What is the ISO 2768 Standard?
ISO 2768 is an international standard that simplifies engineering drawings. It provides default, acceptable variations for linear and angular dimensions, as well as for features like broken edges, chamfers, and radii.
When a designer applies an ISO 2768 callout to the title block of a drawing, the machinist knows exactly how much deviation is allowed for any dimension that lacks a specific +/- value. This reduces drafting time, clarifies manufacturing expectations, and standardizes quality control.
The Four Linear Tolerance Classes
Part 1 of the standard (ISO 2768-1) defines four distinct tolerance classes, which dictate how tight or loose the general manufacturing variations are allowed to be. The choice of class depends on the precision of the manufacturing method and the functional requirements of the part.
The classes are designated by lowercase letters:
The permitted deviation increases as the base dimension gets larger. For example, a 10mm feature in the 'medium' class has a tolerance of ±0.2mm, but a 500mm feature in that same 'medium' class has a tolerance of ±0.8mm.
- f (fine): Extremely tight tolerances, typically used for precision CNC machining.
- m (medium): The most common class, suitable for general machining and sheet metal.
- c (coarse): Looser tolerances, often used for basic fabrication or casting.
- v (very coarse): Very loose, usually reserved for rough cuts or large structural assemblies.
Tolerances for Broken Edges and Angles
ISO 2768 isn't limited to straight linear dimensions. It also provides specific tables for the tolerances of external radii, chamfers, and broken edges. For instance, a 1mm chamfer on a 'medium' class drawing is allowed a deviation of ±0.2mm.
Angular dimensions are also covered. However, unlike linear dimensions where the tolerance is given in millimeters, angular tolerances in ISO 2768 are provided in degrees and minutes based on the length of the shortest leg of the angle.
How to Specify ISO 2768 on a Drawing
To implement these general tolerances, the standard must be explicitly referenced in the title block or general notes of the engineering drawing. If it is omitted, the manufacturer has no legal standard for default tolerances.
A standard callout looks like 'ISO 2768-m'. This tells the manufacturer to apply ISO 2768 Part 1 rules using the 'medium' (m) tolerance class. It is important to remember that if a specific feature requires a tighter fit (like a bearing bore), you simply add a specific tolerance to that specific dimension, which immediately overrides the general ISO 2768 rule for that feature alone.
Frequently asked questions
What happens if a dimension has its own tolerance next to it?
A specific tolerance placed directly on a dimension always overrides the general ISO 2768 title block tolerance for that specific feature.
Does ISO 2768 cover geometric tolerancing like flatness or symmetry?
Yes, but that is covered in Part 2 of the standard (ISO 2768-2). It uses different classes denoted by capital letters: H, K, and L.
Can ISO 2768 be used for plastic injection molded parts?
While it physically can be, ISO 2768 is primarily designed for metal removal (machining) and sheet metal operations. Plastics behave differently, so engineers typically use standards like DIN 16901 or specific plastic molding tolerances instead.
What does a callout like 'ISO 2768-mK' mean?
This is a combined callout. The lowercase 'm' specifies the 'medium' class for linear and angular dimensions (Part 1), and the uppercase 'K' specifies the geometric tolerance class (Part 2).