Topic 4 of 7 · Form 5
Engineering drawing standards, tolerances and fits
Drawing practice follows ISO (and SAZ-adopted) standards for lines, lettering, dimensioning, sections, conventions and title blocks.
Notes for this topic 1 file, in order
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Standards
Drawing practice follows ISO (and SAZ-adopted) standards for lines, lettering, dimensioning, sections, conventions and title blocks.
Sectioning
Full, half, offset, aligned, revolved, removed and part (broken-out) sections; hatching rules; parts not sectioned lengthways (shafts, bolts, ribs, webs, keys).
Conventions
Screw threads (external and internal), knurling, splines, springs, bearings, repeated features, and breaks in long parts.
Dimensioning
Functional dimensioning, chain versus parallel dimensioning, and datum surfaces.
Limits and fits
- Tolerance: permitted variation in a dimension (e.g. 25 ± 0.05 mm).
- Hole-basis system: the hole size is fixed (H7) and shafts vary.
- Fits: clearance (shaft always smaller than hole), transition, and interference (shaft larger than hole – press fit).
Worked example: A hole is 30.000/30.021 mm and a shaft 29.980/29.993 mm. Find the minimum and maximum clearance.
Minimum clearance = 30.000 − 29.993 = 0.007 mm; maximum = 30.021 − 29.980 = 0.041 mm → clearance fit
Surface finish and geometric tolerancing
Machining symbols and roughness values (Ra); geometric tolerances such as flatness, perpendicularity and concentricity.
Check yourself
- What is an interference fit?
- Why are shafts not hatched when sectioned lengthways?
- What does the hole-basis system mean?
Show answers
- A fit where the shaft is larger than the hole, so it must be pressed or shrunk into place.
- Hatching them would not add information and would confuse the drawing.
- The hole tolerance is fixed and different fits are obtained by varying the shaft size.
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