Edges, Seams, Joints and Fastening
This domain tests how sheet edges are finished and how sheets are joined without welding. You must know hemming, wiring, seam types, joint types and the allowances needed for each.
Edge and seam types
- A hem folds the edge over itself for stiffness and safety. A wired edge folds the sheet over a wire to strengthen a thin edge. A false wire is a folded edge without wire.
- Common seams include the lap seam, grooved seam, Pittsburgh seam, standing seam and lock seam. The grooved seam is made by interlocking two folded edges and locking them with a groover. The Pittsburgh seam is a locked joint used on ducts.
- Joints may be butt, lap, corner or tee. Fasteners include rivets, self-tapping screws, bolts, spring clips and adhesives. Space fasteners evenly and keep edge distance adequate.
Worked example
A 2.5 mm diameter wire is to be enclosed in a wired edge. A common allowance is 2.5 times the wire diameter: 2.5 × 2.5 = 6.25 mm. Mark 6.25 mm from the edge, fold the sheet over the wire, and hammer the fold tight without crushing the wire. Reasoning: the sheet must wrap around the wire and still leave enough material for the fold to grip.
Traps that cost marks
Forgetting the seam or wiring allowance; making a grooved seam too tight so it buckles; placing rivets too close to the edge; and joining dissimilar metals without insulation, which causes galvanic corrosion.