How to measure fillet weld using fillet gauge


What is Fillet Weld?

Fillet welding refers to the process of joining two pieces of metal together when they are perpendicular or at an angle to each other. These welds are commonly referred to as tee (T) joints, which are two pieces of metal perpendicular to each other, or lap joints, which are two pieces of metal that overlap and are welded at the edges.

The weld is triangular in shape and may have a concave, flat or convex surface depending on the welder’s technique & called Concave fillet, convex fillet and Mitre weld. Fillet welds and predominately used weld joint types in construction, structural and general fabrication although they are not used in critical weld joint locations.

Dimensioning of Fillet weld

Fillet weld is specified by a size known as either throat size or leg length. So how do you define the fillet weld size?

How you will measure a fillet weld?

Leg length measurement is used mainly in American codes & standards such as AWS D1.1 & ASME Codes. Measurement of fillet using throat size is used mostly in ISO codes, BS, & EN standards. So, an AWS-CWI will measure welds using leg length while a CSWIP or IWIP Inspectors will be taught to measure fillet welds using Throat size.

The figure shown above is a theoretical concept of a filler weld– it depicts an equilateral triangle – whereas a real weld is never flat, and will be either concave or convex, and not necessarily equal-length legs.

For strength calculations, we’re interested in the throat thickness. We get that by simply dividing the leg length by √2 because we’re used to specifying equal-leg welds. It’s therefore pretty important to ensure the specified weld is big enough to achieve the design throat thickness.

Click here to Learn How to find out Minimum & Maximum size of a Fillet weld.

Measuring the Fillet weld size

The leg length refers to distance from the root to toe and should be measured in both directions using fillet gauges.

The leg size is the length of the legs of an imaginary triangle that can be inscribed within the actual weld as shown with feature L1 and L2 in the image below for both a convex and concave fillet weld.

In the case of the convex fillet weld shown on the left, the measured leg (L) is equivalent to the size (S). In the case of the concave fillet shown on the right, the measured leg (L) provides an untrue measure of weld strength which is why these welds are instead assessed based on throat dimension to determine effective weld size (S).

Welding Gauges

A numbers of different types weld gauges are used to perform fillet weld measurements. In AWS-CWI certifications measurements of fillet weld is taught using Standard Fillet weld gauge set shown in below picture.

A standard fillet weld gauge set is used to verify that the leg length matches the specified size. These gauges may not measure exact size but instead verifies that the specified size has been attained.

When using the standard fillet weld set for weld measurements, the weld should first be visually checked and if it appears to be flat or convex, it should be assessed based on leg length only as shown below. The image below shows a fillet weld being inspected to verify the specified 3/8″ (9.5mm) leg size. If the fillet instead appears to be concave, the weld should instead be assessed using the side of the gauge with the center tab which would need to touch the face of the weld.

Measuring Fillet Weld Concavity

In the next scenario, the measured leg length is again 3/8″ but the weld should be rejected as undersize due to its concave profile resulting in an effective weld size of only 1/4″ as shown on the right below. The black lines you see on the gauge points to the effective toe of the imaginary triangular fillet.

Measuring Fillet Weld Convexity

While the above example illustrates how concave fillet profiles should be scrutinized, fillets that are convex may also be rejected due to profile or shape. At first glance, this fillet appears to be acceptable based on a specified leg length of 1/4″. In examining its profile however, noticeable convexity is evident.

To much convexity adds to the stress riser or notch effect at the toe of the fillet that can be a location for crack initiation. Such welds should be avoided in cyclic loading applications. ISO 5817, specify the maximum allowed convexity in welds.

Click here to Download AWS tips for Using Weld Gauges.

Fillet weld measurement using Bridge cam gauge

The Bridge Cam Gauge, commonly known as the Cambridge gauge, is an accurate, sturdy gauge for both shop and field applications. It can measure fillet weld leg length and throat thickness, height of weld reinforcement, depth of undercut, misalignment and the angle of preparation.

The Bridge Cam Gauge includes both imperial and metric measurement scales.

What can the BRIDGE CAM GAUGE measure?

You can use the Bridge Cam Gauge can measure a range of measurements including:

  • Angle of preparation
  • Excess weld metal
  • Depth of undercut
  • Depth of pitting
  • Weld throat size

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