Machining of Castings — Tolerances, Methods and Common Mistakes

Castings rarely leave the foundry in a condition that allows them to go straight into the end customer’s assembly. Between the casting and the finished product, surfaces must be machined to final tolerances, functional faces must be milled flat, holes must be drilled and tapped, and quality inspection must verify that everything falls within drawing requirements.

Machining of castings is a distinct discipline — different from machining forgings or bar stock — with its own challenges, common mistakes and optimisation opportunities.

Achievable tolerances

The tolerances you can hold depend on the casting method, the material and the machining process:

Sand castings: As-cast tolerances CT9–CT11. After machining: IT7–IT9 routinely achievable.
Die castings: As-cast tolerances CT6–CT7. After machining: IT6–IT8.
Investment castings: As-cast tolerances CT5–CT7. Often require minimal machining.

Key machining methods

CNC milling

The workhorse for flat surfaces, pockets and complex 3D geometries. 3-axis for basic operations, 5-axis for complex components with multiple datum faces.

CNC turning

For cylindrical features — bores, outer diameters, faces and grooves. Often combined with milling on mill-turn centres.

Drilling and tapping

Critical for fastener holes and fluid passages. Cast-in pilot holes reduce machining time and improve tool life.

Grinding

For tight tolerances (IT5–IT6) and fine surface finishes (Ra < 0.8 µm) on critical sealing or bearing surfaces.

Common mistakes

Insufficient machining allowance — not accounting for as-cast variation leads to missing material on critical surfaces.
Incorrect datum selection — using as-cast surfaces as primary datums introduces cumulative errors.
Ignoring material structure — cast structures contain porosity and hard spots that affect tool life and surface quality.
Over-specifying tolerances — tighter tolerances than functionally necessary increase cost dramatically.

Design for machinability

Add 2–3 mm machining allowance on critical surfaces
Design clear datum surfaces into the casting
Minimise the number of setups (orientations)
Standardise hole sizes and thread specifications
Discuss machinability requirements with the foundry during the design phase

Traficator’s service

We supply castings with machining included — managing both the foundry and the machining workshop as a single supply chain. This eliminates interface issues and ensures that as-cast quality and machining requirements are aligned from the start.

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