Die Casting Tolerances -A Practical Guide

Engineers specify tolerances based on function. Die casting suppliers quote based on process capability. When these two perspectives are not aligned -when a drawing specifies CNC tolerances (±0.05 mm) on as-cast features -the result is either excessive cost or quality rejection.

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ISO 8062 CT Grades

ISO 8062 defines Casting Tolerance (CT) grades CT1 (most precise) to CT16 (least precise). Die casting achieves CT3-CT6:

Process	Typical CT Grade
Zinc HPDC	CT3-CT5
Aluminum HPDC	CT4-CT6
Magnesium HPDC	CT4-CT6
Low-pressure die casting	CT5-CT7
Sand casting	CT11-CT13

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CT Grade Tolerance Values (±mm on linear dimensions)

Nominal dimension (mm)	CT4	CT5	CT6	CT7
up to 10	±0.16	±0.26	±0.36	±0.52
10-15	±0.20	±0.32	±0.44	±0.64
25-33	±0.26	±0.40	±0.56	±0.80
63-160	±0.36	±0.52	±0.76	±1.10
160-250	±0.44	±0.64	±0.90	±1.30
250-300	±0.52	±0.78	±1.10	±1.60

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Feature-Specific Tolerance Guidance

Parting Line Features

Dimensions crossing the parting line are 1-.5 CT grades worse than same-half dimensions due to die alignment variation. Design critical dimensions to fall within one die half.

• Flash thickness after trimming: 0.05-0.15 mm
• Die mismatch at parting line: 0.10-0.30 mm

Ejector Pin Marks

Depressions or protrusions 0.05-0.20 mm at ejector pin locations. KastMfg places ejector pins on non-functional, non-cosmetic surfaces -identified during DFM review.

Holes and Bores

Feature	As-cast	Post-machined
Hole diameter	±0.10-0.30 mm	H6/H7: ±0.005-0.015 mm
Hole position	±0.20-0.50 mm	±0.05-0.10 mm

Flatness

Condition	Achievable
As-cast flat surface	0.1-0.5 mm
CNC face-milled	0.01-0.05 mm
Ground sealing face	0.005-1.01 mm

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CNC Machining -Extending Tolerance Capability

Feature	As-Cast	Post-Machined
Linear dimensions	±0.15-1.5 mm	±0.02-0.05 mm
Bore diameter	±0.1-0.3 mm	±0.005-0.015 mm
Flatness	0.1-0.5 mm	0.01-0.05 mm
Surface roughness	Ra 1.6-2.2 μm	Ra 0.4-0.6 μm
Thread	Not recommended as-cast	M3-M30, gauge-verified

Economic principle: Specify as-cast tolerances wherever function permits. Reserve machining for features where tighter tolerance adds genuine value. Over-tolerancing a die casting adds 15-20% to part cost unnecessarily.

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How to Specify Tolerances on a Die Casting Drawing

1. Use a general tolerance note: "All untoleranced dimensions per ISO 8062 CT5"
2. Call out tight tolerances explicitly only on features that functionally require them
3. Identify parting line location and ejector pin placement restrictions
4. Mark cosmetic surfaces (for gate placement guidance)
5. Mark pressure-critical surfaces (no porosity)

Common mistakes to avoid:

• Specifying ±0.05 mm general tolerance on a casting drawing
• Not identifying the parting line location
• Specifying flatness of 0.02 mm on an as-cast surface without machining callout

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FAQ

Can die casting achieve ±0.02 mm without machining?

For feature-to-feature dimensions within the same die half, CT3 zinc achieves approximately ±0.05-0.10 mm for small features. Reaching ±0.02 mm consistently requires CNC machining.

What tolerance applies to dimensions crossing the parting line?

Approximately 1-.5 CT grades worse than same-half dimensions. Design critical dimensions within one die half where possible.

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Tolerance consultation -free with every RFQ: yaoqingpu1983@gmail.com | +86 138 1403 4409