Direct answer: Driving the machine to the part removes the lift and replaces it with different conditions. With no fixture, the floor and the timber blocking are the datum: parking position and floor flatness decide what the arm reaches. Part placement varies every time, so every seam start has to be found again, which is schedule time. The cable trails throughout, so routing and access are part of the same layout question.
Who this is for: Written for fabricators and project engineers weighing mobile welding equipment for large structural parts that are awkward or slow to lift.
Scope: This EVST guide covers what changes when a welding unit is driven to the part instead of the part being brought to a fixed cell: floor and blocking as the datum, seam finding on every seam, cable routing and access. It does not quote search accuracy, environmental limits or welding parameters; those come from the equipment proposal and the welding procedure.
When the part stops moving
The standard shop answer is to bring the part to a fixed cell. For large, heavy or widely-distributed parts that answer stops being efficient: a single lift can cost more time and carry more risk than the welding itself.
The alternative shown in the reference footage is a self-contained unit with its power source and gas on board, driven up to a stiffened assembly resting on timber blocks. It sets up, welds and moves on, and the part is never lifted.
This suits large parts, low batch sizes and work that is spread around a shop or a site. It is not a universal replacement for a fixed cell; it exchanges a handling problem for a set-up problem.
The on site robotic welding ground conditions to establish first
In a fixed cell the fixture provides the datum. On site there is no fixture: the part rests on blocking, and the floor plus the blocking provide the datum between them.
So floor flatness and where the unit parks decide directly whether the arm reaches the seam and how much of the working envelope is left over. That has to be worked out for the least favourable position rather than the convenient one.
Block height and seating angle are conditions in the same sense. They set the height and attitude of the seam, and they also set how much variation there is between one part and the next in the same batch.
None of this is unusual for site work; what is unusual is treating it as part of the equipment decision rather than as something the operators will sort out.
An EVST feasibility review therefore starts from the floor and the seam list together, not from the machine specification.
Every seam starts again
Because part placement changes every time, absolute coordinates in a taught program have little value. After the unit parks, the seam start has to be located.
In the footage the stiffener seams are welded one at a time, and each one begins with the unit re-establishing where that seam is. It costs more time than in a fixed cell and it belongs in the schedule rather than in the arc-time estimate.
Search methods differ widely in capability: touch sensing, laser seam finding and vision each suit different joint types and surface conditions. Which one applies, and what accuracy it holds, comes from the equipment proposal rather than from a video.
| Situation | Usual answer | What to verify first |
|---|---|---|
| Large or heavy part, low batch, part hard to lift | Mobile unit driven to the part | Floor flatness, parking positions and reach at the worst seam |
| Repeating part, moderate size, steady volume | Fixed cell with fixture and positioner | Fixture repeatability and loading time |
| Parts spread across several bays or a site | Mobile unit plus a planned set-up sequence | Access route, cable routing and where the unit can safely stand |
| Mixed family, some liftable and some not | Fixed cell for the liftable parts, mobile unit for the rest | Which parts fall on each side and what that does to scheduling |
The cable, the aisle and the people
In the footage the cable trails on the floor for the whole operation. It follows the machine, which means routing, gas supply and the working radius have to be planned together or the unit ends up limited by its own umbilical.
Site work also has an access question that a fixed cell answers with a fence: who may approach, from which direction, and what area is kept clear while the arc is running. On site that is solved by organisation and layout instead.
According to ISO 10218-2:2025, the safeguarded space of an integrated system follows from the movement of the whole system, and for a mobile unit that space moves with the machine.
According to ISO 12100:2010, access during set-up and fault conditions belongs in the same risk assessment, which for site work usually means a documented set-up sequence rather than a physical barrier.
What still belongs to the welding procedure
Where the machine stands does not change what the weld requires. Material, thickness, joint preparation, pass sequence and heat input follow the procedure exactly as they would in a fixed cell.
According to ISO 3834-2:2021, quality requirements for fusion welding cover the arrangements a fabricator has to have in place, and site work does not exempt any of them.
According to ISO 5817:2023, weld imperfections are graded into quality levels; which level applies is a specification decision taken before the equipment is chosen.
In practice an EVST review of a site welding case starts from part size and mass, the seam list, the floor and access conditions, and what the current handling method costs in time.
Every application review opens from the same project inputs – drawing, standard and current method – so selection rests on evidence and acceptance is agreed against your own parts rather than a datasheet.
Frequently asked questions
Is a mobile welding unit less accurate than a fixed cell?
It works from a datum that changes with each set-up, so the accuracy question moves from the fixture to the seam-finding method. The practical difference is set-up and search time per seam, which is why the comparison should be made per part rather than per arc metre.
What does the floor actually have to be like?
Flat enough and firm enough for the unit to park stably in the positions the seams require, with a route to reach them. The requirement follows from the machine and the reach needed at the least favourable seam, so it is set case by case rather than as a general figure.
Does this replace a fixed welding cell?
No. It suits large parts, low batch sizes and work spread around a shop or site. Repeating parts of moderate size are still better served by a fixture and a positioner.
What should we send to get a useful answer?
Part dimensions and mass, a seam list, photographs or a layout of the working area with aisle widths, and how the parts are handled today.
Project inputs for an application review
To have this checked against your own parts and your own floor, send:
- part dimensions, mass and how many of each
- a seam list with position, length and joint type
- working area layout, aisle widths and floor condition
- how the parts are handled and welded today
Send the part size and the site conditions and the arrangement can be worked through against them. Related reading: automatic welding system solutions, large workpiece pose and reach test.