When a part needs to exist, the first answer is usually to build it from pieces. Plate, tube and bar are available in standard sections, cutting them is fast, and joining them is a process every fabrication shop already has. A weldment gets you a functional part quickly, with the minimum of material removed and the maximum of standard stock used.
That default is right more often than not, and it is worth saying clearly before discussing when it is wrong. A fabricated exhaust component is lighter than a solid one, it uses material where the load is, and it can be repaired. Those are real advantages and they do not disappear because a machine tool exists.
The costs that do not appear on a fabrication quote are the ones that follow the part afterwards. Weld distortion has to be corrected or accepted. Joints have to be inspected. Heat-affected zones have to be cleaned and, on stainless, passivated. Fit-up variation means the first assembly and the last assembly of a batch are not the same part.
Then there is the machining that a weldment forces. A flange face has to be faced after welding. A bore has to be opened after welding. A thread has to be cut in position after welding. Each of those is a setup on an awkward assembly, and each of them exists because the part was welded.
There is also the internal geometry problem. A welded box or plenum has seams inside it, and seams inside a gas path disturb flow and collect material. Machining a body from solid produces a smooth internal form with no seams, which is a performance argument rather than a cost argument, and sometimes it is the deciding one.
The clearest case for machining from solid is a small part with complex internal geometry. A manifold block with intersecting passages, a valve body with several ports, a boss with internal steps: each of these is difficult to fabricate and easy to machine. The internal form is the reason, not the external shape.
The second case is a part where the shape itself carries the accuracy. If several faces must be square to each other and several bores must be coaxial, a single setup on one piece of stock produces all of them from one datum. The same part welded from pieces has each face introduced by a different operation and related to the others only by the fixture.
The third case is quantity. Once the program and fixture exist, a machined part repeats with very little variation, and the cost per part falls steeply. A weldment keeps costing roughly the same per unit forever, because the labour does not go away.
There is a volume at which machining becomes cheaper than fabrication, and it is lower than most people assume for small parts and higher than most people assume for large ones. The variable that decides it is not the size of the part but the amount of material that has to be removed to get the shape. A small part with a simple shape machines cheaply. A large part with a simple shape is almost always better fabricated.
Geometry pulls the crossover in the other direction. The more internal passages, the more faces that must relate to each other, and the more the part benefits from being one piece. A part that would need six welded joints to fabricate is usually a candidate for machining even at low volume, because the joints are where the cost and the risk both sit.
The honest way to find the crossover is to cost both routes on the same drawing, including the post-weld machining that the fabricated version will need. That last item is the one most often left out, and it is often the one that decides the answer. You can see the sort of parts where the comparison is live on our titanium fabrication components pages.
The first thing it gives up is material. A part machined from a billet removes most of the billet, and on titanium that is an expensive thing to do. The chips have value as scrap, but nowhere near the value of the stock they came from, and the ratio of bought weight to finished weight is a number worth knowing before committing to the route.
The second thing is time. Removing material takes longer than joining it, and the machine is expensive while it happens. A fabricated part can be built in parallel by several people; a machined part is limited by the number of machines and the length of the program.
The third thing is repairability. A welded part can be cut and re-welded. A machined part that is damaged is usually replaced, and a machined part that is wrong is usually scrapped, because there is no stock left to correct it with.
In practice the answer is rarely one route or the other. A machined body with welded tube stubs gives you a smooth internal form where it matters, accurate faces where they seal, and cheap standard tube where the part is just carrying gas from one place to another. The weld is moved to a place where its distortion does no harm.
The design rule that follows is to put welds where the geometry is simple and the accuracy requirement is low, and to put machining where the geometry is complex or the accuracy requirement is high. A part designed with that rule in mind is usually cheaper than either pure route, and it is much easier to make consistently.
That means the drawing stage is where the money is decided, again. A designer who hands over a welded assembly and asks for it to be made accurate has handed over a problem. A designer who hands over a part with the welds in sensible places has handed over a job.
Large parts, thin-wall parts, parts made from tube, and parts where the material is expensive relative to the labour all belong to fabrication. So do parts that must be repaired in service, and parts where the internal volume matters more than the internal finish.
Welding also wins where the tooling cost of machining would be disproportionate. A one-off bracket that can be cut and welded in an afternoon does not need a fixture, a program and a first article. The economics are not close, and treating them as close is how a simple job becomes an expensive one.
The route decision is a design decision, and it is cheapest to make before anyone quotes.
You can see both routes side by side on our machined and fabricated parts pages, the second group of small components, the brackets and adapters, the aluminium welding work and the titanium welded parts that share the same shop.
