Why does BIS tolerance notation specify hole designation before shaft designation in fits?
To indicate the hole basis or shaft basis relationship used for the fit
In the BIS/ISO system of limits and fits a fit is written by pairing a hole tolerance symbol with a shaft tolerance symbol, for example H7/g6.
By convention the hole designation is always placed first, and that leading symbol declares the basis on which the fit is built.
A capital letter first, such as H, signals the hole-basis system, where the hole tolerance is held constant and different shafts are used to obtain clearance, transition or interference.
Had a shaft symbol led, it would indicate the shaft-basis system, where the shaft is kept constant instead; so the order itself carries the hole-basis-versus-shaft-basis information.
The order is not there to state the shaft's tolerance magnitude first, nor to fix an inspection sequence, nor to give the actual machined size, which is read from the part, not the notation.
The purpose of writing the hole before the shaft is therefore to declare which basis governs the fit.
Hence, the answer is To indicate the hole basis or shaft basis relationship used for the fit.
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Important Questions from Limits and Fits
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\(Hole = 25_{ - 0.01}^{ + 0.02}\;mm\;and\;shaft = 25_{ - 0.02}^{ - 0.004}\;mm\)
Determine tolerance on a shaft.
When tolerance is given on one side of the basic dimension, it is called:
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Which of the following fit is produced with the shaft dia 25.025 and bore dia of 24.994?
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