Choosing Between a Weldment, a Casting and a Part Machined From Solid
Three routes can produce the same structural part. The one that is cheapest is decided mostly by size and quantity, and the decision is usually made before anyone looks at a machine rate.
The three routes in one paragraph each
A machined weldment is plate, tube or section cut to size, welded into shape, usually stress relieved, then machined where the tolerance has to hold. You buy material and labour. There is no tooling to pay for beyond fixtures.
A casting is poured to near net shape in a mould made from a pattern or a die, then machined on the faces that matter. You pay for the pattern once and for a cheaper, faster part every time after that.
A part machined from solid is cut out of one block or one thick plate. Nothing is joined, nothing distorts from welding, and you pay for every kilogram of material that ends up as chips.
Size: where the weldment wins
Above roughly a 300 mm cube, machining from solid starts to look absurd. You are buying a block, paying to remove most of it and paying again for the machine hours that do the removing. A welded structure buys plate at plate prices and only cuts where a cutter is needed.
Castings scale up too, but every increase in size raises the pattern cost, the foundry that can handle it and the risk. A 3 m machine base as a casting is a serious project with a serious lead time. As a weldment it is a normal Tuesday for a shop with a big enough bay and a big enough crane.
There is also a practical ceiling on the machining side. A frame can be welded far larger than it can be machined in one setup, which is why the size envelope and the number of setups drive the quote more than the welding does.
Quantity: where the casting wins
Pattern and die cost is paid before the first good part exists. Spread over 20 parts it is painful. Spread over 2,000 it disappears.
Castings also come out closer to final shape, so there is less to machine per part and less material bought per part. On a repeating, stable design in real volume, that per-part advantage compounds.
The question to ask is not “how many this year” but “how many before the design changes”. A part that will be revised in eighteen months should not be paying for a pattern.
Lead time and the first part
| Route | Typical path to first part |
|---|---|
| Weldment | Cut files, fixtures, weld, stress relieve, machine. Weeks. |
| Casting | Pattern or die, sample casting, approval, production, machine. Months. |
| Machined from solid | Order material, program, cut. Days. |
This is why prototypes and pilot builds so often go out as weldments even when the production intent is a casting. You get a real part, in the real material, with the real stiffness, before the pattern is committed.
Design changes after launch
A design change on a weldment is a new cut file and possibly a new fixture. On a casting it is a pattern modification or a new pattern, plus sample approval again. On a machined part it is a new program, unless the change needs a bigger block.
If the product is young and the design is still moving, that flexibility is worth real money and it rarely appears in a price comparison.
The machine base question
Machine tool bases are where this argument gets interesting, because stiffness is not the only thing being bought.
Steel has roughly twice the elastic modulus of grey cast iron, so a well designed steel weldment can be stiffer than the cast iron part it replaces at the same weight. What cast iron brings is internal damping. Vibration energy that enters a cast iron structure dies away faster than it does in a welded steel one, and on a machine that is cutting metal that matters.
Fabricators work around it. A boxed weldment with internal ribs, filled with a damping material such as an epoxy and aggregate mix, gets the stiffness of the steel and much of the damping back. It is a design decision with real trade-offs, and it is worth settling before the frame is quoted rather than after the first vibration test.
Converting a casting to a weldment
This comes up when a foundry exits, a lead time becomes impossible or a volume forecast collapses. It is doable and it is not a drawing swap.
What changes:
- Wall sections. Cast walls are often thicker than a welded equivalent needs, because a casting has to fill. Carrying cast wall thicknesses into plate wastes weight and welding time.
- Radii and fillets. Generous cast radii become either a machined feature or a weld profile. Neither is free.
- Datums. A casting is usually located from cast surfaces for first operation. A weldment needs datums that a fixture can sit on and that will be machined.
- Distortion. A casting does not weld, so it does not distort from welding. A weldment does, which brings machining allowance and stress relief into the picture.
Budget for engineering time on the conversion. The parts that go badly are the ones where the casting drawing was sent out unchanged and the fabricator was asked to make it work.
A short decision rule
- Large, low to medium volume, design still moving: weldment.
- Small, tight tolerance, no joint permitted: machined from solid.
- Stable design, real volume, complex shape, patience for tooling: casting.
If your part sits on a boundary, send it out as a weldment RFQ and ask the quoting shops what they would change to make it cheaper. Fabricators who also machine see both halves of the cost and they will tell you when the part belongs somewhere else.
Send your drawing and we will match it with shops that weld and machine it in the same building.
Frequently asked questions
At what quantity does a casting beat a weldment?
There is no fixed number, because the crossover moves with the tooling price. A weldment carries no pattern cost and a casting amortises one, so the question is how many parts you will make before the design changes. If the answer is a few dozen over several years, the weldment usually wins.
Is a weldment less stiff than a casting?
Not inherently. Steel has roughly twice the elastic modulus of grey cast iron, so a welded steel structure can be made stiffer per kilogram. What cast iron does better is damp vibration, which is why machine tool bases are still cast in some applications.
Can I convert an existing casting to a weldment?
Often yes, and it is a common move when a foundry drops a part or a lead time becomes impossible. It is a redesign rather than a substitution, because the wall sections, the radii and the datums that suited a casting rarely suit a fabrication.
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