Dimensioning, tolerancing and annotation
This domain tests whether a drawing conveys size, fit and finish without ambiguity. Questions ask you to add dimensions correctly, interpret limits and fits, or explain a feature control frame.
Core rules
- ISO 129: dimension and extension lines are thin continuous. Extension lines start about 1 mm clear of the outline and extend about 2 mm past the dimension line. Arrowheads are roughly 3:1 long to wide.
- Figures are unidirectional, placed above the dimension line, in millimetres with no unit written.
- Symbols: diameter, radius, spherical radius, square section, thickness, countersink, depth, and chamfer shown as 2 x 45 degrees.
- Dimension the view that shows the feature in true shape; never dimension to a hidden line; do not repeat a dimension; keep small dimensions nearest the outline.
- Limits and fits to ISO 286: hole-basis system uses H for the hole, letters g, h, k, p for shafts. Uppercase letters apply to holes, lowercase to shafts. IT grades control tolerance size.
- Geometric tolerancing uses a feature control frame with symbol, tolerance zone and datum letter; surface texture is specified as Ra.
Worked example
A 25 mm hole-basis sliding fit, 25 H7/g6. For 25 mm, H7 gives the hole limits +0.021 and 0, and g6 gives the shaft limits -0.007 and -0.020. Maximum clearance = 0.021 - (-0.020) = 0.041 mm; minimum clearance = 0 - (-0.007) = 0.007 mm. Both positive, so the fit always has clearance.
Traps
Swapping the upper and lower deviation signs; using a capital letter for a shaft; forgetting the diameter symbol before a hole size; chain dimensioning that builds a tolerance stack; leaving a datum letter undefined.