Dimensioning, Tolerancing, and Fits
Dimensioning rules
Dimension from visible geometry and datums, not from hidden lines. Give each feature once; do not repeat a size in two views. Use extension lines with a small gap from the object, dimension lines with arrowheads, and leaders for notes. Text is unidirectional under ISO and reads from the bottom or right. Prefer baseline or ordinate dimensioning to long chain dimensions that accumulate error.
Tolerances and fits
A limit dimension states the maximum and minimum; a plus/minus tolerance states the permitted variation. ISO 286 fits use the hole-basis system: H means the hole's lower deviation is zero, and the letter for the shaft sets the fit. H7/g6 is a sliding clearance fit, H7/k6 is a transition fit, H7/p6 is an interference fit. Always calculate from the actual deviations, not from the letter alone.
Worked example
A Ø25 H7 hole and Ø25 g6 shaft. H7 for this size is +0.021/0, so the hole limits are 25.021 and 25.000 mm. g6 is −0.007/−0.020, so the shaft limits are 24.993 and 24.980 mm. Maximum clearance = 25.021 − 24.980 = 0.041 mm. Minimum clearance = 25.000 − 24.993 = 0.007 mm. The fit is always clearance; the tolerance of the fit is 0.041 − 0.007 = 0.034 mm. The reasoning is to pair maximum hole with minimum shaft for maximum clearance, and minimum hole with maximum shaft for minimum clearance.
Traps
Reversing the deviation signs for a shaft, treating H7 as the same numeric tolerance at every diameter, dimensioning to a hidden line, omitting the datum reference for a geometric tolerance, and double-dimensioning a slot in two views.