| 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:
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.
Remember the golden rule: Use ISO 2768-mh for non-critical features. Reserve explicit tolerances for fits, functions, and failures. Keep a laminated copy of the chart near your CMM and another in your CAD template. When a machinist asks, "What tolerance goes here?", you can confidently reply: "Check the title block—ISO 2768-mH applies."
Next steps for your engineering workflow: iso 2768-mh tolerance chart
By mastering this standard, you stop over-tolerancing the trivial and start focusing on the true functional interfaces that define great product design.
The ISO 2768-mH standard defines general tolerances for parts manufactured by metal removal (machining). The designation "mH" indicates two specific tolerance classes: "m" for linear and angular dimensions (Part 1) and "H" for geometrical features like straightness and flatness (Part 2). These standards are used to simplify engineering drawings by providing "standard" values for any dimension not specifically toleranced. Part 1: Linear & Angular Dimensions (Class "m" - Medium)
Class m (Medium) is the most common tolerance class for CNC machined metal parts. Below are the permissible deviations for linear measures and angles. Nominal Size Range (mm) Linear Dimensions (mm) Ext. Radii & Chamfers (mm) Angular Dimensions ±plus or minus ±plus or minus ±plus or minus ∘raised to the composed with power (up to 10mm) ±plus or minus ±plus or minus ±plus or minus ∘raised to the composed with power 30' (10-50mm) ±plus or minus ±plus or minus ±plus or minus ∘raised to the composed with power 20' (50-120mm) > 30 to 120 ±plus or minus ±plus or minus ±plus or minus ∘raised to the composed with power 10' (120-400mm) > 120 to 400 ±plus or minus ±plus or minus ±plus or minus ∘raised to the composed with power 5' (over 400mm) > 400 to 1000 ±plus or minus Sources: Part 2: Geometrical Tolerances (Class "H")
Class H specifies the allowable deviation for the shape and position of features. Feature Type Range (mm) Tolerance (mm) Straightness & Flatness 100 to 300 Perpendicularity 100 to 300 Symmetry Circular Run-out All ranges Sources: Why Use ISO 2768-mH? | Tolerance Class | Straightness & Flatness (mm)
What is ISO 2768? | CNC Machining Tolerance Standards - Fictiv
| Nominal Size Range (mm) | Tolerance (mm) | |------------------------|----------------| | 0.5 up to 3 | ±0.2 | | >3 up to 6 | ±0.5 | | >6 up to 30 | ±1.0 | | >30 up to 120 | ±2.0 | | >120 up to 400 | ±4.0 |
| Tolerance | |-----------| | 0.1 mm |
Note: Cylindricity, profile tolerances, and positional tolerances are not covered by ISO 2768-2. By mastering this standard, you stop over-tolerancing the
For angles, the tolerance is not a fixed degree value but is given as a deviation per length of the shorter leg.
| Nominal Length Range of the Shorter Leg (mm) | Permissible Deviation for Class 'm' | | :--- | :--- | | up to 10 | ± 1° | | >10 up to 50 | ± 0° 30' | | >50 up to 120 | ± 0° 20' | | >120 up to 400 | ± 0° 10' | | >400 | ± 0° 5' |
Interpretation: A 45° angle on a part with a short leg of 60 mm can vary between 44°40' and 45°20'.
A brief comparative table (conceptual):