All 100 308A Practice Questions & Answers

Parallel line development is used for prisms (rectangular ducts) and cylinders (round ducts). Lines in the pattern are drawn parallel and equally spaced around the perimeter. Radial line is for cones and pyramids. Triangulation is for transition pieces between different shapes.

Triangulation is used when connecting two different shapes (rectangle-to-round, offset transitions). The surface is divided into triangles; true lengths of each diagonal are determined and then the triangles are unfolded into a flat pattern. Parallel line only works when both ends are the same shape. Radial line works only for cones and pyramids with a single apex point.

In orthographic projection, lines that are not parallel to the projection plane appear shorter than they actually are (foreshortened). True length is the actual measurement of a line as it exists on the 3D surface — found by rotating or projecting the line until it is parallel to the projection plane. Using foreshortened lengths produces an incorrect pattern.

In radial line development of a cone, all surface lines radiate from the apex. The radius of the arc in the flat pattern equals the slant height — the true length from the apex to the edge of the base circle. The arc length then equals the circumference of the base (2πr). The height of the cone alone is not the slant height.

Triangulation sequence: 1) Draw plan and elevation views. 2) Divide the surface into triangles (diagonals across each face). 3) Find the true length of each line using a true length diagram (rotate each line to horizontal and measure). 4) Draw each triangle in sequence using the true lengths, building the flat pattern triangle by triangle.

Parallel line development divides the circumference into equal parts (typically 12 or 16) on the stretchout line; more divisions give a smoother curve.

Triangulation is used for non-developable surfaces like square-to-round transitions. The surface is divided into triangles whose true lengths are found on a true-length diagram.

The horizontal base of a true-length diagram is the horizontal plan (top view) distance of the line; the vertical leg is the elevation change. The hypotenuse gives the true length.

Gore lines divide a fitting into equal angular segments for pattern layout. Each gore is a mitre section; together they form the elbow curve when assembled.

An eccentric reducer is an oblique cone — radial lines from the apex are not equal length. Triangulation must be used to find true lengths of each element.

When a cylinder is unrolled flat, its length equals the circumference (πd). The stretchout line represents this unrolled length; heights at each division come from the true-length view.

A 3-piece elbow has two mitre cuts and one centre section, requiring 2 gore lines. The pieces are: end A, centre gore, end B.

Allowances are extra material added to the true pattern for seam laps, wired edges, flanges, or drive cleats — they are not part of the fitting geometry itself.

Trammel points (beam compass) scribe large arcs and circles. Dividers step off equal spaces along curves. Both are essential layout tools for pattern development.

The curved outer and inner faces of a radius rectangular elbow are developed using parallel line development, with each gore being a trapezoidal flat pattern.

A 45° offset cut through a cylinder produces an elliptical mitre. Parallel line development is used: heights at each stretchout division are taken from the front view of the mitre cut.

Compound fittings use combined methods: parallel line for developable flat/cylindrical faces and triangulation for non-developable transition zones.

True length is the hypotenuse of a right triangle whose base is the plan length and whose height is the vertical rise: TL = √(400² + 300²) = √(160000 + 90000) = √250000 = 500 mm. This 3-4-5 relationship is the basis of the true-length diagram used in triangulation.

The seam line is where the flat pattern is joined together after rolling/forming. It is placed at the shortest element (often the throat of an elbow) to minimise seam length.

SMACNA specifies 24 gauge galvanized steel for 1/2" WG pressure class ductwork in sizes up to 24". As duct size or pressure class increases, heavier gauges are required. 14 gauge is structural steel — far too heavy for standard ductwork. 30 gauge is too thin for duct construction.

The Pittsburgh lock (also called Pittsburgh seam) is the standard longitudinal closing seam for rectangular ducts. One edge is formed into a pocket (the "Pittsburgh pocket"), and the mating flat edge is inserted and then clinched (hammered) closed. It provides a strong, airtight closure along the full length of the duct.

The grooved seam (or rolled groove seam) is used on round ductwork — particularly the spiral seam formed by a spiral pipe machine — and for connecting two curved edges. Both edges are formed into matching grooves and then rolled together to lock. It creates a smooth, mechanically tight seam without exposed sharp edges.

SMACNA classifies duct systems by static pressure: 1/2" WG, 1" WG, 2" WG, 3" WG, 4" WG, 6" WG, and 10" WG. The pressure class determines sheet metal gauge, seam types, reinforcement requirements, and joint spacing. Low-pressure residential systems are typically 1/2" or 1" WG.

SMACNA requires heavier gauge for higher pressure classes and larger duct sizes. At 2" WG pressure class, 24"x24" duct requires 22 gauge galvanized steel. The combination of larger size and higher pressure necessitates thicker metal to resist deformation and maintain structural integrity. Always verify with the current SMACNA table.

Drive cleats (also called drive slips) have one open channel — they are hammered (driven) onto one side of a duct joint, with the other duct end inserted first. S-cleats (S-slips) have two channels — both duct ends slide in from opposite sides of the S. Drive cleats are used on the long sides of rectangular duct joints; S-cleats are used on the short sides.

A common rule for mild steel sheet is: minimum inside bend radius ≈ material thickness (1T). Tighter bends risk cracking, especially in harder alloys.

Flat blank = Leg1 + Leg2 + Bend Allowance. The BA accounts for the arc of material consumed in the bend. (Setback is subtracted from each leg when working from outside dimensions.)

SMACNA requires Pittsburgh lock seam (or equivalent structural seam) for 1" WG class. Button punch snap lock is limited to lower pressure classes.

TDC (Transverse Duct Connector) and TDF (Transverse Duct Flange) systems are rated to 4" WG and higher when properly installed per SMACNA tables.

A hem folds the raw cut edge back on itself, removing the sharp edge hazard and adding rigidity to the panel edge.

A hand seamer (seaming pliers) is the correct tool for closing a Pittsburgh lock seam in the field: it grips the folded pocket (P-lock groove) and bends it flat onto the inserted panel edge, completing the longitudinal seam securely. Wrong answers: aviation snips cut sheet metal — they cannot form seams; a pop rivet gun creates mechanical fasteners (not a seam fold); an offset hammer is used to drive and set seams in tight locations but does not grip and fold the lock the way a hand seamer does. For field work, hand seamers are essential because a Pittsburgh machine (power roll former) is shop equipment. Red Seal point: the Pittsburgh machine forms the pocket on the panel edge in the shop; the hand seamer closes it during erection.

SMACNA requires a minimum 1/2" (13mm) lap for seams on negative-pressure ducts to ensure adequate seal and structural integrity.

Yellow-handled aviation snips are the straight-cut snips, used for straight lines and large, gentle curves. Red-handled snips cut to the left (counterclockwise curves) and green-handled snips cut to the right (clockwise curves). This colour code — yellow=straight, red=left, green=right — is standardised across manufacturers.

SMACNA requires intermediate reinforcement (cross-breaking, beading, or standing seams) on large flat panels at elevated pressures to prevent panel flutter and fatigue.

Pop rivets require a hole that matches the rivet shank diameter closely (within +0.1mm) for proper expansion and clamping. Too large a hole prevents full rivet expansion.

Beading rolls a small ridge into the panel surface, greatly increasing stiffness by the same principle as corrugation. It prevents oil-canning (panel flutter) in HVAC ductwork. These linear indentations also stiffen the panel against buckling.

The S-cleat snaps over the standing seam (male) edge of one duct. The second duct's matching seam is then inserted into the open channel and driven closed with a hammer/mallet.

A wire inserted into a folded hem creates a stiff, rounded edge. This adds rigidity to the panel edge, prevents the sharp cut edge from being exposed, and is used on cylindrical fittings.

The waste piece curls downward and away from the cut line under the lower blade. This prevents the curl from interfering with the snip handles and allows a clean, straight cut.

A bar folder (or bar-folding machine) forms a standing seam (S-lock male edge) or short flanges by folding the sheet edge back on itself. The Pittsburgh machine forms the pocket (female side).

SMACNA permits slip-and-drive (S & Drive) transverse connections for low-pressure (up to 2" WG) rectangular ducts at this size. Pocket lock is used at joints; companion angle is for higher pressure or larger ducts.

GTAW (TIG welding) is the preferred process for stainless steel sheet metal because it provides precise heat control, produces clean welds without spatter, and maintains the corrosion-resistant properties of stainless steel. SMAW and FCAW create too much spatter and heat distortion on thin gauge material. Oxy-acetylene may cause carbide precipitation in stainless steel.

Spot welding passes high current through copper electrode tips pressed against both sheet surfaces. Resistance heating at the contact point melts and fuses the sheets at discrete spots. Advantages: fast, no filler metal, minimal distortion, no flux. Limitations: only for lap joints, limited penetration, requires electrode access to both sides.

Solid rivet installation: 1) Drill/punch hole to correct diameter (typically 1/32" larger than rivet diameter). 2) Insert rivet. 3) Support the factory head with a bucking bar (dolly) held firmly against it. 4) Drive rivet shank with pneumatic rivet gun or hammer to upset (flatten) the shank into a shop head. The resulting compression holds the sheets together.

Heat tint on stainless steel is caused by chromium oxide formation when the hot metal is exposed to oxygen. Light gold = slightly exposed; blue/grey = significantly oxidized. Heavy heat tint reduces corrosion resistance in that area. Prevention: proper argon shielding gas coverage, sufficient flow rate, correct torch angle, and allowing the weld to cool under shielding gas before moving the torch. Inadequate shielding gas coverage or slow cooling in open air is what exposes the hot metal to oxygen.

Burning the zinc coating releases zinc oxide fumes that cause metal fume fever. Local exhaust ventilation and a respirator rated for metal fumes are required. Zinc oxide fumes are toxic even at low concentrations.

75/25 Ar/CO2 (C25) gives stable arc, lower spatter, and good penetration for thin mild steel. 100% CO2 causes more spatter; pure Ar is used for aluminium.

GTAW (TIG) provides the most precise heat control and lowest distortion for thin stainless. ER308L filler reduces carbide precipitation risk.

Tack welds temporarily hold parts in correct position and alignment before full seam welding, preventing movement and thermally induced distortion as the joint is welded. Wrong answers: tack welds are NOT the final structural welds — they provide only enough strength to hold position; they should not replace clamps for complex assemblies that require repositioning during fit-up (clamps are removed and repositioned, tacks are not). Tack weld quality matters: a tack that cracks during welding can become a defect in the root of the final weld. Good practice: tack at both ends first to hold square, then space intermediate tacks to control distortion. In sheet metal duct welding, tack spacing typically 150–300mm for light gauge.

Acid (zinc chloride) or rosin flux cleans copper oxide from the surface, allowing solder to wet and flow. Without flux, solder will not bond to oxidised copper.

A minimum 6mm (1/4”) lap is required for soldered sheet metal joints to allow adequate capillary solder flow and develop sufficient joint strength. Capillary action draws molten solder into the joint gap (typically 0.05–0.15mm clearance) — too small a lap leaves insufficient bond area; too large a lap does not improve strength but wastes solder and time. Wrong answers: one sheet thickness is inadequate for most soldered joints; 25mm (1”) is unnecessarily large for standard sheet metal seams. The correct joint clearance for capillary soldering is different from the lap length — a tight fit (clearance 0.05–0.15mm) promotes capillary flow while a loose fit (>0.5mm) results in a cold, porous joint. Solder must be applied to the base metal (not the iron tip) once the seam reaches soldering temperature.

Spot weld nugget size is primarily controlled by current (amperage). Higher current → larger nugget. Weld time and electrode force also affect nugget; current is the dominant variable.

Golden/straw tint = slight oxidation, acceptable but not ideal. Dark blue or black tints indicate heavy oxidation requiring mechanical or chemical cleaning (passivation/pickling).

Brazing: filler melts (liquidus >450°C/840°F) but base metal remains solid. Welding: base metal melts. The key distinction is whether the base metal is melted.

When a bend is released, elastic stress partially straightens the metal. To achieve the required final angle, the sheet must be overbent slightly. The overbend angle depends on material and thickness.

Correct soldering: heat the base metal (the seam), not the solder. When the seam is hot enough, touch the solder wire to the metal — it melts and wicks into the joint by capillary action.

Electrode force ensures intimate electrical contact (reducing contact resistance) and applies forging pressure to contain the weld nugget as it solidifies, preventing porosity and expulsion.

SMACNA Sealing Classes: Class A (most stringent) = all transverse joints, longitudinal seams, AND duct wall penetrations sealed. Class B = all transverse joints and longitudinal seams sealed. Class C (least stringent) = no specific sealing requirement. Class A is required for high-pressure or critical systems.

SMACNA HVAC Duct Construction Standards Table 4-1 permits hanger spacing of 4–10 ft depending on duct size, gauge and hanger type; 8 ft (2.4 m) is the common specified maximum for mid-size low-pressure rectangular duct. Larger or heavier duct, higher pressure class, or seismically active areas may require closer spacing. Hangers must support duct weight without allowing sag that affects airflow.

SMACNA limits flexible duct connections to approximately 6 ft (1.8 m) to minimize pressure drop. Flexible duct has significantly higher friction loss per foot than rigid duct due to its corrugated interior surface. It must be installed fully extended (not compressed), properly supported, and at minimum bending radius. Longer flexible runs cause unacceptable pressure drop and airflow reduction.

A square elbow without turning vanes creates significant turbulence, high pressure drop, and noise as air separates from the outer wall. Turning vanes (sheet metal blades inside the elbow) guide airflow in a smooth arc, reducing separation, pressure drop by up to 75%, and noise. Required by many specifications for critical systems.

SMACNA requires intermediate reinforcement (angle iron, T-bars, or formed stiffeners) for wider ductwork at higher pressure classes to prevent wall deflection. A 30" wide duct at 2" WG exceeds the limit where gauge alone is sufficient. Reinforcement is welded or screwed to the duct at intervals specified in the SMACNA table. Deflection causes noise, leakage, and structural failure.

AHU (Air Handling Unit) conditions and circulates air: it contains a fan, heating/cooling coils, filters, and dampers. Sheet metal ductwork connects to AHU supply and return.

VAV boxes contain a motorised modulating damper controlled by the zone thermostat. They vary airflow from maximum to minimum (or zero) to maintain setpoint temperature.

SMACNA low-velocity design guidelines recommend ≤1,500 FPM (about 7.5 m/s) in main supply ducts to limit noise generation. Branches are typically lower.

As velocity drops at a branch, a regain fitting converts velocity pressure back to static pressure. This static regain partially offsets pressure loss, improving system balance.

Flexible connections (canvas or neoprene) absorb AHU fan vibration, preventing noise and fatigue cracks in the rigid ductwork and building structure.

In a dual-duct system, a mixing box blends hot-deck and cold-deck air in varying proportions controlled by zone thermostats to deliver air at the desired temperature.

Insulation on supply ducts in unconditioned spaces prevents condensation (cold ducts in humid attics), reduces heat gain from the attic, and improves system efficiency.

Commercial kitchen exhaust (grease-laden air) requires minimum 16ga black carbon steel or 18ga stainless with all-welded (external) construction — no internal seams or pockets where grease can accumulate.

A clogged/loaded filter has high resistance, causing elevated pressure drop across the filter section. This also reduces airflow. Filters must be replaced or cleaned.

Balancing dampers are manually adjusted during commissioning to set branch airflows to design values. They are locked in position after balancing is complete.

Transfer air grilles are typically installed high in the partition wall. Warm supply air accumulates near the ceiling and pressure differences drive transfer. Some codes allow door undercuts as an alternative.

A backdraft damper (barometric damper) has gravity-held blades that open with forward airflow from the fan and close by gravity when flow stops, preventing reverse infiltration.

ASHRAE 62.1 sets minimum outdoor air ventilation rates in CFM/person and CFM/ft² of floor area by occupancy type, ensuring adequate dilution of indoor pollutants.

SMACNA specifies intermediate reinforcement (angle iron or flat bar) welded or bolted to large duct panels to prevent buckling and panel flutter under positive or negative pressure.

The WHMIS/GHS flame pictogram indicates a flammable hazard — the material can ignite easily. Flammable liquids (flash point <60°C), flammable gases, and flammable solids all use this pictogram. Sheet metal workers encounter flammable materials including solvents, adhesives, and welding gases. An exclamation mark (!) is a separate pictogram for acute toxicity, skin sensitization, etc. Flammable materials can ignite at ordinary temperatures or when exposed to heat or spark.

Provincial OHS regulations generally require fall protection (personal fall arrest system or guardrails) when working at heights of 3 m (10 ft) or greater. On sloped roofs, a PFAS with shock-absorbing lanyard and a properly rated anchor point is required. The anchor must be capable of supporting 22 kN (5,000 lbs) per worker. Non-slip footwear is an additional requirement, not a substitute for fall protection.

Plasma cutting generates intense UV and IR radiation similar to arc welding. A welding helmet or face shield with appropriate shade lens (typically #5 to #8 for plasma cutting) is required to prevent arc eye (photokeratitis) and retinal damage. The shade number depends on the cutting amperage — higher amperage requires darker shade.

The gas cylinder pictogram (black cylinder silhouette on white background) identifies substances that are gases under pressure — compressed gases (argon, oxygen), liquefied gases (propane, CO₂), refrigerated liquefied gases (LNG), or dissolved gases (acetylene). Wrong answers: the flame-over-circle pictogram indicates an oxidizer (not a compressed gas); the exclamation mark indicates acute toxicity, skin/eye irritation, or other moderate hazards; the skull and crossbones indicates acute lethal toxicity. Red Seal sheet metal workers encounter this pictogram on argon shielding gas cylinders, compressed air cylinders, and refrigerant containers. WHMIS 2015 is aligned with the UN Globally Harmonized System (GHS) — the same pictograms are used internationally, which is why they differ from old WHMIS 1988 symbols.

Plasma cutting produces intense UV/IR radiation. CSA/ANSI recommend shade #8 minimum for plasma cutting. Shades 5-6 are used for light gas cutting; shade 8+ for plasma.

LOTO (Lockout/Tagout) is mandatory before working near or on energised equipment. The sheet metal worker must de-energise, lock out, and verify zero energy state.

Plenum spaces contain energised conduit, communication cables, and the ceiling grid — contact with live wiring or stepping through the grid are key hazards requiring careful footing and awareness.

A large AHU or plenum is a confined space. Entry requires: confined space entry permit, atmospheric testing (O2, CO, combustible gas), standby attendant, and rescue plan per CSA Z1006.

Cut-resistant gloves (ANSI/ISEA level A4+ or equivalent) protect against the sharp edges of cut sheet metal. Standard leather gloves provide limited cut resistance.

Most Canadian OH&S jurisdictions require reporting of injuries requiring medical treatment (beyond first aid) within a set period and a written investigation report.

Sheet metal should be carried vertically with two workers — one on each end — for heavy sheets, keeping the load close to the body and using proper lift technique (back straight, knees bent) to prevent cuts and back strain.

Asbestos disturbance requires a certified abatement contractor. The worker must stop, leave the area, secure the zone, and notify the supervisor. Continuing work violates OH&S asbestos regulations.

In most Canadian provinces (BC, Ontario, Alberta), fall protection (guardrail, safety net, or personal fall arrest system) is required when a worker is exposed to a fall of 3m (10 ft) or more.

A throatless shear (Beverly shear) has a blade mounted without a limiting throat, so sheet metal can be maneuvered in any direction while cutting. This allows cutting curves, circles, irregular shapes, and inside openings — impossible with a standard straight-bar shear. Essential for pattern work in sheet metal fabrication.

A box and pan brake (finger brake) has a segmented upper beam that can be configured to clear flanges already formed on the workpiece. This allows bending multiple sides of a box or pan without removing the previously bent flanges. Individual fingers can be removed to create different-width segments for complex shapes.

A jenny (turning machine / jenny wheel) forms a right-angle flange or turned edge along a curved or straight edge of sheet metal — used for elbow flanges and wired edges.

The rear roller is adjustable up/down. Moving it closer to the front rollers increases curvature (smaller diameter); backing it off reduces curvature. The two front rollers feed/drive the sheet.

The squaring arm (back gauge) positions the sheet at a set distance from the die centreline, ensuring each bend produces an identical flange length without re-measuring.

A throatless (nipper) shear has no throat frame, so the sheet can be rotated freely during cutting, allowing curved and irregular cuts. A guillotine shear only cuts straight lines.

For a Pittsburgh machine, the panel is fed face-up with the edge to be formed entering the rollers. The machine forms the "pocket" (P-lock groove) along the panel edge.

A notcher (corner notcher) cuts square or mitre notches at the corners of flat blanks. Without notching, the corner material would overlap or interfere when the box sides are bent up.

A rotary machine (jenny/rotary) forms wired edges, turned edges, flanges, and similar rolled forms. Punching rivet holes requires a punch press or drill — not a rotary machine.

To form a cone on a slip roll, one end of the rear roller is adjusted higher (or lower) than the other. This creates greater curvature at the narrow cone end and less at the wide end.

The squaring bar (or carpenter's square built into the table) provides a 90° reference. Sheet metal blanks are positioned against it to ensure patterns are laid out perpendicular and square.