Die Casting Tolerance Guide
What you can achieve as-cast vs. with secondary machining
Die casting produces near-net-shape parts, but "near" isn't "exact." Understanding as-cast tolerances helps you design parts that minimize secondary machining — the biggest cost adder in die cast production. This guide covers what's achievable straight from the die vs. what needs post-machining.
Overview
Die casting tolerances depend heavily on the alloy (zinc is tightest, aluminum is mid-range, magnesium varies), part size, and die complexity. As-cast tolerances are wider than CNC, but smart design can eliminate machining entirely for many features.
Tolerance Specifications
| Feature | Standard | Precision | Notes |
|---|---|---|---|
| Linear Dimensions (< 25 mm) | ±0.10 mm (±0.004") | ±0.05 mm (±0.002") | Precision requires premium dies and process control. Zinc holds tighter than aluminum. |
| Linear Dimensions (25–250 mm) | ±0.13 mm + 0.0015/mm | ±0.05 mm + 0.001/mm | Tolerance grows with part size. Each additional 25 mm adds ~0.038 mm. |
| Across Parting Line | Add ±0.15 mm to linear | Add ±0.08 mm | Features spanning the parting line always have extra variation due to die closure. |
| Flatness | 0.10 mm per 25 mm | 0.05 mm per 25 mm | Large flat surfaces warp. Design ribs on the back side to maintain flatness. |
| Draft Angle | 1°–2° per side | 0.5° minimum | Less draft = harder ejection = shorter die life. 1.5° is the sweet spot. |
| Surface Finish (Ra) | Ra 1.6–3.2 µm | Ra 0.8 µm (with polished die) | New dies give Ra 0.8. After 50K+ shots, expect Ra 1.6–3.2. |
| Wall Thickness Variation | ±10% of nominal | ±5% of nominal | Thicker sections shrink more. Uniform wall thickness gives best consistency. |
| Hole Diameter (Cored) | ±0.08 mm | ±0.04 mm | Only for holes cast with core pins. Min diameter ~3 mm for aluminum. |
| Thread (Cast-in) | Not recommended | Not recommended | Always machine or tap threads. Cast threads lack the accuracy for functional fastening. |
Key Considerations
Design for As-Cast
The best die casting design needs zero secondary machining. Use as-cast tolerances for non-critical features and design mating interfaces for post-machining.
Datum Surfaces First
Identify 3 datum surfaces for secondary machining fixturing. Cast these surfaces with extra stock (0.5–1.0 mm) for clean-up machining.
Machine Only Critical Features
Bearing bores, sealing surfaces, and thread holes need machining. Everything else? Design it to work as-cast.
Die Life Affects Tolerance
Tolerances loosen as the die wears. For tight-tolerance production, budget for die refurbishment every 50–100K shots.
💰 Cost Impact of Tolerances
Precision die casting tolerances require premium-grade dies, slower cycle times, and 100% inspection — adding 20–40% to part cost.
Standard tolerances are achievable with normal production dies and cycle times. No cost premium.
Design functional surfaces (bearing bores, sealing faces, thread holes) as machining features with 0.5–1.0 mm stock. Everything else should work at as-cast tolerances.
⚠️ Common Tolerance Mistakes
- ⚠ Specifying CNC-level tolerances on die cast parts — if you need ±0.025 mm everywhere, die casting is the wrong process.
- ⚠ Ignoring parting line effects — features crossing the parting line always have extra variation.
- ⚠ Designing zero-draft walls — minimum 0.5° draft is needed for part ejection, and 1–2° is strongly recommended.
- ⚠ Expecting consistent wall thickness in thick sections — shrinkage porosity occurs in heavy cross-sections.
- ⚠ Tolerancing features on moving die halves relative to features on fixed die halves — these always have parting line variation.
💡 Pro Tips
- ▸ Zinc alloys (Zamak 3, 5) hold 30–50% tighter tolerances than aluminum — consider zinc for small, precision parts.
- ▸ Vacuum-assisted die casting reduces porosity and allows tighter tolerances in critical areas.
- ▸ Our Vietnamese die casting factories produce 100K+ shot dies at 40–60% less than US or European tooling.
- ▸ Request first-article inspection (FAI) with CMM data to validate critical dimensions before production.
- ▸ For parts that need tight tolerances AND complex geometry, consider die casting + CNC finishing — often cheaper than pure CNC for high volumes.
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