Iso 2768-mh Tolerance Chart Extra Quality

The Ultimate Guide to ISO 2768-mh Tolerance Chart: Precision for General Engineering

How to Specify ISO 2768-mh on a Drawing

To legally invoke this standard, you must write it in the title block or general notes. The standard syntax is critical.

Correct notation:

ISO 2768-mH

Alternatively (for clarity):

General tolerances according to ISO 2768-1 (Class m) and ISO 2768-2 (Class H). iso 2768-mh tolerance chart

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.

Part 1: Linear Dimensions (Class 'm' - Medium)

These tolerances apply to dimensions like lengths, diameters, step heights, and radii. The Ultimate Guide to ISO 2768-mh Tolerance Chart:

| Nominal Dimension Range (mm) | Permissible Deviation for Class 'm' (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 | | >2000 up to 4000 | ± 2.0 |

Interpretation: For a shaft with a nominal diameter of 25 mm, the 'm' tolerance allows the actual diameter to be anywhere between 24.8 mm and 25.2 mm. For a longer part of 300 mm, the allowed range is ±0.5 mm (299.5 mm to 300.5 mm). ISO 2768-mH

The Strategic Rationale Behind Using ISO 2768-mh

Why would an engineer specify "mh" instead of a tighter "ff" (fine/fine) or a looser "cL" (coarse/coarse)?

1. Cost Optimization: 'm' for linear dimensions is the most common default because it aligns with the capabilities of standard machining processes (turning, milling, drilling) without requiring secondary operations like grinding. Using 'f' could increase costs by 30-50% due to slower feeds, dedicated tooling, or manual inspection.
2. Balanced Performance: By pairing the 'medium' linear tolerance with 'fine' geometrical tolerance ('h'), the standard acknowledges a key engineering reality: controlling form and orientation (e.g., flatness, perpendicularity) is often more critical for assembly and function than controlling absolute size. Two parts can be slightly oversized but still assemble if they are perfectly flat and square. Conversely, perfect size with poor flatness leads to rocking and stress concentrations.
3. Reduced Drawing Clutter: Without a general tolerance note, an engineer would have to append a specific tolerance to nearly every dimension on a complex drawing. The ISO 2768-mh note instantly applies a rational, pre-defined tolerance regime to all untoleranced dimensions, dramatically simplifying documentation.

ISO 2768-mH Tolerance Chart (Linear Dimensions)

For linear dimensions (lengths, widths, depths, steps, etc.), class m (medium) applies.

ISO 2768‑m / General Tolerance Guidance