CNC Machining vs Die Casting: Choosing by Production Volume

A die-cast part and a machined part can look identical in a photo and behave nothing alike. Die casting gives you cheap complex geometry at volume; CNC machining gives you tight tolerance, no tooling and any material. Most production programmes end up needing both — and the sequence matters. Here is how the two processes compare and how to combine them.

Side-by-Side

Factor	Die casting	CNC machining
Tooling	$10,000–$150,000 per die set	None
Economic volume	5,000+ parts/year	1 – 5,000 parts/year
Unit cost at volume	Very low	Higher — machine time per part
Lead time	6–12 weeks including tooling	3–10 working days
Tolerance	±0.1–0.5 mm typical	±0.01–0.05 mm typical
Wall thickness	0.8–3 mm achievable	Limited by tool access, not material flow
Material	Aluminium, zinc, magnesium, copper alloys	Any metal or engineering plastic
Porosity	Unavoidable to some degree	None — wrought stock is solid
Surface finish as-cast / as-machined	Ra 1.6–6.3 µm, with flash and parting lines	Ra 0.4–3.2 µm, no flash

What Die Casting Is Genuinely Better At

Complexity for free. Once the die exists, a housing with twenty ribs, three bosses, cooling fins and a textured exterior costs barely more than a plain block, because cycle time is driven by part volume and solidification, not by feature count. Casting also produces thin, near-net walls that machining cannot match economically — a 1.2 mm wall in a zinc housing is normal; producing the same shell by machining from solid would take many times the cycle time and a lot of wasted stock.

What Machining Is Genuinely Better At

Tolerance, solidity and freedom from tooling. A machined part has no porosity, no flash, no parting line requiring hand finishing, and no shrink allowance to manage. It can be cut from any material — including 2024 aluminium, 316 stainless or PEEK, none of which are die-castable. And it can change. If your design is not frozen, tooling is a liability; machining lets you iterate in days. See our guide to die casting alongside CNC machining capability.

The Post-Machining Reality Nobody Mentions

Almost no functional die casting ships as-cast. Castings come out of the die with:

• Draft on every vertical wall, which limits how square a face can be.
• Parting line flash that must be trimmed, and often a witness line on a visible face.
• Ejector pin marks on the ejection face.
• Shrinkage mismatch between thick and thin sections, so a bore that must be round and to size needs to be cut after casting.
• Porosity that makes sealing faces unreliable if left unmachined.

So in practice most die-cast parts are cast near-net and then CNC-machined on the critical faces: bearing bores, sealing surfaces, threaded holes, datum faces. This is the single most common production pattern in hardware — and it means the machine shop and the caster are usually the same supplier, or need to be tightly coordinated. Read our note on casting process and die quality for the supply side of this.

Choosing Your Route

1. Volume below ~5,000/year: machine it. Tooling will never pay back.
2. Volume above ~20,000/year with frozen design: cast it, then machine the critical features.
3. Between the two: work the crossover arithmetic — die cost ÷ production years + cast unit cost versus machined unit cost.
4. Design not frozen: machine prototypes first. Cutting steel before validation is the classic expensive mistake.
5. Critical tolerance or sealing face: plan for post-machining regardless of how you form the part.
6. Thin complex walls at high volume: casting earns its tooling; machining cannot compete.

Whichever route you take, run the numbers on the cost factors that drive machined price so you are comparing a real machined cost against a real cast cost, not a guess.

How Renjiu Helps

We machine castings from our customers' casters and from our own partners — face milling, bore cutting, tapping, sealing-face skimming and GD&T-verified inspection — as well as producing parts from solid. If you are weighing casting against machining, send the model and target volume through our RFQ form and we will tell you the crossover volume for your part and quote both routes. Our RFQ checklist shows what to send for an accurate answer on the first pass.