ISO 2768-mh specifies general tolerances for linear and angular dimensions of metal parts produced by machining or metal removal. This standard is designed to simplify drawings by establishing standard tolerances for features that don't require specific, tighter precision. The "mh" designation indicates two separate classes: m: Medium class for linear and angular dimensions.
h: Tolerance class for geometrical tolerances (straightness, flatness, etc.). 1. Linear Dimensions (ISO 2768-m)
The following tolerances apply to linear dimensions such as external sizes, internal sizes, step sizes, diameters, and radii. Nominal Size 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 2. External Radii and Chamfer Heights
These tolerances apply to broken edges, such as external radii and chamfer heights. Nominal Size Range (mm) Tolerance (± mm) Over 3 to 6 3. Angular Dimensions
These tolerances apply to angular dimensions, usually expressed in degrees and minutes. Length of Shorter Leg (mm) Tolerance (±) 1∘1 raised to the composed with power Over 10 to 50 Over 50 to 120 Over 120 to 400 4. Geometrical Tolerances (ISO 2768-h)
The "h" class defines the general tolerances for geometric characteristics. Straightness and Flatness: Up to 10 mm: 0.1 mm 10 to 30 mm: 0.2 mm 30 to 100 mm: 0.4 mm 100 to 300 mm: 0.8 mm 300 to 1000 mm: 1.2 mm
Perpendicularity: Max 0.4 mm (for ranges up to 300 mm) to 0.6 mm (over 300 mm). Symmetry: 0.5 mm. Run-out: 0.1 mm. Usage Summary iso 2768-mh tolerance chart
When you see ISO 2768-mh on a technical drawing, it means that unless otherwise specified, the workshop must adhere to the Medium linear tolerances and High (h) geometric tolerances listed above. This approach reduces drafting time and clarifies which dimensions are critical versus standard.
The designation ISO 2768-mh defines a standard set of general tolerances for engineering drawings that lack individual tolerance specifications. It is divided into two parts: Part 1 (m): (m) tolerance for linear and angular dimensions. Part 2 (h):
class tolerance for geometric features like straightness, flatness, and perpendicularity. ISO 2768-1: Linear Dimensions (Medium Class "m")
These tolerances apply to external/internal sizes, diameters, and distances. Values are in millimeters ( skolarium.com Nominal Size Range ( Medium (m) Tolerance plus or minus 0.1 plus or minus 0.1 plus or minus 0.2 plus or minus 0.3 plus or minus 0.5 plus or minus 0.8 plus or minus 1.2 plus or minus 2.0 ISO 2768-1: Broken Edges (External Radii & Chamfers) Nominal Size Range ( Medium (m) Tolerance plus or minus 0.2 plus or minus 0.5 plus or minus 1.0 ISO 2768-2: Geometric Tolerances (Class "H")
Geometric tolerances control the shape and orientation of features. The Basics of General Tolerance Standard – ISO 2768-mK
To legally invoke this standard, you must write it in the title block or general notes. The standard syntax is critical. ISO 2768-mh specifies general tolerances for linear and
Correct notation:
ISO 2768-mH
Alternatively (for clarity):
General tolerances according to ISO 2768-1 (Class m) and ISO 2768-2 (Class H).
What to avoid: Do not write just "Tolerances: mH" without referencing ISO 2768. The standard implies specific rules for radii, chamfers, and how to treat zero values.
Do not rely on ISO 2768-mH for:
| Tolerance Class | Straightness & Flatness (mm) | Perpendicularity (mm) | Symmetry (mm) | Circular Runout (mm) | | :--- | :--- | :--- | :--- | :--- | | H (Medium) | 0.2 per 100mm length | 0.3 per 100mm length | 0.5 | 0.2 |
Interpretation of Table 2:
A frequently overlooked part of the standard concerns external radii (r) and chamfer heights.
| Dimension Range for Radii/Chamfer (mm) | Tolerance Class 'm' (mm) | | :--- | :--- | | 0.5 up to 3 | ±0.2 | | >3 up to 6 | ±0.5 | | >6 up to 30 | ±1.0 |
Example: A 5mm chamfer callout with no tolerance is allowed to be 4.5mm to 5.5mm. That is a massive 20% variation. If you need a cosmetic 45° x 1.5mm chamfer, you must explicitly tolerance it as max 1.6mm/ min 1.4mm.