A fillet weld rests between two perpendicular pieces of metal. The fillet weld should be the size of the thinnest metal thickness coming to the weld joint. Before measuring the fillet weld, you must determine whether the fillet weld is convex or concave. The difference between a convex and concave weld is immediately visible, as a concave weld is lower in the middle and higher on the sides. A convex weld is the reverse, and the centre of the fillet weld is the highest point of the weld. A fillet weld gauge measures the thickness of the fillet weld.
Determine the thickness of the metals joined by the fillet weld. Place the metal gauge onto each piece of metal. Use the thickness of the thinnest metal in the weld joint as the determining factor for the fillet weld size. For example, if one metal thickness is 11 gauge (.125 or 1/8 inch) and the other metal thickness is .25 or 1/4-inch plate, the fillet weld should be no smaller than 1/8 inch.
Examine the weld to determine if the throat of the fillet weld is convex or concave. The throat of the fillet weld is the face of the weld facing out from the weld joint. Use the appropriate notched end of the fillet weld gauge to determine the correct throat width of the fillet weld. The throat of the fillet must touch the entire notch in the fillet weld gauge to be the correct size.
Slide the fillet weld gauge along one side of the metal entering the weld. The side of the fillet weld must touch the height notch of the fillet weld gauge to pass. Place the fillet weld gauge on the other side of the fillet weld, and check that side as well.
Use a larger fillet weld gauge if the weld is larger than the minimum fillet weld size. Gradually increase the gauge size until the fillet weld completely fills the notch in the fillet weld gauge. This is the actual size of the fillet weld.
Mark the size of the fillet weld on one of the pieces of metal coming into the weld joint with the paint marker. Make note if the weld needs to be enlarged or if the fillet weld is too large.