308A Fabrication Techniques 3 — Questions and Answers
Question 1: What is the recommended method for cutting galvanized steel to minimize exposure to zinc fumes?
- Always use an oxy-fuel torch
- Use mechanical cutting methods such as shears, snips, or plasma with proper ventilation (Correct answer)
- Use a grinding wheel without ventilation
- Heat the metal first to burn off the zinc
Correct answer: Use mechanical cutting methods such as shears, snips, or plasma with proper ventilation
Mechanical cutting methods do not produce zinc fumes. When thermal cutting is necessary, proper ventilation and respiratory protection are required to protect against zinc oxide fumes which cause metal fume fever.
Question 2: What is the function of a 'turning machine' (or elbow edging machine) in sheet metal fabrication?
- To spin metal into round shapes
- To turn or flange the edges of round fittings like elbows and tees for assembly (Correct answer)
- To rotate ductwork during installation
- To polish the metal surface
Correct answer: To turn or flange the edges of round fittings like elbows and tees for assembly
A turning machine creates flanged edges on round fittings such as elbows and tee sections. This turned edge allows the pieces to overlap and be fastened together during assembly.
Question 3: When using a rotary machine for crimping round duct, how deep should the crimp typically be?
- The full thickness of the material
- Approximately 1/4 inch deep, enough to reduce the diameter for insertion into the next piece (Correct answer)
- At least 1 inch deep
- The crimp should go all the way through the metal
Correct answer: Approximately 1/4 inch deep, enough to reduce the diameter for insertion into the next piece
Crimping reduces the diameter of one end by approximately 1/4 inch, creating a slight taper that allows the crimped end to slide inside the adjacent duct section for a friction fit connection.
Question 4: What is the advantage of using self-drilling screws (Tek screws) for sheet metal assembly?
- They are cheaper than all other fasteners
- They drill their own pilot hole and tap threads in one operation, speeding assembly (Correct answer)
- They can be reused multiple times
- They are invisible when installed
Correct answer: They drill their own pilot hole and tap threads in one operation, speeding assembly
Self-drilling (Tek) screws have a drill-point tip that pierces and drills through the sheet metal, then the threads tap into the hole — all in one operation. This eliminates the need for pre-drilling.
Question 5: What is a 'cleat' in sheet metal duct fabrication?
- A type of rivet
- A narrow strip of metal used to connect two duct sections together at their transverse joints (Correct answer)
- A tool for measuring angles
- A protective coating for ductwork
Correct answer: A narrow strip of metal used to connect two duct sections together at their transverse joints
Cleats are narrow strips of sheet metal formed into specific profiles (drive cleats, S-cleats) used to mechanically join the ends of rectangular duct sections together at transverse joints.
Question 6: What is the purpose of 'cross-breaking' a sheet metal panel?
- To weaken the metal for easier cutting
- To add rigidity by creating a slight diagonal crease across the panel surface (Correct answer)
- To clean the surface of the panel
- To test the metal for defects
Correct answer: To add rigidity by creating a slight diagonal crease across the panel surface
Cross-breaking puts a slight bow or crease diagonally across a flat panel, which dramatically increases its rigidity and resistance to vibration and oil-canning (flexing or popping).
308A Fabrication Techniques 3
What is the recommended method for cutting galvanized steel to minimize exposure to zinc fumes?