Limits, Fits, Tolerances and Surface Finish Symbols
Definitions
Basic size is the nominal dimension; the limits are the two permissible sizes and the tolerance is their difference (T = UL − LL). Deviation is the algebraic difference from basic size; the fundamental deviation nearest the zero line gives the fit letter - capitals for holes (H), lower case for shafts (h). IT grades IT01 to IT16 scale with size and grade: IT7 is typical of a reamed precision bore, IT8–IT9 of general turning. Hole basis (H) is preferred because holes are made with fixed reamers while shafts are turned to suit.
Fits
Clearance: maximum clearance = hole max − shaft min. Interference: the shaft is always larger. Transition: either may result. Recognise common designations - 40 H7/g6 and H7/h6 are clearance (H7/h6 with zero minimum), H7/k6 and H7/js6 transition, H7/p6 and H7/s6 interference.
Surface finish
The tick symbol carries Ra in micrometres or an N-grade: N7 = 0.8, N8 = 1.6, N9 = 3.2, N10 = 6.3, N11 = 12.5 µm. A figure above the symbol is the Ra value; a tick with a circle means any production method is permitted; a tick with a bar means material must be removed.
Worked example
Hole 40 H7 (+0.025/0) with shaft 40 g6 (−0.009/−0.025). Maximum clearance = 0.025 − (−0.025) = 0.050 mm; minimum clearance = 0 − (−0.009) = 0.009 mm. Hole tolerance is 0.025 mm, shaft tolerance 0.016 mm, and because the minimum clearance is positive the fit is clearance throughout.
Traps
Subtracting deviations with the wrong signs (two negatives); quoting tolerance as allowance; calling H7/h6 an interference fit; giving Ra without µm; drawing the finish symbol without a value where the drawing requires one.