Mixed Deck — All 308A Topics Flashcards

What is the purpose of a 'flexible connector' between the air handling unit and the ductwork?

Answer: To isolate fan vibration from the duct system and accommodate thermal movement

Flexible connectors (typically fabric or rubber) isolate mechanical vibration from the AHU fans, preventing it from transmitting through the rigid ductwork. They also accommodate thermal expansion.

What is the standard third-angle projection used in Canadian and North American drawings?

Answer: The front view is centered, top view is above, right side view is to the right — each view shows what you see looking at that side

Third-angle projection places the top view above the front view, the right side view to the right of the front view, and the left side view to the left. Each view shows the object as seen from that direction.

What happens to accuracy if too few division points are used when developing a curved surface pattern?

Answer: The pattern will have flat spots and will not form a smooth curve

Fewer division points mean the curved surface is approximated with fewer straight-line segments, resulting in visible flat spots. More divisions improve accuracy but increase layout complexity.

What is the advantage of using self-drilling screws (Tek screws) for sheet metal assembly?

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.

What is the purpose of a 'heat sink' when welding thin sheet metal?

Answer: To absorb excess heat and prevent distortion or burn-through by drawing heat away from the weld zone

Heat sinks (copper bars, aluminum blocks, or wet rags) placed near or behind the weld joint absorb excess heat, reducing the risk of burn-through and minimizing warping on thin sheet metal.

What is the purpose of 'tacking' before welding a sheet metal joint?

Answer: To hold the pieces in alignment with small spot welds before completing the full weld

Tack welds are small spot welds placed at intervals along the joint to hold the pieces in correct alignment and prevent distortion during the full welding process. They become part of the final weld.

What is the purpose of 'annealing' sheet metal?

Answer: To soften work-hardened metal by heating it to a specific temperature and cooling it slowly, restoring ductility

Annealing heats work-hardened metal to a temperature that allows the crystal structure to recrystallize, then cools it slowly. This restores the metal's ductility and formability, allowing further work without cracking.

What is the thickness in inches of 24-gauge galvanized steel?

Answer: 0.0239 inches

24-gauge galvanized steel has a thickness of approximately 0.0239 inches (0.607 mm). This is a common gauge used for residential and light commercial ductwork.

What is TIG welding and when is it preferred for sheet metal work?

Answer: Gas Tungsten Arc Welding using a non-consumable tungsten electrode, preferred for precision work on stainless steel and aluminum

TIG (GTAW) welding uses a non-consumable tungsten electrode with separate filler rod and argon shielding gas. It provides superior control and weld quality, making it ideal for stainless steel, aluminum, and critical applications.

What visual indicators show a properly made solder joint on sheet metal?

Answer: A smooth, shiny surface with solder visibly drawn into the joint by capillary action with no gaps or voids

A good solder joint has a smooth, shiny appearance indicating proper temperature and flux action. Solder should be drawn into the seam by capillary action, with a slight concave fillet at the edges and no voids or cold spots.

What is the difference between hot-rolled and cold-rolled steel sheet?

Answer: Hot-rolled is formed at high temperature with a rougher finish; cold-rolled is further processed at room temperature with tighter tolerances and smoother finish

Hot-rolled steel is formed above the recrystallization temperature, resulting in a scaled, rougher surface. Cold-rolled steel is further processed at room temperature, producing a smoother surface, tighter thickness tolerances, and increased hardness.

What is the significance of the 'minimum bend radius' for a particular sheet metal material?

Answer: It is the tightest bend the material can make without cracking, expressed as a multiple of material thickness

Minimum bend radius (often expressed as a multiple of thickness, like 1T or 2T) is the tightest bend a material can make without cracking on the outside of the bend. It varies by material, temper, and grain direction.

What is 'sweat soldering' and when is it used in sheet metal work?

Answer: Pre-tinning both surfaces with solder, assembling them, then reheating to fuse the solder layers together

Sweat soldering involves pre-tinning (applying a thin coat of solder to) both mating surfaces, placing them together, then reheating until the solder reflows and bonds the surfaces. It is used for flat seams and patches.

What is the purpose of a 'bar folder' in sheet metal fabrication?

Answer: To make narrow folds and hems on sheet metal edges for seams and safe edges

A bar folder creates narrow folds on the edges of sheet metal, typically for lock seams, hems, and edges. It produces consistent, uniform folds along the entire length of the metal edge.

What is the seam allowance typically added for a drive cleat connection?

Answer: 3/8 inch on each side

A drive cleat connection typically requires approximately 3/8 inch seam allowance on each edge where the cleat will be applied. This provides enough material to fold over and lock into the cleat.

What is the difference between a return air duct and an exhaust air duct?

Answer: Return air recirculates air back to the AHU for reconditioning, while exhaust air is expelled outside the building

Return air ducts bring room air back to the air handling unit to be reconditioned and recirculated. Exhaust air ducts remove air from the building entirely, venting it outdoors.

What is the purpose of 'cross-breaking' a sheet metal panel?

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).

In radial line development, what type of object is typically being patterned?

Answer: A cone or frustum

Radial line development is used for objects that taper to a point or apex, such as cones, frustums, and pyramid shapes. All layout lines radiate from a common center point.

What is the purpose of a 'submittal' drawing in the sheet metal project process?

Answer: A shop drawing or product data submitted to the engineer/architect for review and approval before fabrication begins

Submittals are shop drawings, product data, and samples submitted to the engineer or architect for review before fabrication. This process verifies that the contractor's proposed methods and materials meet the design intent.

What is the maximum working temperature for a standard 50/50 tin-lead soldered joint?

Answer: Approximately 200°F (93°C) for continuous service

Standard 50/50 tin-lead solder joints have a maximum continuous service temperature of approximately 200°F (93°C). Above this temperature, the solder begins to lose strength and may eventually fail.