Guide To General Tolerance – ISO 2768 1&2 Standard

Table of Contents

I. Introduction

Overview of ISO 2768-1 & 2 standard

In the day-to-day activities in various industries, there is the designing and manufacturing of different parts which are either metallic or plastic. All parts that are to be produced vary in terms of their sizes and physical appearances. Tolerances are applied to ensure the right size and shapes are adhered to. The absence of standard tolerance to monitor the different measurements of the parts makes work strenuous for both the designers and the engineers. Tolerance can be explained as the span of measurement that is determined or various physical characteristics that make a certain product look and perform appropriately as desired. Tolerance can either be in the form of measurements, shape, texture, or even color. Tolerance has proven to be important in the designing and making of parts of a CNC machine. For easier and faster designing and production processes, the International Organization for Standardization (ISO), came up with ISO 2768. ISO 2768 is usually divided into two categories, that is, ISO 2768-1 and ISO 2768-2, whereby ISO 2768-1 deals with dimensions of lines and angles while ISO 2768-2 focuses on the geometric requirements for various features.=

B. Importance of understanding general tolerances

It is crucial to understand general tolerances to know the range within which measurements should range. This helps to ensure accuracy and consistency, come up with products of high quality and avoid expensive mistakes which might occur in the production process.

II. ISO 2768-1: General Tolerances for Linear and Angular Dimensions

Linear dimensions include the diameters, radii, external and internal sizes, distances, and any other measurements that are vertical or horizontal of various items. There are certain ranges given for parts depending on their varying textures; whether they are fine, medium, coarse, or very coarse.

There is a table that shows the allowed differences allowed for linear measurements and angular measurements. They are shown below.

Permissible deviations for ranges in nominal lengths in mmfmcv
0.5 to 3±0.05±0.1±0.2
Over 3 to 6±0.05±0.1±0.3±0.5
Over 6 to 30±0.1±0.2±0.5±1.0
Over 30 to 120±0.15±0.3±0.8±1.5
Over 120 to 400±0.2±0.5±1.2±2.5
Over 400 to 1000±0.3±0.8±2.0±4.0
Over 1000 to 2000±0.5±1.2±3.0±6.0
Over 2000 to 4000±2.0±4.0±8.0
This is whereby f stands for fine, m stands for medium, c stands for coarse, and v for very coarse. The values used are usually in millimeters and not any other metric unit.

For external radii and heights of chamfers, the following table is adhered to.
Permissible deviations in mm for ranges in nominal lengthsfmcV
0.5 to 3±0.2±0.2±0.4±0.4
Over 3 to 6±0.5±0.5±1.0±1.0
Over 6±0.1±0.1±2.0±2.0

The angular measurements also have a table that helps to give the right ranges of the dimensions as shown below.
The allowed deviations in degrees and minutes for ranges in nominal lengthsfmcV
Up to 10±10±10±1030’±30
Over 10 to 50±0030’±1030’±10±20
Over 50 to 120±00.20’±00.20’±0030’±10
Over 120 to 400±0010’±0010’±0015’±00.30’
Over 400±005’±005’±0010’±00.20’

C.  Application of ISO 2768-1

ISO 2768-fH has been of great significance in most industries since it helps in CNC machining whereby it is used in the drawing of the specific design of an item. It helps in ensuring that the designer and engineer come up with a product that is of the required size, angles, and measurements to ensure the efficiency of the end product. CNC machining is a process that is part of the manufacturing process and is becoming popular in the modern world. The engineers in charge of CNC machining put together various designs, drawings, measurements, and computer skills for programming to put together a certain kind of metallic or plastic part. ISO 2768-1 cannot be used in some measurements such as;

  • Additional measurements are indicated using brackets.
  • Accurate measurements are indicated in the frames of rectangles.

Rapid prototyping also uses ISO 2768-1 in ensuring that the prototype produced is of the right tolerances before going on with the production of products for the public. There is usually a difference between the original measurements and the range within which other measurements may still be fit for the product to be good enough. This is defined by the tolerance ranges. The automobile industry is one of the industries that make use of ISO 2768-1 in the production of various parts in an automobile. Mold manufacturing also utilizes this aspect to ensure the right mold design so that the production and product are a success.

III. ISO 2768-2: General Tolerances for Geometrical Tolerances

A.  Geometrical Tolerances

On the other hand, ISO 2768-2 represents the exact manner in which a part has to look at engineering drawings. It might be circular, straight, cylindrical, or even flat. ISO 2768-2 helps to make drawing easy and consists of three tolerance classes which include H, K, and L. For straight or flat parts the table below is followed.

Ranges in nominal lengths in mmHKL
Up to
Over 10 to 300.050.10.2
Over 30 to 1000.10.20.4
Over 100 to 3000.20.40.8
Over 300 to 10000.30.61.2
Over 1000 to 30000.40.81.6

Also ISO 2768- 2 is used in cases of perpendicularity whereby the followed table is adhered to.

Ranges of nominal lengths in mmHKL
Up to
Over 10 to 300.30.61.0
Over 30 to 1000.40.81.5
Over 100 to 3000.51.02.0

In cases of symmetry the table below is used.

Ranges of nominal lengths in mmHKL
Up to
Over 10 to 300.50.61.0
Over 30 to 1000.50.81.5
Over 100 to 3000.51.02.0

In situations with circularity, in all ranges of nominal length they use the same dimension in all the categories H, K, and L which are 0.1, 0.2, and 0.5 respectively. This allows the manufacturer to decide on which tolerance level satisfies their requirements. The manufacturers get to choose to have in mind that a small level of tolerance would be more expensive than a higher tolerance range.

 B.  Application of ISO 2768-2

ISO 2768-2 is important in the production process when it comes to where two surfaces come into contact with each other. The flatness of the two surfaces is stated in the drawing that is done before the production process. When two rough surfaces are measured, one may come up with several values and that is how the set dimensions are applied. ISO 2768-2 helps to ensure that a prototype is correct before mass production too. It also helps to determine how much a certain part can bend or twist.

IV. Relationship between ISO 2768-1 & 2 and Other ISO Standards

Geometric Product Specifications can be explained as the symbolic language that is set globally that is used to indicate different tolerances in drawings. It explains the actual form of certain parts and the range in which the parts are allowed to change to still suit their roles as that of the actual form. The GPS language is applied by many ISO standards and has been of great significance to engineers and designers. ISO 14638 is an essential standard of ISO GPS. It gives details on the whole ISO GPS notion and gives a conceptual structure on how the current and future ISO GPS work in sync with the system. The conceptual structure intends to ensure that the users understand completely the various standards of ISO GPS and how they correlate with each other. ISO 1101 is responsible for the language used in the specified geometry of a part and how the language is interpreted.

 B.  ISO 8015 – Fundamental Tolerancing Principle

ISO 8015 helps to show how the dimensional and geometrical tolerances relate to each other. This makes it applicable in ISO 2768-1 and 2 which involves the linear, angular, and geometrical tolerances respectively.

V. Importance of Tolerance in Manufacturing and Quality Control

Tolerances play a big role in ensuring that each part is made according to the right specifications. In the manufacturing process, tolerances play a major part and are very crucial. These important roles include the following;

  • They ensure that there is clarity in various instructions. In the production process, every manufacturer wants to come up with the best product, however, unclear specifications on what needs to be met can make this unachievable. Tolerances help a manufacturer to know within which range a product should exist in terms of measurements, shapes, and many others
  • They are cost-friendly. This is whereby when the tolerance is tight, the cost of production tends to be high and vice versa. Therefore, knowing that a less tight or loose tolerance can be applied on a part saves you the cost that would otherwise be spent on tighter tolerances. Some parts however need tight tolerances and discovering that early helps the manufacturer save on delays that might be more costly.
  • Tolerances also help to enhance how the end product turns out. By knowing what is needed for a particular product to come out and work efficiently, then the manufacturer gets to work towards meeting the needs.
  • They help to allow a gap for error. As discussed earlier, tolerances help to provide the manufacturer with the maximum and minimum measurement within which a product can still work efficiently similar to the one using the original measurements required. This helps to ensure that other further errors can be avoided hence a faster production process.
  • They help to ensure that different parts fit properly and work in the required way. Some products are formed by two parts connecting and therefore tolerance helps to see to it that the parts are compatible enough.
  • They help to ensure that the market gets access to the end products quickly. Tolerances help to identify the various areas that need corrections and which suit the end product. Discovering the tolerances early makes it possible for the manufacturer to not waste time in the production process hence the products reaching the market faster. Having the wrong measurements or other incorrect features delays the process due to the repetitive corrections needed.
  • Tolerances help to get rid of ambiguity and hence make the production process simpler for the manufacturers.
  • They ensure that there are more productive communications done through the drawings designed by designers. This helps to ensure the product is of the agreed quality by use of the right tolerances.
  • In addition, using the tolerance range the right way makes the manufacturer the liability of manufacturing the part in the right way. However, if the manufacturer manufactured a certain part within the set tolerance and yet the part refuses to fit properly, the manufacturer is not supposed to be answerable.

VI. Conclusion

A.  Summary of ISO 2768-1 & 2 standard and its applications

In conclusion, we have seen that ISO 2768-1 and 2 are standards with different features which help each of them be suitable for different applications. Tolerance is also another aspect of the manufacturing process which helps the manufacturer know within which ranges to stick within. In the production process, the manufacturer should be familiar with the kind of materials that are being used in the process. This helps to know the dos and don’ts when using them.

B.  Importance of adhering to general tolerances for successful manufacturing and quality control

The measure of a part’s tolerance is usually dependent on how the part will be utilized and what the actual application entails. Again, when manufacturers decide to calculate the tolerances of parts, they have to put the costs involved, the materials, and the whole process in general into consideration. For instance, metallic parts that need to be accurate and with tight tolerance are preferably manufactured through CNC machining due to their ability to produce products with those features. Adhering to the right tolerances helps with the production process since the right ranges are known and therefore end products have high quality and efficiency in their performance. It is also important to ensure that a manufacturer has the knowledge and skills to use tolerances in the right way. In cases where the customers have not been given a certain tolerance level or if a company does not have any, there is a standard tolerance to be applied which is usually between ±0.005’’ and ±0.030’’.


Gary Liao

Gary Liao

Gary Liao is the Engineering Manager of TDL Company and has more than 20 years of mold design experience.

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