How each gauge is read
Fillet weld gauge: leg size
A fillet gauge set has one blade per size. G.A.L. Gage's fillet weld gauge instructions say to pick the blade of the size noted in the specifications, check the leg length with one end, then turn the gauge over to check the throat. Its 7-piece set covers 1/8 in to 1 in and lists both uses: leg size and throat size.
In the trainer, the leg end sits against one member with its tip on the other, one blade size out from the root. If bare plate shows between the weld toe and the tip, that leg is short of the blade size. If weld metal runs past the tip, the leg is longer. Check both legs. For an equal-leg callout, the weld meets the largest blade that both legs reach, so the shorter leg sets the size. Convexity adds nothing to the size.
Fillet weld gauge: throat on a concave weld
A concave face can leave long legs but a thin throat, so the leg end alone can overstate the size. The throat end checks the throat directly. In our drawing its tip sits on the 45° line, at the flat-face throat of that blade size from the root. If the shoulders seat on both plates and a gap shows at the tip, the throat is too thin for that size. If the tip meets the face before the shoulders seat, the throat is larger. Our throat end is simplified to show only that one comparison.
V-WAC gauge: undercut and crown height
G.A.L. Gage's V-WAC instructions describe both readings. For undercut, set the bottom of the gauge on the base material, put the pointer tip in the undercut and read the depth from the scale. For the reinforcement of a butt weld, set the bottom on the base material, put the tip on the top center of the weld and read the height. The locking screw holds a reading. Its Cat # 5 sheet gives the inch range as undercut and crowns to 1/4 in, in 1/64 in increments. The same gauge also has a porosity scale and two pore-size holes, which this trainer does not cover.
Our drawn scale reads undercut on the left of zero and crown on the right, with a small mark every 1/64 in and a label every 1/16 in. Check which side is which on your own gauge before reading it.
The geometry behind the scoring
These are first-principles relations for a 90° T-joint. Let a be a leg, s a blade size, c the concavity (how far mid-face sits below the straight line joining the toes) and t the actual throat (the shortest distance from root to face).
- Flat-face throat of an equal-leg fillet: t = s ÷ √2, the altitude of a right isosceles triangle. TWI gives the rounded form, the throat being 0.7 of the leg.
- Concave face, equal legs, face deepest at mid-face: t = a ÷ √2 − c. The root lies on the face's axis of symmetry, so the shortest distance runs along the 45° line.
- Size a concave weld supports: √2 × t = a − √2 × c. A weld meets blade s when √2 × t ≥ s.
- Flat or convex face, single fillet size called for: size = the shorter leg. A fillet called out with two leg sizes is checked leg by leg.
- V-WAC style pointer as a lever: indicator travel = tip travel × (indicator arm ÷ tip arm). The scale is engraved in tip travel, so the mark under the indicator reads the depth or height directly. Our drawing uses a ratio of 1.52.
AWS A3.0 governs welding terms, including how fillet weld size and throat are defined. Check those definitions in your copy, by section, rather than relying on this summary.
Worked examples
- Unequal legs. Legs of 5/16 in and 3/8 in. The 5/16 in leg end meets the toe of the shorter leg; on the longer leg, weld metal runs past its tip. The 3/8 in end shows a gap on the shorter leg. The weld meets 5/16 in.
- Concave throat. Legs of 1/2 in, concavity 1/32 in. t = 0.5 ÷ √2 − 0.03125 = 0.35355 − 0.03125 = 0.32230 in. Supported size = √2 × 0.32230 = 0.45581 in. The 7/16 in tip needs 0.4375 ÷ √2 = 0.30936 in, which the throat exceeds. The 1/2 in tip needs 0.35355 in, so it shows a gap. The weld meets 7/16 in, not 1/2 in, although both legs are 1/2 in.
- Undercut on the lever scale. Undercut depth 1/32 in with a ratio of 1.52: the indicator moves 0.03125 × 1.52 = 0.0475 in, and each 1/64 in mark is spaced 0.015625 × 1.52 = 0.02375 in. That is two marks, so the reading is 1/32 in.
Score: each case earns one point, and percentage = correct ÷ 10 × 100, rounded. Eight correct gives 80%. This scores gauge reading only; it is not an exam pass prediction.
Discontinuity calls
The second mode draws generic weld profiles with undercut, overlap, excess convexity, underfill or misalignment. A dashed line runs from the discontinuity to a drawn scale in 1/32 in steps. Read where it lands, then call the weld accept or reject against the limit printed in that case. Each case's limit is made up for the practice drawing, so it tests the habit of measuring first and calling second, not any code's numbers. Overlap cases print no limit: overlap is a reject under common criteria, so you measure it to describe it in your report. Real criteria differ by code, joint and job. Look yours up in your own copy, by section.
Limits of this trainer
Real welds are not perfect curves, and real gauges differ by maker and model. This trainer does not decide whether a real weld is acceptable. The governing code, the drawing and the WPS set the required size and any undercut or reinforcement limit; look them up by section in your own copy. For leg-to-throat arithmetic on other sizes, use the fillet weld calculator.
Gauge readings are Part B work. For the Part A side of the same joints, the weld position trainer drills 1G to 6G and 1F to 4F on original plate and pipe drawings.