Pillar guide

Heavy steel frame fabrication: a guide for industrial equipment buyers

Published 21 September 2026

Most buying guides for fabrication are written for parts that fit on a pallet. Heavy frames do not. Once a weldment passes a couple of tonnes, the decisions stop being about the steel and start being about the building it is made in, how much the crane lifts, how long the stress-relief oven is, whether there is a mill with a table big enough to face the finished piece.

This guide is the version we wish buyers had before their first heavy frame RFQ. It covers what the category includes, the four constraints that decide which shops can even bid, how to write a specification that produces comparable quotes, and where the cost actually sits.

What counts as a heavy frame

There is no formal threshold. In practice, the category starts where a jobbing machine shop stops being able to handle the piece and ends where you would go to a structural steel contractor instead. That awkward middle contains:

Machine bases and beds: welded structures that carry a machine tool, test rig or production line, usually stress-relieved and then machined flat
Equipment skids and package frames: structural bases for pumps, compressors, dosing units and modular process packages
Press and column frames: plate weldments where geometry has to hold under load
Structural weldments: conveyor frames, platforms, support steel, welded chassis for heavy equipment
Energy and marine subassemblies: where weld procedure and documentation matter as much as the steel

What these share is a process chain that cannot be compressed: cut and prep plate, fit, weld, relieve the stress you just put in, machine the faces that matter, finish, and move something very heavy to a port.

The four constraints that decide who can bid

Before anyone quotes, a heavy frame job gets filtered by four numbers. Knowing yours saves a fortnight.

1. Crane capacity

The single-piece weight has to be liftable at every station, fit-up, welding, into and out of the oven, onto the machine table, onto the truck. A shop with a 20-tonne crane in the welding bay and a 10-tonne crane over the mill can only machine 10-tonne pieces, whatever the brochure says.

2. Stress-relief oven length

Welding puts residual stress into a structure. Machine a frame without relieving it and the geometry moves after you have paid for the tolerance, see stress relief for welded machine bases. Thermal stress relief means an oven long enough for the whole piece; where the oven is the constraint, vibratory stress relief is the usual alternative, and it is worth agreeing which one you are buying before the quote lands.

3. Machining envelope

A 7-metre frame with four machined pads needs a gantry mill whose table and travel cover it in one setup. Multiple setups on a long weldment mean re-datuming, which means the flatness figure you specified gets harder and more expensive to hit, see flatness tolerance on large machined weldments.

4. Weld standard and certification

If your drawing calls for AWS D1.1 or ISO 9606, the shop needs current welder certificates and documented procedures to match, not a claim in an email. This is the constraint buyers most often discover late, usually when the inspection report is due. The three documents to ask for are in AWS D1.1 in plain language.

How to write a specification that gets comparable quotes

The most common reason three quotes are unusable is that they priced three different jobs. A specification that prevents that is short:

Item	Why it changes the price
Overall dimensions and weight per piece	Decides which shops can lift and machine it
Plate thickness and grade	Drives cutting, prep and consumables
Weld standard, plus any NDT	Certification and inspection hours
Which faces are machined, to what flatness	Often the largest single line
Stress relief method	Thermal and vibratory are not interchangeable
Surface finish	Blast and primer, 2K paint and galvanising are far apart
Quantity and delivery date	Sets nesting efficiency and overtime
Destination port and Incoterm	EXW, FOB and CIF are not comparable numbers

If you leave one of these out, you have not saved time, you have moved the assumption into the quote, where you cannot see it.

Where the money actually goes

On a typical multi-tonne machine base, material is rarely the dominant cost. The rough shape of it:

Plate and sections: significant but competitive, since most Asian shops buy from a similar pool of mills
Fit-up and welding hours: usually the largest block, and the one most affected by joint design
Stress relief: modest in itself, expensive when it forces a subcontract move
Machining: often the second-largest line, and highly sensitive to how many faces and setups you ask for
Finishing and packing: small unless galvanising or a marine-grade system is specified
Inland transport and port handling: real money on anything oversize

The practical implication: design changes that reduce welding hours or machined faces move the number far more than shopping for cheaper steel. That is why it is worth sending a drawing before it is frozen, the detail is in what drives the cost of heavy frames and skids.

Why Vietnam shows up in this category

Heavy fabrication is a geography question. The process chain is fixed, so what varies is the cost of hours under an arc and the freight at the end.

Vietnamese shops in this segment run skilled welding and machining labour well below coastal China, buy plate from the same regional pool, and sit close to deep-water ports with a shorter ocean leg to the US West Coast than an inland Chinese fabricator. Crane capacity in the better shops now reaches 20 to 50 tonnes, with ovens and gantry mills scaled to match.

What is genuinely harder is the vetting. The capability is real but unevenly distributed, and a shop’s website tells you very little about its crane. That is the gap we work in: we do not own a factory, which is why we can put a drawing in front of the shop whose equipment actually fits it. Fabricators who want that work can apply to the supplier network.

A realistic timeline

For a single multi-tonne frame, plan on:

Week 0: drawing review, feasibility, shortlist
Week 1: shop quotes in, scope agreed
Weeks 2–4: material procurement and plate prep
Weeks 4–8: fit-up and welding, with in-process dimensional checks
Weeks 8–10: stress relief and machining
Weeks 10–12: finishing, FAI, packing, inland transport

Eight to twelve weeks ex-works is normal. Repeat orders against approved drawings usually land at six to eight. Ocean freight is on top.

Before you send an RFQ

Three questions worth answering on your own side first:

What is the real tolerance? A flatness figure copied from an old drawing can add a machining setup nobody needed.
Does it have to be one piece? Split-and-bolt designs with machined mating faces are frequently cheaper landed, because oversize freight is expensive.
What documentation will your QA actually ask for? Mill certs, weld map, NDT report and dimensional FAI are cheap to specify up front and awkward to reconstruct afterwards.

Go deeper

This guide is the overview. The detail lives in these:

Specifying the part

Design and cost

Quality and inspection

Sourcing and logistics

With those settled, a heavy frame RFQ is a straightforward thing to run. Send us the drawing, even a partial one, and within 24 business hours we tell you whether it fits the network and which shops clear the envelope. Up to three of them quote you directly, with the Incoterm fixed.

See also: types of heavy steel frames, a matrix of skids, bases, structures, modules and lifting frames.