Introduction: A round duct elbow takes shape through an ordered sequence of flat sheet layout, rolling, lock seam formation, and segment assembly that any workshop learner can follow.
A flat sheet does not look like a curved air fitting on day one. It looks like a rectangle with scribed lines and a pair of seam allowances cut into its edges. The transformation from that flat blank to a finished elbow happens in a clear, repeatable order, and each step depends on what the previous step did. Understanding that order matters when you are learning the workshop, checking a quote, or watching a dedicated round elbow forming machine run for the first time. this guide walks through the sequence from layout to finished fitting.
A round elbow is not one curved tube. It is a stack of short tapered segments, and each segment starts as its own flat blank. The blank for one segment is wider at one end than the other, because the segment has to wrap into a cone-like slice rather than a straight cylinder. A layout mark on the flat sheet shows where the seam allowance sits on each edge, and that allowance is what the later lock seam will use to join the two ends. Without a correct allowance, the rolled segment will not close cleanly, and the finished elbow will show a gap or a buckle at the seam line. The pattern for one segment depends on the elbow angle, the number of segments, and the segment position. The two end pieces usually differ from the middle pieces, so a shop building a 90° elbow in five segments cuts five different blanks, not five identical ones. Diameter also drives layout: a 125 mm segment uses a much shorter developed length than a 1250 mm segment, even if both sit on the same machine bed. Workshops handle this by keeping a pattern set or a table of developed dimensions for each common elbow size, so layout is a check against a known value rather than a fresh calculation every time. When a nonstandard angle comes in, the pattern is recalculated and the blank is trimmed to the new developed outline. At the end of layout, the metal is still flat, but every later step is locked to choices made here. The blank is deburred, the seam allowance is confirmed, and the segment is ready to move to the rolls. A workshop learner who can describe this blank accurately - developed length, end widths, allowance - already understands half of what makes the finished elbow fit together.
Rolling turns the flat blank into an open cylindrical shell. The metal is fed between rolls that bend it progressively into the target curvature, and the two long edges come around until they meet. On thin sheet metal - typically 0.4 to 1.2 mm for galvanized or ordinary steel and down to 0.7 mm for stainless - the roll pressure is light, but the curvature has to stay even along the full length or the finished segment will look oval from one end. The seam allowance prepared at layout is now the pair of edges waiting at the rolls for the next step.
Feed speed and roll alignment decide whether the segment comes off round. If an operator pushes one edge faster than the other, or the sheet is not square to the rolls, curvature drifts and the seam ends will not line up. Skilled operators watch for the sheet slipping toward one side and correct it early, before the blank has been half-rolled and the drift is harder to remove. A dedicated electric round elbow making machine such as the COORIG Order Code 4003 runs its forming rolls at a nominal 7 r/min, giving the operator time to guide the sheet without chasing the machine.
The two edges of the rolled segment carry the seam allowance the layout defined. One edge is folded into a flange, the other into a matching hook, and the two are pressed together and closed. The closed seam becomes a mechanical joint that holds the segment round and carries part of the fitting's structural load. A seam that is closed too loosely lets the segment relax out of round; a seam that is closed too tightly can split the folded edge and force a rework of the whole blank. Seam quality feeds directly into duct airtightness expectations used in ventilation systems, such as those in Eurovent guidance and the strength and leakage framework outlined in EN 12237.
A finished elbow is the stack of segments joined end to end along their short edges. Segments are oriented in the correct order - the two end pieces at the ends, the middle pieces between them - and their long seam lines are usually staggered so the seams do not line up in one continuous line along the elbow. That staggering is not cosmetic: a continuous seam line becomes a weak path along the fitting, while staggered seam lines spread the joint load across the whole elbow. The finished elbow reads as one curved duct from the outside, but a workshop learner who knows the sequence can still see the individual segments inside it. Every segment-to-segment joint is itself a seam, made in the same family of techniques as the long seam: edges are prepared, brought together, and closed. After assembly, the elbow is checked for roundness at each end, alignment along the outer and inner curves, and a straight overall profile when laid on its side. A shop that has never built elbows before tends to underestimate how much of the finished part's quality comes from this final step - a perfectly formed segment joined at the wrong rotation still produces a twisted elbow, and the fix is at assembly, not at the rolls. The 125-1250 mm diameter coverage and the material range on a dedicated machine such as the COORIG Order Code 4003 describe the working envelope for this process; the sequence itself is what turns sheet into fitting.
Round duct elbows are built from a flat sheet in a fixed order, and each step depends on the one before it. Layout and blank preparation set the seam allowance and the segment shape, rolling and lock seaming use that allowance to close a segment, and segment assembly stacks those closed segments into the fitting. Reading that order is the fastest way for a workshop learner to move from "I know what an elbow is" to "I can explain how one is made." A dedicated machine such as the COORIG 4003 electric round elbow making machine handles the rolling and lock seam steps within its stated working range, while layout, seam choice, and assembly judgment still sit with the operator.
A: The sequence runs through flat sheet layout, blank cutting, rolling into a curved segment, lock seam folding and closing, and joining segments end to end into the finished elbow. Each segment is its own blank, and the segment stack is what produces the final elbow angle.
A: The lock seam is the mechanical joint that holds each rolled segment closed and round. Without a closed seam, the segment would spring open under its own curvature. The seam also carries part of the fitting's structural load and affects how tight the finished elbow stays in a ventilation system.
A: A dedicated machine with a nominal working range such as 125-1250 mm covers a wide spread of diameters on the same unit. The exact setup for each diameter follows the machine's forming rolls and the segment pattern, so a workshop should confirm the specific size against the machine's stated range before committing a blank.
Guidebooks, Recommendations and Reports - Eurovent
EN 12237 - Ventilation for buildings - Ductwork - Strength and leakage of circular sheet metal ducts
