By Albert J. Moore Jr. Feature Other than the measuring tape, the fillet weld gauge is the most used measuring device in the welding inspector’s kit. That is understandable considering the majority of the welds deposited are fillet welds. Yet many people measure the size of the fillet weld incorrectly. The principle of the fillet gauge is pretty simple. The fillet weld has a cross section that is essentially a triangle. If the two members being joined are at right angles to each other, the cross section of the fillet weld resembles an isosceles right triangle — Fig. 1. One basic assumption is that the fillet weld is fused to the joint root, but not necessarily beyond. With those two assumptions in place, i.e., the cross section of the fillet weld is an isosceles right triangle and it has fusion to the root, we can establish a relationship between the length of the fillet weld legs and the theoretical throat. We need to know the dimension of the throat because it is the “weak link”; that is, it represents the shortest failure path through the cross section of the weld. With the two conditions noted in place, 14 Inspection Trends / January 2011 we can define the size of the fillet weld as the largest inscribed isosceles right triangle. Inspectors usually use a leaf-type fillet weld gauge to measure the size of fillet welds. The gauges come as a set of individual gauges that are used to size fillet welds. I did not say “measure” fillet welds because the leaf-type gauge only indicates whether the weld is less than, larger than, or the same size as the gauge. The inspector checks the size of the actual weld by using one or more gauges to compare the size of the weld to the size of the individual gauges. The gauges typically come in 1⁄16-in. increments for weld sizes ½ in. and smaller, and in 1⁄8-in. increments for sizes of 5⁄8 in. and larger. The basic leaf-type gauge has two ends, one for sizing the weld leg and the other for checking the throat dimension. Well, that’s not exactly accurate. The gauge does not directly measure or size the throat. Instead, it sizes the throat for the equivalent leg size for an isosceles right triangle. As shown in Fig. 2, the gauge has two ends. The left end is used to size the fillet weld leg; the right end is used to size the weld throat. If we were to check an ideal fillet weld, one that is a perfect isosceles right triangle that matched the size of the gauge, both the weld legs would be exactly 1⁄2 in. The throat of the fillet weld would measure exactly 0.354 in. (0.707 × 0.5). Rather than imprinting 0.354 on the right end of the gauge, the manufacturers imprint it with 1⁄2 because the fillet throat is the proper size for a fillet weld with 1⁄2-in. legs. That saves the inspector the time needed to calculate the required throat dimension for each leg dimension. Figures 3 and 4 show that the fillet weld legs are exactly ½-in. each. Figure 5 depicts how to measure the vertical fillet leg. As indicated by the figure, the gauge has to be turned to verify the horizontal fillet leg is the proper size. After all, the two legs may not be the same size by design or as dictated by the welder’s skill, so both the horizontal and vertical legs must be checked. Checking the perfect fillet weld is Tips for Using Fillet Weld Gauges The following advice will help you consistently size fillet welds properly Fig. 1 — A typical fillet weld, such as the one shown here, has a cross section that resembles an isosceles right triangle. Fig. 2 — Example of a gauge used to size 1⁄2-in. fillet welds.
Inspection Trends - January 2011 - Winter
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