Tweeler AB is a precision aluminium die casting company based in Stockholm, Sweden, specialising in custom mould design, HPDC, re-engineering, and global logistics.
Tolerances are one of the most misunderstood aspects of aluminium die casting. Specify them too loosely and your parts won't fit or function. Specify them too tightly and you drive up cost unnecessarily with machining operations that aren't needed.
As-Cast vs Machined Tolerances
The most important distinction is between as-cast tolerances (what the casting process achieves directly) and machined tolerances (achieved by CNC machining selected features after casting). As-cast is far cheaper because it requires no secondary operation, but it is limited in precision. A well-designed part uses as-cast tolerances everywhere possible and reserves tight machined tolerances only for the few features that genuinely need them.
Typical As-Cast Tolerance Capability
| Dimension type | Typical as-cast tolerance |
|---|---|
| Linear, within one die half | ±0.1 mm up to 25 mm |
| Linear, larger dimensions | ±0.1 mm + 0.002 mm/mm |
| Across the parting line | Add ±0.1–0.2 mm |
| Involving moving cores / slides | Add ±0.1–0.15 mm |
| Flatness | ~0.3% of dimension |
| Wall thickness | ±0.1–0.25 mm |
Why the Parting Line Matters
A die casting mould is made of at least two halves. Features formed entirely within one half can hold tight tolerances relative to each other. But features split across the two halves have additional tolerance stack-up because the two halves never close in exactly the same position every cycle. When you specify a critical dimension, it is far better if both reference surfaces are in the same die half.
Linear Tolerances
Linear tolerances grow with dimension size because of thermal contraction during cooling. International standards such as ISO 8062 (casting tolerance grades, CT) provide standardised tolerance grades. HPDC typically achieves grades CT4–CT6 depending on size and complexity.
Geometric Tolerances (GD&T)
Flatness, parallelism, perpendicularity, and position tolerances are achievable as-cast within limits, but tight geometric tolerances usually require machining. A flat sealing surface that must be flat within 0.05 mm will need to be machined — as-cast flatness is typically around 0.3% of the dimension.
When to Add Machining
- Sealing or mating surfaces requiring flatness better than ~0.1 mm
- Bores and bearing seats requiring H7 or tighter fits
- Threaded holes (cast-in threads are coarse; tapped threads are precise)
- Hole positions that must align precisely with a mating component
- Any dimension tighter than ±0.1 mm
How to Specify Tolerances on Your Drawing
- Apply a general tolerance note (e.g. ISO 8062-CT6) for all as-cast dimensions
- Call out individual tight tolerances only on the specific features that need them
- Use GD&T datums to define which surfaces are the functional references
- Mark which dimensions are critical-to-function
The Cost Impact of Over-Specifying
Every feature you mark with a tight tolerance that exceeds as-cast capability adds a machining operation, a fixture, and inspection time. Reviewing your drawing critically — and relaxing tolerances on non-functional features — is often the fastest way to reduce the cost of a die cast part.
Send Tweeler AB your drawing and we'll tell you which tolerances are achievable as-cast and which require machining — with a clear breakdown of the cost impact of each, so you can make informed decisions before tooling.
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