H7 and g6 fits are commonly discussed in precision mechanical design because they help engineers define the relationship between a hole and shaft. The correct fit should always be selected from the functional requirement and the applicable standards.
What Is a Fit?
A fit describes the relationship between the size limits of mating parts such as a shaft and hole. Depending on the tolerances, the assembly may have clearance, transition or interference.
Why H7 Is Common for Holes
In a hole-basis system, the hole tolerance zone is commonly used as the reference while the shaft tolerance is selected to obtain the desired fit. H7 is one standardized hole tolerance designation.
What Does g6 Mean?
g6 identifies a shaft tolerance zone. The lowercase letter describes the position of the shaft tolerance zone relative to the nominal size, while the number indicates the tolerance grade.
Clearance Fit
A clearance fit ensures that the shaft remains smaller than the hole over the specified size limits. This is useful where parts must slide or rotate relative to each other.
Transition and Interference Fits
Transition fits can produce either small clearance or interference depending on actual sizes. Interference fits intentionally create a tight assembly and may require pressing or thermal methods.
How to Choose a Fit
Consider movement, lubrication, operating temperature, assembly method, material, manufacturing capability and required service life. Do not select H7/g6 simply because it is familiar.
Practical CAD Drawing Tip
When a fit is functionally important, specify the hole and shaft tolerances clearly and ensure the mating features use compatible datum and inspection requirements.
Understanding ISO fits and tolerances helps mechanical designers communicate functional assemblies more accurately and avoid unnecessary trial-and-error during production.