Engineering Drawing and Graphics
What you must command
You need the conventions of orthographic projection (first-angle and third-angle), the standard arrangement of the three principal views, sectioning practice, dimensioning rules, and the meaning of symbols for surface finish, welding, and fits. Understand isometric and oblique projection, auxiliary views for inclined surfaces, and the geometry of development of surfaces (cones, cylinders, prisms).
Traps that appear in questions
- First vs. third angle: in first-angle projection the view from the front is placed below the front view when projected; in third-angle it is placed above. Questions test the symbol (truncated cone) and the layout.
- Hidden lines: dashed lines represent edges not visible in that view; they are omitted where a centre line or a visible line coincides.
- Dimensioning: never dimension to a hidden line; avoid duplicate dimensions; chain vs. baseline dimensioning.
- Section lines: hatch at 45° to the principal outline, spacing uniform, and adjacent parts hatched in opposite directions.
- True length: a line inclined to both planes requires an auxiliary view or rotation to find true length.
Worked rule of thumb
For a line of true length L inclined at angle α to the horizontal plane and β to the vertical plane, the front view length is L·cos β and the top view length is L·cos α. If a 60 mm line is inclined at 30° to the HP, its top view is 60·cos30° = 51.96 mm. Use this to check whether a drawn projection is consistent - a quick arithmetic check catches many errors.
For fits, remember the hole-basis system: H is the fundamental hole, and the shaft letter (h, g, f, k, p) sets the clearance or interference. H7/g6 is a sliding fit; H7/p6 is an interference fit.