Sheet Metal Fabrication and Shop Practices
Sheet Metal Fabrication and Shop Practices
This domain tests layout, cutting, forming, seaming, and joining in the shop. You must know which seam and joint suits a duct and how material thickness affects the pattern.
Core rules
- Longitudinal seams: Pittsburgh lock, snap lock, button lock, and standing seam. Pittsburgh is common for rectangular duct; snap lock speeds assembly on smaller work.
- Transverse joints: drive slip, double S, standing S, and TDC/TDF flanges. TDC/TDF stiffen the duct and speed hanging.
- Stiffening: cross breaks, beads, and V-grooves control oil canning and noise.
- Joining: rivets, self-tapping screws, spot welds, solder, and sealant. Deburr edges before handling.
- Bend allowance: the flat pattern must account for bend radius, thickness, and setback; do not simply add outside dimensions.
Worked example
Fabricate a rectangular duct 18 x 10 in. The perimeter is 2(18 + 10) = 56 in. With a 3/8-in. Pittsburgh lock allowance, the blank width is 56 + 0.375 = 56.375 in. The reasoning: the flat pattern wraps the perimeter, and the lock consumes additional metal, so the blank must be longer than the perimeter.
Traps
Forgetting thickness when bending; over-tightening screws and crushing the duct; using the wrong sealant on a high-temperature job; failing to stagger seams; and cutting galvanized with an abrasive that later cuts stainless without cleaning.