How to read H7/g6
A complete fit designation combines a hole tolerance and a shaft tolerance. Capital letters identify holes; lowercase letters identify shafts. The letter positions the tolerance zone relative to the nominal size. The number identifies the tolerance grade: a lower grade number generally means a narrower tolerance zone.
H7/g6 therefore describes an H7 hole and a g6 shaft at the same nominal diameter. It does not describe one fixed clearance for every diameter. The numerical deviations change at defined diameter ranges, so the nominal diameter is required before a machinist can turn the designation into limit sizes.
Clearance, transition and interference
The fit type comes from the two extreme assemblies. Minimum clearance is the smallest hole minus the largest shaft. Maximum clearance is the largest hole minus the smallest shaft. A positive result is clearance; a negative result represents interference.
- Clearance fit: both extreme assemblies retain clearance.
- Transition fit: tolerance zones overlap, so assembly may have clearance or interference.
- Interference fit: both extreme assemblies require interference.
Worked example: Ø25 H7/g6
For the supported Ø25 mm reference case, the H7 hole limits are 25.000 to 25.021 mm. The g6 shaft limits are 24.980 to 24.993 mm. Minimum clearance is therefore 25.000 − 24.993 = 0.007 mm. Maximum clearance is 25.021 − 24.980 = 0.041 mm.
The result is a 7–41 µm clearance fit. Dimvo shows these limits together with deviations and a tolerance-zone diagram. For contractual or safety-critical work, verify the result against the controlled drawing and authorized standard used for the job.
Choosing a fit by function
Start with what the parts must do, not with a memorized designation. A freely rotating or sliding part needs reliable running clearance. A precise locating feature may need small clearance or a transition zone. A permanent torque-transmitting joint may require controlled interference and an assembly process.
Temperature, lubrication, surface finish, geometry, material, wall thickness, load, assembly force and service access can all change the correct choice. A common-fit suggestion is a starting point for design discussion, not automatic approval.
What to put on the drawing
State the nominal size and the required tolerance designation for each controlled feature. Do not rely on a fit name alone. Make sure the drawing also defines the applicable standard edition, temperature/reference conditions and any geometry or surface requirements needed for function and inspection.