An inside corner weld between two perpendicular pieces of metal is called a fillet (fill-it) weld. The fillet weld needs to be at least the thickness of the thinnest material being joined by the weld. For example, if you are welding a 1/4-inch metal plate to a 3/16th-inch metal plate, the fillet weld will need to be 3/16ths of on inch across and have a 3/16th-inch throat depth. A fillet gauge measures the size of the fillet weld to ensure its structural integrity.
Read the weld specification sheet to see if there are any specifications for the fillet weld size. Unless otherwise specified, assume that the fillet weld needs to be the same size as the thinnest piece of metal being welded.
Measure the metal welded with the fillet weld. Place the metal gauge on each piece of metal to determine the thickness of each piece. Select the fillet weld gauge that matches the thinnest piece of metal.
Hold the gauge flat against one surface. Slide the gauge to the weld. Inspect the area where the top of the notch of the gauge meets the edge of the weld. If the fillet gauge notch and weld edge are even, the weld is the proper size in that direction.
Flip the gauge and place it flat on the other piece of metal. Slide the gauge to the weld and check the height of the weld on that side as well. Mark the weld with the paint marker if the edge of the weld falls below the notch of the gauge.
Place the concave side of the gauge against the weld, with the 45 degree sides of the gauge resting on each piece of welded metal. Mark the weld with the paint marker, if the weld does not touch the arm of the fillet weld gauge.
Cap weld any fillet welds that do not measure properly. If the welds are larger than the fillet weld gauge, they will pass as they are larger than the thinnest piece of metal being joined by the fillet weld.