Understanding CNC machining tolerances helps engineers produce parts that fit together correctly, perform reliably, and avoid unnecessary manufacturing costs.

In this guide, we’ll explain what machining tolerances are, why they matter, common industry standards, and how choosing the correct tolerance can reduce production costs without compromising quality.

When designing or ordering a machined component, one of the most important specifications you’ll encounter is the machining tolerance. Tolerances determine how much variation is acceptable from the nominal dimensions shown on an engineering drawing.

What Are CNC Machining Tolerances?

A machining tolerance defines the allowable variation from a specified dimension.

For example:

Nominal Diameter
20.00 mm

Tolerance
±0.05 mm

This means any finished part measuring between:

19.95 mm
20.05 mm

would be considered acceptable.

Without tolerances, it would be impossible—and unnecessary—to manufacture every component to an exact mathematical size.

Why Are Tolerances Important?

Every manufacturing process introduces small variations.

These can be caused by:

Machine accuracy
Tool wear
Material movement
Cutting forces
Temperature changes
Operator setup
Measurement uncertainty

Tolerances define what level of variation is acceptable while ensuring the part still functions correctly.

They are essential for:

Assembly
Interchangeability
Product reliability
Safety
Consistent quality

Understanding Nominal Size and Tolerance

Consider a shaft specified as:

25.00 ±0.02 mm

Acceptable sizes are:

Maximum: 25.02 mm
Minimum: 24.98 mm

Any measurement outside these limits would be rejected.

Common CNC Machining Tolerances

Most CNC machining companies work to standard tolerances unless a drawing specifies otherwise.

Typical achievable tolerances include:

Feature	Typical Tolerance
Linear dimensions	±0.10 mm
Precision machining	±0.05 mm
High precision	±0.02 mm
Grinding operations	±0.005 mm
Ultra precision applications	±0.001–0.003 mm

Actual capability depends on:

Machine tool quality
Material
Component size
Geometry
Inspection requirements

General Tolerances vs Critical Dimensions

Not every feature requires extreme precision.

Good engineering drawings identify:

General Dimensions

These use default tolerances such as:

±0.1 mm

or ISO 2768 standards.

These dimensions are usually:

Overall lengths
External profiles
Non-critical holes
Cosmetic features

Critical Dimensions

Critical dimensions directly affect:

Bearing fits
Seal performance
Thread engagement
Assembly
Concentricity
Alignment

These often require tighter tolerances such as:

±0.02 mm
±0.01 mm
or even tighter

ISO 2768 General Tolerances

Many engineering drawings use ISO 2768 for unspecified dimensions.

This avoids placing individual tolerances on every dimension.

Typical drawing note:

ISO 2768-mK

This tells the manufacturer which tolerance class applies unless otherwise stated.

Benefits include:

Cleaner drawings
Faster inspection
Reduced programming time
Lower manufacturing costs

Geometric Tolerances (GD&T)

Size alone doesn’t guarantee a part will function correctly.

Geometric tolerancing controls:

Flatness
Perpendicularity
Parallelism
Concentricity
Position
Runout
Circularity

For example:

A hole may be the correct diameter but positioned incorrectly.

GD&T ensures the feature is both the correct size and in the correct location.

Why Tight Tolerances Increase Cost

Many designers assume tighter tolerances automatically produce a better product.

In reality, unnecessarily tight tolerances often increase manufacturing costs significantly.

Reasons include:

Slower Machining

The machine may require:

Smaller depth of cut
Reduced feed rates
Additional finishing passes

More Inspection

Tight tolerances require:

Additional measurement
More quality control
Higher accuracy inspection equipment

Increased Scrap Risk

The smaller the allowable variation:

the higher the chance of rejected parts

More Tool Wear

Precision finishing often increases:

cutting time
tool wear
tooling costs

Specialist Equipment

Very tight tolerances may require:

Grinding
Jig grinding
Honing
Wire EDM
Climate-controlled inspection

Typical Tolerance Requirements by Industry

General Engineering

Typically:

±0.10 mm

Suitable for:

Brackets
Covers
Mounting plates

Automotive

Commonly:

±0.02–0.05 mm

Required for:

Engine components
Gearbox parts
Hydraulic systems

Aerospace

Often:

±0.01 mm or tighter

Combined with extensive inspection and material traceability.

Medical

Medical components frequently require:

Extremely tight tolerances
Excellent surface finishes
Complete inspection records

Oil & Gas

Precision sealing surfaces often require:

Tight diameters
Controlled surface finish
Excellent concentricity

Factors That Affect Achievable Tolerances

Several factors influence machining accuracy.

Material

Different materials behave differently.

Examples:

Aluminium can move during machining.
Stainless steel generates heat.
Plastics expand significantly with temperature.
Titanium experiences spring-back.

Component Size

Maintaining ±0.01 mm over:

20 mm

is much easier than maintaining it across:

800 mm.

Machine Condition

Modern CNC machines offer:

Better thermal stability
Higher spindle accuracy
Improved repeatability

Older equipment may not consistently achieve the same precision.

Cutting Strategy

Toolpaths, speeds and feeds all influence:

Surface finish
Deflection
Final accuracy

Inspection Methods

Manufacturers verify tolerances using equipment such as:

Digital callipers
Micrometers
Bore gauges
Height gauges
Thread gauges
Coordinate Measuring Machines (CMM)
Vision inspection systems
Surface roughness testers

Critical components often receive full inspection reports before delivery.

How Designers Can Reduce Manufacturing Costs

One of the easiest ways to reduce machining costs is to specify only the tolerances that truly matter.

Good design practice includes:

Tight tolerances only where function requires
Standard hole sizes
Standard thread sizes
Avoid unnecessary GD&T
Use general tolerances where appropriate
Discuss critical features with your machining supplier early

This approach often reduces machining time while maintaining product performance.

Frequently Asked Questions

What is a standard CNC machining tolerance?

Most CNC machined components are manufactured to around ±0.10 mm unless tighter tolerances are specified.

Can CNC machines achieve ±0.01 mm?

Yes. Modern CNC equipment can achieve ±0.01 mm on suitable features when the machine, material, tooling and inspection process are all optimised.

Why do tighter tolerances cost more?

Tighter tolerances increase machining time, require more precise inspection, lead to greater tool wear, and raise the likelihood of rejected parts.

What is ISO 2768?

ISO 2768 is an international standard that defines general tolerances for dimensions and geometric features when no individual tolerance is specified on a drawing.

Should every dimension have a tight tolerance?

No. Applying unnecessarily tight tolerances increases manufacturing costs without improving performance. Only dimensions that affect the function, fit or safety of the part should have close tolerances.

Conclusion

CNC machining tolerances are fundamental to producing reliable, high-quality components. While modern CNC equipment is capable of exceptional accuracy, specifying tighter tolerances than necessary can significantly increase production costs and lead times.

By understanding the difference between general and critical tolerances, making appropriate use of standards such as ISO 2768, and applying GD&T where it adds value, engineers can achieve the right balance between performance, manufacturability and cost.

Working closely with an experienced CNC machining supplier during the design stage can help identify where precision is essential—and where more economical tolerances can be used—resulting in better parts, faster production, and lower overall manufacturing costs.