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iso 2768

ISO 2768: A Complete Guide to CNC Machining Tolerance Standards

Manufacturing metal and plastic parts occur in large numbers every day. However, fabricating these parts without deviating from original designs can be very complicated. Sometimes, it is almost impossible to get precisely the exact dimensions for a batch process. The ISO 2768 international tolerance standards help to simplify designs and production processes.

Machinists and designers can decide the level of deviation from the original specifications, giving a controlled margin for error. The tolerance limits provided by the ISO 2768 define a range of variation acceptance between nominal dimensions and other dimensional values that qualifies as fit.

In this article, we will discuss the details of ISO 2768 to help you better understand this tolerance standard. We also have the latest revised ISO 2768 PDF for download. Stay with us.

What is ISO 2768?

ISO (International Organization for Standardization) created ISO 2768 as an international manufacturing standard that simplifies drawing specifications for CNC machining tolerances.

what is iso 2768
What is iso 2768

Creating this international standard and its implementation make designing and manufacturing more accessible and convenient. Partnerships between manufacturing companies are also more seamless. 

International organizations and other agencies liaise with the ISO for this standardization.

ISO 2768 comes in two parts — ISO 2768-1 and ISO 2768-2. These parts define mechanical precision levels to simplify technical drawings. 

  • Part 1 – General Tolerances for linear and angular dimensions. The precision here is defined as fine, medium, coarse, and very coarse. 
  • Part 2 – Geometrical tolerances for features. Precision levels or tolerance classes here are H, K, and L. 

For instance, a drawing specified to be ISO 2768-mK means that such a component must meet the tolerance range of “medium” for Part 1 and the tolerance class “K” for Part 2. 

The ISO 2768-mK is often more popularly used for manufacturing sheet metal parts. However, manufacturers also choose ISO 2768-fH for machined components.

Why ISO 2768 is Important

Every feature on a component always has a geometrical shape and size. Deviating from theoretically exact dimensions often impair the function of the part. This is why it is important to complete the tolerancing on technical drawings.

Including ISO 2768 specifications in your manufacturing process ensure adequate control of every element of geometry and size of a part. With this standard, there will be no need to leave anything to judgment. Using general tolerances for geometry and size allows you to avoid writing tolerances for every feature and dimension.

There are general rules to this standard. However, there can be exceptions if some dimensions need tighter general geometrical tolerances than the set standards.

In such cases, you should never forget to check title blocks on drawings for general tolerance requirements. After checking, note special project requirements or part specifications. 

ISO 2768 Part 1: Linear and Angular Dimensions

The ISO 2768-1 intends to simplify drawing indications. This part specifics the general tolerances in four different classes, namely:

  • f – Fine
  • m – Medium
  • c – coarse 
  • v – very coarse

ISO 2768 part one applies for linear and angular dimensions. These dimensions include internal sizes, external sizes, radii, external radii, diameters, chamfer heights, and step sizes. They are dimensions of components produced using metal removal. They are also relevant for parts formed using sheet metal fabrication techniques.

Thus, the designer will indicate “ISO 2768-m” in the title block if general tolerance standards following this standard shall apply.

The following tolerance tables correspond to the different precision levels. This part of the international tolerance standards has three tables with respect to ideas involving general dimensional tolerances.

You can select the most suitable class according to design requirements and machining capabilities. 

Table 1 – Linear Dimensions

iso 2768 table 1 linear dimensions
iso 2768 table 1 linear dimensions

Designers shall indicate nominal sizes lesser than 0.5 mm adjacent to the appropriate size.

Table 2 – External Radii and Chamfer Heights

Similarly, Table 2 indicates tolerance standards for external radii and chamfers. 

iso 2768 table 2 external radii and chamfer heights
iso 2768 table 2 external radii and chamfer heights

Designers shall indicate nominal sizes lesser than 0.5 mm adjacent to the appropriate size.

Table 3 – Angular Dimensions

This table defines tolerance standards for angular dimensions. The tolerances in this table are in minutes and degrees, as you would expect for angular dimensions.

iso 2768 table 3 angular dimensions
iso 2768 table 3 angular dimensions

ISO 2768 Part 2: Geometrical Tolerances for Features

This part includes general geometrical tolerances of straightness and flatness. It also includes circularity and cylindricity. ISO 2768-2 simplifies draws, fixing general tolerances in three classes — H, K, and L. 

It also mainly applies to parts created by material removal. There are four tables here with respect to general geometric tolerances of dimensions and further information.

These are pretty different from clearance and fitting tolerance grades that use numbers and letters. As with the ISO 2768-1, Part 2 also has nominal ranges and deviations. However, how we define these deviations differs. 

The tolerance charts are available below:

Table 4 – General Tolerances on Straightness and Flatness

This table defines straightness and flatness ranges. Let’s take a compressor base as an example. First, it is essential to consider the contact surface between the engine and the base and the contact surface between the base and compressor. This will help you specify their flatness ranges in the drawing. 

Straightness tolerances will control the level at which a surface varies from a specified line. They also help control the number of twists or bends allowed.

iso 2768 table 4 general tolerances on straightness and flatness
iso 2768 table 4 general tolerances on straightness and flatness

Table 5 – General Tolerances on Perpendicularity

Perpendicularity comes in distance units like inches or millimeters. Its general tolerance standards are summarized below.

iso 2768 table 5 general tolerances on perpendicularity
iso 2768 table 5 general tolerances on perpendicularity

Table 6 – General Tolerances on Symmetry 

This table shows general geometrical characteristics for Symmetry with permissible deviations for some features which may be uniform across a plane.

iso 2768 table 6 general tolerances on symmetry
iso 2768 table 6 general tolerances on symmetry

Table 7 – General Tolerances on Circular Run-Out 

Table 7, which happens to be the final table of the ISO 2768, corresponds to Run-out. This is the total permissible variation of a surface when you rotate the part around an axis.

iso 2768 table 7 general tolerances on circular run out
iso 2768 table 7 general tolerances on circular run out

FAQ

What is ISO 2768-mK Meaning? 

The tolerance class “m” for Part 1 means medium. The “K” class, on the other hand, is part of ISO 2768-2. Therefore, ISO 2768-mK means that such a component must meet the “medium” tolerance range for Part 1 and the tolerance class “K” for Part 2.

Other tolerance levels in Part 1 are fine, coarse, and very coarse, while those in Part 2 include H, K, and L. 

Why is ISO 2768 Important? 

ISO 2768, as an international tolerance standard, is essential for simplifying drawing specifications used in geometrical tolerances. It also puts you on an equal page with designers and manufacturers worldwide to prevent manufacturing misunderstandings. 

The Latest Revision PDF of ISO 2768 for download

Conclusion 

Tolerance is a vital aspect of product design. ISO 2768 covers the most crucial geometric features and tolerances used in the manufacturing industry, defining tolerance values to simplify work for designing and production. 

AT-Machining understands every aspect of ISO 2768 tolerance, and we are ready to help you save time and cost with effective services. So contact us today, and let’s get your project started!

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