Guide
Welded assemblies: when to choose a weldment over a machined part
A welded assembly joins cut, bent and welded parts into one structure. Here is when that choice beats machining, and how it is built properly.
Updated
What a welded assembly is
A weldment combines mechanical operations with welding. Sheets and profiles are cut, bent, positioned, then welded.
The result is a single structure: a chassis, a machine base, a bracket, a walkway, a column. The geometry comes from the assembly, not from removing material.
It is the core process of a metal fabrication shop. At PacoMetal, most industrial and institutional projects are welded assemblies.
Welded or machined: the simple rule
A machined part starts from a solid block and removes material. A welded assembly starts from standard sheets and profiles and joins them. The right choice depends on size, quantity and tolerances.
| Criterion | What tips the balance |
|---|---|
| Size | Beyond 0.5 m in one direction, welded wins: the solid block becomes unaffordable. |
| Quantity | One-off or small run: welded. Large runs of small parts: machining or casting. |
| Tolerances | To the millimetre: welded. To a hundredth of a millimetre across the whole part: machined. In between: welded, then machined at the critical points. |
| Material | A welded assembly uses 80 to 95% of the material bought. Machining a block often throws away more than half. |
| Changes | A welded assembly can be reinforced or modified afterwards. A machined part, rarely. |
| Lead time | Standard profiles and sheets in stock: a few days. A large solid block: special order. |
The most common combination in industry: a welded structure, then local machining of the surfaces that must be precise.
The five steps in the shop
- 01
Preparation
Plasma or laser cutting of sheets, profiles cut to length, bending. Each part comes out with its holes and bevels.
- 02
Positioning
Parts are set on an assembly table or in a jig. Squareness and dimensions are checked before tacking.
- 03
Welding
MIG for the structure, TIG for visible joints and stainless, stick or flux-cored for heavy sections. The sequence is chosen to limit distortion.
- 04
Inspection
Overall dimensions, flatness, squareness. Straightening if heat has distorted the part. Visual inspection of the welds.
- 05
Finishing
Grinding of visible welds, blasting, primer, paint or galvanizing depending on use, then machining of critical surfaces if the drawing calls for it.
What makes a good welded assembly
- A design made for welding: accessible joints, similar thicknesses, no overhead welding when the part can be turned.
- Realistic tolerances. Welding distorts. ± 1.5 mm on overall dimensions is normal; tighter needs machining after welding.
- A shop drawing before the first cut. A 3D plan approved on screen avoids rework on site.
- The right process for the right metal: TIG on stainless and aluminum, MIG on steel, flux-cored for thick reinforcements.
- An inspection before finishing. A distortion found after painting costs twice as much.
Three examples from the shop
The ten columns of the Verdun fire station were built to the millimetre in the shop, because no welding was allowed on site. The Atlas Copco mining compressor structure combines MIG and flux-cored welding to withstand a bulldozer push. The Old Port bollards are 316L stainless assemblies, TIG-welded then galvanized.
When it is not the right choice
A small part produced by the thousand is cast or machined at a better price. A part held to ± 0.05 mm throughout comes off a machining centre, not a welding station.
If your project falls into those cases, we say so at the quote and point you elsewhere. That is the shop rule.