Iso 2768-mh Tolerance Chart
In practice, "ISO 2768-mH" means: All unspecified linear dimensions follow the 'm' (Medium) tolerance band, and all unspecified geometrical tolerances follow the 'H' grade. You might ask: Why not just tolerance every single hole and edge?
"The 'H' means the same as the 'm'." Reality: No. 'H' is specifically from ISO 2768-2. It defines geometry, not size. A part can be the wrong size (within 'm') but perfectly flat (within 'H'). Creating Your Own ISO 2768-mh Reference Card For your workshop wall or CAD template, condense the ISO 2768-mh tolerance chart into this quick-reference table:
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. Radii and Chamfers Under ISO 2768-m A frequently overlooked part of the standard concerns external radii (r) and chamfer heights. iso 2768-mh tolerance chart
If you design a shaft with a length of 50mm and do not write a tolerance, ISO 2768-mh applies. Looking at the chart, 50mm falls into the ">30 up to 120" row. Therefore, the acceptable length is 50mm ±0.3mm (49.7mm to 50.3mm). Angular Dimensions (ISO 2768-m) Angles are treated slightly differently. For Class 'm', the tolerance is expressed in millimeters per millimeter of the shorter side.
Introduction: Decoding the Alphabet Soup of Engineering Standards In the world of mechanical engineering and manufacturing, a drawing is a legal contract between the designer and the machinist. But what happens when a dimension on that drawing has no explicit tolerance value listed next to it? In practice, "ISO 2768-mH" means: All unspecified linear
"If I write ISO 2768-mh, I never have to tolerance diameters." Reality: Diameters (like a Ø10mm hole) follow the SAME linear chart. A Ø10mm hole could be Ø10.2mm. That is an H11 tolerance loose fit. For a running fit, you still need an explicit H7 or G6.
| Type | Condition | Tolerance | | :--- | :--- | :--- | | Linear (0.5-6mm) | Size | ±0.1mm | | Linear (6-30mm) | Size | ±0.2mm | | Linear (30-120mm) | Size | ±0.3mm | | Linear (120-400mm) | Size | ±0.5mm | | Flatness | Form | 0.2mm per 100mm | | Straightness | Form | 0.2mm per 100mm | | Perpendicularity | Orientation | 0.3mm per 100mm | | Symmetry | Location | 0.5mm | | Circular Runout | Runout | 0.2mm | | Angle (<10mm leg) | Angle | ±1° | The ISO 2768-mh tolerance chart is the silent workhorse of modern mechanical drawings. By adopting "m" for linear and "H" for geometric tolerances, you strike an optimal balance between manufacturing cost and functional reliability. 'H' is specifically from ISO 2768-2
| Class | Linear (6-30mm) | Geometric (Flatness per 100mm) | Typical Application | | :--- | :--- | :--- | :--- | | | ±0.05mm | 0.1mm | Precision instruments, aerospace backups | | m (Medium) | ±0.2mm | 0.2mm | General machining, welding fixtures | | c (Coarse) | ±0.5mm | 0.4mm | Sand castings, agricultural equipment | | v (Very Coarse) | ±1.0mm | 0.8mm | Rough fabrications, concrete molds |
