Drawing and Estimation
Scope
Drawing and Estimation tests orthographic views, reading a working drawing, preparing a cutting list, and calculating material quantity and cost.
Drawing
- India follows first-angle projection: the view from the front is placed below the plan, and the view from the left is placed on the right.
- Lines: continuous thick for outlines, dashed for hidden detail, chain for centre lines, thin for dimension and projection lines, cutting-plane line with arrows for sections.
- Isometric drawings show three faces at 30° to the horizontal; they are pictorial, not for dimensioning production work.
- Dimension in millimetres, from datum faces, and never repeat a dimension.
Estimation
- Read the drawing and write a cutting list: part, quantity, finished size, material.
- Volume = length × width × thickness in metres, giving m³; multiply by 35.315 for cubic feet.
- Add wastage: 10–15% for solid timber, 5–10% for sheet material.
- Add consumables (glue, abrasive, polish, screws), labour at the workshop rate, overheads and profit.
Worked example
A stool: 4 legs 40 × 40 × 450 mm = 4 × 0.04 × 0.04 × 0.45 = 0.00288 m³; 4 rails 25 × 75 × 350 mm = 4 × 0.025 × 0.075 × 0.35 = 0.00263 m³; top 350 × 350 × 25 mm = 0.00306 m³. Total 0.00857 m³ ≈ 0.30 cft; with 12% wastage 0.34 cft; at ₹1,200 per cft, timber cost ≈ ₹407.
Traps
Mixing first- and third-angle views; forgetting to convert millimetres to metres before multiplying; omitting wastage or labour; taking part sizes from the drawing without checking that they add up to the overall dimensions.