The ISO 2768-mK standard is an international specification used to simplify technical drawings by providing "general tolerances" for parts manufactured by machining or metal forming. Instead of specifying a tolerance for every single dimension on a drawing, designers can simply reference "ISO 2768-mK" in the title block to cover all non-toleranced dimensions. Breakdown of "mK"
The designation consists of two parts that refer to different precision levels:
m (Medium): Refers to Part 1 of the standard, covering linear and angular dimensions (e.g., lengths, radii, and angles).
K (Geometric): Refers to Part 2 of the standard, covering geometrical characteristics such as straightness, flatness, and perpendicularity. ISO 2768-1: Linear & Angular (Class m)
This section defines the permissible deviations for dimensions like lengths, diameters, and external radii. The "m" (medium) class is the most common for standard industrial machining. Table 1: Linear Dimensions (Permissible deviations in mm) Nominal Range (mm) Class f (fine) Class m (medium) Class c (coarse) ±0.05plus or minus 0.05 ±0.1plus or minus 0.1 ±0.2plus or minus 0.2 Over 3 to 6 ±0.05plus or minus 0.05 ±0.1plus or minus 0.1 ±0.3plus or minus 0.3 Over 6 to 30 ±0.1plus or minus 0.1 ±0.2plus or minus 0.2 ±0.5plus or minus 0.5 Over 30 to 120 ±0.15plus or minus 0.15 ±0.3plus or minus 0.3 ±0.8plus or minus 0.8 Over 120 to 400 ±0.2plus or minus 0.2 ±0.5plus or minus 0.5 ±1.2plus or minus 1.2 Over 400 to 1000 ±0.3plus or minus 0.3 ±0.8plus or minus 0.8 ±2.0plus or minus 2.0 ISO 2768-2: Geometrical Tolerances (Class K)
This part limits how much a feature can deviate in shape or orientation. Class K is the intermediate level between H (tightest) and L (loosest). Key Geometric Controls (Class K) Straightness and Flatness: Ranges from for small parts up to for lengths over Perpendicularity: Maximum deviation of depending on the length of the shorter leg. Symmetry: Standardized at for class K. Circular Run-out: Fixed at for class K. Core Benefits
Cleaner Drawings: Eliminates "dimension clutter" by removing repetitive ±plus or minus Tolerance Iso 2768 Mk Pdf
Cost Efficiency: Avoids unnecessarily tight tolerances that drive up manufacturing costs.
Manufacturing Readiness: Provides a clear baseline that matches standard workshop capabilities.
📍 Application Note: If a specific feature requires higher precision (e.g., a bearing fit), that specific dimension must be toleranced individually, which then overrides the general ISO 2768 standard.
For full technical charts, you can reference the ISO 2768-mK Overview or specialized guides from ZEISS Quality Forum.
If you tell me the material or manufacturing process you're using (e.g., CNC milling vs. sheet metal), I can help you decide if class mK is the right choice for your project.
The designation ISO 2768-mK is an international standard used to define general tolerances for parts manufactured by material removal (such as CNC machining). It streamlines engineering drawings by providing a default "medium" precision level, eliminating the need to specify tolerances for every single dimension. Breaking Down the "mK" Designation The ISO 2768-mK standard is an international specification
The designation consists of two parts that refer to different sections of the ISO standard:
m (Part 1 - Linear & Angular Dimensions): Represents the "medium" tolerance class for linear dimensions (lengths, diameters, radii) and angular dimensions.
K (Part 2 - Geometrical Tolerances): Represents the "K" class for geometric features like flatness, straightness, and perpendicularity. ISO 2768-1: Linear Dimensions (Class m)
Part 1 defines four classes: f (fine), m (medium), c (coarse), and v (very coarse). Class m is the industry standard for roughly 80% of CNC machined parts because it balances cost and quality. Nominal Size Range (mm) Tolerance (± mm) for Class m Over 3 to 6 Over 6 to 30 Over 30 to 120 Over 120 to 400 Over 400 to 1000 Source: Derived from ISO 2768-1 Tables. ISO 2768-2: Geometrical Tolerances (Class K)
Part 2 defines three classes: H, K, and L. These control the shape and position of features without individual callouts.
The Basics of General Tolerance Standard – ISO 2768-mK - Eurotools covering linear and angular dimensions (e.g.
If a drawing states ISO 2768-mK, the following limits apply automatically unless a specific tolerance is written next to the dimension.
| Class | Description | Typical Use | |-------|-------------|--------------| | f (Fine) | High precision | Instruments, small mechanisms | | m (Medium) | General machining | Brackets, housings, structural parts | | c (Coarse) | Fabrication | Welded frames, sheet metal, casting | | v (Very coarse) | Rough cuts | Forging, sand casting, clearance-only |
ISO 2768-mK is the default for CNC-machined parts without tight tolerances.
The tolerance varies depending on the nominal size of the dimension. Below is the reference table for the Medium (m) class:
| Nominal Dimension (mm) | Tolerance (mm) | | :--- | :--- | | 0.5 up to 3 | $\pm 0.1$ | | Over 3 up to 6 | $\pm 0.1$ | | Over 6 up to 30 | $\pm 0.2$ | | Over 30 up to 120 | $\pm 0.3$ | | Over 120 up to 400 | $\pm 0.5$ | | Over 400 up to 1000 | $\pm 0.8$ | | Over 1000 up to 2000 | $\pm 1.2$ | | Over 2000 up to 4000 | $\pm 2.0$ |
Note: For nominal dimensions below 0.5 mm, the tolerances shall be stated directly on the drawing.