ISO 2768-mK is an international manufacturing standard used to define general tolerances for mechanical parts, specifically for dimensions that don't have individual tolerance callouts on a drawing. Using this standard simplifies technical drawings by removing the need to label every single dimension with a plus/minus value. Breaking Down "mK"
The designation "mK" combines two specific tolerance classes from different parts of the ISO 2768 standard: m (Medium) : Defined in ISO 2768-1 , this class specifies permissible deviations for linear and angular dimensions (like lengths, radii, and diameters). K (Geometric) : Defined in ISO 2768-2 , this class specifies tolerances for geometrical features
such as flatness, straightness, perpendicularity, and run-out. Key Tolerance Tables
The following tables outline the permissible deviations (in mm) for the "m" and "K" classes. Linear Dimensions (Class m)
For basic sizes from 0.5mm up to 4000mm, these are the standard deviations for the medium (m) class: Nominal Length (mm) Permissible Deviation (± 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 Geometrical Tolerances (Class K)
These deviations apply to the form and position of features under the "K" designation: Feature Type Tolerance Range (mm) Straightness/Flatness 0.05 (up to 10mm) to 0.8 (over 1000mm) Perpendicularity 0.4 (up to 100mm) to 0.8 (over 1000mm) 0.6 (up to 100mm) to 1.0 (over 1000mm) Circular Run-out 0.2 (fixed value) Why Use ISO 2768-mK?
What is ISO 2768? | CNC Machining Tolerance Standards - Fictiv
ISO 2768 is the international standard for general tolerances, designed to simplify engineering drawings by providing standardized limits for dimensions without individual tolerance markings . When you see ISO 2768-mK
on a drawing, it specifically combines two different parts of the standard: m (Medium) : Refers to ISO 2768-1 , covering linear and angular dimensions. : Refers to ISO 2768-2
, covering geometrical tolerances such as flatness, straightness, and symmetry. Key Reference Resources (PDF & Articles) Comprehensive Overviews ISO 2768-1 Official Summary
provides the fundamental concepts and tables for linear dimensions. Technical Breakdown Fictiv's Guide to ISO 2768
explains why this standard is used in CNC machining to reduce costs and prevent misunderstandings. Geometric Tolerances : For details on the "K" part (geometry), this ISO 2768-2 Sample iTeh Standards outlines rules for parallelism, perpendicularity, and more. Quick Reference Charts : A widely used General Tolerances Chart PDF ZEISS Quality Forum
provides a one-page lookup for "m" (medium) and other classes. Summary Tables for ISO 2768-mK
These values apply to dimensions where no specific tolerance is listed. waterson.com Part 1: Linear Dimensions (Class m) Nominal Size Range (mm) Tolerance (± mm) Over 3 to 6 Over 6 to 30 Over 30 to 120 Over 120 to 400 Part 2: Geometrical Tolerances (Class K) Nominal Length Range (mm) Straightness/Flatness (mm) Perpendicularity (mm) Symmetry (mm) 100 to 300 (Part 2) or tips on how to properly indicate these on your drawings Quality Assurance Auditor CNC Machinist General Tolerance - ISO 2768 1 & 2 - ZEISS Quality Forum
Understanding ISO 2768-mk: The Standard for General Geometrical Tolerances Tolerance Iso 2768 Mk Pdf
ISO 2768-mk is an international quality standard designed to simplify technical drawings by providing general tolerances for linear and angular dimensions, as well as geometrical specifications (such as flatness and symmetry). It is the backbone of efficient manufacturing, ensuring that parts fit together correctly without requiring every single dimension to be individually toleranced. The Purpose of ISO 2768
In engineering design, specifying tolerances for every dimension is time-consuming and often leads to cluttered drawings. ISO 2768 solves this by establishing a "default" level of precision. When a designer specifies ISO 2768-mk on a drawing, they are instructing the manufacturer to follow a specific set of accuracy rules for any dimension that doesn't have its own unique tolerance. Breaking Down the "mk" Designation The designation is split into two distinct parts:
"m" (General Tolerances for Linear and Angular Dimensions): This letter refers to Part 1 of the standard. The "m" stands for Medium. Other classes include 'f' (fine), 'c' (coarse), and 'v' (very coarse).
"k" (General Geometrical Tolerances): This letter refers to Part 2 of the standard, which covers features like straightness, flatness, and circularity. The "k" represents a specific tolerance class for these geometric characteristics. Other classes in Part 2 include 'H' and 'L'. Key Components of the Standard
Linear Dimensions: These cover the permissible deviation in length, such as external sizes, internal sizes, and step sizes. The tolerance increases as the size of the part increases.
Angular Dimensions: These define the allowed error in degrees and minutes for angles, also scaled based on the length of the shorter leg of the angle.
Geometrical Tolerances: This includes limits on how much a surface can warp (flatness) or how much two features can be off-center (symmetry/run-out). Why ISO 2768-mk is the Industry Favorite
The "mk" combination is widely considered the industry standard for general machining. It provides a balance that is precise enough for most mechanical assemblies while remaining achievable for standard CNC machines and manual tools without driving up production costs.
By using this standard, companies reduce the time spent on "dimensioning and tolerancing" (GD&T), minimize disputes between designers and machinists, and ensure a baseline level of quality across global supply chains.
| 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: A shaft length of 50mm with no individual tolerance is acceptable between 49.7mm and 50.3mm.
False. This standard explicitly excludes:
Imagine you design a motor mount bracket with a hole pattern. The critical dimension is the center distance (100mm) for the motor bolts. You specify that as 100 ± 0.05 mm (tight tolerance).
However, the overall length of the plate (200mm) is non-critical. You leave it blank. Because of ISO 2768-mK, the machinist knows the 200mm length can vary by ±0.5mm. If the length comes back at 199.6mm, you cannot scrap the part. It is technically compliant. ISO 2768-mK is an international manufacturing standard used
No. The standard is case-sensitive. You will often see it written as mK (lowercase m, uppercase K) or Mk (rare). Technically, "M" implies the linear class, "K" implies the geometry class. Writing "MK" in all caps is common in file naming but does not change the numerical values.
To apply this standard, add a note in the title block of your engineering drawing:
"GENERAL TOLERANCES ISO 2768-mK"
This single line replaces dozens of individual tolerance boxes, cleans up the drawing, and clearly defines the acceptable limits for the manufacturer.
Conclusion: ISO 2768-mK is the "Goldilocks" of machining tolerances—not too tight (expensive), not too loose (non-functional). It is perfect for 80% of standard mechanical parts.
Disclaimer: This article is for informational purposes. Always verify tolerances against the official ISO 2768:1989 standard for your specific application.
The Tolerance Tango: A Story of Precision and Collaboration
In the world of engineering and manufacturing, precision is key. One of the most widely used standards for tolerances is ISO 2768, and its variant, ISO 2768-MK. But what does it mean, and how can it help you create high-quality products?
The Challenge
Meet Alex, a young engineer working for a company that produces precision machinery. Alex's team was tasked with designing and manufacturing a critical component for a high-stakes project. The client required a very specific set of tolerances to ensure the component would fit perfectly into the larger assembly.
As Alex began to work on the design, she realized that she needed to specify the tolerances carefully. Too loose, and the component might not fit; too tight, and it might be impossible to manufacture. That's when she stumbled upon ISO 2768-MK.
The Discovery
ISO 2768-MK is a standard that provides guidelines for general tolerances in engineering. It defines the acceptable limits of variation for linear and angular dimensions. The "MK" variant specifically provides a set of tolerances for medium- and high-precision applications.
Alex downloaded the ISO 2768-MK PDF and began to study it. She learned that the standard provided a range of tolerance classes, each with its own set of limits. She realized that by specifying the correct tolerance class, she could ensure that her design was both precise and manufacturable. Worn or obsolete parts
The Collaboration
As Alex worked on the design, she collaborated with her colleagues, including a seasoned machinist named Jack. Jack had years of experience working with tolerances and knew the practical implications of specifying the right limits.
Together, Alex and Jack reviewed the ISO 2768-MK standard and selected the most suitable tolerance class for their design. They considered factors such as the material, manufacturing process, and the component's function.
By working together and using the ISO 2768-MK standard as a guide, Alex and Jack were able to create a design that met the client's requirements and was also easy to manufacture.
The Outcome
The final product turned out to be a huge success. The component fit perfectly into the larger assembly, and the client was thrilled with the result. Alex and her team had not only met but exceeded the client's expectations.
The experience taught Alex the importance of collaboration and the value of using established standards like ISO 2768-MK. By working together and using the standard as a guide, engineers and manufacturers can create high-quality products that meet the most demanding requirements.
The Takeaway
If you're working on a design or manufacturing project, remember that tolerances are crucial to its success. Consider using the ISO 2768-MK standard as a guide, and collaborate with your colleagues to ensure that you're specifying the right limits.
By doing so, you'll be able to create products that are both precise and manufacturable, and that meet the most demanding requirements.
You can download the ISO 2768-MK PDF from various online sources or purchase a copy from the International Organization for Standardization (ISO) website.
Tolerance classes for general use:
The ISO 2768-MK standard provides a range of tolerance classes, each with its own set of limits. By selecting the correct class, you can ensure that your design is both precise and manufacturable.
Under the "m" class: