308A Fabrication Techniques 4 — Questions and Answers
Question 1: What is an S-cleat and where is it typically used?
- A decorative border for ductwork
- An S-shaped metal strip used to join the longer sides of rectangular duct sections together (Correct answer)
- A type of solder
- A round duct connector
Correct answer: An S-shaped metal strip used to join the longer sides of rectangular duct sections together
An S-cleat is a formed strip of sheet metal with an S-shaped profile. It slides over the edges of two adjoining duct sections, connecting them along the longer (non-drive-cleat) sides of rectangular duct.
Question 2: When fabricating a round duct elbow, what is the 'throat' radius?
- The radius of the outer curve
- The radius of the inner (tighter) curve of the elbow (Correct answer)
- The radius of the duct itself
- The length of the elbow
Correct answer: The radius of the inner (tighter) curve of the elbow
The throat radius is the inside radius of the elbow — the tighter curve. Combined with the duct diameter, it determines the centerline radius and the overall size and airflow characteristics of the elbow.
Question 3: What is the recommended overlap for a longitudinal lap seam on round duct?
- Flush with no overlap
- 3/8 to 1/2 inch (Correct answer)
- 2 inches
- The full diameter of the duct
Correct answer: 3/8 to 1/2 inch
A longitudinal lap seam on round duct typically has a 3/8 to 1/2 inch overlap, which provides enough surface for rivets or screws while minimizing material waste and maintaining a smooth interior.
Question 4: What is the purpose of a 'standing seam' in sheet metal roofing and architectural work?
- To lie flat against the surface
- To create a raised, interlocking seam that sheds water and allows thermal movement (Correct answer)
- To hold insulation in place
- To attach flashing to a wall
Correct answer: To create a raised, interlocking seam that sheds water and allows thermal movement
Standing seams project above the panel surface, creating a water-shedding joint that also accommodates thermal expansion and contraction. They are the premium seam type for metal roofing.
Question 5: What tool is used to set a Pittsburgh lock seam after the edges have been formed?
- A soldering iron
- A hand seamer and mallet or a closing machine (Correct answer)
- A drill press
- A wire brush
Correct answer: A hand seamer and mallet or a closing machine
After the Pittsburgh pocket and lock edges are formed and engaged, a hand seamer is used to close the seam flat, and a mallet is used to set it. Production shops may use a closing machine.
Question 6: What is the standard method for reinforcing large rectangular duct to prevent flexing?
- Adding additional layers of sheet metal
- Installing angle iron or channel reinforcement (tie rods and angles) at specified intervals (Correct answer)
- Painting the duct with a rigid coating
- Reducing the gauge of the metal
Correct answer: Installing angle iron or channel reinforcement (tie rods and angles) at specified intervals
Large rectangular ducts require external reinforcement using angle iron, channel, or tie rods at intervals specified by SMACNA standards. This prevents the duct walls from flexing under pressure.
308A Fabrication Techniques 4
What is an S-cleat and where is it typically used?