Dimensioning, Tolerancing and Surface Finish
This domain tests whether a drawing can actually be manufactured from: complete, unambiguous dimensions, correct symbols, an expressed tolerance and a specified surface texture.
Dimensioning rules
Every feature is dimensioned once, on the view where it appears as a visible outline - never on a hidden line. Use thin extension lines clear of the outline, with dimension lines terminating in arrowheads. Figures are read unidirectional, placed above the dimension line, and are in millimetres with no unit symbol except where another unit is used. Group dimensions outside the outline, avoid crossing dimension lines, and never close a chain of dimensions onto an overall dimension unless it is bracketed as a reference.
Symbols
Ø diameter, R radius, SR spherical radius, □ square, with depth and pitch noted. Countersinks are specified by symbol, diameter and angle (e.g. 90°), counterbores by symbol and diameter, and chamfers as 1 × 45°.
Tolerances and fits
A tolerance is the difference between the upper and lower limits. Limits may be stated directly (25.00/24.95) or as deviations from a nominal size. Hole-basis fits keep the hole at H; H7/h6 gives a close running fit with negligible clearance. Clearance, transition and interference fits describe whether the shaft is always smaller, sometimes larger, or always larger than the hole. Geometric tolerances use framed symbols with a datum letter, controlling form, orientation and position.
Surface finish
Surface texture is specified by the tick symbol carrying a roughness value, Ra in micrometres (typical values 0.4, 0.8, 1.6, 3.2, 6.3, 12.5), with machining process or lay added where needed.
Worked example
A shaft is dimensioned Ø25.00/24.95. Tolerance = 25.00 − 24.95 = 0.05 mm. Paired with a hole Ø25.00/25.02, the minimum clearance is 25.00 − 25.00 = 0 mm and the maximum is 25.02 − 24.95 = 0.07 mm - a clearance fit.
Traps
Dimensioning to hidden lines; repeating a dimension on two views; writing 'mm' after every figure; reversing upper and lower limits; confusing Ra with Rz; omitting the datum letter from a geometric tolerance frame.