Interpretation of Drawings, Plans and Specifications
Reading the document set
A welding candidate must work across a drawing set, not a single sheet: general arrangement, assembly, detail and welding symbol sheets, plus the specification and bill of materials. Learn to read the title block (drawing number, revision, scale, units), the notes, and the revision cloud before touching the weld data. A superseded revision is a common trap: always confirm revision status against the work pack.
Welding symbols
Understand the reference line, arrow, tail, and the symbol placed above (other side) or below (arrow side) the line. Know the common symbols: fillet (triangle), butt/v-groove, square butt, plug, spot, seam, and backing. Supplementary info sits in the tail (process, standard). Size is to the left of the symbol, length and pitch to the right; a circle at the junction means weld all around; a flag means field weld.
Dimensions and tolerances
Read joint preparation dimensions (root gap, root face, included angle), fit-up tolerances, and any NDT requirements. Watch for dual dimensions (metric/imperial), datum-based dimensions, and symbols for surface finish or profile (flat, convex, concave).
Traps
- Misreading arrow side versus other side.
- Ignoring a note that overrides the symbol (notes take precedence).
- Confusing a weld-all-around circle with a field-weld flag.
- Assuming a dimension is in millimetres when the drawing states inches.
Worked example
A fillet symbol has a triangle below the reference line with '6' to its left, a circle at the junction, and a flag in the tail. Meaning: 6 mm fillet, arrow side, weld all around the joint, performed in the field. If a note says 'seal weld only', the note governs.