Aluminum Casting Tolerances Explained: CT6 to CT8 per ISO 8062 (Design Engineer's Guide)

Introduction

Dimensional tolerances are where casting design meets manufacturing reality. Specify too tight and you force unnecessary machining, increasing cost and lead time. Specify too loose and the part may not fit or function correctly in its assembly.

For aluminum castings, tolerance capability depends on the casting process, part geometry, alloy, and thermal behavior during solidification. The international standard ISO 8062-3 provides a systematic framework for specifying casting tolerances using Casting Tolerance (CT) grades. This guide explains how the system works and how to apply it effectively when sourcing aluminum castings.

What Is ISO 8062 and How Do CT Grades Work?

ISO 8062-3 (Geometrical Product Specifications �?Dimensional and geometrical tolerances for moulded parts) defines 16 tolerance grades from CT1 to CT16. Lower CT numbers mean tighter tolerances.

For aluminum castings, the practical range is CT6 to CT10, depending on the process:

These values increase with nominal dimension. A 250 mm dimension at CT8 has a wider tolerance band than a 50 mm dimension at CT8. The standard provides complete tables for all dimension ranges.

ISO 8062 Tolerance Table (Excerpt)

The following table shows tolerance values in millimeters for common CT grades across different nominal dimension ranges:

Values represent total tolerance band (plus and minus combined). For example, CT8 at 100�?60 mm nominal = 1.40 mm total tolerance, which can be expressed as �?.70 mm if centered.

Tolerance Capability by Casting Process

Each casting process has a natural tolerance capability determined by its physics:

High-Pressure Die Casting: CT6鈥揅T7

Die casting achieves the tightest as-cast tolerances because:

• •Steel dies are precision machined to �?.02 mm
• •High injection pressure ensures complete fill and consistent reproduction
• •Rapid solidification minimizes thermal distortion
• •Short cycle times mean less thermal variation between shots

Typical achievable tolerance: �?.1 to �?.3 mm for most features, depending on dimension and geometry.

However, die casting has a caveat: parting line mismatch and ejector pin witness marks can add �?.1�?.2 mm to local tolerances. These must be considered in GD&T specifications.

Gravity Casting (Permanent Mold): CT7鈥揅T8

Gravity casting offers good dimensional control with some additional considerations:

• •Metal molds provide repeatable cavity dimensions
• •Slower solidification allows more thermal contraction variation
• •Manual or semi-automated pouring introduces slight fill variation
• •Larger parts experience more thermal gradient effects

Typical achievable tolerance: �?.3 to �?.5 mm for most features at Bohua.

At Bohua, our automated gravity casting lines with controlled pouring and consistent mold temperature management achieve the tighter end of this range. For critical dimensions, we often achieve CT7 performance.

Low-Pressure Casting: CT7鈥揅T9

Low-pressure casting falls between gravity and sand casting:

• •Controlled bottom-fill reduces turbulence but introduces riser-related variation
• •Excellent for cylindrical and axially symmetric parts
• •Pressure control helps with consistent fill but adds complexity

Typical achievable tolerance: �?.3 to �?.7 mm depending on geometry.

Sand Casting: CT9鈥揅T12

Sand casting has the widest tolerances due to:

• •Expendable molds with sand surface irregularity
• •Core positioning variation
• •Higher thermal contraction in sand (slower cooling)
• •Pattern wear over time

Typical achievable tolerance: �?.7 to �?.0 mm depending on dimension and complexity.

Factors That Affect As-Cast Tolerances

Beyond process selection, several design and manufacturing factors impact achievable tolerances:

1. Part Size

Larger parts experience more total thermal contraction. A 500 mm dimension contracts more than a 50 mm dimension, so the absolute tolerance must be wider. This is already built into the ISO 8062 table, but designers should understand why: thermal contraction is proportional to dimension length.

2. Wall Thickness Variation

Sections with large thickness differences cool at different rates, creating internal stress and distortion. A 3 mm wall next to a 15 mm boss will distort more than a uniform 6 mm wall. Design for as uniform a wall thickness as practical.

3. Parting Line Location

Features that cross the mold parting line are subject to mold alignment tolerance (typically �?.1�?.3 mm in addition to the casting tolerance). Where possible, keep tight-tolerance features on one side of the parting line.

4. Draft Angles

Casting draft (typically 1�?�?for permanent mold, 0.5�?�?for die casting) means that as-cast surfaces are not perfectly parallel or perpendicular. This adds effective dimensional variation on drafted surfaces, especially across long features.

5. Core Position

If the part uses sand cores (common even in permanent mold casting for internal passages), core positioning accuracy becomes a tolerance factor. Core-to-mold relationships typically add �?.3�?.5 mm of variation.

6. Mold Temperature and Wear

Over a production run, mold temperature stabilizes but never reaches perfect steady state. Additionally, mold surfaces gradually wear, shifting dimensions. Good foundries monitor this and maintain molds on schedule. At Bohua, we track dimensional trends by shot count and perform preventive maintenance accordingly.

How to Specify Tolerances on Your Casting Drawing

Best Practice: Tiered Tolerance Approach

Not every dimension on a casting needs the same tolerance. Using a tiered approach reduces cost and manufacturing complexity:

Tier 1 �?Machined Features (tightest)

Functional interfaces, bearing bores, sealing faces. These are CNC machined after casting, achieving �?.02�?.05 mm. Leave 1.5�? mm machining stock on as-cast dimensions.

Tier 2 �?Critical As-Cast Features (medium)

Mounting surfaces, location datums, mating interfaces that will not be machined. Specify per ISO 8062 CT grade appropriate to the process (CT7 for gravity casting, CT6 for die casting).

Tier 3 �?Non-Critical As-Cast Features (widest)

Non-functional surfaces, cosmetic areas, general envelope dimensions. Use default CT grade for the process or specify "as-cast per ISO 8062 CT8" as a general note.

Drawing Notes Example

A well-specified casting drawing might include:

• •General tolerance note: "Unless otherwise specified, casting tolerances per ISO 8062-3, CT8"
• •Machined features: Dimensioned with standard ISO 2768 machining tolerances and machining symbol
• •Critical as-cast features: Individually toleranced where needed (e.g., 85.0 �?.5)
• •Machining stock: Indicated on sections to be machined (e.g., "Machine stock: 2.0 mm per side")

Common Over-Tolerancing Mistakes

Mistake 1: Applying machining tolerances to as-cast surfaces

If a non-machined wall dimension is specified as 8.0 �?.1 mm, no casting process can hold that. The part will either be rejected constantly or the supplier will add machining (and cost) without telling you.

Mistake 2: Tolerancing every dimension

Over-dimensioning creates conflicts and makes inspection reports unnecessarily complex. Tolerance only the dimensions that matter functionally.

Mistake 3: Not specifying datum structure

Without defined datums, the foundry and the buyer may measure from different references, creating apparent non-conformances that are actually measurement disagreements.

Mistake 4: Ignoring draft angle effects

A 200 mm deep pocket with 2�?draft means the bottom is 14 mm wider than the top. If the drawing dimensions the pocket width without referencing a specific depth, the measurement is ambiguous.

Tolerance vs Cost: The Real Trade-Off

Tighter tolerances always cost more. Here is a rough guide to the cost impact:

The key insight: every dimension you over-tolerance either adds machining cost or adds scrap cost. A part with 50 toleranced dimensions at CT7 will have a higher rejection rate than the same part with 5 critical dimensions at CT7 and the rest at CT8.

How Bohua Manages Dimensional Quality

At Bohua Casting, our dimensional quality system includes:

Process Control

• •Mold temperature monitoring and documentation per production run
• •Automated pouring systems for consistent fill behavior
• •Preventive mold maintenance by shot count, not just visual condition
• •First-article dimensional validation before production release

Measurement Capability

• •CMM (Coordinate Measuring Machine) �?NANO Metrology CMM for 3D dimensional verification of critical features and GD&T relationships
• •In-process gauging �?Go/no-go gauges and checking fixtures at workstations for real-time monitoring
• •Contour scanning �?Profile measurement for complex curved surfaces

Reporting

• •First article inspection reports with full dimensional layout
• •Statistical process capability studies (Cpk) for critical dimensions on serial programs
• •Dimensional trend monitoring across production runs to detect and correct drift early

Practical Checklist for Design Engineers

When specifying tolerances for an aluminum casting:

• •Identify which dimensions are truly functional �?tolerance only those
• •Use ISO 8062 CT grades appropriate to your chosen casting process
• •Add machining stock (1.5�? mm) for features requiring tight tolerances
• •Define clear datums for measurement reference
• •Account for draft angles in your tolerance analysis
• •Discuss tolerance expectations with your foundry during DFM review �?before tooling release
• •Consider total part cost (casting + machining + inspection) not just casting cost

Conclusion

Aluminum casting tolerances are not arbitrary numbers �?they reflect the physics of solidification, thermal contraction, and tooling precision. The ISO 8062 CT grade system provides a clear, standardized framework for specifying what your part needs without over-constraining what the process can deliver.

The best outcomes happen when design engineers and foundry engineers collaborate early. At Bohua, our DFM review process specifically addresses tolerance feasibility, machining strategy, and datum structure before any tooling is cut. This prevents costly surprises and ensures that the parts you receive match the parts you designed.

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Related Resources

• •Casting Capabilities �?Review Bohua's machining, QC, and dimensional control resources
• •How to Choose a Casting Supplier in China �?Evaluate process control and measurement capability
• •Gravity Casting vs Die Casting �?Match tolerance expectations to the right process