| Parameter | Tolerance Class K | |-----------|-------------------| | Symmetry (mm) | 0.5 mm | | Circular runout | 0.1 mm |
Cylindricity and circularity are generally covered under straightness/roundness of 0.1 mm for Class K.
| Nominal Length Range | Tolerance | |----------------------|-----------| | ≤ 10 | 0.05 | | >10 to 30 | 0.1 | | >30 to 100 | 0.2 | | >100 to 300 | 0.4 | | >300 to 1000 | 0.6 | | >1000 to 3000 | 0.8 |
This ensures that surfaces intended to be at 90 degrees are within a certain limit. The tolerance is defined by the longer of the two sides forming the angle.
| Length of the longer side (mm) | Tolerance (Class K) | | :--- | :--- | | Up to 100 | 0.20 mm | | Over 100 up to 300 | 0.30 mm | | Over 300 up to 1000 | 0.40 mm |
The standard ISO 2768-mK represents a pragmatic approach to engineering. By applying medium linear tolerances and Class K geometric tolerances, engineers ensure that parts are functional and interchangeable without demanding unnecessary precision. Understanding how to read and apply this standard is fundamental knowledge for anyone involved in mechanical design or CNC machining.
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). Tolerance Iso 2768 Mk Pdf
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
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. and symmetry. These are "general" tolerances
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 ISO 2768-mK standard provides general tolerances for linear and angular dimensions (m) and geometrical features (K), ensuring consistent quality in manufacturing without requiring individual dimension callouts. This standard serves as a cost-effective, universally recognized guide for machining and fabrication. A detailed breakdown of these standards is available from SogaWorks. General Tolerance - ISO 2768 1 & 2 - ZEISS Quality Forum
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 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. covering linear and angular dimensions (e.g.
The Basics of General Tolerance Standard – ISO 2768-mK - Eurotools
ISO 2768-mK is a standard for general tolerances used on engineering drawings when no specific tolerances are indicated. It combines two parts: "m" (medium) from ISO 2768-1 for linear and angular dimensions, and "K" from ISO 2768-2 for geometrical features like flatness and symmetry. Part 1: Linear Dimensions (Class "m")
These tolerances apply to lengths, diameters, radii, and distances. Nominal Range (mm) Tolerance (±mm) over 3 to 6 over 6 to 30 over 30 to 120 over 120 to 400 over 400 to 1000 over 1000 to 2000 over 2000 to 4000 Sources: RpProto PDF, RivCut. Part 2: Geometrical Tolerances (Class "K")
These apply to the shape and position of features without individual GD&T callouts. Up to 100mm 100–300mm 300–1000mm 1000–3000mm Straightness & Flatness Perpendicularity Symmetry Circular Run-out Sources: Runsom PDF, Fictiv. Angular Dimensions (Class "m") Nominal Length (Short Side) up to 10 mm 10 to 50 mm 50 to 120 mm 120 to 400 mm over 400 mm Sources: Xometry, RpProto. Usage & Standards Understanding ISO 2768-mK Tolerances for Engineers
The code mK is a combination of the two parts of the standard:
Therefore, ISO 2768-mK means: "Use Medium tolerances for linear measurements and Class K tolerances for geometric deviations."
While Part 1 deals with "size" (how long or wide something is), Part 2 deals with "shape" (how flat, straight, or square something is).
The Class K provides specific limits for straightness, flatness, perpendicularity, and symmetry. These are "general" tolerances; if a specific GD&T (Geometric Dimensioning and Tolerancing) callout is not on the drawing, these values apply.
The "K" class in ISO 2768 MK is the medium precision for form and position. You do not need to add individual flatness or perpendicularity callouts unless the part requires tighter control than the K class.