Why Die Castings Almost Never Ship As-Cast
Ask for a die-cast part off the machine and you'll notice the parting line bead, the ejector pin pads standing slightly proud, and gate vestige where the metal rushed in. That's normal. As-cast tolerances on a 100 mm feature are typically ±0.1 to ±0.2 mm, but the moment your design puts two machined bores in a functional relationship — a bearing seat and a seal face, say — you're into second operations. The skill is in designing the casting so those second operations are cheap and repeatable, not an afterthought that doubles the part cost.
Know What the Process Holds — and What It Doesn't
Die casting earns its keep by net-forming geometry that would eat machining hours: ribs, bosses, thin walls, internal threads in the die (for large threads only). Typical ADC12/A380 capability:
- Linear tolerance on the first 25 mm: about ±0.1 mm; add roughly ±0.02–0.03 mm per additional 25 mm. Across a 200 mm part you're realistically at ±0.2–0.3 mm.
- Flat across the parting line: not holding. Flash and thermal bowing make the die-halves' seam the least stable surface on the part.
- Cored holes: excellent roundness, but position tolerance is set by die steel movement and thermal cycling — expect drift, and don't chain tolerances through cast datums.
The rule of thumb: features that mate, seal, or locate bearings get machined. Everything else stays as-cast.
Stock Allowance: Less Is More, But Not Zero
Bigger machining stock feels safer, and it's usually wrong. Die cast surfaces have a dense skin and a porous interior; cutting deeply into the casting exposes porosity exactly where you need a sealed surface. Practical numbers:
- Faced surfaces: 0.3–0.5 mm total stock is plenty for cleanup of flash and ejector pads.
- Bored holes: 0.4–0.7 mm radial stock, enough to true the hole without breaking into the porosity band that concentrates 0.5–1.0 mm below the skin.
- Never specify a machining allowance that reaches past the skin on both sides of a thin wall — a 2.5 mm wall with 0.5 mm stock each side leaves a 1.5 mm core full of shrinkage and gas porosity.
Datum design matters more than the stock number itself. Carry the as-cast datum through to the machining fixture so machining corrects nothing but local error. If machining datums jump to cast features that weren't held tightly, your fixture will chase drift all day. Clamping on ejector pads or support ribs opposite the cut keeps a 3 mm wall from buzzing under the cutter.
The Usual Second Operations
- Face mill parting line and seal faces: one light pass, Ra 1.6 µm or better for O-ring seats. Carbide tooling, high speed, small depth of cut to avoid grabbing flash remnants.
- Finish boring/tapping: leave threads over M5 to the tap for a clean, full-form thread; internal die threads below that are rarely economical.
- Reaming dowel holes: ±0.02 mm is achievable when the pilot hole was cored in the die rather than drilled.
- Deburr and vibratory finish: budget it. Flash removal is real labor, and hidden burrs inside cored passages cause assembly issues you'll only find on the line.
DFM Details That Save Real Money
A few design habits reduce both casting and machining cost at once:
- Put machined features on the same die half where possible. Cross-parting-line machining datums force fixturing that references two unstable surfaces.
- Add 0.5 mm tall, 8–10 mm diameter machinist pads as extra ejector bosses — they cost nothing in the die and give the fixture clean clamp-and-loc points.
- Avoid machining through cored holes into thick sections; the exit side is where porosity lives. If a through-bore must cross a rib, expect rejects and say so on the drawing.
- Anodizing after machining is a trap on high-silicon alloys — the surface comes out mottled. Machine, then finish, and choose a coating compatible with ADC12 (chem-film and powder coat, usually).
None of this is exotic. It's mostly a willingness to mark up the casting drawing with what machining will touch, before the die is cut. A molder who sees machined datums, stock targets, and clamp pads on the print will quote faster and scrap less. If you're weighing a die-cast design against a fully machined one, our die casting team can review the casting-to-machining handoff, and for high-precision features that outgrow as-cast capability, our CNC machining service handles the second operations in-house.
