Workshop Drawing Interpretation and Geometric Calculations
Drawing interpretation questions ask you to read views, symbols, tolerances and scales, then calculate a missing dimension. First-angle and third-angle projection, line conventions and geometric tolerancing carry easy marks.
Reading the drawing
- First angle: object between observer and plane; the view from the front appears above the top view in the layout used in India. Third angle: plane between observer and object; top view above front view.
- Visible edges: continuous thick; hidden: dashed; centre lines: chain; dimension lines: thin continuous with arrowheads.
- Tolerances: bilateral (50 ±0.05), unilateral (50 +0.10/0), limits (50.00/49.95). Fit = hole size − shaft size; positive gives clearance, negative gives interference.
- Surface finish Ra is in micrometres; lower Ra means smoother.
- Scale 1:2 means drawing dimension = actual ÷ 2.
Worked example
Eight holes are equally spaced on a pitch circle diameter of 120 mm. Radius R = 60 mm. Angle between adjacent holes = 360° ÷ 8 = 45°. Chord = 2R sin(45°/2) = 120 × sin 22.5° = 120 × 0.3827 = 45.92 mm. This is the straight-line centre distance to set with dividers, not the arc length.
Traps
- Swapping first-angle and third-angle view positions.
- Treating a diameter as a radius in bolt-circle calculations.
- Adding a unilateral tolerance on the wrong side.
- Confusing Ra with Rz or quoting surface finish without units.
- Applying the scale in the wrong direction.