Fillet & Groove
Every weld you will make is one of two types: a fillet laid into a corner, or a groove filling a prepared opening between edges. · 12 min
Last folio gave you the five ways two pieces of metal can meet. This folio gives you the two kinds of weld that join them. A fillet weld (a triangle of weld metal laid into a corner) serves lap, tee, and corner joints. A groove weld (weld metal filling a prepared opening between two edges) serves butt joints. Learn to size the first and prepare the second, and you can read almost any welded assembly ever drawn.
Guess before you learn
An equal-leg fillet weld is sized by its leg — say 10 mm. How thick is the weld at its thinnest working section?
The load crosses the throat — the shortest line from the weld's root to its face. For an equal-leg fillet that diagonal is about 0.7 times the leg, so a 10 mm fillet is really a 7 mm weld where it counts. If you guessed the leg, keep the pencil mark: this folio is about measuring welds where they actually carry.
9–12
3–5
Look at how the pieces meet. If they form a corner — one plate lapped over another, or standing on another — you fill the corner with a triangle of weld metal: a fillet weld. Each straight side of that triangle is called a leg.
If the pieces meet edge to edge, there is no corner to fill. Instead you slant the edges back so the arc can reach all the way down, then fill the opening you made. That is a groove weld, and the slanting is called beveling.
6–8
A fillet weld is a weld with a roughly triangular cross-section, laid where two surfaces meet near 90° — lap, tee, and corner joints. Its legs are the distances it reaches up each plate; its face is the exposed surface; the root is the deepest point of fusion; the toes are where the face meets each plate.
A groove weld fills an opening cut between two edges — the standard weld for butt joints. Preparing that opening is called beveling: each edge is cut back at a bevel angle, a flat root face is kept at the bottom, and a root gap spaces the plates so the first pass can fuse clear through.
9–12
A fillet's strength is not its leg but its effective throat — the shortest distance from root to face. For equal legs, the throat is the leg times cos 45°, about 0.707 × leg. Doubling a leg doubles the throat but quadruples the weld metal, which is why oversizing fillets wastes rod and pours in needless heat.
Groove preparation is an access problem: the arc must fuse the full edge, so the bevel angle opens a path to the bottom, the root gap lets the first pass burn through, and the root face leaves just enough metal that it does not fall through. Full-thickness fusion makes a complete-joint-penetration weld; anything less is partial.
K–2
Two plates can meet in a corner. To join them, you melt a small triangle of metal into that corner. That triangle is called a fillet weld.
Two plates can also meet end to end. You cut their edges at a slant, leave a small gap, and fill it with weld. That is a groove weld.
Undergrad
In design terms a fillet carries load in shear across the plane of its effective throat: allowable force per unit length equals throat times allowable shear stress. Since deposited volume grows with the square of the leg, the economical answer to a stronger joint is usually a longer fillet, not a fatter one.
Groove geometry is a three-way compromise: wider bevel angles improve access and sidewall fusion but demand more filler and more heat; tighter angles save both but risk lack of fusion. Root face and root gap are set as a pair — a larger face needs a larger gap, or the root pass will not penetrate.
Postgrad
Structural codes such as AWS D1.1 define effective throat precisely and let deep-penetrating processes claim throat beyond the theoretical triangle. Fatigue design treats fillet toes as the governing detail: the toe is a geometric stress raiser, and toe grinding or profiling buys measurable fatigue life.
Complete-joint-penetration grooves are the default where cyclic or through-thickness tensile loads cross the joint, since an unfused root face is a built-in planar discontinuity — a crack starter under fatigue. Partial-penetration grooves and fillets remain legitimate for static, shear-dominated, or light service; specifying beyond need is its own defect, paid in cost, heat, and distortion.
effective throat
The shortest distance from a fillet's root to its face — the section that actually carries load. For equal legs, about 0.707 × leg.
A groove weld earns its strength before the arc starts, in the preparation. Butt two thick plates squarely together and the arc can only melt the top — the bottom of the joint stays cold and unfused. So you bevel: cut each edge back at an angle, commonly 30° per side for 60° included, to give the arc a path down. You keep a small flat root face at the bottom so the first pass does not burn through, and you set a root gap between the plates so that same pass can fuse the joint completely.
Why is this true?
Why does a butt joint need edge preparation when a tee joint does not?
A tee joint offers the arc an open corner to weld into. Butted edges hide their depth from the arc, so the edges must be cut back to give it access all the way down.
One more piece of vocabulary completes the picture: welding position — how the joint sits in space while you weld it. A digit names the position and a letter names the weld type: F for fillet, G for groove. Position 1 is flat: the work lies below the arc and gravity holds the puddle in the joint. Position 2 is horizontal: the weld runs level along a vertical surface, and gravity starts to argue with you. Positions 3 (vertical) and 4 (overhead) belong to later courses. This course welds 1F, 2F, 1G, and 2G.
Size the working section of a 12 mm fillet — the steps fade as you master them
throat ≈ 0.707 × leg
throat ≈ 0.707 × 12 mm
throat ≈ 8.5 mm
Two weld types, five joints, four positions — the grammar is complete. Next folio you learn to read it the way a fabricator writes it: as a symbol on a drawing that names the weld, the size, the side, and the site, all on one line.
Practice — new ink and old, interleaved
1.Without looking back: what is a fillet weld, what is a groove weld, and which joints does each serve?
A fillet weld is a triangular weld laid into a corner — it serves lap, tee, and corner joints. A groove weld fills a prepared opening between edges — it serves butt joints.
How close were you? Grade yourself honestly — it sets your review date.
2.Match each defect to its cure.
3.A square tube post welded upright onto a flat baseplate?
4.An equal-leg fillet has 8 mm legs. What is its effective throat, in millimetres?
5.Above roughly what crosswind, in miles per hour, does MIG shielding start to fail?
6.The drawing calls for a 6 mm fillet. A beginner runs 12 mm to be safe. What did the joint actually gain?
7.Why can a sound weld outperform a bolted or glued joint in the same steel?
8.You weld a tee joint with its base plate flat on the bench and the web standing upright. Which position code?
9.One plate overlaps another and you weld along the overlapped edge. Name the joint and the weld.