3.Sheet Metal Work

1. Common Materials & Gauges

• Galvanized Iron (GI): The most common material for HVAC ducts. The zinc coating prevents rusting from condensation.

• Aluminum: Used where weight is a factor or in highly corrosive environments (e.g., coastal areas).

• Stainless Steel: Used for cleanrooms, hospitals, or food processing plants due to high hygiene and corrosion resistance.

• Copper: Used for specialized heat exchanger fins and small high-end fittings.

• Sheet Gauge (SWG):

  • Standard Rule: Higher the Gauge number, thinner the sheet.

  • Residential Ducts: 24 to 26 Gauge.

  • Commercial/High Pressure: 18 to 22 Gauge.

2. Essential Sheet Metal Tools

3. Joints and Seams (Duct Fabrication)

• Lap Joint: Simplest joint; edges overlap and are joined by rivets, spot welding, or soldering.

• Grooved Seam: Two folded edges (hems) hooked together and flattened. Used for longitudinal pipe seams.

• Standing Seam: Used for large plenums; provides structural rigidity to the flat surface.

• Pittsburgh Lock: The “Gold Standard” for rectangular HVAC ducts. It provides a leak-proof, high-pressure mechanical lock.

• S-Cleat and Drive Clip: The primary method for connecting two sections of rectangular ductwork together.

4. Patterns and Allowances

• Parallel Line Development: Used for square, rectangular, and cylindrical shapes (standard ducts).

• Radial Line Development: Used for tapering shapes like cones and funnels.

• Triangulation: Used for complex “transition” pieces (e.g., square-to-round offsets).

• Bend Allowance: The extra material needed to compensate for the metal stretching during a bend.

  • Formula Hint: Calculation involves the Neutral Axis, where the metal neither stretches nor compresses.

5. Key Manufacturing Processes

• Hems: Folding the edge of a sheet back on itself to eliminate sharp edges and add strength.

  • Single Hem: One fold.

  • Double Hem: Two folds for maximum safety and rigidity.

• Wiring: Folding the edge of a sheet over a wire (like the rim of a bucket) for extreme reinforcement.

• Notching: Cutting out portions of the metal (V-notch, Square notch) so the edges don’t overlap awkwardly when folded.

• Standard GI sheet size: Usually $8′ \times 4’$ or $10′ \times 4’$.

• Soldering: Soft solder (Lead-Tin) is used for sealing sheet metal seams to make them airtight, but not for structural strength.

• Zinc Coating: GI sheets are coated using the Hot-dip Galvanizing process.

• Safety: Always wear leather gloves to prevent cuts from “burrs” (sharp edges left after cutting).