of-straightness condition is seen as a "bow" form within the bore that was introduced as part of the manufacturing process. The air straightness plug attempts to measure the depth of its curve. Usually this out-of-straightness condition is all within one orientation along the axis of the hole.

Design: A typical air plug has four measuring jets in two opposing sets—two near the middle and two near the ends, as seen in the diagram below. This allows the plug to look at both extremes of the bow condition.

There are no rules for the exact positioning of the jets relative to each other, as is sometimes the case with taper or squareness checks. Nor are ratios involved. The air jets at the extreme of the plug are positioned to inspect for the out-of-straightness condition, usually specified over the total length of the bore. In order to understand how a straightness plug works, we have to look at the combinations of jets typical in air tooling.

Differential Measurement: A two-jet plug is a differential measuring system. Imagine a two-jet air plug inside a master ring with the indicator reading zero. Now move the plug so one jet is pressed against the side of the ring. This increases the back pressure on one jet and decreases it on the other. The indicator reading does not change because the combined pressure remains the same. When you insert the plug into a smaller or larger test part, however, the pressure changes, and the gage reads the differential.

An extension of the two-jet air plug is a four-jet system. Four jets are added together, and if the plug is moved in any direction, an average (differential) reading is made. The four jets see four pressure changes and add them all together. If there is a change in any of the measured dimensions, the total—and the reading on the indicator dial—changes.

The four jets are normally at the same level or plane on the plug. In theory, the four jets could be moved anywhere along the length of the plug independently, and if they are positioned at 90 degree angles to each other, they will measure an average diameter of the bore. If we move the jets so that two are on the same side at the extremes of the plug, and the other two are moved to the center on the other side of the plug, we have the straightness plug described above. If the part being measured is perfectly straight and the plug is moved up and down, it acts like a two-jet piece of tooling. The two jets on the top are offset by the two jets on the bottom, and the result is no change on the display. But if the bore is not perfectly straight, then the combined pressure changes and the differential is shown on the instrument.

Dynamic Measurement: But don't think you've got it down so fast. If the straightness plug is simply inserted into the bore, the display shows a number. What does that number mean? Straightness needs to be made as a dynamic measurement, similar to a squareness check. With both of these form errors, the out-of-form condition is at its maximum along the axis at two positions, 180 degrees opposite each other.

Looking at the diagram, we can see what the inner and outer jets see. When the jets are in line with the bow (up and down), they are seeing either their maximum or minimum reading, depending on the orientation. When moved 180 degrees, the inner and outer jets reverse roles. The same value is seen, and the plug is working in its differential mode.

As the plug is rotated through 180 degrees exploring the bore, the jets experience maximum clearance, then find minimum clearance, usually at right angles to each other. The difference between the two is the out-of-straightness condition as seen over the total length of the plug measurement length.

Think of it this way: If you looked at the bore from the end and drew a line around the extremes of its path, you would end up with an ellipse. If you had an ellipse in a hole, a two-jet air plug could measure the variation in size by rotating it in the part. Think of the straightness plug as a stretched-out air plug with four jets doing the same thing.

The air straightness plug, though more complicated, maintains the advantages of a standard air plug—easy setup, easy use and high precision results.

Schuetz, George. "The Three Ds Of Straightness Plugs." Modern Machine Shop. Modern Machine Shop, 04 Sept. 2002. Web. 06 June 2017.

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