Direct answer: Section bending and repositioning exists because cold forming removes the flat face the part was located from. An arch line therefore needs a second locating operation: the formed part is clamped again on a rotating fixture, re-aligned to its actual shape, and turned so each seam is welded in position. Skip it and the path references a datum that no longer exists.
Who this is for: Fabrication engineers and production managers specifying automated lines for curved structural sections such as tunnel arch ribs, frames and curved beams.
Scope: This EVST guide is written from one filmed line: a structural section fed to a length stop, sawn, cold formed into an arc over several passes, then clamped on a rotating fixture where a robot welds the circumferential seams. Three stations of that line are used. Nothing here states a cycle time, a springback value or an acceptance result for the filmed work, because none of those can be read from footage.
Length and cut face are settled before forming
On the reference line the section is fed along a roller conveyor to a length stop and sawn to length before anything else happens. It looks like ordinary cutting, and it is easy to treat it as a support operation rather than part of the welding decision.
It is not. Length is the last variable on this line that can be corrected cheaply. Every operation after it works on a part that is harder to change, so an inconsistent length is absorbed by compensation further down: extra alignment, extra grinding, extra fit-up time at the joint between two arch segments.
Cut face quality carries the same weight. An uneven face means the butt joint between two formed segments has to be brought together by hand work rather than by the fixture, and hand work is exactly what an automated line is meant to remove.
According to ISO 5817:2023, quality levels for fusion-welded joints are defined against imperfection limits rather than against the method used, so section bending and repositioning is judged by the joint it produces, not by the sequence itself.
Forming happens in steps, not in one stroke
The bending machine does not press a straight section into an arc in a single stroke. It advances the section and presses repeatedly, building the radius in stages, and the reference footage shows the same section with the curve deepening across successive windows.
Stepping exists so the material can follow. Pressed to final radius in one action, a flange tends to wrinkle and a web tends to buckle locally; distributed across several passes, the deformation stays inside what the section can take.
How many steps and how much each step presses depends on the section profile and the material. Once that set is established for a profile it is applied to every part of that size, which is what makes the output consistent enough to weld with one program.
According to ISO 15614-1:2017, a welding procedure is qualified against a defined range of conditions, and in practice that range is what section bending and repositioning has to keep the incoming part inside.
Section bending and repositioning: why the datum moves
This is the step the article exists for. Once the section is an arc, the flat face it was originally located from has curved with it. The reference geometry used before forming is not there afterwards.
Locating the formed part on that face means aligning to a surface that no longer matches the drawing. It can be done, and on a single prototype it usually passes, because one part can be adjusted until it looks right.
Batches behave differently, and the reason is springback. Material and rolling batch variation mean the same press sequence produces slightly different final radii, so the residual difference between parts is real. Locating on an already deformed face passes that difference directly into seam position, which is why the second locating operation is not optional.
According to ISO 13920:1996, general tolerances for welded constructions are specified against defined reference features; when the feature itself has been reshaped by forming, the tolerance frame has to be re-established rather than assumed.
Clamping again, and turning the seam into position
The formed section is transferred to a rotating fixture, clamped, re-aligned to its actual shape, and turned so the seam to be welded sits in a favourable position.
Welding in position is steadier than reaching for a seam at an awkward angle: the pool is easier to control and bead formation is more repeatable along the joint. The fixture exists to bring the work to the torch, not to hold the part still while the robot compensates.
The order of equipment on the line follows from this. The rotating fixture sits after the bending machine, not before it, because its job is to reference the part as it is now, not as it was when it was straight.
A decision table for an arch line
The decisions in this order are what an application review works through, and each of them is settled from the customer’s own drawing and parts rather than from a catalogue.
| Decision | Settled from | What goes wrong if skipped |
|---|---|---|
| Length stop and cut face | Drawing length, joint type between segments | Compensation accumulates at every later station |
| Press sequence | Section profile, material, allowable local deformation | Wrinkled flanges or buckled webs at final radius |
| Second locating operation | Actual formed geometry, springback spread across a batch | Welding path referenced to a datum that no longer exists |
| Index positions on the fixture | Seam layout around the arch, attitude the process tolerates | Seams welded out of position, inconsistent bead |
| Inspection points | Quality level, radius tolerance from the drawing | Deviation found after the assembly is complete |
What the footage cannot show
Radius tolerance, springback value and how much each press step should advance are not visible in a moving picture. They come from the drawing and from measurement on the shop floor.
Weld quality level, inspection results and whether a batch of arches is acceptable are equally outside what footage can carry. ISO 5817 defines quality levels for fusion-welded joints and ISO 17637 covers visual testing; neither is settled by watching a station run.
The three segments used here are three stations of one line, not three lines, and they are shown together to make the sequence between operations legible.
The order EVST works in is the same each time: settle length and cut face, establish the press sequence, then re-establish the datum before any seam is welded.
Frequently asked questions
Why can the part not simply be welded after bending?
Because the surface it was located from has been reshaped by the bending operation. Welding straight after forming means the path is referenced to geometry that no longer exists on the part.
Does springback have to be measured for every batch?
It has to be known well enough to size the tolerance band. Material and rolling batch variation move it, so a value taken once and reused indefinitely is the usual source of drift between batches.
Can one fixture handle several radii?
Often yes, if the clamping points are chosen where every radius in the family has material. The locators change; the fixture body does not.
Is a rotating fixture always necessary?
Not always, but where the seam runs around the section it is what allows each sector to be welded in position instead of at whatever attitude the arm can reach.
Project inputs for an application review
To assess an arch line, an EVST application review opens from the following inputs:
- section profile, material and length per part
- radius requirement and the allowable tolerance
- existing sawing and bending equipment, and how length is set
- seam layout, quality level and output target
Send the section size, the radius requirement and the batch size, and we will work through cutting, forming and the second locating operation against your own drawing. Related reading: large workpiece robotic welding pose and reach, headstock and tailstock positioners for long parts.