Quick Answer
Casting tolerances — each casting process has inherent tolerance capability. Specifying tighter tolerances than necessary increases cost; specifying too loose tolerances causes fit-up problems. The right approach: understand what each process can achieve naturally, then specify tighter tolerance only where function requires it. Here is how tolerances work in sand casting, investment casting, and die casting.
What Are Casting Tolerances?
Casting tolerance is the dimensional variation that occurs naturally in the casting process due to:
- Pattern wear: Pattern dimensions change slightly with use
- Mold expansion: Sand molds expand slightly when heated by molten metal
- Metal shrinkage: Metal shrinks as it cools from liquid to solid (typically 1–2% linear)
- Core shift: Sand cores can move slightly during pouring
- Machining allowance: Surface that will be machined can be cast oversize
Tolerance Capability by Casting Process
| Process | Standard Tolerance | Best Achievable | Notes |
|---|---|---|---|
| Sand Casting | ±0.5–1.0 mm per 100 mm | ±0.3 mm per 100 mm | Coarse sand surface limits precision |
| Investment Casting | ±0.1–0.3 mm per 100 mm | ±0.05 mm per 100 mm | Ceramic shell is dimensionally stable |
| Gravity Die Casting | ±0.2–0.5 mm per 100 mm | ±0.1 mm per 100 mm | Steel die, but gravity fill limits |
| Low Pressure Die Casting | ±0.1–0.3 mm per 100 mm | ±0.05 mm per 100 mm | Precision die, controlled fill |
| High Pressure Die Casting | ±0.1–0.25 mm per 100 mm | ±0.05 mm per 100 mm | Best dimensional precision |
Tolerance Hierarchy in a Casting
Not all dimensions on a casting are equal. Some are critical, some are not:
Critical Dimensions
Critical dimensions directly affect function — fit, assembly, sealing, or structural integrity:
- Bearing seats and bore diameters
- Sealing surfaces
- Mounting holes and bolt patterns
- Threaded areas
- Critical wall sections
Buyer action: Mark critical dimensions on the drawing. Do not assume the foundry knows which dimensions matter.
Non-Critical Dimensions
Non-critical dimensions do not affect function if they vary slightly:
- Overall envelope dimensions (unless it affects packaging or fit)
- Appearance features
- Draft angle taper (within reason)
- Wall thickness (unless minimum wall is critical)
Specifying Tolerances on Drawings
Default Casting Tolerances
Specify a default tolerance for all non-marked dimensions:
Example drawing note:
“Unless otherwise specified, casting tolerances shall be ±0.5 mm per 100 mm for sand castings and ±0.2 mm per 100 mm for investment castings.”
Individual Feature Tolerances
Mark tighter tolerances on critical features:
- Use GD&T (ASME Y14.5) symbols for critical position, flatness, perpendicularity
- Specify tolerance per feature on the drawing
- Use a tolerance table for repeated features (e.g., hole sizes, boss diameters)
The Cost of Over-Specifying Tolerances
Many buyers specify tighter tolerances than necessary, which drives up cost:
| Tolerance Tightening | Cost Impact | When Justified |
|---|---|---|
| Sand cast: ±1.0 mm → ±0.5 mm | +15–30% on machining cost | Critical fit surfaces |
| Investment: ±0.3 mm → ±0.1 mm | +20–40% on casting cost | Precision gear seats |
| Add machining for tight tolerance | Usually cheaper than tight casting tolerance | Better than tight casting tolerance |
Rule: If a dimension needs precision tighter than the casting process can achieve, specify a machining allowance on that surface and machine it to final tolerance. It is almost always cheaper than trying to hold tight tolerances in the casting.
Machining Allowance vs. Casting Tolerance
Machining allowance is the extra material left on a surface for machining to final dimension:
| Surface Type | Machining Allowance | Effect |
|---|---|---|
| Datum/machining setup surfaces | 2.0–3.0 mm | Provides clean, flat reference |
| Precision fit surfaces | 1.5–2.5 mm | Allows finish machining to tolerance |
| General machined surfaces | 1.5–2.0 mm | Standard allowance |
| Non-machined surfaces | 0 mm | Cast to final dimension |
Buyer tip: Only add machining allowance where you need it. Every mm of excess material adds machining time and material cost.
Tolerance for Specific Features
| Feature | Sand Casting | Investment Casting | Die Casting |
|---|---|---|---|
| Bore diameter (uncut) | ±0.5–1.0 mm | ±0.1–0.3 mm | ±0.1–0.25 mm |
| Boss diameter | ±0.3–0.5 mm | ±0.1–0.2 mm | ±0.05–0.15 mm |
| Wall thickness | ±0.5–1.0 mm | ±0.2–0.5 mm | ±0.1–0.2 mm |
| Flatness (per 100 mm) | 0.3–0.5 mm | 0.1–0.2 mm | 0.05–0.1 mm |
| Concentricity | ±0.5–1.0 mm | ±0.1–0.3 mm | ±0.05–0.15 mm |
Pattern and Core Tolerances
The pattern (tool that forms the mold) determines casting dimensions:
- Pattern tolerance: ±0.05–0.2 mm per 100 mm (depending on pattern material and method)
- Core print tolerance: ±0.1–0.3 mm (core location tolerance)
- Pattern wear allowance: Patterns wear over time; the foundry adjusts to compensate
Tolerance and Surface Finish Relationship
Tighter tolerance requires better surface finish, which costs more:
- ±1.0 mm tolerance → Ra 12.5–25 µm surface (sand casting, as-cast)
- ±0.3 mm tolerance → Ra 3.2–6.3 µm surface (investment casting, as-cast)
- ±0.05 mm tolerance → Ra 0.8–1.6 µm surface (machined surface)
Design implication: If you need ±0.1 mm tolerance, you almost certainly need to machine the surface. Casting to ±0.1 mm in sand or investment is possible but expensive and inconsistent.
Before finalizing the sourcing decision, many OEM buyers also compare Surface Finish Guide, Machining Allowance, DFM for Sand Casting, and Sand Casting Process to clarify process fit, cost trade-offs, tolerance expectations, and supplier risk.
If you need application-specific guidance, drawing review, or a quotation, you can Contact YCUMETAL.
Buyer Checklist
- Do you understand the natural tolerance capability of your casting process?
- Are critical dimensions marked on the drawing?
- Is the default tolerance specified for non-marked dimensions?
- Have you added machining allowance only where machining is needed?
- Is GD&T used for position, flatness, and other geometric tolerances where needed?
- Have you avoided over-specifying tolerances on non-critical features?
- Have you reviewed the drawing with the foundry before ordering tooling?
FAQ
Can sand casting achieve ±0.1 mm tolerance?
No — standard sand casting cannot reliably achieve ±0.1 mm. You would need to specify a machining allowance on that surface and machine it. Sand casting natural tolerance is ±0.5–1.0 mm per 100 mm.
What is the most cost-effective way to achieve tight tolerance on a casting?
Specify a machining allowance on the surface that needs tight tolerance. Cast it slightly oversize (with enough allowance), then machine to final tolerance. This is almost always cheaper than trying to hold tight tolerance in the casting itself.
How does GD&T apply to castings?
GD&T symbols (position, flatness, perpendicularity, etc.) apply to the finished part dimensions, including after any machining. Specify datum references that are cast surfaces (not machined, unless those surfaces are also machined). The foundry produces the casting to meet the GD&T callouts.