Understanding Casting Tolerances: Complete Guide to ISO 8062 for International Buyers

Quick Answer

Casting tolerances per ISO 8062 range from CT1 (most precise) to CT16 (least precise), with typical production castings at CT7-CT11. Green sand castings achieve CT10-CT13, resin sand CT8-CT11, investment casting CT5-CT8, and die casting CT4-CT7. Machining allowances typically range from 2-5mm for ferrous castings. Tighter tolerances increase cost significantly — specify only where functionally required.

Overview: Why Tolerance Understanding Matters

Casting tolerances directly affect part fit, function, machining requirements, and cost. Overly tight tolerances increase casting cost without adding value; overly loose tolerances create assembly problems or excessive machining. Understanding tolerance standards enables appropriate specification and cost optimization.

Tolerance impact:

Key principle: Specify tolerances based on functional requirements, not arbitrary precision. Match tolerances to casting process capability.

ISO 8062 Tolerance System

Tolerance Grade Designations

ISO 8062 defines 16 tolerance grades:

Grade selection factors:

• Casting process capability

• Part size (larger parts = coarser tolerances)

• Production volume (higher volume = tighter achievable)

• Pattern equipment quality

• Cost targets




Tolerance Values by Grade




Typical tolerance values (in mm):




Basic Dimension (mm)	CT7	CT8	CT9	CT10	CT11	CT12	CT13
10-16	0.28	0.40	0.56	0.80	1.1	1.5	2.2
16-25	0.30	0.44	0.62	0.88	1.2	1.7	2.4
25-40	0.34	0.50	0.70	1.0	1.4	1.9	2.7
40-63	0.38	0.56	0.78	1.1	1.5	2.1	3.0
63-100	0.44	0.64	0.90	1.3	1.8	2.4	3.5
100-160	0.52	0.74	1.0	1.5	2.0	2.8	4.0
160-250	0.62	0.88	1.2	1.7	2.3	3.2	4.5
250-400	0.74	1.0	1.4	2.0	2.7	3.7	5.0
400-630	0.88	1.2	1.7	2.4	3.2	4.3	6.0
630-1000	1.0	1.5	2.0	2.8	3.8	5.0	7.0
1000-1600	1.2	1.7	2.4	3.3	4.4	6.0	8.0

Note: Tolerances increase with part size — larger parts cannot achieve same absolute tolerances as small parts.




Tolerance Grade Selection




Recommended grades by application:




Application Type	Recommended Grade	Rationale
General engineering (pumps, valves)	CT8-CT10	Balance of cost and accuracy
Heavy equipment (construction, agriculture)	CT10-CT12	Large parts, less critical fits
Large structural castings	CT12-CT14	Size makes tight tolerances impractical
Prototype/low volume	CT11-CT13	Wood patterns, less precise tooling

Typical Tolerances by Casting Process




Green Sand Casting




Process characteristics:

• Most common casting process (70%+ of all castings)

• Manual or automated molding

• Reusable sand




Achievable tolerances:




Production Type	Typical CT Grade	Notes
Manual molding (medium volume)	CT10-CT12	Standard production
Wood patterns (prototype)	CT12-CT14	Pattern limits accuracy
Large castings (>500 kg)	CT11-CT14	Size affects capability

Typical tolerance example:

Part dimension: 100 mm
Process: Green sand, automated production
Tolerance grade: CT10
Tolerance value: ±0.65 mm (total 1.3 mm)

Part dimension: 300 mm
Process: Green sand, automated production
Tolerance grade: CT10
Tolerance value: ±0.85 mm (total 1.7 mm)




Resin Sand Casting




Process characteristics:

• Chemically bonded sand

• Better dimensional stability than green sand

• Suitable for larger, more complex parts




Achievable tolerances:




Production Type	Typical CT Grade	Notes
Precision resin sand	CT7-CT9	Enhanced capability
Large castings (>1000 kg)	CT9-CT12	Size affects capability

Comparison to green sand:

• Resin sand: CT8-CT10 (tighter)

• Green sand: CT10-CT12 (looser)

• Improvement: 1-2 CT grades




Investment Casting (Lost Wax)




Process characteristics:

• Wax pattern, ceramic shell

• Excellent surface finish

• High dimensional accuracy




Achievable tolerances:




Part Size	Typical CT Grade	Notes
Medium (50-200 mm)	CT5-CT7	Standard investment
Large (>200 mm)	CT6-CT8	Size affects capability

Typical tolerance example:

Part dimension: 50 mm
Process: Investment casting
Tolerance grade: CT5
Tolerance value: ±0.30 mm (total 0.6 mm)

Note: Investment casting can achieve tolerances
approaching machining for small parts.




Die Casting




Process characteristics:

• Metal mold (die)

• High pressure injection

• Non-ferrous only (aluminum, zinc, magnesium)




Achievable tolerances:




Part Size	Typical CT Grade	Notes
Medium (50-200 mm)	CT5-CT6	Standard die casting
Large (>200 mm)	CT6-CT7	Size affects capability

Note: Die casting achieves tightest tolerances among casting processes, but limited to non-ferrous materials.




Centrifugal Casting




Process characteristics:

• Rotating mold

• Cylindrical parts (pipe, tubes, rings)

• Good dimensional accuracy for roundness




Achievable tolerances:




Dimension Type	Typical CT Grade	Notes
Inner diameter	CT8-CT10	Slightly looser
Length	CT9-CT11	Less critical

Machining Allowances




What Are Machining Allowances




Definition: Extra material added to casting dimensions to allow for machining cleanup.




Purpose:

• Remove surface scale and decarburization

• Achieve final dimensions and tolerances

• Improve surface finish on critical surfaces

• Correct minor casting variations




Typical Machining Allowances




Ferrous castings (iron and steel):




Casting Size	Typical Allowance (per surface)	Notes
Medium (100-500 mm)	3-4 mm	Standard allowance
Large (500-1000 mm)	4-5 mm	Additional for distortion
Very large (>1000 mm)	5-8 mm	Compensation for movement

Non-ferrous castings (aluminum, bronze):




Casting Size	Typical Allowance (per surface)	Notes
Medium (100-500 mm)	2-3 mm	Standard allowance
Large (>500 mm)	3-5 mm	Additional for distortion

Machining Allowance by Process




Allowance recommendations:




Casting Process	Typical Allowance	Rationale
Resin sand	2.5-4 mm	Better surface finish
Investment casting	0.5-2 mm	Excellent surface, may not require machining
Die casting	0.5-1.5 mm	Very good surface, minimal machining
Shell molding	2-3 mm	Good surface finish

Machining Allowance Calculation




Example calculation:




Finished part dimension: 100 mm
Machining required: Both sides

Allowance per side: 3 mm
Total allowance: 6 mm

Casting dimension: 100 + 6 = 106 mm

After machining:
- Remove 3 mm from side A
- Remove 3 mm from side B
- Final dimension: 100 mm




Important considerations:

• Specify which surfaces require machining

• Non-machined surfaces can have tighter as-cast tolerances

• Machined surfaces must have adequate allowance

• Consider part location during machining for allowance distribution




Tolerance and Cost Relationship




Cost Impact of Tolerances




Tighter tolerances increase cost:




Tolerance Change	Cost Impact	Rationale
One grade tighter (CT9)	+10-20%	More careful production
Two grades tighter (CT8)	+20-40%	Enhanced process control
Three grades tighter (CT7)	+40-70%	Precision production required
Investment/die casting	+50-200%	Different process required

Cost drivers for tight tolerances:

• Higher-quality pattern equipment

• More frequent tooling maintenance

• Enhanced process control

• Increased inspection frequency

• Higher rejection rates

• Possible secondary operations




When Tight Tolerances Are Justified




Appropriate applications:




Application	Tolerance Justification
Mating parts	Assembly requirements
Sealing surfaces	Leak prevention
Precision components	Performance requirements
Interchangeable parts	Standardization needs

When Standard Tolerances Suffice




Acceptable applications:




Application	Rationale for Standard Tolerances
Clearance fits	Loose fit acceptable
Machined surfaces	Final dimension achieved by machining
Aesthetic surfaces	Appearance only, no fit required
Internal features	Not visible, no assembly impact

Drawing Specification




How to Specify Tolerances on Drawings




Standard tolerance callout:




GENERAL TOLERANCES:
Cast dimensions per ISO 8062-CT10
Machined dimensions per ISO 2768-m

SPECIFIC TOLERANCES:
[Bearing bore] Ø50 H7 (machined)
[Mounting surface] Flatness 0.1 mm (machined)
[Bolt holes] Ø12 ±0.2 mm (machined)

MACHINING ALLOWANCE:
3 mm on all machined surfaces
(as indicated by finish symbols)




Tolerance Symbols




Common symbols:




Symbol	Meaning	Application
H7, H8	ISO hole tolerance	Machined holes
h7, h8	ISO shaft tolerance	Machined shafts
±0.5	Direct tolerance	Specific dimensions
⌖	Position tolerance	Hole patterns
⏥	Flatness	Surface flatness
◎	Concentricity	Cylindrical features

Drawing Notes




Essential drawing notes:




CASTING REQUIREMENTS:

1. Material: [Specification, e.g., EN-GJS-500-7 per ISO 1083]

2. Tolerances:
- Cast dimensions: ISO 8062-CT10
- Machined dimensions: ISO 2768-m
- Unless otherwise specified

3. Machining Allowance:
- 3 mm on surfaces marked with finish symbol
- 0 mm on as-cast surfaces

4. Quality Requirements:
- Visual inspection per [standard]
- Dimensional inspection: [sampling level]
- Material testing: [requirements]

5. Pattern Equipment:
- Pattern type: [metal/wood]
- Pattern ownership: [customer/supplier]




Common Tolerance Issues




Issue 1: Unrealistic Expectations




Problem: Specifying tolerances beyond process capability




Example:

Unrealistic:
- Green sand casting, 500 mm dimension, ±0.2 mm
- This requires CT5, but green sand achieves CT10-11

Solution:
- Relax to ±0.9 mm (CT10) or
- Change to investment casting




Issue 2: Inconsistent Tolerances




Problem: Mixing tolerance standards without conversion




Example:

Inconsistent:
- Some dimensions in inches with ±0.010"
- Some dimensions in mm with ±0.2 mm
- These are not equivalent!

Solution:
- Use single measurement system
- Convert consistently if mixing required




Issue 3: Missing Machining Allowance




Problem: Drawing shows finished dimensions without allowance indication




Example:

Problem:
- Drawing shows 100 mm
- No indication of machining allowance
- Casting may be produced at 100 mm
- No material for machining

Solution:
- Show casting dimensions with allowance, or
- Show finished dimensions with machining symbols, or
- Add note specifying allowance




Issue 4: Over-Tolerancing




Problem: Applying tight tolerances to all dimensions




Example:

Over-toleranced:
- Every dimension ±0.1 mm
- Cost 3-5x necessary
- Most dimensions non-critical

Solution:
- General tolerances for non-critical features
- Specific tight tolerances only where functionally required




Tolerance Specification for Casting Projects




Tolerance over-specification increases cost without adding value. Tiegu works with a network of qualified foundries across different regions in China. We evaluate suppliers based on execution capability to help buyers specify appropriate tolerances with confidence.




Submit drawings and requirements for engineering review and factory-direct pricing.




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Submit drawings and requirements for engineering review and factory-direct pricing.




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Summary: Key Takeaways




1. ISO 8062 defines 16 tolerance grades (CT1-CT16) — CT7-CT11 typical for production castings

2. Process capability varies significantly — Investment CT4-CT8, green sand CT10-CT13

3. Tolerances increase with part size — Larger parts cannot achieve same absolute tolerances

4. Machining allowances typically 2-5mm — Ferrous castings require more than non-ferrous

5. Tighter tolerances increase cost 10-70%+ — Specify only where functionally required

6. Match tolerances to process capability — Don't specify CT7 for green sand casting

7. Clearly specify on drawings — Include tolerance grades, machining allowances, and inspection requirements




Further reading topics:

• Surface finish requirements for castings

• Machining guidelines for cast iron and ductile iron

• Casting defect types and acceptance criteria




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