Should I choose die casting or CNC machining for high-volume aluminum parts?

The short answer

The decision is one division, so run it on your own numbers

The whole comparison collapses into a single line: crossover quantity equals tooling cost divided by the difference between the machined and the cast unit price. Published comparisons put that crossover anywhere from about 500 pieces for a small aluminium housing to 3,571 pieces in a worked example with a USD 30,000 die, and to 1,176 pieces where a USD 50,000 die offsets a USD 45 machined part against a USD 2.50 casting. The spread is not sloppiness, it is the arithmetic: change the die price or the machining rate and the answer moves.

What is stable is the shape of the two cost curves rather than their crossing point. Machining spreads almost nothing up front and holds a roughly flat unit cost at any quantity. Casting spends a five-figure sum before the first part exists and then produces parts at a fraction of the machined price, so the per-part figure falls steeply as volume climbs. A published comparison puts a 100-piece order at about USD 220 per part once tooling is amortised, 10,000 pieces at about USD 4 per part against USD 15 machined, and a 100,000-piece programme at USD 1.28 million total against USD 2.17 million for the machining route.

Seven lines where the two processes do not overlap

Volume decides the price, but capability decides whether the part can be made that way at all. The seven rows below are the ones that most often override a favourable volume calculation, and they are worth checking before the tooling question is asked.

The first two rows are why so many "die casting" parts still visit a machining centre. A cast body can hold plus or minus 0.2 millimetres on a general feature, but a bearing bore, a sealing groove or a mating face at plus or minus 0.02 millimetres cannot be cast. The practical answer is a hybrid: a near-net casting plus a light machining pass on the ten or twenty percent of the part that has to be accurate. That route keeps most of the casting economy and buys the tolerance where it is needed.

The casting quote is not the delivered cost

This is the mistake that ruins the comparison. A die-cast quotation usually covers the shot, the runners and the flash trimmed off, and nothing else. Machining quotations usually bundle every feature into one number, which makes the casting look far cheaper than it is. Published secondary-operation ranges add USD 0.50 to 2 for trimming, USD 0.50 to 3 for deburring, USD 5 to 30 for machining the critical features and USD 2 to 10 for finishing, which is how an as-cast figure of USD 8 reaches USD 15 to 30 delivered.

Three costs sit outside the unit price and often decide real programmes. Lead time is one: a production die takes six to ten weeks while a machined first article takes days, so a launch deadline can force machining even where the volume maths favours casting. Design maturity is another: a part whose geometry is still changing is cheap to iterate in aluminium and expensive to iterate in hardened steel. Cash flow is the third, because casting asks for a five-figure tooling payment before the first part, and a buyer without that budget may rationally choose machining even where the long-run cost favours the die.

Quality cost belongs in the model too. Compare the cost per good part rather than the cost per produced part: a casting lot with porosity on a machined face carries scrap, rework and sorting costs that no unit price shows, and a machining programme with a bad fixture carries the same burden in reverse. Ask both suppliers for their scrap and rework history alongside the price.

When to choose die casting, and when not to

• Choose die casting above roughly 5,000 pieces a year, or from about 1,000 to 2,000 where the tooling is cheap and the part is simple. Above 20,000 pieces the advantage is large and durable.
• Choose CNC machining below about 500 pieces a year, and for one-offs. The tooling cannot be recovered at that volume, and the unit price gap does not exist yet.
• Choose a hybrid when a cast body needs two accurate features. Cast to near net shape, then machine the bores, grooves and mating faces. This is the most common real answer above 5,000 units.
• Do not die cast a part with deep undercuts, fine threads or a tight flatness callout. Slides and lifters can be added to the die, but each one costs several thousand dollars and adds a failure mode.
• Do not die cast a geometry that is still moving, or a product with a short life. Bigger up-front money needs a longer, more stable run to repay it.
• Do not compare a casting quote against a machining quote without loading both. Match the scope of work first, then compare the numbers.

What to send for a volume-based decision

Send the STEP model, the toleranced drawing, the annual volume and the expected product life, the material callout and the two or three tightest features. Those five items are enough to run the crossover arithmetic and to say whether the part is a casting, a machining job or a hybrid. See aluminium die casting for the casting route, CNC machining for the subtractive route and the die casting capability overview for tooling scope.