Iso — 2768-mh Tolerance Chart
A brief comparative table (conceptual):
ISO 2768 defines tolerance grades for linear dimensions:
Each grade is applied to ranges of nominal sizes. The tolerance for a dimension equals the tolerance value from the chart corresponding to the nominal size and selected grade. The “m” grade gives moderate tolerances suitable for general machining.
ISO 2768‑1 also provides default angular tolerances (e.g., ±1° for coarse grades, ±0.5° for medium — see the standard for exact figures) and for unspecified radii/chamfers.
ISO 2768‑2 provides default values for geometrical tolerances when such symbols are omitted, but it is safer to specify geometric controls directly when required.
To legally invoke this standard, you must write it in the title block or general notes. The standard syntax is critical. iso 2768-mh tolerance chart
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. A brief comparative table (conceptual): ISO 2768 defines
| 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:
Example 1 — Shaft diameter called Ø25 mm on a drawing with "ISO 2768‑m" specified.
Example 2 — Hole 50 mm on drawing with ISO 2768‑m:
Example 3 — Angular dimension 90° with ISO 2768‑m: Each grade is applied to ranges of nominal sizes
While powerful, the ISO 2768-mh chart is not a universal panacea. Engineers must apply it with intelligence.
| Nominal Hole Size (mm) | Tolerance Grade (implied) | Tolerance (± or +/–) | |------------------------|---------------------------|----------------------| | ≤3 | IT13 | +0.14 / 0 | | >3–6 | IT13 | +0.18 / 0 | | >6–10 | IT13 | +0.22 / 0 | | >10–18 | IT13 | +0.27 / 0 | | >18–30 | IT13 | +0.33 / 0 | | >30–50 | IT13 | +0.39 / 0 | | >50–80 | IT13 | +0.46 / 0 | | >80–120 | IT13 | +0.54 / 0 | | >120–180 | IT13 | +0.63 / 0 | | >180–250 | IT13 | +0.72 / 0 | | >250–315 | IT13 | +0.81 / 0 |
✅ Example: A 20 mm diameter hole with no tolerance is allowed to be 20.00 mm to 20.33 mm. It can never be 19.98 mm.
⚠️ Important: H means only positive deviation (hole gets larger). This ensures assembly with shafts, pins, or bolts.