308A Fabrication Techniques 1 — Questions and Answers
Question 1: What is the primary purpose of a Pittsburgh lock seam in sheet metal ductwork?
- To provide a decorative edge
- To create a strong, airtight mechanical joint without welding (Correct answer)
- To reduce material usage
- To increase airflow velocity
Correct answer: To create a strong, airtight mechanical joint without welding
The Pittsburgh lock seam creates a strong mechanical connection that is airtight and does not require welding. It is the most common longitudinal seam used in rectangular duct fabrication.
Question 2: What is the maximum gauge of galvanized steel that can typically be formed on a standard hand brake?
- 28 gauge
- 22 gauge (Correct answer)
- 16 gauge
- 10 gauge
Correct answer: 22 gauge
Standard hand brakes are typically rated for forming galvanized steel up to 22 gauge. Heavier gauges require power brakes or press brakes to achieve clean, accurate bends.
Question 3: When using a slip roll former, what determines the minimum diameter of a cylinder that can be formed?
- The length of the rolls
- The diameter of the upper (adjustable) roll (Correct answer)
- The speed of the machine
- The colour of the metal
Correct answer: The diameter of the upper (adjustable) roll
The minimum cylinder diameter is limited by the diameter of the top roll. The sheet metal wraps around this roll, so the cylinder cannot be formed smaller than approximately the roll's diameter.
Question 4: What is the purpose of 'beading' on a round duct fitting?
- Decorative appearance only
- To add rigidity and strength to the fitting and provide a point for connection (Correct answer)
- To reduce the weight of the fitting
- To mark the fitting for identification
Correct answer: To add rigidity and strength to the fitting and provide a point for connection
Beading adds structural rigidity to round duct fittings and creates a crimped edge for inserting into the next duct section. It reinforces the fitting against collapse and provides a positive connection point.
Question 5: What is the correct procedure for notching a corner on a sheet metal pattern before forming?
- Cut a random shape at the corner
- Remove a precise amount of material based on the seam type and material thickness to allow clean folding (Correct answer)
- Do not notch; fold the extra material over
- Drill a hole at each corner
Correct answer: Remove a precise amount of material based on the seam type and material thickness to allow clean folding
Proper notching removes exactly the right amount of material so that when edges are folded, they meet without overlapping or leaving gaps. The notch size depends on the seam allowance and bend radius.
Question 6: What type of shear is best suited for cutting internal openings in sheet metal?
- Squaring shear
- Throatless shear or aviation snips (Correct answer)
- Ring and circle shear
- Power guillotine shear
Correct answer: Throatless shear or aviation snips
Throatless shears allow the metal to be fed from any direction without obstruction from a throat, making them ideal for cutting irregular internal openings. Aviation snips also work for smaller openings.
308A Fabrication Techniques 1
What is the primary purpose of a Pittsburgh lock seam in sheet metal ductwork?