ISO 2768 Tolerance Chart for CNC Machining (2026 Guide)

Introduction

In CNC machining, tolerances directly influence manufacturing cost, machining strategy, inspection requirements, and assembly performance. However, not every dimension on a drawing requires an individually specified tolerance.

This is where ISO 2768 becomes important.

ISO 2768 is one of the most widely used international standards for general tolerances in mechanical manufacturing. Instead of applying custom tolerances to every feature, engineers can use ISO 2768 to define acceptable dimensional variation for unspecified dimensions.

For procurement teams, ISO 2768 simplifies supplier communication and reduces drawing complexity. For machinists and manufacturing engineers, it creates a standardized expectation for machining accuracy without overcomplicating production.

This guide explains how ISO 2768 works, how tolerance classes are interpreted in CNC machining, and how engineers can use the standard effectively to balance precision and manufacturing cost.

What Is ISO 2768?

ISO 2768 is an international tolerance standard used for general machining tolerances where individual tolerances are not specifically indicated on technical drawings.

The standard helps avoid excessive tolerance specifications while maintaining acceptable manufacturing consistency.

ISO 2768 is commonly divided into two sections:

• ISO 2768-1 → General tolerances for linear and angular dimensions
• ISO 2768-2 → General geometrical tolerances

In CNC machining, ISO 2768-1 is used more frequently because it applies directly to standard dimensional tolerances.

Rather than assigning tolerances to every dimension manually, engineers can simply specify a tolerance class on the drawing.

For example:

ISO 2768-mK

This notation immediately communicates the general tolerance requirements for the entire part unless otherwise specified.

Why ISO 2768 Matters in CNC Machining

Without a standardized tolerance system, engineering drawings can become unnecessarily complex.

Over-dimensioning and over-tolerancing often lead to:

• Higher machining cost
• Slower production speed
• Increased inspection requirements
• Longer lead times

ISO 2768 helps manufacturers avoid these problems by applying reasonable default tolerances to non-critical dimensions.

This creates several advantages:

For Engineers

• Cleaner technical drawings
• Faster design workflow
• Easier communication with suppliers

For Procurement Teams

• More consistent quotations
• Reduced risk of misunderstanding
• Easier supplier comparison

For CNC Manufacturers

• More efficient machining strategy
• Better process consistency
• Reduced unnecessary precision requirements

In practice, ISO 2768 improves manufacturability by aligning engineering intent with realistic machining capability.

ISO 2768 Tolerance Classes Explained

ISO 2768 defines several tolerance classes depending on required precision.

The most common classes include:

In CNC machining, the most commonly used class is:

ISO 2768-m

because it provides a practical balance between machining cost and dimensional accuracy.

For higher precision assemblies, engineers may specify:

ISO 2768-f

However, tighter tolerance classes increase machining complexity and inspection requirements.

ISO 2768 Linear Tolerance Chart

Below is a simplified reference chart based on ISO 2768-1 for linear dimensions.

These tolerances apply only when individual tolerances are not specified separately on the drawing.

Critical dimensions should still use dedicated tolerancing.

What Does ISO 2768-mK Mean?

Many engineering drawings include combined notations such as:

ISO 2768-mK

This notation combines:

• m → General dimensional tolerance class
• K → Geometrical tolerance class

In practice:

• “m” controls unspecified linear dimensions
• “K” controls general geometrical tolerances such as flatness and perpendicularity

This combined notation simplifies drawings while maintaining acceptable manufacturing standards.

When ISO 2768 Should NOT Be Used

One of the biggest engineering mistakes is assuming ISO 2768 can replace all functional tolerances.

It cannot.

ISO 2768 is intended for non-critical dimensions only.

Critical features should always have explicitly defined tolerances, especially for:

• Bearing fits
• Sealing surfaces
• Precision assemblies
• Thread interfaces
• Datum-related features

Functional dimensions should never rely solely on general tolerance standards.

Experienced engineers apply precision strategically rather than uniformly.

ISO 2768 and CNC Machining Cost

Tolerance requirements directly affect manufacturing cost.

Tighter tolerances require:

• Slower machining parameters
• More stable fixturing
• Additional inspection
• Higher scrap risk
• More experienced operators

This is why unnecessarily tight tolerances can dramatically increase production cost without improving part functionality.

ISO 2768 helps reduce this issue by allowing standard tolerances on non-functional dimensions.

In many projects, using ISO 2768 appropriately can reduce both machining time and inspection cost.

Common Mistakes Engineers Make with ISO 2768

Even though ISO 2768 is widely used, several common mistakes still appear in RFQs and production drawings.

Applying Tight Tolerances Everywhere

Many inexperienced designers over-tolerance entire parts instead of identifying truly functional dimensions.

This often creates unnecessary manufacturing cost.

Missing Functional Tolerances

Some drawings rely entirely on ISO 2768 without defining critical assembly dimensions separately.

This creates risk during assembly and inspection.

Confusing ISO 2768 with GD&T

ISO 2768 is not a replacement for GD&T.

Complex geometric relationships may still require dedicated GD&T controls.

Ignoring Supplier Capability

Not all suppliers can consistently achieve fine tolerance classes across complex geometries or difficult materials.

Tolerance strategy should match realistic machining capability.

How Experienced CNC Suppliers Evaluate ISO 2768 Drawings

Professional CNC suppliers do not simply read tolerance notes and start machining.

They review:

• Functional dimensions
• Critical assemblies
• Material behavior
• Machining accessibility
• Inspection feasibility

If tolerance requirements appear unrealistic or unnecessarily restrictive, experienced suppliers typically provide DFM feedback before production begins.

This early engineering communication often prevents future production problems.

Why Buyers Choose Kachi Precision

At Kachi Precision Manufacturing, we help customers balance precision requirements with practical manufacturability.

Our engineering team supports:

• ISO 2768 drawing review
• DFM optimization
• Tolerance risk analysis
• Process planning for precision features
• Stable CNC machining for prototype and production parts

We focus on reducing unnecessary machining complexity while maintaining the functional requirements that matter most.

FAQ

What does ISO 2768-mK mean?

ISO 2768-mK combines general dimensional and geometrical tolerances.

• “m” = medium tolerance class for dimensions
• “K” = general geometrical tolerance class

It is commonly used on CNC machining drawings to simplify unspecified tolerances.

Is ISO 2768 suitable for precision parts?

Partially.

ISO 2768 works well for non-critical dimensions, but precision features such as bearing fits or sealing surfaces should always have individually specified tolerances.

What is the most common ISO 2768 class in CNC machining?

ISO 2768-m is the most commonly used class because it balances machining cost and dimensional accuracy for standard CNC parts.

Does tighter tolerance increase machining cost?

Yes.

Tighter tolerances require slower machining, additional inspection, and more stable setups, which increases both production time and manufacturing cost.

Can ISO 2768 replace GD&T?

No.

ISO 2768 covers general tolerances only. Complex geometric relationships may still require GD&T controls such as position, runout, or profile tolerances.

Conclusion

ISO 2768 remains one of the most practical and widely used tolerance standards in CNC machining because it simplifies drawings and improves manufacturing efficiency.

However, effective use of ISO 2768 requires engineering judgment.

The goal is not to maximize precision everywhere. The goal is to apply tolerances intelligently based on actual functional requirements.

For engineers and procurement teams, understanding how ISO 2768 affects machining cost, inspection strategy, and supplier capability can significantly improve sourcing decisions and production consistency.

Call to Action

If you are preparing CNC machining drawings and want help evaluating tolerance strategy, manufacturability, or ISO 2768 application, early engineering review can help reduce cost and avoid production issues.

At Kachi Precision Manufacturing, we support customers with professional DFM analysis, tolerance evaluation, and precision machining solutions for both prototypes and production orders.

Send your drawings today and receive engineering feedback within 24 hours.