Because ISO standards are universally recognized, a drawing created under ISO 2768-mK can be seamlessly manufactured by any shop worldwide without interpretation errors. 5. When to Override ISO 2768-mK
Geometrical tolerances control the form, orientation, and location of features relative to one another. Under class , the variations are strictly regulated based on the length of the longest relevant feature. Straightness and Flatness Length of Longest Surface (mm) Tolerance (mm) Over 10 to 30 Over 30 to 100 Over 100 to 300 Over 300 to 1000 Over 1000 to 3000 Perpendicularity
| Nominal Size Range (mm) | Class f (Fine) | Class m (Medium) | Class c (Coarse) | Class v (Very Coarse) | | :--- | :---: | :---: | :---: | :---: | | | ±0.20 | ±0.20 | ±0.40 | ±0.40 | | 3 up to 6 | ±0.50 | ±0.50 | ±1.00 | ±1.00 | | 6 | ±1.00 | ±1.00 | ±2.00 | ±2.00 |
: Governed by ISO 2768-1 , this defines permissible deviations for linear and angular dimensions, such as lengths, radii, and chamfers.
ISO 2768-mK is an international standard used to define general tolerances for parts manufactured by machining or other material removal processes. It simplifies technical drawings by providing a default set of tolerances for dimensions that do not have an individually specified tolerance. The designation combines two specific precision classes: m (Medium): ISO 2768-1 general tolerance iso 2768-mk
Choosing the correct tolerance class is a critical design decision that directly impacts manufacturing cost and feasibility.
These tolerances apply to dimensions like length, width, and diameter when not specified. All values below are in . Nominal Range (mm) Tolerance (±) 120 to 400 400 to 1000 1000 to 2000 2000 to 4000 Additional "m" class values:
ISO 2768 is an international standard developed by the International Organization for Standardization (ISO) to regulate general tolerance requirements for mechanical parts. Its primary purpose is to simplify engineering drawings . Instead of cluttering a drawing with a tolerance value next to every single dimension, an engineer can add a single note — for example, "ISO 2768-mK" — in the title block. This single callout automatically defines the permissible variation for all features that do not carry an explicit individual tolerance.
Complete Guide to General Tolerance ISO 2768-mK In mechanical engineering and manufacturing, the standard serves as the universal benchmark for general tolerances, streamlining engineering drawings by establishing default acceptable limits for dimensions and geometry without requiring individual labeling for every feature. Because ISO standards are universally recognized, a drawing
Note: For nominal sizes below 0.5 mm, the deviation must be indicated adjacent to the size on the drawing.
These values define how much a surface or edge can bow or warp over its total length: Nominal Length Range (mm) Straightness and Flatness Tolerance (mm) Perpendicularity
Section D — Problem solving & design considerations (40 marks) 13. (10) You are designing a bracket with multiple features. Explain, with brief justification, which features you would: a) apply ISO 2768‑m to (3 examples), b) require specific tighter tolerances (3 examples), and c) select ISO 2768‑k for (2 examples). 14. (8) Calculate cumulative tolerance stack-up for three aligned features in series: A, B, and C, nominal lengths 15 mm, 25 mm, and 40 mm respectively, all unspecified on the drawing and ISO 2768‑m applies. Use the simplified table above to compute worst‑case total length tolerance and resulting possible total length range. 15. (8) For the same features as Q14 but B is specified with a tighter machining tolerance of ±0.05 mm (explicit), while A and C remain under ISO 2768‑m, compute the worst‑case total length range. 16. (6) Explain how note “ISO 2768‑m unless otherwise specified” can reduce drawing clutter but also identify two risks associated with relying on general tolerances. 17. (8) A customer requires interchangeable parts with consistent function across suppliers. Propose a concise set of drawing practices (6 actionable items) to ensure interchangeability while using ISO 2768‑m where appropriate.
saves you time and money. It tells your machinist: “I don’t need a micrometer for every single edge. Just machine it cleanly and consistently.” Under class , the variations are strictly regulated
Tolerances for straightness and flatness are selected based on the length of the corresponding line or the larger surface dimension. Perpendicularity
Highlighting individual tolerances immediately signals to the machinist where extreme precision is critical (e.g., bearing fits). When to Use Specific Tolerances Instead
(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