Two routes, two cost curves
Machining starts from solid stock and removes everything that is not the part. Casting pours molten metal into a mould and forms the shape. Machining carries almost no setup cost and a high cost per part; casting carries a pattern or die cost and a low cost per part once that tool exists.
The two cost curves cross at a volume that depends on the size, the complexity and the tolerance of the part. Below the crossover, machining is cheaper and reaches a first part far sooner. Above it, casting spreads the tooling over enough pieces to win.
What casting buys, and what it costs
Casting produces a near-net shape, so less material is removed and less machining time is required. It can form internal passages a cutter cannot reach, and it suits large or intricate parts. In exchange it needs a pattern for sand casting or a die for die casting, and its as-cast tolerance and surface finish are coarser than any machined face.
Porosity is the characteristic casting defect. Gas or shrinkage voids can sit below the surface and reveal themselves as a leak, a poor finish when the skin is machined away, or a weak point under load. A casting specification that states an acceptable porosity level, and how it will be verified, is doing real work rather than quoting boilerplate.
Tolerance and finish: the honest comparison
An as-cast tolerance is measured in fractions of a millimetre and an as-cast surface would not pass as a bearing bore or a sealing face. The usual engineering answer is therefore to cast the part oversize and machine the features that matter, which combines the two processes instead of choosing between them.
That combination is where the real decision usually lies. The question is rarely 'cast or machine' but 'which features must be machined after casting, and does the casting leave enough stock to clean them up'. A casting that arrives with insufficient material on a critical face is a classic and expensive cause of scrap.
Volume, and where the crossover actually sits
The crossover moves with part size and with how much machining the casting still needs. A small, simple part can be cheap enough to machine outright that casting never pays. A large part with a complex internal form can justify a pattern from the first hundred pieces.
Sand casting has a low pattern cost and a higher per-part cost; die casting has a high die cost and a very low per-part cost. Between them, investment casting and permanent-mould casting shift the balance again. The route that wins is the one whose cost structure best matches the quantity the program actually expects, not the quantity in the first forecast.
Material and mechanical behaviour
A casting's properties depend on how it solidified, not only on its chemistry, so the same alloy in cast and wrought form does not behave identically. A casting can carry a coarser grain and a directional structure, while a part machined from wrought stock has a finer, more uniform grain. For a part under fatigue or impact load, that difference belongs in the material specification.
Cast aluminium and wrought aluminium are different materials with different design allowables. Treating them as interchangeable because both are described as 'aluminium' is a common and expensive simplification.
Choosing, and combining
For prototypes, low volume, tight tolerance, or a design that is still changing, machine. For high volume with a stable design, a coarse-as-cast tolerance and a shape that suits a mould, cast. For most serious production parts, do both: cast or form near-net, then machine the features that carry the tolerance.
Where the volume justifies it and the geometry suits a moulded production route rather than a cast one, the conversation shifts from casting to moulding. A production partner such as MadeInDayin, which builds injection moulds and runs moulding, is the natural next step once the part's shape is fixed and the annual quantity is known - and arriving with a machined part that has already passed field trials is a much stronger position than arriving with a concept.
References
The process family is described under metal casting, and the most common route for larger parts under sand casting. The subtractive alternative is set out under machining. Material and test standards come from ASTM International, and the measurement traceability behind an inspection result from the US National Institute of Standards and Technology.
Frequently asked
Is casting always cheaper at high volume?
No. Casting trades tooling cost for a low per-part cost, so it only wins once the volume is large enough to amortise the pattern or die. If the part also needs tight tolerances or fine finishes, the machining that follows narrows the gap, and for a small simple part machining can stay cheaper at any realistic quantity.
Can a casting be held to a tight tolerance?
Not as-cast. The standard route is to cast oversize and machine the critical features, so the casting supplies the shape and the machining supplies the accuracy. The casting specification has to leave enough stock on every face that will be machined.
Is a machined part stronger than a casting?
Not automatically. Strength depends on the alloy and on how it solidified. A wrought part machined from rolled or extruded stock usually has a finer, more uniform grain than a casting of similar chemistry, which favours fatigue and impact performance, but the design and the alloy matter as much as the route.