ISO 2768 Tolerances Explained for CNC Machining (With Practical Examples)
Introduction
ISO 2768 is widely used on mechanical drawings to define general tolerances for dimensions and geometry. However, in CNC machining, many quality issues and misunderstandings come from unclear expectations around ISO 2768 tolerances.
In this article, we explain ISO 2768 from a CNC supplier’s practical perspective, including how it affects machining accuracy, quotation decisions, and real production risks.
What Is ISO 2768?
ISO 2768 is an international standard that defines general tolerances when individual tolerances are not specified on technical drawings. It allows engineers to avoid over-dimensioning every feature while maintaining functional manufacturing accuracy.
The standard consists of two parts:
ISO 2768-1 — General tolerances for linear and angular dimensions
ISO 2768-2 — General tolerances for geometrical features
It is commonly applied to CNC machined parts, sheet metal components, and general mechanical assemblies.
ISO 2768 Tolerance Classes Explained
ISO 2768 defines four tolerance classes:
f — Fine tolerance (precision parts)
m — Medium tolerance (standard CNC machining)
c — Coarse tolerance (fabricated parts)
v — Very coarse tolerance (non-critical structures)
From practical machining experience, ISO 2768-m is the most frequently specified class in industrial drawings.
Typical ISO 2768-m Linear Tolerances
Example reference values:
0.5 – 6 mm → ±0.1 mm
6 – 30 mm → ±0.2 mm
30 – 120 mm → ±0.3 mm
120 – 400 mm → ±0.5 mm
400 – 1000 mm → ±0.8 mm
These tolerances apply only when no explicit tolerance is stated on the drawing.
Common CNC Machining Issues Related to ISO 2768
From a supplier perspective, common situations include:
• Functional dimensions relying only on general tolerance
• Designers expecting tighter fit than the standard allows
• Assembly misalignment due to tolerance stack-up
• Critical features missing explicit specification
Clarifying these points before production helps avoid rework and delivery delays.
When ISO 2768 Should Not Be Used Alone
ISO 2768 should not be relied upon for:
• Bearing seats
• Press fits or sliding fits
• Alignment surfaces
• Reference datums
• High-precision automation parts
These features should always include explicitly defined tolerances.
How CNC Suppliers Evaluate ISO 2768 During Quotation
When reviewing drawings that reference ISO 2768, suppliers typically:
• Identify critical functional dimensions
• Assess manufacturability risks
• Confirm tolerance expectations
• Suggest clarification where necessary
This reduces production uncertainty and improves communication between engineering teams.
Frequently Asked Questions
What does ISO 2768-m mean?
It specifies medium general tolerances applied when individual tolerances are not provided.
Is ISO 2768 suitable for CNC machining?
Yes, for non-critical dimensions. Functional features should be specified separately.
Does ISO 2768 replace ISO fit standards?
No. Shaft and hole fits should follow ISO 286.
Should all dimensions have tolerances?
Critical functional dimensions should always be specified explicitly.
Conclusion
ISO 2768 simplifies drawing preparation and communication, but understanding its limitations is essential for reliable CNC production. Applying the standard correctly helps balance cost, manufacturability, and functional performance.