Tolerances and fits
Tolerances and fits questions ask you to read a fit designation, calculate limits and clearances, classify the joint as clearance, transition or interference, and interpret geometric tolerance frames.
The ISO 286 system
A tolerance zone is fixed by a fundamental deviation (letter) and a grade (IT number). Holes use capitals, shafts lower case; H means the lower deviation is zero, h means the upper deviation is zero. Grades run IT01–IT18, and for a given nominal size the IT value is the total width of the zone, not a ± value. The hole-basis system (an H hole against a lettered shaft) is preferred because holes are harder to machine to a lettered deviation.
Common fits
H7/g6 - close running or sliding fit. H7/h6 - location with minimal clearance. H7/k6 and H7/m6 - transition fits, located and easily dismantled. H7/p6 and H7/s6 - interference fits, pressed on and transmitting torque by friction. Rolling-bearing seats follow the same logic: shaft k5/m5, housing H7 for a stationary outer ring.
Geometric tolerancing
A frame carries the symbol, the tolerance and the datum, e.g. perpendicularity 0.05 to A. Flatness, straightness and cylindricity need no datum; parallelism, perpendicularity, angularity and position do. The Ⓜ symbol (maximum material condition) allows bonus tolerance as the feature departs from maximum material.
Worked example
Ø40 H7/g6. For the 30–50 mm range, IT7 = 25 µm and IT6 = 16 µm; the g deviation is es = −9 µm. Hole: EI = 0, ES = +25 µm, giving 40.000/40.025 mm. Shaft: es = −9 µm, i.e. 39.991 mm; ei = −9 − 16 = −25 µm, i.e. 39.975 mm. Minimum clearance = EI − es = 0 − (−9) = 9 µm; maximum clearance = ES − ei = 25 − (−25) = 50 µm. The fit is a clearance fit of 0.009–0.050 mm.
Traps
Treating IT6 = 16 µm as ±16 µm, swapping upper and lower case, forgetting that clearances are hole minus shaft (EI − es and ES − ei), measuring at a temperature other than 20 °C, and calling a joint clearance when the drawing specifies H7/p6.