Die Casting Tolerances - CT Classes & Dimensions
Complete die casting tolerances guide: ISO 8062 CT grades, linear dimension tables, hole position, flatness, and machining allowance recommendations.
Mike Yao
Technical Director, Die Casting Engineering
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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.
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 |
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 |
| 10-25 | +/-0.20 | +/-0.32 | +/-0.44 | +/-0.64 |
| 25-63 | +/-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 |
| 250-400 | +/-0.52 | +/-0.78 | +/-1.10 | +/-1.60 |
Feature-Specific Tolerance Guidance
Parting Line Features
Dimensions crossing the parting line are 1-1.5 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 | 0.005-0.01 mm |
CNC Machining -Extending Tolerance Capability
| Feature | As-Cast | Post-Machined |
|---|---|---|
| Linear dimensions | +/-0.15-0.5 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 1.6-3.2 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 15-30% to part cost unnecessarily.
How to Specify Tolerances on a Die Casting Drawing
- Use a general tolerance note: "All untoleranced dimensions per ISO 8062 CT5"
- Call out tight tolerances explicitly only on features that functionally require them
- Identify parting line location and ejector pin placement restrictions
- Mark cosmetic surfaces (for gate placement guidance)
- 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
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 1-1.5 CT grades worse than same-half dimensions. Design critical dimensions within one die half where possible.
Tolerance consultation -free with every RFQ: casting@kastdiecast.com | +86 138 1403 4409
Related
- DFM die casting
- surface finish chart - Ra values and process capability
About The Author
Mike Yao on die casting tolerances
Technical Director, Die Casting Engineering
Technical Director at KastMfg with 20+ years in high-pressure die casting, leading engineering and program management across aluminum, zinc, and magnesium programs for automotive, EV, electronics, and medical OEMs worldwide.
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