Why fittings deserve their own production strategy
Fittings are a minority of the ductwork by linear metre but the majority of the fabrication labour, because each one is unique to its location and orientation. Straight duct runs continuously — round on a spiral tubeformer, rectangular on an auto duct line — but fittings are built one at a time from cut blanks, formed panels and closed seams. So the fitting bench, not the straight-duct line, is usually the real throughput limit of a duct shop.
Two construction methods recur across every fitting type. A mechanical lock seam interlocks the sheet edges with no heat, keeps the galvanised coating intact, and is standard for low- and medium-pressure galvanised work. A weld is used where the joint must be sealed or hygienic — high pressure, stainless, cleanroom or kitchen exhaust. Which one a fitting uses is set by its pressure class and material, not by the fitting type.
Elbows — gore segments and lobster-backs
A round elbow changes direction with gores: ring segments cut at an angle so that, stacked, they sweep through the bend. The gore blanks are cut from sheet or coil, rolled round, and joined gore-to-gore on a gorelocker, which folds and closes a circumferential lock seam at each joint. A 90-degree elbow commonly uses three to five gores. A lobster-back is the same idea taken further — many narrow gores giving a near-smooth, large-radius sweep on big or industrial duct. More gores mean a smoother bend and lower pressure loss, but more cuts and more seams, so the gore count is a direct trade between airflow and labour.
The SBKJ Gorelocker SBEM-1250 (hydraulic) closes the gore seam from diameter 100 to 1250 mm at up to 120 m/min, in galvanised, black steel and aluminium 0.4–1.5 mm or stainless 0.4–1.0 mm, and rolls a 5, 7.5 or 10 mm flange on the gore edge in the same pass. Electric Gorelockers (SBWT-1000 and SBWT-1500) extend the range to 1500 mm. Below roughly 400 mm the companion machine is the Fitting Shaper SBJQJ-500, which forms the ends of small round fittings — elbows, tees, reducers, branches — from 80 to 400 mm. Rectangular elbows are made differently — a square-throat elbow with turning vanes, a radius elbow, or a mitred elbow, all built from flat panels rather than gores. The full elbow picture, gore counts and machine specs are in the dedicated duct elbow machine specs page.
Reducers and transitions — developed blanks, rolled or folded
A reducer is a cone that steps one duct size down to another. Its flat development is cut as a blank, then rolled (round) or folded (rectangular) to shape and closed with a lock seam or a weld. A concentric reducer keeps the centreline aligned and is used in vertical risers; an eccentric reducer keeps one wall flat, so on horizontal runs condensate can drain or the soffit line stays level. Keep the included angle shallow — around 30 degrees total is the low-loss target; steeper cones separate the flow and add resistance.
A square-to-round transition blends a rectangular end to a round end. Because the surface is irregular, its flat pattern is a developed set of triangular facets (triangulation); the blank is cut, folded along the facet lines, and seamed. Transitions and reducers are the fittings most worth driving from a CAM flat-pattern file rather than hand layout, because the developed geometry is unforgiving. The Fitting Shaper SBJQJ-500 is built specifically to form reducers, transitions and custom profiles, and the Laser Cutting Machine page notes the laser is suited to complex elbow gores and fitting blanks. For flat-oval duct — an in-between profile that ships flatter than round but flows better than rectangular — the Ovalizer SBHF-3100 presses spiral round duct into an oval section from a minimum diameter of 330 mm at up to 3100 mm length, in 0.4–1.2 mm material.
Tees, branches, offsets and end caps
A tee or branch takeoff joins a smaller run into a main. The branch is fabricated as a stub, and a profiled hole — a saddle — is cut into the main to receive it; takeoff geometry sets the loss, from a low-loss 45-degree lateral tap to a higher-loss 90-degree straight tap. A bullhead tee splits a main into two opposed branches. An offset shifts a run sideways without changing its size, built from two opposed bends — on round duct two short gored elbows, on rectangular duct two opposed mitres. End caps close a run: flat caps are sheared and seamed, domed caps pressed. Branch, saddle and cap blanks all start on a cutting table.
Cutting the blanks — plasma and laser
Every fitting starts as a flat blank, and the blank decides its accuracy. Profiled gore, reducer, saddle and transition blanks are cut on a CNC table. The SBKJ Plasma Cutter cuts 0.4–4 mm on a 1550×4000 mm or 1550×5000 mm bed at 7–8 m/min — the workhorse for galvanised and mild-steel blanks. The Laser Cutting Machine (2000 W or 3000 W, 0.5–8 mm) gives a cleaner, narrower kerf for stainless fittings and tight nested patterns. Round elbow ends and stiffening rings for large fittings are formed on the Auto Metal Sheet Hoop Machine, which rolls strip hoops and bands for duct connections from diameter 80 to 500 mm.
Joining the fitting — lock seam or weld
Once formed, the fitting is closed. Galvanised low- and medium-pressure fittings are lock-seamed — the gorelocker for round elbows and reducers, a folding and seaming pass for rectangular work — keeping the coating unbroken. Where the joint must be sealed or hygienic, the fitting is welded: high-pressure, stainless cleanroom, pharmaceutical or kitchen-exhaust, and heavy industrial fittings. SBKJ duct welding machines cover that work, from seam and stitch welding to handheld laser welding for stainless. Fix this choice early — it changes the blank allowances and bench process for every fitting on the job.
Fitting-fabrication machines at a glance
The machines above, with the catalog figures that matter when you scope a fitting bench:
| Machine | Makes | Range | Material | Power |
|---|
| Gorelocker SBEM-1250 | Round elbow / reducer / tee lock seam | Φ100–Φ1250 mm, 120 m/min | GI/black/Al 0.4–1.5 mm, SS 0.4–1.0 mm | 4 kW + 1.25 kW fan |
| Fitting Shaper SBJQJ-500 | Small round fittings: elbows, tees, reducers, branches | Φ80–Φ400 mm | GI, stainless, aluminium | 3.5 kW |
| Auto Hoop Machine | Stiffening hoops & connection bands | Φ80–Φ500 mm, strip 20–50 mm | GI 1.5–4.0 mm, SS 1.5–3.0 mm | — |
| Ovalizer SBHF-3100 | Flat-oval duct from round | min Φ330 mm, length to 3100 mm | 0.4–1.2 mm | 18.5 kW |
| Plasma Cutter | Gore / reducer / saddle blanks (GI, mild steel) | bed 1550×4000–5000 mm, 7–8 m/min | 0.4–4 mm | 12 kW |
| Laser Cutting | Clean fitting blanks, stainless, tight nesting | bed 1500×4000 mm | 0.5–8 mm (2000/3000 W) | — |
Source: SBKJ Product Catalog 2026, manufacturer nameplate specifications. All machines run 380 V / 50 Hz / 3-phase (60 Hz on request). Round fittings conform to EN 1506 / SMACNA round-duct construction; rectangular fittings to EN 1505. Diameters and gauges are the machine's catalogued range; achievable gauge depends on material temper.
Pressure drop drives which fitting you choose
Fitting type is mostly an airflow-loss decision. A many-gore or smooth elbow loses far less than a three-gore quick elbow, a 45-degree branch tap far less than a 90-degree tap, a vaned square-throat rectangular elbow far less than an unvaned one, and a shallow reducer cone far less than a steep one. The authoritative coefficients are in the ASHRAE Duct Fitting Database; the practical rule is to spend low-loss fittings where air moves fastest — the supply trunk — and accept higher-loss, cheaper-to-make fittings on low-velocity return and exhaust.
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FAQ
How is a round duct elbow fabricated?
From gores — ring segments cut at an angle, rolled round, and joined gore-to-gore on a gorelocker with a mechanical lock seam. A 90-degree elbow commonly uses three to five gores; more gores give a smoother sweep and lower pressure loss. The SBEM-1250 closes the seam from Φ100–Φ1250 mm; the Fitting Shaper SBJQJ-500 covers small fittings from Φ80 mm.
How is a duct reducer made, concentric vs eccentric?
A reducer is a cone: its flat development is cut as a blank, rolled or folded, and seamed or welded. Concentric keeps the centreline aligned (vertical runs); eccentric keeps one wall flat (horizontal runs, for drainage or a level soffit). Keep the included angle near 30 degrees to stay low-loss.
How are tees, branches and offsets made?
Tees and branches join a stub into a saddle cut in the main — a 45-degree tap flows better than a 90-degree tap. An offset shifts a run with two opposed bends (two short gored elbows on round, two mitres on rectangular). Branch and saddle blanks are cut on a plasma or laser table.
How is a square-to-round transition made?
The irregular surface is triangulated into a developed pattern of facets, cut as a blank, folded along the facet lines, and the seam closed. It is the fitting most worth driving from a CAM flat-pattern file rather than hand layout.
When are fittings welded instead of lock-seamed?
Lock seams suit galvanised low- and medium-pressure fittings and keep the coating intact. Welding is for sealed or hygienic joints — high pressure, stainless cleanroom, pharmaceutical or kitchen exhaust, and heavy industrial duct. SBKJ duct welding machines cover that, including handheld laser welding for stainless.
