The Engineer’s Guide to Structural Fabrication and Machined Weldments

Structural fabrication – what is it and how is it used?

Structural fabrication is widely used to manufacture vehicle structures, equipment frames, enclosures and structural assemblies across defence, transportation, energy and industrial engineering sectors. Understanding how fabrication, welding and machining processes interact allows engineers to design structures that are strong, dimensionally accurate and efficient to manufacture. The fabrication combines cutting, forming and welding processes to create strong assemblies from plate and structural sections. These assemblies are often then combined with precision machining operations to produce critical interfaces such as mounting surfaces, bearing locations or sealing faces. This combination of fabrication and machining is widely used in sectors including defence vehicles, heavy equipment, energy systems and industrial machinery. 

While fabrication provides structural strength and efficient use of material, machining ensures dimensional accuracy where tight tolerances are required. For engineers designing structural components, understanding how these manufacturing processes interact can significantly improve manufacturability, reduce distortion risk and ensure final assemblies meet functional requirements.

Welded fabrication or machining?

Fabrication allows engineers to create large structures efficiently without machining them from solid material.  Large machined components require significant amounts of raw material and long machining times. Therefore production timelines, cost and weight become part of the decision making process.

Fabricated structures instead use plate sections that are cut, formed and welded together to create the final geometry.  This approach reduces material waste while maintaining structural strength.

Examples of fabricated structures include:

  • vehicle chassis and frames
  • equipment support structures
  • battery enclosures
  • machine frames
  • structural housings

In many cases, fabrication provides the most efficient manufacturing method for large structural components.

How welding influences structural behaviour

Welding introduces localised heat into the material which causes expansion and contraction during the heating and cooling cycle. This thermal movement can create residual stresses within the structure. As welds cool and the material contracts, small dimensional movements can occur. This effect is known as welding distortion. Distortion does not necessarily indicate poor manufacturing quality. It is a natural physical behaviour of welded structures.

Manufacturers manage distortion through several methods, these techniques help maintain alignment during fabrication and minimise structural movement:

  • controlled weld sequencing
  • fabrication fixtures
  • heat input management
  • balanced weld design

Why machining is often required after fabrication

While fabrication creates the primary structure, many assemblies require precision interfaces that must meet tight dimensional tolerances. Examples of this are such features as mounting faces, bearing housings, alignment features and sealing surfaces. Because welding may introduce small dimensional changes, these features are frequently machined after fabrication. Machining ensures that critical surfaces meet tolerance requirements once the structure has stabilised after welding. This hybrid manufacturing approach combines the strength of fabrication with the precision of machining.

Managing distortion in welded structures

Distortion control is an important aspect of structural fabrication. Manufacturers use several strategies to manage distortion during production. Fabrication fixtures are commonly used to hold components in position while welds are applied.  These fixtures maintain alignment and resist movement caused by thermal stresses. Weld sequencing also plays a key role.  By carefully planning the order in which welds are applied, heat can be distributed more evenly throughout the structure.

In many cases, critical interfaces are intentionally machined after welding so that final tolerances are achieved regardless of minor structural movement.

Designing structures for manufacturability

Engineers can significantly improve manufacturing outcomes by considering fabrication processes during the design stage.

Design strategies may include:

  • distributing welds across the structure rather than concentrating them in one location
  • designing symmetrical joints that balance thermal stresses
  • utilising multiple or different welding methods such as laser, MIG, TIG, CMT and Robotic Welding
  • allowing machining allowances on critical interfaces
  • simplifying weld access for manufacturing

Collaboration between engineering teams and manufacturing specialists during development often leads to more efficient production and improved structural performance.

Robotic welding cell manufacturing large fabrications at Universal Fabrications in Coventry

Inspection and quality control

Inspection is an essential part of manufacturing structural assemblies. Fabricated structures are typically inspected throughout production to verify dimensional accuracy and structural integrity. Inspection methods may include dimensional measurement using precision instruments, verification of alignment and flatness, weld inspection and material certification and documentation.

Weld inspection types

In terms of weld inspection, it splits into two categories. Non-destructive testing (NDT) checks a weld without damaging it. Destructive testing physically breaks a test sample to see how it holds up. Inspectors use both at different points in the job: before welding starts, while it’s underway, and once it’s finished.

Non destructive testing

Where every weld check starts. An inspector looks over the joint, sometimes with a magnifying glass or a few basic gauges, for cracks, porosity, undercut, or a bead that just doesn’t sit right.

  • Visual inspection – Where every weld check starts. An inspector looks over the joint, sometimes with a magnifying glass or a few basic gauges, for cracks, porosity, undercut, or a bead that just doesn’t sit right.
  • Ultrasonic testing – Sends sound waves through the metal to pick up flaws you can’t see from the outside, things like cracks, voids, or lack of fusion buried in the joint.
  • Radiographic testing – Uses X-rays or gamma rays to get a look inside the weld. It’s good for catching porosity or cracking that’s sitting deep in the material.
  • Liquid dye penetrant testing – A dye is brushed or sprayed onto the surface and left to soak into any cracks. Wipe off the excess, apply a developer, and the flaws show up as the dye bleeds back out.
  • Magnetic particle testing – It only works on ferromagnetic metals. You run a magnetic field through the weld and dust it with iron particles, which bunch up wherever there’s a surface or near-surface crack.

Destructive Testing

These tests aren’t done on the actual finished part, they’re run on test coupons to prove out a welding procedure or check that a welder is qualified.

  • Bend testing – The sample is bent around a set radius. If there’s a hidden crack in the weld or the heat-affected zone, this is usually when it shows up.
  • Tensile testing – The sample is pulled apart on a machine until it fails, giving you a number for how much load the weld can actually take.
  • Macro-etch testing – The sample is has a cross-section cut out of the weld, polishing it, and etching it with acid so you can see the penetration and internal structure clearly.

These key inspection processes ensure fabricated assemblies meet both engineering specifications and quality requirements.

Prototype to production manufacturing

Many structural assemblies begin as prototype components during development programmes. During this stage, engineers may evaluate structural performance, assembly behaviour and manufacturing feasibility. Once designs are validated, production manufacturing requires stable processes that can deliver repeatable results. Manufacturers capable of supporting both prototype and production manufacturing provide continuity during programme development and help reduce supply chain complexity.

Conclusion

Structural fabrication combined with precision machining provides an efficient and reliable manufacturing method for large engineering assemblies. By understanding how welding behaviour, distortion control and machining operations interact, engineers can design structures that achieve both structural strength and dimensional accuracy.

Frequently Asked Questions

What is structural fabrication?

Structural fabrication involves cutting, forming and welding metal components together to create structural assemblies.

Why are fabricated structures machined afterwards?

Machining ensures critical interfaces meet tight tolerances after welding distortion has stabilised.

Does welding always cause distortion?

Small dimensional movement is common due to heat expansion and contraction, but it can be managed through manufacturing techniques.

Why is fabrication used instead of machining large structures?

Fabrication reduces material waste and machining time when producing large structural components.

What industries use fabricated structures?

Fabricated assemblies are widely used in defence, transportation, heavy equipment, energy and industrial engineering sectors.