ISO 2768-mK standard defines general tolerances for dimensions and geometric features on technical drawings where specific tolerances are not indicated. It simplifies drawings by providing a default "medium" level of precision, ensuring parts fit together without over-specifying every dimension. Meaning of "mK" "m" (Medium) : Refers to ISO 2768-1 , covering general tolerances for linear and angular dimensions (lengths, diameters, radii, chamfers). "K" (Medium) : Refers to ISO 2768-2 , covering general geometrical tolerances
(straightness, flatness, perpendicularity, symmetry, and circular run-out). Tolerance Tables for ISO 2768-mK All values are in millimeters (mm) unless otherwise stated. ALFA MIMtech 1. Linear Dimensions (Class m) Applies to external sizes, internal sizes, and diameters. Range (Nominal Size) Tolerance (±) 0.5 to 3 mm >3 to 6 mm >6 to 30 mm >30 to 120 mm >120 to 400 mm >400 to 1000 mm >1000 to 2000 mm >2000 to 4000 mm 2. External Radii and Chamfer Heights (Class m) Applies to broken edges and rounded corners. ALFA MIMtech Range (Nominal Size) Tolerance (±) 0.5 to 3 mm >3 to 6 mm 3. Angular Dimensions (Class m) Applies to angular measurements. ALFA MIMtech Length of Short Side Tolerance (±) Up to 10 mm >10 to 50 mm >50 to 120 mm >120 to 400 mm Over 400 mm 4. Geometrical Tolerances (Class K) Applies to the form and position of features. waterson.com Feature Type Range (Nominal Length) Straightness & Flatness Up to 10 / 30 / 100 / 300 / 1000 / 3000 mm 0.05 / 0.1 / 0.2 / 0.4 / 0.6 / 0.8 mm Perpendicularity Up to 100 / 300 / 1000 / 3000 mm 0.4 / 0.6 / 0.8 / 1.0 mm Up to 100 / 300 / 1000 / 3000 mm 0.6 / 0.6 / 0.8 / 1.0 mm Circular Run-out All lengths Important Considerations Understanding ISO 2768-mK Tolerances for Engineers
ISO 2768-mK is an international manufacturing standard used to simplify technical drawings by providing a "default" set of tolerances for features that do not have specific values written next to them. This callout ensures that every part of a drawing is controlled without the need to clutter it with repetitive data, balancing precision with manufacturing efficiency. Understanding the "mK" Notation
The suffix "mK" combines two distinct parts of the ISO 2768 standard:
m (Medium): Refers to ISO 2768-1. This part defines general tolerances for linear and angular dimensions, such as lengths, diameters, radii, and chamfers.
K (Tolerance Class K): Refers to ISO 2768-2. This part covers geometrical tolerances (GD&T) for features like flatness, straightness, parallelism, and perpendicularity.
ISO 2768-1: General Tolerances for Linear and Angular Dimensions (The "m") general tolerance iso 2768-mk
ISO 2768-1 uses four classes: f (fine), m (medium), c (coarse), and v (very coarse). The "m" class is the most common industry standard for CNC machining and general metalwork. Nominal Dimension Range (mm) Tolerance for "m" Class (mm) ±plus or minus Over 3 to 6 ±plus or minus Over 6 to 30 ±plus or minus Over 30 to 120 ±plus or minus Over 120 to 400 ±plus or minus Over 400 to 1000 ±plus or minus Data sourced from standard ISO 2768-1 charts. ISO 2768-2: General Geometrical Tolerances (The "K")
| Nominal Size Range (mm) | Tolerance ‘k’ (mm) | | :--- | :--- | | Up to 100 | 0.1 | | >100 to 300 | 0.2 | | >300 to 1000 | 0.3 | | >1000 to 3000 | 0.4 |
Note: For symmetry and runout (ISO 2768-2), the ‘k’ class generally allows 0.2mm for most common part sizes.
The letters 'm' and 'k' are not arbitrary; they denote specific tolerance classes.
Key Insight: Using ISO 2768-mk tells the manufacturer: "Unless a specific tolerance is written on a dimension, assume the 'medium' class for size and the 'medium' class for geometry."
ISO 2768-mk provides a cost-effective, industry-standard default tolerance for machined parts where precision is not critical. It balances manufacturing ease with acceptable quality. However, engineers must explicitly specify tighter tolerances for mating, safety, or high-precision features. This report shall be appended to the company’s quality management system (QMS) as the reference for general tolerance compliance. Note: For symmetry and runout (ISO 2768-2), the
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A feature for ISO 2768-mk establishes a "medium" precision standard for parts, ensuring they are manufactured within acceptable limits for both size and shape without requiring individual tolerance callouts for every dimension. The designation breaks down into two parts:
m (Medium): Governed by ISO 2768-1, this defines permissible deviations for linear and angular dimensions, such as lengths, radii, and chamfers. specific dimensions override general note.
k (Class K): Governed by ISO 2768-2, this covers geometric characteristics like flatness, straightness, and circular runout. Tolerance Tables for ISO 2768-mk
The following values apply based on the nominal size of the feature: Linear Dimensions (Class m)
For linear measurements like external/internal sizes, heights, and distances. Nominal Range (mm) Tolerance (± mm) 120 to 400 400 to 1000 Geometric Tolerances (Class K)
For shape and position characteristics without individual indications. Feature Type Range (mm) Tolerance (mm) Straightness/Flatness 100 to 300 Perpendicularity Symmetry Run-out (Circular) All ranges Key Implementation Details
The Basics Of General Tolerance Standard - ISO 2768-mK - LEADRP