Robotic welding for repeat sheet metal assemblies

Robotic welding for repeat assemblies is decided at design, not at the program. A canopy frame with 14 seams, six variants and a gusset 32 mm from the longest weld is not a cell part yet. The first program will weld nine seams, stall at the tenth because the torch body meets the gusset, and the fix is a drawing change or a second fixture — not a better program.

The five conditions are measurable. Gap under 1.0 mm along the whole seam, mismatch under 1.0 mm at the joint face, 80 mm of clearance behind the nozzle on every approach, tacks inside 50 mm of a seam end and ground flush, and a fixture that locates the assembly on formed faces rather than on cut edges. A part that meets all five welds the same way at part 5,000 as at part 5.

Joint type decides more than the robot does

A fillet on a lap or a T-joint is the easiest thing to automate: the joint self-locates, the gap is set by the fixture, and the torch has a clear line to the root. That is why most repeat enclosures are designed as a frame with an overlapping panel rather than a butt-jointed box.

A square butt joint needs a backing bar, tight gap control, or both. Without backing, a 1.0 mm gap in 2 mm sheet at 240 A opens into a hole, and the operator who would have bridged it by hand is now a fitter holding a scrapped panel.

A plug weld through a punched or laser-cut hole adds a tolerance the cell cannot see: hole position. If the hole wanders 1.5 mm, the arc lands on the edge of the hole and the plug fills unevenly. Laser cutting sets that position to about ±0.2 mm, which is why a laser-cut blank is the usual feed for a plug-welded assembly and a sheared blank is not.

Fixture design for a repeat weldment

A weld fixture does three jobs: it holds the parts in the relationship the drawing specifies, it holds them there against the pull of the weld, and it lets the finished assembly out without springing. A fixture that does the first two and not the third produces a good part that will not release.

Location follows 3-2-1. Three points on the primary face, two on the secondary, one on the tertiary, with the primary points as hard stops rather than clamps. Clamps hold the part against the stops; they are not the datum. When a clamp doubles as a stop, the part position depends on clamp force, and clamp force depends on the operator.

Wear plates belong under the stops, not on them. A mild steel stop face peens over 20,000 cycles and the part sits 0.2 mm lower than it did at first article. A hardened wear plate can be replaced in minutes and restores the datum without re-machining the fixture.

Clamps need to sit at least 60 mm from the seam, measured along the surface. Any closer and the clamp body disturbs the gas flow, blocks the torch on the approach, or both.

Left-hand and right-hand variants need a poka-yoke. A mirrored bracket that fits the fixture the wrong way round will weld, and it will be discovered at assembly. An asymmetric pin or a blanked-off location hole costs nothing and prevents the whole class of error.

Programming from the fixture, not from the drawing

The program is taught on the fixture with a representative part, not derived from the CAD model alone. The CAD gives the nominal path; the fixture gives the path that matches the parts the fixture actually locates. On a 2,000 mm frame, the difference between the two is usually 1–2 mm, which is enough to put the wire on the edge of a 5 mm flange.

Three parameters are recorded with the program and not changed without a re-prove:

• Torch angle and work angle per seam, typically 10–15° push for a flatter bead.
• Stick-out, 10–15 mm for short-arc; beyond 20 mm the current falls and the root loses fusion.
• Travel speed and the weave, if any, with the stitch pattern for long seams.

Teaching is the cheap part. Proving takes five parts and a record: gap and mismatch on the fixture, leg size and length per seam, the diagonal of the assembly, and the flatness of any visible panel. If a seam moves more than 0.5 mm across those five parts, the cause is the fixture or the incoming fit-up, and a program offset will hide it rather than fix it.

The audit that keeps a cell honest

A cell drifts slowly. Spatter builds on a stop, a liner kinks, a tip erodes, and none of it shows in a single part. The audit is what catches the drift before a release does.

Measure one part in twenty-five on the fixture: gap at the two seams that vary most, leg size at three points, and the diagonal of the assembly. Check the torch tip, the gas nozzle and the fixture stops at the start of every shift, and replace a tip that has lost its bore rather than cleaning it. A contact tip that has worn from 1.2 mm to 1.6 mm changes the arc and the bead without any change to the program.

Sub-assemblies and the closing panel

Most repeat enclosures are welded as a frame, a panel set, and then a closing panel. The order matters for the cell because it decides which seams are reachable when.

Weld the sub-assembly with the most seams first, while it is still open. Weld the frame square and check the diagonal before any panel goes on, because a frame that goes out of square after the panel is welded cannot be corrected by welding the panel harder.

The closing panel goes on last, and its seam is usually the one that is visible. Run it as a stitch with a short arc and a push angle, and accept that it will be the seam that needs the most attention at first article. A closing seam that runs the full perimeter in one pass puts the most heat into the least restrained edge of the assembly, which is why it is the first place a 2 mm panel shows an oil-can.

Galvanised or pre-coated sheet changes the parameters. Zinc boils at 907 °C, well below the arc temperature, so a galvanised joint produces porosity and fume unless the coating is removed from the weld zone or the parameters are adjusted. Ganesh welds bare steel and sends the assembly through seven-tank pretreatment before coating, which avoids the problem entirely; a customer-supplied galvanised part is reviewed before it is committed to a cell.

Distortion, and why a cell holds thin panels better

A hand welder varies travel speed by 10–20% between the first and last panel of a shift, and heat input varies with it. A cell puts the same current, the same speed and the same sequence into every part, so the distortion is repeatable — and repeatable distortion can be designed out.

Heat input is the number to control. For MIG, it is (volts × amps × 60) ÷ (travel speed in mm/min × 1000) in kJ/mm. At 24 V, 240 A and 300 mm/min that is 1.15 kJ/mm. On 2 mm sheet, 1.15 kJ/mm is enough to bow a 600 mm panel by several millimetres. At 200 A and 450 mm/min the same joint takes 0.64 kJ/mm, and the panel stays flat.

Sequence does the rest. Weld from the centre outward, alternate sides, and use a stitch pattern — 60 mm of weld on a 120 mm pitch is a common starting point for a panel seam — rather than a continuous run. On a frame, weld the diagonally opposite corners before the adjacent ones, so the shrinkage pulls evenly.

The weld distortion control guide covers the mechanics of shrinkage and the clamping that resists it without locking the part into a stressed shape.

How Ganesh qualifies a repeat weldment

Ganesh runs three CLOOS robotic MIG cells and more than 30 manual MIG/MAG, spot and SPM stations, with welding fixtures designed and built in-house; the welded assemblies capability sets out the full route. That matters for a repeat assembly because the fixture is where the tolerance is held: a fixture built by the same team that reviewed the drawing can be changed when the drawing changes, instead of being sent back to a third party.

Parts arrive from the 12 kW fibre laser on its 40 ft × 16 ft bed with a cut edge inside ±0.2 mm, which is the fit-up the cell needs, and from the 200 T, 5-axis brake with flanges formed over a 4,450 mm bed to 10 mm. After welding, assemblies go through seven-tank pretreatment and the 18 ft × 14 ft powder booth, with DFT and cross-hatch adhesion checks.

Ganesh has manufactured sheet metal since 1992 across five units in Mumbai (Vasai) and Nagpur, holds ISO 9001:2015 and ISO 14001:2015, and offers PPAP and first article inspection. Published customer work includes elevator frames for TKE and Eros Elevators, genset base frames for Mahindra Powerol and railway assemblies for Indian Railways.

What robotic welding for repeat assemblies needs from the drawing

• A numbered weld map with ISO 2553 symbols, leg size, length and pitch per seam.
• Which seams are structural, which are cosmetic and which need to seal.
• Gap and mismatch the design assumes at each joint type.
• Locating faces, marked as formed or machined, since the fixture will use them.
• Clamp zones, or at least the surfaces that must stay clear for clamping.
• Stitch pattern and weld sequence, where distortion is a risk.
• Diagonal, flatness and twist limits, with the measuring method.
• The surfaces that must stay spatter-free or uncoated.

The hub for this cluster, bending and welding assemblies, links the rest of the guides, and the welded assembly tolerances post sets the callouts that follow the weld.

When a repeat assembly should stay manual

Repetition alone does not justify a cell. A repeat assembly stays on a bench when the seams are shorter than about 40 mm, when the part is a one-piece fabrication with a weld inside a closed box, or when the family has more variants than the volume supports.

It also stays manual when the design is still moving. Every engineering change to a critical feature is a fixture change and a path re-teach, and a programme that changes monthly spends its time in setup rather than in production.

Ganesh does not offer laser welding, does not weld on site, and does not claim that a cell can fix a part that will not hold its fit-up. If the gap cannot be held under 1.0 mm on the fixture, the honest answer is a bench route with a checking fixture, and better blanks.

Send the assembly drawing with the weld map and the quantity per release. The engineering team will return the fixture concept, the seam-by-seam feasibility and the first article plan, and the supplier audit checklist lists the evidence that should come back with the parts.