CNC Machining Die Castings - Tolerances & Cost Impact

CNC machining die castings is used when as-cast geometry cannot meet functional tolerances, sealing requirements, bearing fits, threads, flatness, or assembly interfaces. Die casting creates the near-net shape; machining finishes the critical surfaces.

The best cost result comes from machining only the features that need precision. Over-machining turns an efficient casting into an expensive hybrid part.

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When CNC Machining Is Needed

Feature	Why Machining Is Needed
Bearing bores	Roundness, diameter, and alignment control
Sealing faces	Flatness and surface roughness control
Threaded holes	Reliable assembly and torque control
Connector openings	Accurate fit and clean edges
Gasket grooves	Controlled width, depth, and surface finish
Datum pads	Repeatable assembly position
Fluid ports	Leak-tight geometry and clean surface

Some holes can be cast as pilot features and finished by machining. This reduces material removal and improves location control.

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As-Cast vs Machined Tolerances

Feature	As-Cast Typical	Machined Typical
General linear dimensions	+/-0.1-0.5 mm	+/-0.02-0.1 mm
Hole diameter	+/-0.1-0.3 mm	+/-0.02-0.05 mm
Flatness	0.1-0.5 mm	0.02-0.1 mm
Threaded holes	Usually machined	Controlled by tap or thread mill
Bearing fits	Usually machined	Based on drawing requirement

Final tolerances depend on part size, fixture strategy, alloy, wall stability, and inspection method.

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Cost Drivers

Cost Driver	Why It Adds Cost
Number of setups	More fixtures, handling, and inspection
Tight tolerances	Slower machining and more inspection
Deep holes	Tool wear and cycle time
Thin walls	Distortion during clamping
Poor datum strategy	More rework and inconsistent dimensions
Coating after machining	Masking and protection requirements

Designing clear datums early is one of the simplest ways to reduce machining cost.

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DFM Rules for Machined Die Castings

1. Mark critical machined surfaces on the drawing.
2. Use realistic tolerances only where function requires them.
3. Add machining allowance to sealing faces and bores.
4. Avoid clamping on thin flexible walls.
5. Keep datum surfaces accessible.
6. Decide whether machining happens before or after surface finishing.
7. Protect machined faces during packaging.
8. Define burr limits for holes and edges.

For related tolerance guidance, see die casting tolerances.

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RFQ Checklist

Send the supplier:

• 3D model and 2D drawing
• Critical dimensions and datums
• Tolerance requirements
• Surface roughness requirements
• Thread specifications
• Annual volume
• Surface finish requirements
• Inspection report requirements

KastMfg provides die casting, CNC machining, inspection, and finishing support in one workflow. Submit drawings through the RFQ page.

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FAQ

Do all die castings need CNC machining?

No. Many non-critical surfaces can remain as-cast. CNC machining is used for functional surfaces such as bores, threads, sealing faces, gasket grooves, and datum pads.

What tolerance can CNC machining hold on die castings?

Common machined tolerances are around +/-0.02-0.1 mm depending on feature, part size, fixture design, and alloy stability.

Does machining increase porosity risk?

Machining can expose internal porosity if pores are close to the machined surface. Critical zones should be reviewed during DFM.

How can buyers reduce machining cost?

Use realistic tolerances, limit machining to functional surfaces, design good datums, and avoid unnecessary tight controls on cosmetic or non-critical features.