The Core Difference Between H and h Tolerances
Within the ISO system of limits and fits, the fundamental rule of thumb is: capital letters are used for holes, while lowercase letters are used for shafts. H and h serve as the respective datum elements for the basic hole system and the basic shaft system:
Capital H (Datum Hole in the Basic Hole System):
Its lower deviation is always zero.
This means that the machined hole can only be larger than the basic size; it can never be smaller. On technical drawings, its tolerance zone is situated entirely above the zero line.
Lowercase h (Datum Shaft in the Basic Shaft System):
Its upper deviation is always zero.
This means that the machined shaft can only be smaller than the basic size; it can never be larger. On technical drawings, its tolerance zone is situated entirely below the zero line.
Tolerance Values for Different Diameter Ranges (Size Segments)
In actual manufacturing practice, the magnitude of a tolerance-that is, the permissible range of error-is not static. As the basic dimension (diameter) increases, the difficulty of machining rises; consequently, the tolerance ranges prescribed by ISO standards are correspondingly broadened.
To provide a clear visual illustration of this pattern, the following section presents the specific variations in tolerance values across different diameter ranges, using H7 (high-precision holes) and h6 (high-precision shafts)-two of the most commonly used tolerance classes in industry-as examples (Units: micrometers [μm]; 1 μm = 0.001 mm):
comparison of product parameters
| Diameter (Basic Size) Range (mm) | H7 Hole Tolerance (μm) (Upper Deviation / Lower Deviation) |
h6 Shaft Tolerance (μm) (Upper Deviation / Lower Deviation) |
| ≤ 3 | +10 / 0 | 0 / -6 |
| > 3 ~ 6 | +12 / 0 | 0 / -8 |
| > 6 ~ 10 | +15 / 0 | 0 / -9 |
| > 10 ~ 18 | +18 / 0 | 0 / -11 |
| > 18 ~ 30 | +21 / 0 | 0 / -13 |
| > 30 ~ 50 | +25 / 0 | 0 / -16 |
| > 50 ~ 80 | +30 / 0 | 0 / -19 |
| > 80 ~ 120 | +35 / 0 | 0 / -22 |
Key Summary and Practical Tips
Fixed Boundaries: Regardless of how the diameter changes, the lower limit for H always remains at 0 (representing a positive deviation), while the upper limit for h always remains at 0 (representing a negative deviation).
Tolerance Values Increase with Diameter: A comparison of the tables reveals that for diameters under 3 mm, the H7 tolerance is a mere 10 microns; however, as the diameter approaches 100 mm, the H7 tolerance widens to 35 microns.
Lower Numbers Indicate Higher Precision: The number following the letter (e.g., H7, H8, h6, or h7) denotes the tolerance grade. A lower number (for instance, grade 6 is more precise than grade 7) signifies a narrower tolerance zone and, consequently, more stringent manufacturing requirements. Since machining shafts is generally easier than machining holes, a "hole one grade lower than the shaft" fit is commonly adopted (e.g., H7/h6 or H8/h7).
