Joint Design, Fit-Up, and Weld Preparation
Core Concepts
Know the five basic joints (butt, lap, tee, corner, edge) and the groove types (V, bevel, U, J, square) with their included angles, root openings, root faces, and land. Understand the terminology of a weld cross-section: root, face, toe, leg, throat, reinforcement, and effective throat. Fit-up topics include root gap control, alignment, tack welding, back gouging, backing bars, and cleaning of joint surfaces. Distortion control methods - balanced welding, backstep, skip welding, presetting, and restraint - are frequently tested, as are the causes of lack of fusion and incomplete penetration from poor fit-up.
Traps
Questions often confuse root opening with root face, or included angle with bevel angle (a 60° included V-groove is two 30° bevels). Remember a larger root gap can cause burn-through; a smaller gap risks incomplete penetration. Throat vs. leg in a fillet weld: effective throat equals 0.707 × leg size for equal-leg fillets.
Worked Example / Rule of Thumb
Fillet throat rule: effective throat = 0.707 × leg length. A 1/4-in. equal-leg fillet has a throat of about 0.177 in. For distortion, use backstep and balanced sequences; "weld toward the free end" to let contraction pull the part straight.