What is ISO 2768: A Guide to CNC Machining Tolerances

When designing a part for manufacturing, especially with CNC machining, one of the most important decisions is how tightly to control each dimension. Tolerances define how much a finished part can differ from its intended dimensions without becoming unusable.

Without tolerances, manufacturers and designers would constantly struggle with inconsistent parts and higher rejection rates. One widely used international standard that helps define these acceptable variations is ISO 2768. In this article, we’ll explore what ISO 2768 is, why it matters in CNC machining, and how to apply it effectively in your technical drawings.

What is ISO 2768?

ISO 2768 is an international standard for general tolerances. It provides guidelines on how much a part can deviate from its nominal dimensions without compromising function. Instead of specifying tolerances for every single dimension on a drawing, engineers can apply ISO 2768 to cover most features unless a specific tighter tolerance is required.

This standard is especially useful in mass production where some variation between parts is inevitable. It ensures that designers, manufacturers, and inspectors are aligned on what is acceptable and what isn’t, even if they’re working from different countries or companies.

The standard comes in two parts:

1. ISO 2768-1 covers general tolerances for linear and angular dimensions.
2. ISO 2768-2 covers geometric tolerances, such as flatness, perpendicularity, and symmetry.

By referencing ISO 2768 on a drawing, a designer avoids having to list individual tolerances for every dimension or geometric feature. Instead, a single note like “ISO 2768-mK” sets the default expectations across the board. In this example:

• “m” indicates medium precision for linear/angular dimensions.
• “K” represents a geometric tolerance level suitable for general mechanical parts.

Why Tolerances Matter in CNC Machining

To understand the importance of tolerance, let’s consider a simple scenario. Suppose you’re ordering 200 machined parts designed to be 50 mm in diameter and 100 mm long. When the batch arrives, you measure individual components and discover diameters ranging from 47 mm to 53 mm. Some parts aren’t perfectly round, and their lengths vary by a fraction of a millimeter.

Without a clear tolerance specification, you face uncertainty. Can you install a part that measures 53 mm in diameter? Does a 47 mm piece still meet your requirements? How much out-of-roundness is acceptable?

By applying ISO 2768, you remove the guesswork. The standard defines the maximum and minimum variations allowed for your dimensions and form. If each part falls within those limits, you know it will function and fit as intended. If any component exceeds the tolerance, you have an objective basis to request corrections or reject the shipment.

Next, we’ll explore ISO 2768 Part 1—linear and angular tolerances—and then move on to Part 2’s geometric controls.

ISO 2768 Part 1: Linear and Angular Dimensions

This part of the standard defines four tolerance classes for size-related dimensions:

• f – Fine
• m – Medium
• c – Coarse
• v – Very coarse

The grades cover dimension ranges from 0.5 mm up to 4000 mm. The table below shows key intervals.

Tolerance for Linear Dimensions

Basic Size Range (mm)	f (Fine)	m (Medium)	c (Coarse)	v (Very coarse)
0.5 – 3	±0.05	±0.10	±0.20	–
>3 – 6	±0.05	±0.10	±0.30	±0.50
>6 – 30	±0.10	±0.20	±0.50	±1.00
>30 – 120	±0.15	±0.30	±0.80	±1.50
>120 – 400	±0.20	±0.50	±1.20	±2.50
>400 – 1000	±0.30	±0.80	±2.00	±4.00
>1000 – 2000	±0.50	±1.20	±3.00	±6.00
>2000 – 4000	–	±2.00	±4.00	±8.00

The tighter the tolerance class, the smaller the allowed variation. However, tighter tolerances usually mean higher costs. Achieving a fine tolerance often requires better machines, more skilled labor, more inspections, and more time.

Tolerance for Angles

Angular dimensions follow the same four classes. ISO 2768 converts degree and minute units. The table shows permissible angular deviation based on the shorter side of the angle.

Shorter Side (mm)	f (Fine)	m (Medium)	c (Coarse)	v (Very coarse)
up to 10	±1°	±1°	±1°30′	±3°
10 – 50	±0°30′	±0°30′	±1°	±2°
50 – 120	±0°20′	±0°20′	±0°30′	±1°
120 – 400	±0°10′	±0°10′	±0°15′	±0°30′
over 400	±0°5′	±0°5′	±0°10′	±0°20′

Engineers select one tolerance grade for all dimensions without individual notes. For a typical machined part, the medium class (m) often balances cost and precision.

ISO 2768 Part 2: Geometrical Tolerances

Part 2 sets default limits on form and location features. It splits into three tolerance classes:

• H – Highest precision
• K – Standard precision
• L – Least precision

The tables below list the default deviations for key features.

Flatness and Straightness Tolerances

Nominal Length (mm)	H	K	L
up to 10	0.02	0.05	0.10
>10 – 30	0.05	0.10	0.20
>30 – 100	0.10	0.20	0.40
>100 – 300	0.20	0.40	0.80
>300 – 1000	0.30	0.60	1.20
>1000 – 3000	0.40	0.80	1.60

Perpendicularity Tolerances

ISO 2768 does not list a separate table for parallelism. Instead, it uses whichever is larger: the size tolerance from Part 1 or the flatness/straightness tolerance from Part 2.

Shorter Side (mm)	H	K	L
up to 100	0.20	0.40	0.60
>100 – 300	0.30	0.60	1.00
>300 – 1000	0.40	0.80	1.50
>1000 – 3000	0.50	0.80	2.00

Symmetry Tolerances

Symmetry tolerance checks whether two features mirror across a datum plane.

Nominal Length (mm)	H	K	L
up to 100	0.5	0.6	0.6
>100 – 300	0.5	0.6	1.0
>300 – 1000	0.5	0.8	1.5
>1000 – 3000	0.5	1.0	2.0

Run-out (Circular) Tolerances

Run-out controls the total wobble of a rotating surface. It measures deviation as the part spins around a datum axis.

Feature	H	K	L
Circular run-out	0.1	0.2	0.5

BOYI TECHNOLOGY specializes in precision CNC machining services that align with ISO 2768 standards. Our experienced engineers review your drawings to ensure proper tolerance classes.

If your design calls for finer or custom tolerances, our team works with you to update machining processes and inspection plans. Upload your CAD files to get an instant quote that reflects your tolerance requirements.

Choosing Your ISO 2768 Class

A project starts by defining critical features. Engineers ask:

1. Which dimensions link parts together?
2. Which surfaces seal against fluids?
3. Which features mainly support strength?
4. Which surfaces drive bearings or shafts?

Once critical features are clear, engineers pick the tolerance classes:

Tolerance Need	Part 1 Class	Part 2 Class
Extremely tight, critical fit	f (fine)	H (high)
Typical production balance	m (medium)	K (medium)
Low-cost, non-critical parts	c (coarse)	L (low)
Very loose for large parts	v (very coarse)	—

How to Use ISO 2768 in Your Engineering Drawings

When preparing a CNC machining parts drawing, you can simplify your documentation by adding a note such as:

“Unless otherwise specified, all dimensions follow ISO 2768-mK.”

This means:

• Use medium tolerance class for dimensions and angles (from Part 1).
• Use K-level geometric tolerances (from Part 2).

However, not all features can share the same tolerance level. For example:

• Mounting holes may need fine tolerances for alignment.
• Ribs or non-critical supports can be assigned coarse tolerances.
• Reference surfaces or datums should have tighter controls for accuracy.

If a specific dimension needs a tighter tolerance, the designer adds a note directly. For example, a critical hole can show “Ø10 ±0.02”. That exception takes precedence.

Comparing ISO 2768 with GD&T

Geometric Dimensioning and Tolerancing (GD&T) can specify form, orientation, location, and run-out in more detail. ISO 2768 Part 2 overlaps with basic GD&T symbols like flatness and perpendicularity. However, GD&T offers additional controls:

• True position tolerance
• Profile of surface
• Concentricity
• Circularity

When parts need advanced control, designers may combine ISO 2768 with GD&T notes. In that workflow:

• Keep the drawing logical by grouping notes.
• Use ISO 2768 for general tolerances.
• Apply GD&T symbols on critical features.

ISO 2768 works well for simple tolerance control. When you need to control form, orientation, and location precisely, you may use GD&T (Geometric Dimensioning and Tolerancing). GD&T follows ASME Y14.5 or ISO 1101 for feature controls, datums, and tolerance zones.

Another approach is Model Based Definition (MBD). With MBD, you embed all tolerance info directly into the 3D CAD model. That method reduces reliance on 2D drawings. Some shops accept MBD models with full tolerance data. However, 2D drawings remain the most widely used authority in many industries.

When to Override ISO 2768

ISO 2768 covers most straightforward dimensions and features. Designers should override defaults when:

• A press fit or slip fit needs precise clearance.
• A sealing interface requires a high-precision finish.
• A tight bearing fit demands special roundness control.
• A safety-critical component needs verified geometry.

In those cases, add explicit tolerances next to the dimension. Always clarify which note applies first: the specific note or the general ISO 2768 class.

What ISO 2768 Doesn’t Cover

ISO 2768 is a great starting point, but it doesn’t replace all dimensioning needs. It doesn’t apply to:

• Threads
• Fits and clearances (use ISO 286 for those)
• Surface roughness
• Tolerances tighter than the specified class

Also, some industries or applications may require a different standard, such as ASME Y14.5 or specific GD&T symbols.

Conclusion

ISO 2768 is an essential tool for modern engineering. It brings clarity, consistency, and cost-efficiency to CNC machining by simplifying how tolerances are defined. Whether you’re working with a global supplier or a local machine shop, applying ISO 2768 properly ensures everyone understands what’s acceptable.

If you’re interested in advanced tolerance control, dive deeper into GD&T or consider using Model-Based Definition (MBD) where your CAD file carries all the tolerance data.

Want help with your CNC machining tolerances? BOYI TECHNOLOGY is here to help. We follow international standards like ISO 2768 to ensure consistent quality and performance across all your parts. Reach out today to learn more about our precision CNC machining services.