What Each Process Actually Does
Die casting and CNC machining solve the same problem—making a metal part—in opposite ways. In die casting, molten aluminum or zinc alloy is injected into a hardened steel die at high pressure, and the part takes shape in 30 to 90 seconds. In CNC machining, a cutting tool removes material from a solid billet, chip by chip, until the geometry appears. One is an additive-by-mold process optimized for repetition; the other is subtractive and optimized for precision.
That single difference drives almost every practical trade-off between them: tolerance, unit cost, tooling investment, lead time, and design freedom.
Tolerances and Surface Finish
This is where the two processes differ most sharply. Well-maintained CNC machining centers hold ±0.005 mm (±0.0002") on critical features as a matter of routine, and tighter tolerances are achievable when the part justifies extra care. Most metal machining shops treat ±0.01 mm as standard machining tolerance for milled features.
Die casting, by contrast, is a net-shape process. As-cast dimensional tolerances typically run around ±0.10 mm for the first 25 mm of nominal size, growing with projected area. Achievable as-cast surface finish lands in the Ra 0.8–1.6 μm range on die faces, but the part still usually needs secondary machining for bores, threads, and sealing surfaces.
The common solution is a hybrid: cast the bulk geometry, then machine the 5–10% of features that actually need precision. Most production die cast parts you'll see in industry follow exactly this route.
Where the Money Goes
CNC machining has essentially no tooling cost. You pay for machine time and material—so unit cost stays flat as volume grows. If a part takes 20 minutes to machine, it takes 20 minutes whether you order 10 or 10,000.
Die casting inverts the curve. A production die costs anywhere from a few thousand dollars for a simple single-cavity tool to well over $30,000 for a complex multi-slide mold—but it's good for tens of thousands to over 100,000 shots. Per-part cost then drops to mostly metal plus a short cycle.
The breakeven point between the two depends heavily on part size and complexity, but for typical aluminum housings it lands somewhere between a few hundred and a few thousand units. Below that, machined-from-billet is often cheaper once you count tooling amortization. Above it, die casting wins decisively.
Geometry: What Each Process Does Well
Die casting rewards designs with:
- Uniform thin walls, typically 2–4 mm
- Draft angles of 1–3° so the part ejects cleanly
- Integrated bosses, ribs, and brackets that would need assembly in a machined design
- Complex 3D contours that would demand 5-axis setups in machining
It punishes thick sections (porosity and sink marks), sharp internal corners, and undercuts that require sliding cores—each slide adds tooling cost and maintenance risk.
CNC machining rewards different things: tolerance-critical features, thin or variable wall sections that a die would struggle to fill, and design changes. Changing a machined part costs one CAM edit; changing a cast part may cost a die modification.
Material Selection
Die casting alloys are a short list. A380 and ADC12 dominate aluminum die casting—good fluidity, decent strength, excellent castability. Zamak 3 and 5 cover zinc applications where thin walls and as-cast surface quality matter. Magnesium alloys like AZ91D appear where weight savings justify the handling precautions.
Machining opens the full metals catalog. 6061-T6 is the workhorse aluminum—strong, weldable, affordable, and easy to machine. 7075-T6 offers substantially higher strength for aerospace and high-load applications, at the cost of weldability and a somewhat harder, more expensive cut. Add brass, stainless steels, tool steels, and titanium, and metal machining covers materials die casting simply cannot touch.
Don't Cut a Die Before You Need To
The most expensive mistake in this decision is committing to a production die before the design is frozen. The standard approach: prototype the part first—CNC machining the geometry from 6061 or a comparable alloy, or printing casting patterns for form-and-fit checks. Test, iterate, and only when the design is stable, release it to tooling.
Between prototype and full production volume, soft tooling and vacuum casting bridge the gap economically, delivering a few hundred parts without a hard tool commitment.
A Practical Decision Rule
If your part is small-to-medium, has stable geometry, needs thousands of units, and only a few features need tight tolerance—die cast it, then machine the critical faces. If volumes are low, the design may still change, tolerances are tight throughout, or the material is steel or titanium—machine it from billet.
And when in doubt at prototype stage, start with machining. It is the only one of the two processes that forgives you for being wrong.
