Engineering Drawing, Reading and Sketching
The exam tests line conventions, first and third angle projection, dimensioning rules, sections and freehand sketching.
Line work
Continuous thick lines show visible outlines; continuous thin lines are for dimension, projection and hatching; short dashes show hidden edges; thin chain lines mark centre lines and pitch circles; chain lines thickened at the ends mark cutting planes; wavy lines show breaks.
Projection
India follows first angle projection (IS 696): the object lies between the observer and the plane, so the top view is placed below the front view and the left side view appears on the right. In third angle the top view is above and the left side view on the left. The truncated cone symbol distinguishes the two systems. Views stay aligned and depth is transferred with a 45 degree miter line.
Dimensioning
Dimensions are either aligned or unidirectional, with arrowheads and a small gap between the object and extension line. Use R for radius, the diameter symbol for circles and a square symbol for square sections. Give each dimension once only, never on a hidden line, and quote tolerances as limits such as 25.02/25.00 or as 25 plus or minus 0.01. Surface roughness is given as Ra in micrometres, and a fit may appear as 25 H7/g6.
Sections and sketching
Full, half, revolved, offset and removed sections. Hatching uses thin lines at 45 degrees, same direction within one part and opposite for adjacent parts. Isometric sketches use three axes at 30 degrees with true lengths along them; oblique sketches keep one axis at 90 degrees.
Worked example
A stepped shaft is drawn in first angle with an overall length of 120 and a step dimensioned 40 plus or minus 0.1. The top view must be placed below the front view. The dimension 40 plus or minus 0.1 accepts 39.9 to 40.1 mm, giving a total tolerance of 0.2 mm, not 0.1 mm.
Traps that cost marks
- Putting the top view above the front view in first angle.
- Dimensioning to a hidden line.
- Repeating the same dimension in two views.
- Reading plus or minus 0.1 as a 0.1 mm tolerance band.