Engineering Drawing
Engineering Drawing
The exam checks whether you can read an Indian Standards drawing: projection method, line conventions, dimensioning rules, sectional views and geometric tolerance frames. Most marks come from interpreting a symbol or spotting a drawing that breaks a convention.
Core conventions
- IS drawings use first-angle projection: the object sits between the observer and the plane, so the view of the left side is placed on the right of the front view, and the top view goes below the front view. Third-angle puts the top view above.
- Line types: continuous thick for visible outlines, continuous thin for dimension and projection lines and hatching, dashed for hidden edges, chain thin for centre lines and pitch circles, wavy thin for break lines.
- Dimensioning: dimension lines thin with arrowheads, numerals placed above the line and read from the bottom or right, no dimension on a hidden line, no dimension repeated in two views, and dimensions taken from a common datum to avoid tolerance stack-up.
- Section views use 45° hatching; adjacent parts are hatched in opposite directions.
Geometric tolerancing
A feature control frame reads left to right: tolerance symbol, tolerance value, datums. Common symbols include straightness (a straight line), flatness (a parallelogram), circularity (a circle), cylindricity, perpendicularity, parallelism and position (a circle with a cross).
Worked example
A frame reads: position, diameter 0.2, M in a circle, then A, B, C. Reasoning: the axis of the hole must lie within a cylindrical zone 0.2 mm in diameter, oriented and located from datums A (primary), B and C; the M modifier means bonus tolerance is added when the hole is larger than maximum material condition. Datum A controls orientation, B and C control location.
Traps
Placing the top view above the front view out of third-angle habit; dimensioning to hidden lines; using a centre line where a hidden line belongs; misreading the M modifier as a maximum limit rather than a bonus condition.