Limits, Fits and Tolerances
This domain tests the language of interchangeable manufacture: limits, deviations, tolerances and the fit selected for a hole and shaft.
Definitions
Basic size is the nominal size; actual size is measured. Limits are the maximum and minimum permitted sizes. Tolerance is the difference between them. Deviation is the algebraic difference between a limit and the basic size. Allowance is the intentional difference between the hole and shaft limits; it may be positive (clearance) or negative (interference).
IS 919 / ISO 286
Tolerance grades run IT01, IT0, IT1 to IT16. Fundamental deviations are lettered A–ZC for holes and a–zc for shafts. Hole basis uses H (lower deviation zero) and is preferred. Shaft basis uses h (upper deviation zero). Clearance fits: H7/g6, H7/h6. Transition: H7/k6, H7/n6. Interference: H7/p6, H7/s6. Maximum material condition is the largest shaft and smallest hole; least material condition is the opposite.
Worked example
Hole Ø25 H7 (+0.021/0), shaft Ø25 g6 (−0.007/−0.020). Maximum clearance = 0.021 − (−0.020) = 0.041 mm. Minimum clearance = 0 − (−0.007) = 0.007 mm. Both are positive, so it is a clearance fit. Hole tolerance = 0.021 mm; shaft tolerance = 0.013 mm.
Traps
Subtracting the wrong limits; treating a negative deviation as a clearance; confusing MMC with LMC; using a shaft-basis fit when the drawing specifies hole basis; forgetting that H7/h6 has zero minimum clearance but is still a clearance fit.