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newSchoolWorks.com › VoTechWorks › Building & Construction › HVAC/R › Ductwork: Design, Fabrication, and Installation › Lesson 5
BLD-HVAC07-05 · Lesson 5 · 20 clock hours · lab

Sheet-Metal Fabrication: Cutting, Forming, and Joining

Objective. Students will cut, fold, and assemble a complete rectangular duct section with Pittsburgh lock seams and drive-cleat cross-joints that are square, tight, and free of sharp external edges.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional engineer or master tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Sheet-Metal Fabrication: Cutting, Forming, and Joining

Fabricating a rectangular duct section by hand is a core HVAC/R skill. In this lab you will take the scribed blank from Lesson 4 and turn it into a finished duct piece complete with Pittsburgh lock seams and drive-cleat cross-joints. Work carefully: every cut and fold affects fit, airflow, and installation time on the job.

Safety First

Sheet-metal edges are razor-sharp from the moment a shear or snip touches them. Wear heavy leather or cut-resistant gloves throughout this lab — no exceptions. Safety glasses must be on before the first cut and stay on until you leave the workbench. Never reach over or behind a blade on the shear. Keep fingers clear of the throat of the notcher. When closing a Pittsburgh seam with a mallet, keep your free hand on the side of the duct, not in the seam path. File or deburr every exposed edge before handling without gloves, and before the instructor inspection: a single sharp external edge is an automatic rework.

Tools and Their Roles

Hand shear / bench-top shear — makes long straight cuts. The bench-top shear handles thicker gauges with less effort and gives a cleaner edge. Use it for your main cross-cuts.

Hand notcher — removes a small square of metal at each corner so the folded flanges do not overlap and bind.

Pittsburgh lock former — a specialized roll-forming machine that creates the male and female profiles of the Pittsburgh seam along the long edges of the blank.

Cornice brake — bends the duct body into its rectangular shape. The brake's clamping beam holds the metal while the apron folds it to the desired angle.

Drive-cleat bender — forms the S-shaped or drive-cleat strip used to join duct sections end to end.

Dead-blow mallet — the weighted, non-rebounding head seats the Pittsburgh seam without denting the duct face.

Framing square and steel rule — verify squareness and dimensions at every stage.

Understanding the Pittsburgh Lock Seam

The Pittsburgh seam is the industry-standard longitudinal seam for rectangular ductwork. One long edge of the blank is formed into a pocket (the female side) and the opposite long edge is formed into a flat tab (the male side). After the duct body is folded on the brake, the tab slides into the pocket and is driven closed with a mallet. A properly closed Pittsburgh seam has no visible gap greater than 1/16 inch and lies flat without a ridge that could catch air or create turbulence.

The lock former produces the pocket in one pass. Feed the blank edge squarely into the machine, hold steady lateral pressure, and keep a consistent feed rate. Rushing causes the pocket to collapse or flare. The opposite edge receives a simple hem or flat tab — follow your instructor's demonstration for the exact profile your machine produces.

Step-by-Step Fabrication Sequence

1. Cut the blank. Align your scribed lines with the shear blade. Make cross-cuts first, then rip cuts. After each cut, immediately place the piece on the bench with cut edges away from you and your partner.

2. Notch the corners. Use the hand notcher to remove the corner squares you marked during layout. Accurate notching prevents corner bunching when you fold.

3. Form the Pittsburgh edges. Run both long edges through the Pittsburgh lock former per the instructor's sequence — pocket edge first, then tab edge. Inspect each edge before moving on.

4. Fold the duct body. Set the cornice brake for 90-degree bends. Fold in sequence — typically the two short sides first, then the remaining two — so the brake can reach each bend line. After each fold, check the corner with your framing square.

5. Close the seam. Hook the tab into the pocket. Starting at one end, drive the seam closed along its length with the dead-blow mallet, working in short overlapping strokes. Check the seam gap with a rule; it must be 1/16 inch or less.

6. Attach drive cleats. Slide a pre-formed drive cleat onto each open end of the duct. The cleat must seat fully against the flange on both sides. Tap lightly with the mallet if needed.

7. Connect and inspect. Slide the pre-made mating section into the drive cleats. Check the assembled joint for squareness and seam tightness by feel and by framing-square check at each corner.

8. Deburr all exposed edges. Run a file along every external cut edge. The inspector will fail any sharp edge immediately.

Why Squareness Matters

A duct that is out of square will not align with fittings in the field. It creates gaps at connections that bypass insulation, allow conditioned air to escape into unconditioned spaces, and reduce system efficiency. The 1/16-inch tolerances used in this lab reflect real shop and field standards for commercial ductwork quality.

Drive-Cleat Connections

Drive cleats are one of the fastest cross-joint methods for straight rectangular duct runs. The cleat is formed with two flanges: one slides over each duct end, locking them together. Proper seating requires that the duct ends are square and that the cleat length matches the duct perimeter closely. A cleat that is too short will not close; one that is too long will buckle. Always inspect the assembled joint with your hand — if you can feel a step or gap, the joint needs adjustment before the inspector sees it.

Key terms8

Pittsburgh lock seamA longitudinal sheet-metal seam in which a pocket formed on one edge receives a flat tab from the opposite edge; the tab is then driven closed with a mallet to create a tight, low-profile joint along the length of a rectangular duct.
Pittsburgh lock formerA roll-forming machine that shapes the pocket profile of a Pittsburgh seam along the edge of a sheet-metal blank in a single pass.
Cornice brakeA manually operated sheet-metal machine that clamps the workpiece along a straight line and bends it to a precise angle, used here to fold the duct body into its rectangular cross-section.
Drive cleatA formed metal strip with two flanges that slides over the open ends of two adjoining duct sections to join them end-to-end; also called a drive slip.
Dead-blow malletA mallet filled with shot or sand that eliminates rebound; used to close Pittsburgh seams and seat drive cleats without denting the duct surface.
Hand notcherA punch-style tool that removes a small square of metal at the corner of a blank so that folded flanges fit together cleanly without overlapping or binding.
DeburringThe process of removing sharp burrs and rough edges left by cutting tools, typically using a file or deburring tool, to prevent injury and ensure proper fit of mating parts.
Squareness toleranceThe maximum allowable deviation from a true 90-degree corner; in this lab, corners must be within 1/16 inch of square as checked with a framing square.

Worked examples4

1. Your scribed blank is laid out for a duct with a 10-inch height and a 14-inch width. Before folding, you need to confirm that the four fold lines divide the blank correctly. List the four panel widths across the blank (Pittsburgh tab edge to Pittsburgh pocket edge) and their sum.
  1. A rectangular duct has four sides: two matching the height and two matching the width. For a 10 × 14-inch duct the four panels are: 10 in, 14 in, 10 in, 14 in.
  2. Add them: 10 + 14 + 10 + 14 = 48 inches.
  3. This 48-inch dimension is the developed blank width before the lock-seam allowances are added. Confirm your scribed blank measures 48 inches across the four fold panels (plus seam allowances at both edges per the lock-former spec).
Answer: The four panels are 10 in, 14 in, 10 in, and 14 in, totaling 48 inches of developed width.
2. After closing the Pittsburgh seam you measure the gap at three points: 0.040 in, 0.068 in, and 0.055 in. Does the seam pass the 1/16-inch (0.0625 in) standard? Which measurement(s) fail?
  1. Convert 1/16 inch to a decimal: 1 ÷ 16 = 0.0625 inches. This is the maximum allowable gap.
  2. Compare each measurement to 0.0625 in: 0.040 in — PASS (less than 0.0625); 0.068 in — FAIL (greater than 0.0625); 0.055 in — PASS (less than 0.0625).
  3. Because one point exceeds the tolerance, the seam fails and requires rework at the 0.068-inch location.
Answer: The 0.068-inch measurement fails the 1/16-inch (0.0625-inch) maximum gap requirement; the seam must be reworked at that location.
3. You are cutting corner notches for a duct with 1/2-inch flanges. What square size must the notcher remove at each corner, and what is the total area of metal removed from all four corners?
  1. Each corner notch must clear both flanges. With 1/2-inch flanges on both intersecting sides, the notch is a 1/2 in × 1/2 in square.
  2. Area of one notch: 0.5 × 0.5 = 0.25 square inches.
  3. There are four corners on a rectangular blank, so total area removed: 4 × 0.25 = 1.00 square inch.
Answer: Each notch is a 1/2 × 1/2-inch square (0.25 sq in); total metal removed from all four corners is 1.00 square inch.
4. Your drive cleats must span the full outside perimeter of one end of a 10 × 14-inch duct. If the cleat material comes in 10-foot (120-inch) sticks, how many full sets of four cleats (one per side) can you cut from one stick, with no cleat shorter than its respective duct side?
  1. Each set of four cleats requires: two at 10 inches and two at 14 inches. Total length per set: 10 + 10 + 14 + 14 = 48 inches.
  2. Number of full sets from one 120-inch stick: 120 ÷ 48 = 2.5. Round down to 2 full sets.
  3. Remaining material: 120 − (2 × 48) = 24 inches, which is not enough for a third full set (which would need 48 inches).
Answer: Two complete sets of four drive cleats can be cut from one 10-foot stick, with 24 inches of material left over.

The math in this lesson2

Learners compute duct perimeter for cleat length and the area of corner notches removed.
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Brush up in the math halls at newSchoolWorks.com:
In class: Cut the scribed blank from Lesson 4 on the hand shear (or bench-top shear where appropriate) and form the Pittsburgh lock edges using the lock former, following the step sequence demonstrated by the instructor. · Fold the duct body on the cornice brake, square corners with a framing square, and close the Pittsburgh seam by hand driving with a mallet; inspect for gap greater than 1/16 inch. · Attach drive cleats to both ends, slide a pre-made mating section together, and perform a visual and tactile inspection of the assembled joint for squareness and seam tightness.
Materials: Scribed sheet-metal blanks from Lesson 4, cornice brake, hand notcher, Pittsburgh lock former, drive-cleat bender, tin snips, dead-blow mallet, framing square, steel rule, file for deburring, gloves and safety glasses
Check: Instructor inspects assembled duct: seam gap must not exceed 1/16 inch, corners within 1/16 inch of square on a framing-square check, and all exposed edges deburred; any sharp edge is an immediate fail requiring rework.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

HVAC Excellence / NCCER HVAC — Ductwork Fabrication
1. What is the maximum allowable gap in a closed Pittsburgh lock seam according to the lab standard used in this course?
NCCER HVAC — Sheet-Metal Tools and Equipment
2. Which tool is used to create the pocket profile of a Pittsburgh seam along the edge of a sheet-metal blank?
NCCER HVAC — Ductwork Quality Standards
3. A technician folds a duct blank and checks one corner with a framing square. The inside corner measures 91.5 degrees. What should the technician do?
NCCER HVAC / HVAC Excellence — Sheet-Metal Layout and Fabrication
4. Why must corner notches be cut in a duct blank before folding on the cornice brake?
OSHA / NATE — Shop Safety
5. Which personal protective equipment (PPE) is required at all times during sheet-metal cutting and forming operations?
NCCER HVAC — Ductwork Fabrication Techniques
6. When closing a Pittsburgh seam with a dead-blow mallet, what technique produces the most uniform closure along the seam length?
NCCER HVAC / HVAC Excellence — Sheet-Metal Layout and Mathematics
7. A rectangular duct measures 8 inches × 12 inches. What is the total developed blank width across the four side panels, not including seam allowances?
HVAC Excellence / NATE — Safety and Workmanship Standards
8. After assembly, an inspector finds a sharp external edge on a finished duct section. According to the lab check sheet, what is the correct outcome?
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