The Ultimate Guide to ISO 2768-mK: Streamlining Your Manufacturing Drawings
In the world of precision engineering and CNC machining, time is money—and clarity is king. If you’ve ever looked at a technical drawing and seen "ISO 2768-mK" in the title block, you’re looking at one of the most powerful tools for simplifying design and communication.
But what exactly does it mean, and why is it the industry standard? Let’s break it down. What is ISO 2768-mK?
ISO 2768-mK is an international standard that provides a simplified system of general tolerances for linear and geometrical dimensions. Instead of manually labeling every single dimension on a complex drawing, engineers use this shorthand to define acceptable levels of precision for all non-critical features.
The notation "mK" combines two distinct parts of the standard:
"m" (Medium): Refers to ISO 2768-1, which governs linear and angular dimensions (like lengths, radii, and diameters).
"K" (Medium Geometrical): Refers to ISO 2768-2, which controls geometrical deviations such as straightness, flatness, and perpendicularity. Why "Medium" (mK) is the Industry Favorite
While there are other classes—such as "f" (fine) for high precision or "c" (coarse) for loose fits—the mK combination is the most widely used, representing about 80% of typical manufacturing requirements.
Cost Efficiency: Tighter tolerances (like ISO 2768-f) can be 2 to 2.5 times harder and more expensive to machine. The "mK" class offers a sweet spot between precision and production speed.
Reduced Complexity: It prevents drawings from becoming cluttered with hundreds of individual tolerance notes, making them much easier to read.
Global Language: Because it's an international standard, a part designed with ISO 2768-mK in Germany can be manufactured in Australia or the US with zero ambiguity. Breaking Down the Tables
To use the standard effectively, you need to know how the nominal size of a feature dictates its allowable deviation. Here is a look at the "m" and "K" standards: Linear Dimensions (ISO 2768-m)
For standard linear measurements like length or diameter, the permissible deviation increases as the part gets larger. Nominal Length Range (mm) Tolerance (± mm) 120 to 400 Source: Derived from General Tolerances ISO 2768-1 Geometrical Tolerances (ISO 2768-K)
This part controls the "form" of the part. For example, "K" ensures a surface stays reasonably flat or straight without needing a specific GD&T callout for every face. The General CNC Machining Tolerance: ISO 2768-mk
This report outlines the application and specifications of the ISO 2768-mK general tolerance standard, which is widely used in mechanical engineering to simplify technical drawings by providing standard tolerances for dimensions and geometric features that do not have individual callouts. 1. Scope and Application
ISO 2768-mK is an international standard that defines general tolerances for machining processes, such as CNC machining. It is typically indicated in the drawing's title block as "General Tolerance: ISO 2768-mK".
ISO 2768-1 (m): Represents the "medium" tolerance class for linear and angular dimensions (external sizes, diameters, radii, distances).
ISO 2768-2 (K): Specifies general geometric tolerances, primarily for flatness, straightness, parallelism, and symmetry. 2. Tolerance Class "m" (Linear & Angular) general tolerance iso 2768-mk
The "m" (medium) class is the most common choice for metal parts. It sets permissible deviations based on the nominal size of the dimension: Nominal Size (mm) Tolerance (± mm) Over 3 to 6 Over 6 to 30 Over 30 to 120 Over 120 to 4000 0.5 to 2.0 (variable) Data sourced from LEADRP and ZEISS Quality Forum. 3. Tolerance Class "K" (Geometric)
The "K" class defines general limits for the form and position of features. For example, surfaces with fitting dimensions must remain flat and parallel within the specified K-class range to ensure assembly functionality. 4. Technical Advantages
Simplification: Reduces drawing complexity by eliminating the need to label every single dimension with a specific tolerance.
Cost Efficiency: Using a standard "medium" tolerance helps avoid unnecessarily tight specifications that drive up manufacturing costs.
Consistency: Provides a baseline for quality inspection across different suppliers and manufacturing facilities. General Tolerance - ISO 2768 1 & 2 - ZEISS Quality Forum
ISO 2768-mK is an international standard for general tolerances used to simplify technical drawings by providing default limits for dimensions and geometric features that do not have specific tolerance markers. 🛠️ What "mK" Means
The designation combines two different parts of the standard:
m (Part 1): Represents the Medium tolerance class for linear and angular dimensions (lengths, radii, angles).
K (Part 2): Represents the K tolerance class for geometrical features (flatness, straightness, perpendicularity, symmetry). ISO 2768-1: Linear Dimensions (Class m)
These tolerances apply to dimensions like length, width, and diameter when not specified. All values below are in mm. Nominal Range (mm) Tolerance (±) 120 to 400 400 to 1000 1000 to 2000 2000 to 4000 Additional "m" class values:
External Radii & Chamfers: ±0.2 mm (for 0.5–3mm) to ±1.0 mm (over 30mm).
Angular Dimensions: ±1° (up to 10mm length) to ±0°10' (over 400mm). ISO 2768-2: Geometrical Tolerances (Class K)
This part controls the shape and position of features to ensure they fit correctly. Feature Type Tolerance Rule for Class K Straightness / Flatness
Ranges from 0.05 mm (up to 10mm length) to 0.6 mm (over 1000mm). Perpendicularity Max 0.6 mm for lengths up to 300mm. Symmetry Max 0.6 mm up to 300mm length. Run-out 0.2 mm (standard for class K). 🎯 When to Use ISO 2768-mK Understanding ISO 2768-mK Tolerances for Engineers
You will rarely see ISO 2768-f (Fine) because it is too expensive to machine for generic features. You will rarely see ISO 2768-c (Coarse) because it usually looks sloppy on a drawing.
‘MK’ is the Goldilocks zone: It is tight enough to ensure parts fit together without wobbling, but loose enough that a standard 3-axis CNC mill can hit it every time without inspection headaches.
The "K" designation refers to the general geometrical tolerances. It controls features such as straightness, flatness, perpendicularity, and symmetry when specific Geometric Dimensioning and Tolerancing (GD&T) callouts are not used. The Ultimate Guide to ISO 2768-mK: Streamlining Your
Obtain ISO 2768‑1 and ISO 2768‑2 (latest edition) for authoritative tables and exact angular/form tolerances.
Would you like a printable one‑page PDF with the exact ISO table values filled in (I will include the numeric tables from the standard)?
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.
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
Understanding ISO 2768-mk: The Standard for General Tolerances
In the world of precision manufacturing, specifying every single dimension with a dedicated tolerance would make technical drawings cluttered and nearly impossible to read. To solve this, engineers use general tolerance standards. The most common among these is ISO 2768-mk.
If you’ve seen "ISO 2768-mk" in the title block of a blueprint, What is ISO 2768?
ISO 2768 is an international standard created by the International Organization for Standardization. It provides a set of general tolerances for linear and angular dimensions without individual tolerance indications.
The goal is simple: to simplify drawings. By referencing ISO 2768, a designer tells the machinist, "Unless I specify otherwise, follow these standard accuracy levels." Breaking Down the "mk" Suffix
The designation "mk" consists of two separate parts that define the accuracy level for different features:
'm' (Lower Case): Refers to Part 1 of the standard, covering Linear and Angular dimensions. The 'm' stands for Medium.
'k' (Lower Case): Refers to Part 2 of the standard, covering Geometrical tolerances (like flatness, symmetry, and run-out). The 'k' is the class for general geometrical tolerances. ISO 2768-1: Linear Dimensions (The 'm') Why ‘Medium’ is the industry default You will
Under ISO 2768-1, there are four tolerance classes: f (fine), m (medium), c (coarse), and v (very coarse). The Medium (m) class is the most frequently used in general mechanical engineering. Linear Dimensions (mm)
For the 'm' class, the allowable deviation depends on the size of the dimension: Nominal Size (mm) Tolerance (± mm) 120 to 400 400 to 1000 External Radii and Chamfer Heights Nominal Size (mm) Tolerance (± mm) ISO 2768-2: Geometrical Tolerances (The 'k')
Part 2 focuses on the "form" of the part. There are three classes: H, K, and L. The K class is the medium-level requirement for geometry.
Flatness and Straightness: For a length up to 100mm, the 'k' class allows a 0.2mm deviation.
Perpendicularity: For a side up to 100mm, the limit is 0.4mm.
Symmetry: The 'k' class generally allows a symmetry deviation of 0.6mm. Run-out: Circular run-out for class 'k' is typically 0.2mm. Why use ISO 2768-mk?
Clarity: It keeps drawings clean. Only critical dimensions (like a bearing fit or a sealing surface) need specific ± tolerances.
Cost-Efficiency: It signals to the machine shop that standard workshop accuracy is sufficient for non-critical areas, preventing over-processing and reducing costs.
Consistency: It provides a universal language between designers and manufacturers worldwide, ensuring that a part made in Germany fits a part made in the USA. When NOT to use it
ISO 2768-mk is a "general" standard. You should never rely on it for: High-precision fits (e.g., H7/g6).
Parts made from plastics or materials with high thermal expansion (the standard is primarily designed for metal removal/machining). Dimensions where safety or critical function is at stake.
ISO 2768-mk is the "Goldilocks" of manufacturing tolerances—not too tight, not too loose. It ensures that parts are functional and interchangeable without unnecessary manufacturing expenses. When you see it on a drawing, you’re looking at a standard of Medium Linear accuracy and Medium Geometrical control.
This report is designed as a practical guide for engineers, drafters, and quality control inspectors to understand, apply, and inspect tolerances based on this standard.
These tolerances apply to features like external lengths, step heights, and diameters.
| Nominal Dimension Range (mm) | Tolerance (mm) | | --- | --- | | 0.5 up to 3 | ±0.1 | | >3 up to 6 | ±0.1 | | >6 up to 30 | ±0.2 | | >30 up to 120 | ±0.3 | | >120 up to 400 | ±0.5 | | >400 up to 1000 | ±0.8 | | >1000 up to 2000 | ±1.2 |
Example: If a shaft length is specified as 50 mm with no individual tolerance, ISO 2768-mk permits a length between 49.7 mm and 50.3 mm.
| 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.