ISO 2768 Tolerance Standards for CNC Machining: The Complete Guide

As an engineer at DSR Company, one of the tables I use most frequently is ISO 2768. It can be said that it is the most important “default protocol” in CNC machining. Without it, every dimension of all my drawings needs to be marked with tolerances, which not only wastes my design time but also increases the machining cost of the project. And I believe that many of you are here because you want to know various contents about ISO 2768. Don’t worry, this article will tell you all the knowledge and data you want to know, so that you can become familiar with my “martial arts secrets” in one reading.

What is ISO 2768?

ISO 2768 is an international standard developed by the International Organization for Standardization (ISO) on the basis of unspecified tolerances. It is a default consensus between the designer and the manufacturer. In fact, many times, your mechanical drawings do not need to be marked with separate tolerances for each size, such as ±0.01mm. For these unmarked dimensions, ISO 2768 specifies a default precision range. Therefore, as long as there are no special requirements on the drawings, the dimensions of the parts subsequently processed by cutting will be deemed to be qualified as long as they are within the range specified by ISO 2768.

Breakdown of ISO 2768 Parts

The ISO 2768 part standard is divided into two separate parts, which give you a tolerance range for dimensions and shape positions.

1. ISO 2768-1: Linear and Angular Dimensions

ISO 2768-1 specifies the tolerances for linear dimensions such as length, diameter, chamfer radius, and angle of parts. It is divided into four classes according to the accuracy level:

• f (Fine): Precision class(high-precision machining)
• m (Medium): Medium class
• c (Coarse): Coarse class
• v (Very coarse): The coarsest class

Among them, Class m is commonly used as a general tolerance class for many CNC-machined metal parts when tighter tolerances are not functionally required.

Table 1: Permissible Deviations for Linear Dimensions

The most commonly used table, mainly used for shape length and width, step height, inner hole diameter, etc. (excluding chamfers).

NOTE:

1. Since nominal feature lengths only start at 0.5 mm, you will need to indicate any nominal dimensions less than 0.5 mm next to the corresponding dimensions.
2. Most CNC precision parts (aluminum alloy parts, steel machining parts) look at the m column by default. If it is a sheet metal or cast part, column c may be used.

Table 2: Permissible Deviations for External Radii and Chamfer Height

This table is specifically for chamfers or radius marked on drawings.

NOTE:

When you design parts with external fillets or chamfers, you must specify acceptable deviations from these dimensions to ensure the part will fit correctly and function as intended.

Table 3: Permissible Deviations for Angular Dimensions

NOTE:

Angular tolerance is based on the length of the short side.

2. ISO 2768-2：Geometrical Tolerances

This part mainly stipulates the uninjected shape and position tolerances of parts, including straightness, flatness, verticality, symmetry and circular runout. It is divided into three classes:

• H (High): Precision class
• K (Medium): Mediumclass
• L (Low): Rough class

Here, I want to talk about an industry practice. In the title block of the drawing, the most common form of annotation is “ISO 2768-mK”. This annotation indicates that dimensional tolerances for the part follow ISO 2768-1 Class m, while geometric tolerances follow ISO 2768-2 Class K.

Important: ISO 2768-2:1989 has been withdrawn and replaced by ISO 22081:2021. The H, K, and L tables below are retained as a reference for interpreting legacy engineering drawings.

Table 4: General Tolerances on Straightness and Flatness

This table can help you measure whether a surface is flat or a line is straight.

NOTE:

These tolerances ensure that features such as the edges or surfaces of the part remain aligned and functional even with slight deviations.

Table 5: General Tolerances on Perpendicularity

The following table can help you to check to see if the two faces are 90 degrees apart. You should be aware that perpendicularity is a geometric tolerance that allows to control the two features to be at a right angle to each other. Excessive deviation in verticality can cause misassembly, misalignment and eventually result in failure of the part to function.

NOTE:  

You need to use the longer side as the base length.

Table 6: General Tolerances on Symmetry

This table is used for keyways, symmetrical hole positions, etc.

NOTE:

Symmetry is also an important geometric feature of the part, which ensures that the features of the part are evenly distributed around the central axis. This tolerance is important for parts whose performance depends on even distribution.

Table 7: General Tolerances on Circular Run-out

This table is mainly used for rotary parts, such as stainless steel shaft, drive shaft and aluminum gear.

NOTE: 

1. Coaxiality is not a specific table in ISO 2768-2. Therefore, if there is no special annotation, the unmarked tolerance value of coaxiality can directly refer to the value of circular runout.
2. Cylindricity is also not specifically defined and is usually controlled by the combined error of roundness, straightness, and parallelism.

What is the Difference Between ISO 2768-1 and ISO 2768-2?

This is a place where many novice engineers are often confused. To say it as simply as possible, ISO 2768-1 is for size, and ISO 2768-2 is for shape.The most important difference between the two is the difference in focus.

ISO 2768-1 specifies permissible deviations in feature size, position spacing, and angles. It is suitable for parts with clear design requirements for dimensional values and specific tolerance limits are set for them.

ISO 2768-2 involves geometric characteristics such as flatness, straightness, verticality, symmetry and circular runout. It specifies the allowable deviations of part geometry.

ISO 2768 Latest Status in 2026

As of September 2026, ISO 2768-1:1989 is still a current published standard. It is primarily used to specify general tolerances for linear dimensions and angular dimensions that are not individually toleranced on engineering drawings.

However, ISO is already advancing a new version of ISO 2768. The new ISO 2768 2nd edition has entered the final publication stage and will subsequently replace the current ISO 2768-1:1989.

This means that you may currently encounter two situations simultaneously:

• In existing drawings and old projects, ISO 2768-1:1989 is still widely used, as well as general tolerance classes such as f, m, c, v.
• For newly designed part drawings, in addition to paying attention to the official release status of the new version of ISO 2768, you need to determine which version is actually referenced in the drawings.

It can be said that for CNC machining, the core role of ISO 2768 has not changed. Through general tolerances, you can avoid individually marking tolerances for each non-critical dimension on drawings, thereby simplifying drawings and improving manufacturing communication efficiency.

But now, engineers should not simply see “ISO 2768” and assume its meaning is completely consistent. Before actual production, you should still confirm the ISO standard version referenced in the drawings. Similarly, you should also confirm whether there are individually marked critical dimensions or geometric tolerances on the drawing.

Is ISO 2768-2 Still Valid?

Strictly speaking, ISO 2768-2:1989 has been withdrawn and is no longer part of the current valid ISO general geometric tolerance standards. After ISO 2768-2 was withdrawn, the related general geometric specifications have been succeeded by ISO 22081:2021.

However, this does not mean that the ISO 2768-2 annotations on old drawings can be ignored. In fact, in the mechanical processing industry, a large number of historical drawings, long-term production parts and old projects still use annotations such as ISO 2768-mK, ISO 2768-fH.

For such old drawings, you still need to understand and evaluate according to the standards originally specified on the drawings. For new design drawings, I do not recommend that you continue to use ISO 2768-2 as the current geometric tolerance standard, but should refer to ISO 22081:2021 according to the current ISO GPS system.

Why Use ISO 2768 Tolerance Standards for CNC Machining?

Regarding this issue, my perspective can be divided into the following key points.

1. Simplifies Drawings & Design

This is our most direct feeling as engineers. Just imagine that if your drawing has a tolerance value of ±0.1mm for every dimension, then the drawing will appear dense and cluttered, which can easily lead to the oversight of the core dimension data.

Then, after using ISO 2768, your drawings only need to retain the tolerances of the key fitting dimensions, and all other dimensions only need to be written “General Tolerance ISO 2768-mK” to summarize them all. In this case, doesn’t the whole drawing look neater? It also allows processors to see at a glance which areas are most important.

2. Reduces Cost

Reduce costs, including labor costs and time costs. Without ISO 2768, machinists would be careful about some insignificant parts of the part because there are no clear tolerance requirements on the drawings, thereby extending the machining time.

But with ISO 2768, processors only need to control these non-critical dimensions within the specified range, and do not need to use finishing equipment for detailed processing. It can be said that it is precisely because of ISO 2768 that the factory’s scrap rate and processing time can be significantly reduced, thereby directly reducing the unit price of parts.

3. Clarifies Acceptance Criteria

For quality inspection, ISO 2768 is also very critical. Since both the buyer and the seller have acquiesced in this technical agreement, when the parts produced deviate, everyone only needs to take out the caliper to measure the data and check the corresponding table. Then it is very quick to judge whether the product is qualified or unqualified, and the attribution of fault is also very clear. There will also be no disputes between the two parties due to a lack of basis.

In addition, here is a reminder to the quality inspectors: when conducting quality inspections on parts, you must also strictly follow the ISO 2768 standards to ensure that there are no quality problems when the parts leave the factory.

4. Reduces Design Oversights

Engineers design parts drawings with hundreds of dimensions, and it is inevitable that a few tolerances will be missed. If there are no universal tolerances, these dimensions without marked tolerances may be made into various specifications, resulting in a sharp increase in the scrap rate. However, with ISO 2768, at least the tolerances of the parts can be kept within acceptable limits.

5. Improves Interchangeability & Consistency

For manufacturers who need to produce parts in batches, ensuring interchangeability and consistency of parts is critical. You know, the premise of interchangeability is that all parts are within a controllable range. If there is no standard tolerance, then among these parts, it is possible that some parts have large tolerances and some parts have small tolerances. When they are assembled together, the consistency of the product is poor.

ISO 2768 specifies a maximum allowable range for the industry. So even if parts are made in different factories or at different times, the non-critical dimensions of these parts are within a standard range, ensuring their interchangeability.

6. Global Interchangeability

This is easier to understand. Because ISO 2768 is an international tolerance standard, whether your drawings are sent to a factory in China, Germany, or the United States, as long as you write ISO 2768, engineers all over the world will know what level of accuracy it is. You don’t need to communicate with the manufacturer too much.

What Industries and Companies Is ISO 2768 Relevant for?

ISO 2768 is widely used in industries with high precision and high-quality components, such as our CNC machining industry.

1. Key Industries

(1) General Machinery & Automation

It is arguably the most widely used area of ISO 2768. Such as packaging machines, non-standard automation equipment, robot arm terminals, etc., these equipment contain many parts. If all parts are marked with tight tolerances, the cost increases. If not, there will be problems with assembly. Therefore, using ISO 2768-mK can perfectly solve these problems.

(2) Automotive Aerospace

The automotive industry is also a commonly used field. Because this industry is extremely sensitive to cost control, the management of tolerance grading is very strict. Non-mating surfaces such as housings and brackets, generally use ISO 2768-m. Chassis and suspension generally use casting services, so the looser ISO 2768-c or v is used. However, you should note that the key mating surfaces of the car (such as the piston bore) will definitely not use ISO 2768, but extremely high-precision special tolerances.

(3) Aerospace

The aerospace industry, even though when we hear it, we think it will be associated with “high precision”, but in fact this industry also uses ISO 2768. For example, components such as cabin interior structural parts, seat brackets, handles, etc. do not need to use strict tolerances to specify dimensions. Moreover, aircraft manufacturers also want to control costs well and leave more budget for key components such as engine blades. Those non-critical components only need to use ISO 2768, but usually use the more sophisticated ISO 2768-f.

(4) Medical Devices

We know that precision is demanding in the medical industry. However, many components in the medical industry, such as the handles of surgical instruments and the internal frames of large medical equipment, are only for support. So for components like these, ISO 2768 provides sufficient accuracy and ensures their interchangeability.

(5) Consumer Electronics

In the consumer electronics industry, ISO 2768 is also widely used. For example, our mobile phones, laptops, and smart wearable devices all contain internal metal structural parts, which generally use ISO 2768. However, due to the crowded interior space of electronics, even if these components use ISO 2768, they tend to favor ISO 2768-f (precision class) rather than the ordinary m class.

2. Types of Companies

ISO 2768 connects both ends of the supply chain, and naturally enterprises are also its heavy users. For example, the following two types of companies:

(1) OEMs

Companies such as Tesla, Siemens, Apple, etc., all have their own design engineers, and they all use ISO 2768 when creating drawings. They can either use it to lower the project budget, or they can use it to avoid liability (as mentioned above, factory parts are not done well, but the designer has standards to follow).

(2) Contract Manufacturers

There are countless small and medium-sized CNC machining manufacturers around the world, such as Foxconn and DSR, which also use ISO 2768. When our engineers see ISO 2768-m, they know that the factory can machine with a common milling machine and can begin quoting. When engineers see ISO 2768-f, they know that finishing machines need to be used, and the corresponding quotation will be higher. At the same time, before all goods are shipped, quality inspection personnel also need to inspect the final products according to ISO 2768 to determine whether these parts are qualified.

What are the Challenges and Limitations of ISO 2768?

Although ISO 2768 is widely used and has many advantages. But it also has certain limitations, and I will explain them to you one by one.

1. Accuracy Trap

This is a common misunderstanding in everyone’s daily life. Even the most precise f-class in ISO 2768 has a tolerance range that is too large for a modern precision class. For example, if you design a bearing hole with a diameter of 20 mm and use ISO 2768-mK, the result may be that the bearing will either not be able to fit in or will shake due to the large tolerance range. Therefore, for the bearing position and locating pin holes, you still have to mark the tolerance separately.

2. Material Bias

You should note that ISO 2768 is based on the logic of metal cutting (material removal) and cannot be directly applied to other materials.

• Plastic: Plastic has the characteristics of shrinkage and moisture absorption. If you require plastic parts to meet the metal-level accuracy of ISO 2768, costs will often skyrocket. Moreover, for CNC plastic machining, we should usually refer to ISO 20457.
• Rubber: Rubber is soft. We generally use ISO 3302.
• 3D Printing: Thermal stress deformation of additive manufacturing is difficult to control within the linear tolerance range of ISO 2768.

3. Tolerance Accumulation

ISO 2768 can only control the accuracy of a single part, but problems may arise when multiple parts that reference the ISO 2768 standard are assembled together. Because the total length error of the final assembly may reach a relatively large tolerance value, which is unacceptable for many precision equipment.

To avoid this problem, you need to perform a tolerance analysis on the multi-part assembly chain. At the same time, it is also necessary to reduce tolerances for critical dimensions.

4. Lack of Contour Control

ISO 2768 mainly targets regular geometric features. However, in modern industry, there are already many components with 3D freeform surfaces, and ISO 2768 lacks effective control values for them. So, if your parts are quite complex, just write ISO 2768-mK, I’m afraid the factory will not give you a very good result. At this time, we have to control it separately with the contour symbol in GD&T.

5. Misinterpreting Tolerance Classes

This is a common mistake that many novice engineers make. They can easily confuse the various tolerance classes in ISO 2768, resulting in false deviations in part tolerances. So here, I also suggest that when designing drawings, these engineers should find the most appropriate tolerance class based on the function and accuracy requirements.

How to Specify on Your Drawings?

Although I have said so much about ISO 2768, some people may still have a question: How to apply ISO 2768 to specific work when designing drawings? Next, I will provide a practical guide to help you understand the entire usage process more intuitively and accurately.

1. The Formula

First of all, you need to know that the most standard writing method includes three elements: standard number, part 1 class (linear), and part 2 class (shape).

 Formula: ISO 2768 – [Linear Class] [Shape Class]

• Linear Class(Lowercase): f (Precise), M (Medium), C (Rough), V (Coarse)
• Shape Class(Uppercase): H (Precision), K (Medium), L (Rough)

Correct examples: ISO 2768-mK (most commonly used), ISO 2768-fH (precision machining parts), ISO 2768-cL (sheet metal fabrication)

2. Where to Put It

As for where to place the label, we usually label it in two locations.

(1) The Title Block

The engineering drawing templates we use now, such as SolidWorks and AutoCAD, have a special grid in their lower right corner title bar, marked:

• General Tolerance
• Unspecified Tolerance

 You can directly fill in the corresponding annotations here. This approach is the most professional and the annotation is not easily ignored.

(2) Technical Notes

If there is no special box for you to fill in, or you want to emphasize the manufacturing requirements with the manufacturer. Then you can write it directly in the text description in the upper left corner of the drawing. Directly indicate the technical requirements.

3. The Golden Rule

Another thing you should pay attention to is that the priority of dimensional tolerances must be emphasized in bold or warning symbols in the article. The default rule for a factory is specific dimensions override general tolerances. This is because ISO 2768 is just the default setting, and your specific dimension next to the dimensions is the custom setting. Your custom settings are always greater than the default settings.

When to Use Tight Tolerances vs. ISO 2768?

In other words, this is a comparison of cost and performance. Next, I will explain to you from an engineer’s perspective, when is it most reasonable to use tight tolerances and ISO 2768?

1.  When to use tight tolerances?

Tight tolerances, such as ±0.01mm, ±0.005mm, and H7/g, etc., are all special values used in the precision parts manufacturing process. You will only use it when the accuracy of ISO 2768 (usually around ±0.1mm ~ ±0.3mm) cannot meet the functional needs.

• Fits & Assembly: Anything that needs to be plugged, stuck, or slid. For example: bearing holes, dowel holes, keyways.
• Moving Parts: Surfaces that involve rotating, sliding direction. If the tolerance is too large, the part will shake, wear or jam. For example: pistons, sliders, stainless steel shafts.
• Sealing Surfaces: O-ring grooves, gasket contact surfaces. If the surface is uneven or the size is wrong, oil and air leakage will occur.
• Datums for Automation: If the part is to be picked up by a robot or used as a datum for subsequent machining, it must be very accurate.

2.  When to use ISO 2768?

If you need design drawings, then you use ISO 2768 as the default design. For dimensions that do not have special functional requirements, you do not need to specify tolerances.

• Non-mating Surfaces: The outer wall, side, or any surface suspended in air that does not touch other parts.
• Aesthetic Features: Chamfers, rounded corners, and deburred edges. All these areas only need to look nice and do not require micron precision.
• Clearance Holes & Reliefs: Holes that a screw passes through (usually with gaps), grooves dug to reduce weight.
• Rough Positioning: For example, the position of the lettering, the position of the label slot.

3. Decision Making Tips: The 80/20 Rule

• ISO 2768-m: A common machine tool can be done in one fell swoop, no additional checks are required, and the cost is relatively basic.
• ±0.05mm: Operators need to be careful with knife setting, occasional spot checks, cost moderately increased.
• ±0.005mm: Grinder or slow wire is required, must be in a constant temperature workshop, 100% full inspection, high scrap rate, cost significantly increased.

Quick Decision Reference Table

How Does ISO 2768 Improve Quality Control in Machining?

ISO 2768 provides quality inspectors with an international standard basis. They can inspect the parts according to the tolerance table and directly judge the quality of the product. They can inspect the components according to the tolerance table and directly judge the quality of the product.

Because ISO 2768 covers approximately 80% of non-critical dimensions on drawings, quality inspectors can use rapid sampling of these non-critical parts, allowing high-precision inspection instruments (such as CMMs) to be used more on critical tolerances that affect component functionality.

At the same time, ISO 2768 can dictate that factories produce with consistent precision due to uniform standards, ensuring that these components can maintain repeatability.

Conclusion

That’s all I have to say about ISO 2768. I believe that you must have gained a lot through reading the entire article. As long as you understand and use ISO 2768 properly, it is the fastest way to reduce CNC machining costs.

If you are now looking for a CNC partner who understands high standards? Then our DSR company is the best answer. We have been deeply involved in the CNC machining industry for 14 years and have provided high-quality CNC machining services to customers from all over the world. I lead a team of professional engineers who can provide perfect solutions for your products. So don’t hesitate, upload your CAD files today to get a free quote!

※ Here, we also provide you with the PDF file of ISO 2768 Tolerance Standards for viewing and use: ISO 2768 Tolerance Standards.pdf

FAQ

Q1: Is ISO 2768 still valid?

A: Yes, in actual industrial applications, ISO 2768 is still mainstream and effective.

Q2: What is the difference between ISO 2768 and ASME Y14 5? 

A: The main difference between the two is the area of application and focus. ISO 2768 is a tolerance standard that is used to define unindicated tolerances. It provides you with a set of internationally accepted default standards. On the other hand, ASME Y14.5 is the fundamental standard of GD&T within the American standard system. It defines a comprehensive set of symbolic language and rules. You can use it to precisely control key features.

Q3: What are the tolerances for manual machining?

A: The tolerances for manual machining are highly dependent on the skill level of the operator and the condition of the machine tool. Generally speaking, skilled machinists can control the tolerance of parts to around 0.1mm, and extremely fine operations can reach 0.05mm.

Q4: How much tolerance is acceptable?

A: In general, the tolerance range that meets ISO 2768-m (Intermediate Grade) is an acceptable level of standard acceptance in the industry.

Q5: Is 0.005 a tight tolerance? 

A: It depends on the unit. 0.005 inches is a standard tolerance, but 0.005 mm is a tight tolerance for very high precision.

Q6: What is the calibration tolerance limit?

A: Calibration tolerance limits generally do not have a specific fixed value, but are based on the “10:1 rule” in metrology. In other words, the error of the measuring instrument itself must not exceed 10% of the tolerance of the part being inspected.

Q7: What are the three basic tolerances?

A: The three basic tolerances usually refer to Bilateral, Unilateral, and Limit. 

Q8: What does 2% accuracy mean? 

A: “2% accuracy” means that the allowed error range is 2% of the target value.

Q9: What is a good precision to tolerance ratio? 

A: In quality control, the ideal precision/tolerance ratio (P/T Ratio) should typically be less than 30%, with an even better target of 10%.

Q10: How to check the straightness and flatness tables?

A: It’s very simple actually. For straightness, according to the length of the line you want to measure, and look up the table. For flatness, according to the surface you want to measure, what is the longest side, or what is the diameter of the circular surface, and look up the table according to the largest size. For example, if you have a rectangular surface of 100mm x 50mm, you can press 100mm to check the flatness tolerance.

Q11: How to calculate roundness?

A: In fact, there is a rule, the tolerance of roundness = the dimensional tolerance of diameter. But the number you calculate cannot exceed the coaxiality/circular runout value specified in the table.

Q12: How to obtain ISO 2768 certification for my manufacturing process? 

A: In fact, there is no such thing as “ISO 2768 certification.” ISO 2768 is a technical standard, not a management system certification like ISO 9001.

Q13: What CAD programs support automatic ISO 2768 dimensioning and tolerances?

A: Almost all mainstream advanced CAD software is supported, such as SolidWorks, Autodesk Inventor, Siemens NX, PTC Creo, CATIA.