Steel Structure Welding Retrofit: Can the Part Move?
Direct answer: Ask first whether the part can be moved. If it can be handled routinely, a fixed station with proper fixturing is usually simpler and more repeatable. If it cannot, the equipment travels to the part, and three constraints follow: floor and aisles become hard conditions, the part has to be located again every cycle, and cable, gas and consumables travel with the machine. Seam finding method and any accuracy figure belong in the proposal, not in a video.
Who this is for: Written for engineers and buyers in steel fabrication deciding between a fixed welding station and equipment that travels to the workpiece.
Scope: This EVST guide covers the question that comes before equipment selection in steel fabrication, which is whether the part can be handled at all, and the three constraints that follow when it cannot: floor and access conditions, re-positioning on every cycle, and the services that travel with the machine. It does not quote cycle times, positioning accuracy or quality results.
Start from whether the part can move
In most automation discussions the part comes to the machine. In heavy steel fabrication that assumption is often the expensive part of the plan, because moving a large fabricated assembly needs cranes, floor space and time that the cycle then has to carry.
In the reference footage the workpiece is not lifted at all. It stays on the shop floor and a self-propelled welding unit drives to it, positions itself and welds where the part already is.
That is a legitimate arrangement, not a compromise, and it suits large or heavy assemblies and batches that are not big enough to justify dedicated handling. What it does is trade one set of constraints for another, and the trade has to be made deliberately.
What a steel structure welding automation retrofit has to establish
If the equipment travels, the floor stops being background. Aisle width, floor flatness, ramps, drainage channels and anything crossing the route become part of the specification, because the machine has to reach every place a part might sit.
The second constraint is positioning. A part that was set down by crane is never in exactly the same place twice, so the cell cannot rely on a taught position alone. Something has to establish where the seam actually is before the arc starts.
The third is what travels with the machine. Power, gas, wire and control all follow it across the floor, and how they are routed affects both the working radius and what the shop has to keep clear.
EVST treats those three as specification items for a travelling arrangement, not as site details to sort out later.
Positioning happens on every cycle, not once
On a fixed station the fixture solves positioning once and the program is taught against it. When the machine travels, positioning is repeated for every part and every setup.
In the footage the arm approaches the workpiece, settles, and only then begins. That sequence is the visible part of a positioning method whose details, whether by tactile sensing, vision, laser seam finding or a combination, are not visible at all.
Which method is used, and what tolerance it achieves on your joints, is a proposal question. Accepting a number for it from footage is exactly the kind of claim that cannot be supported.
The table below sets out the arrangement choice as a starting point for your own product mix.
| Can the part be moved? | Usual arrangement | What to verify first |
|---|---|---|
| Yes, handled routinely by crane or truck | Fixed station with dedicated fixturing | Fixture repeatability and load and unload access |
| Yes, but handling dominates the cycle | Fixed station with a buffer or second fixture | Whether handling time already exceeds arc time |
| No, too large or already positioned in the shop | Equipment travels to the part | Aisle width, floor flatness and the re-positioning method |
| Mixed, depending on the product | Both, with an explicit routing rule | Which products go to which arrangement, and who decides |
Services travel with the machine
In the footage the supply cable trails on the floor for the whole sequence. That is not a detail of the video; it is a design constraint that decides where the machine can go and how far it can work from its supply point.
Routing, working radius, gas supply and consumable changes are planned together with the movement, because a machine that can reach a part but cannot drag its services there has not solved the problem.
The same applies to what the shop has to keep clear. A travelling machine needs its route free at the moment it needs it, which is a scheduling and housekeeping commitment as much as an engineering one.
Execution class and what the footage cannot show
Structural steelwork is welded to a specification, not to a general standard of care. EN 1090-2:2018 sets out the execution requirements for steel structures, including the execution class that applies to the work and the requirements that follow from it.
According to EN 1090-2:2018, the execution class determines the requirements applied to the work, so the class has to be established before the cell is designed rather than after it is running.
According to ISO 5817:2023, weld imperfections are graded into quality levels, and which level applies to your joints is a specification decision taken up front.
None of that is visible in footage of a machine welding a box section, and no reading of the video should be taken as evidence about it.
In practice an EVST application review starts from the same project inputs each time: part size and mass, the shop’s access conditions and the joints to be welded. Selection follows from that evidence, and acceptance is agreed against the specification rather than against a datasheet.
Safety when the equipment moves through the shop
A machine that drives across a working floor introduces hazards a fenced cell does not have, because the safeguarded space moves with it and can include areas where people normally walk.
According to ISO 10218-2:2025, the safeguarded space follows from the movement of the integrated system, which for travelling equipment means the route as well as the working position.
According to ISO 12100:2010, the assessment covers the tasks actually performed, including positioning, setup and recovery from a fault, rather than only steady-state welding.
Frequently asked questions
When is a fixed station still the better answer?
Whenever the part is handled routinely anyway. If a crane already moves it, a fixed station with good fixturing gives better repeatability and removes the per-cycle positioning problem entirely.
What makes the floor a hard condition?
The machine drives on it. Flatness, aisle width, thresholds and anything crossing the route decide whether it can reach the places parts are set down, so those become specification items rather than site details.
Can the video tell us the positioning accuracy?
No. The approach and settle are visible; the sensing method behind them and the tolerance achieved on your joints are not. Both belong in the proposal.
What should we send to get a useful answer?
Typical part size and mass, the joints to be welded, and the shop’s aisle widths and floor condition along the routes a machine would use.
Project inputs for an application review
To have this checked against your own shop rather than a generic cell, send:
- typical part size, mass and how it is currently set down
- the joints to be welded, with position and length
- aisle widths, floor condition and any obstructions on the route
- product mix and how often it changes
Send part size, weight and the access conditions in your shop and the arrangement can be worked through against them. Related reading: robot welding workstation, welding system with seam tracking.