Insights · Reference
A sheet of coated steel becomes a length of duct in five stages — cut, form, seam, weld, flange. This reference walks the full HVAC duct fabrication process end to end and names the machine that does the work at each step, so you can see exactly how round and rectangular ductwork is manufactured.
Every piece of HVAC ductwork, round or rectangular, goes through the same sequence. The machine changes with duct shape and material, but the stages do not.
| Stage | What happens | Machine | SBKJ reference |
|---|---|---|---|
| 1. Cut | Coil to strip, sheet to blank, or CNC profile cuts | Slitter, shear, plasma, laser | Cutting machines |
| 2. Form | Strip to round tube, or blank to rectangular box | Spiral tubeformer / auto line / lockformer | Forming machines |
| 3. Seam | Close the longitudinal lock seam | Lockformer / integrated line station | Pittsburgh vs snap lock |
| 4. Weld | Continuous, spot or stitch weld (airtight duct) | Seam / spot / stitch welder | Duct welders |
| 5. Flange | Form the bolt-together connection | TDF flange line / angle flange | TDF flange machines |
Fabrication starts with mill coil or flat sheet. Slitting cuts a wide coil lengthwise into the narrow strip a spiral tubeformer needs (a 137 mm strip is standard). Shearing cuts flat sheet into square-edged blanks for rectangular duct. Plasma and laser machines make CNC profile cuts — holes, fittings, elbow gores and custom shapes. The operation chosen depends on whether you are making round duct, rectangular duct or fittings. See the cutting machine comparison for thickness and bed-size ranges.
Round duct is formed on a spiral tubeformer: a continuous strip is wound helically and lock-seamed into a tube of any length, from Φ80 to Φ2500 mm. It is the fastest, lowest-leakage way to make round duct.
Rectangular duct is formed in sections. An auto duct line takes coil and notches, folds and seams it into four-sided duct in one pass; a lockformer and folders do the same job station by station for lower volumes. This is also where the material’s gauge matters — heavier galvanized needs a machine rated for it (see the gauge & thickness chart).
The longitudinal seam is what makes a formed section rigid and airtight. Round spiral duct closes with a rolled lockseam as it is wound. Rectangular duct closes with a Pittsburgh lock (a mechanically continuous interlocking corner) or a faster snap lock. The seam is folded, not welded, so the zinc coating survives intact. The Pittsburgh vs snap lock guide covers when each is used.
Most low-pressure duct never needs welding — a locked seam is enough. But high-pressure, industrial, kitchen-exhaust or stainless duct calls for a gas-tight weld. A seam welder runs a continuous resistance weld along the lap; spot and stitch welders tack round duct and fittings; a medium-frequency or laser welder handles heavier or stainless work. Welding is the step that turns a mechanically-joined duct into a sealed one.
Finally the duct needs a way to join to the next section on site. The modern standard is the TDF (Transverse Duct Flange) — an integral flange rolled directly onto the duct ends by a TDF flange line, so no separate angle iron is needed. Older or heavier duct uses a bolted angle flange. With the flange formed, the section is complete and ready to ship.
The default material through all five stages is galvanized steel — zinc-coated for corrosion resistance, usually G90/Z275 in 26–20 gauge (0.55–1.0 mm). It cuts, forms, seams and flanges cleanly at full speed. Stainless or aluminium is reserved for wet, coastal or corrosive air. The whole line — cutting through flanging — is sized to the gauge you run; SBKJ machines form 0.4 mm up to 2.0 mm galvanized.
Planning a duct line? Tell us your output and we’ll map the machines →
In five stages: cut the coil/sheet, form it into round or rectangular duct, close the lock seam, weld where an airtight joint is needed, and form a TDF or angle flange so sections bolt together on site.
Cutting machines (slitter, shear, plasma, laser), forming machines (spiral tubeformer, auto duct line, lockformer, folders), seam/spot/stitch welders, and TDF flange-forming lines. See the full machine specifications hub.
Mostly galvanized steel (G90/Z275, 26–20 gauge), with stainless or aluminium for wet, coastal or corrosive environments.
Round duct is wound continuously on a spiral tubeformer; rectangular duct is folded and seamed in sections on an auto line or lockformer, then TDF-flanged. Round is faster and lower-leakage; rectangular suits tight spaces.
Setting up to fabricate duct? Tell us your duct type, gauge and daily output and an SBKJ engineer maps the machines for each stage — within 12 hours, not a salesperson.
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