Pattern Development and Surface Layout
This domain tests how a three-dimensional sheet metal article is opened out into a flat pattern. You must choose the development method and calculate true lengths, stretch-outs and seam allowances.
Development methods
- Parallel-line development suits prisms and cylinders, where all elements are parallel. Radial-line development suits cones and pyramids, where elements radiate from an apex. Triangulation suits transition pieces such as square-to-round ducts.
- Find true lengths before drawing the development. A line that is inclined to two planes is not shown in true length in either view. Use auxiliary views or construction.
- Add seam allowance, wiring allowance and notches. Mark bend lines and match edges. For a truncated cone, develop the full cone first, then subtract the truncated portion.
Worked example
A cone has base radius 60 mm and vertical height 80 mm. The slant height is L = √(60² + 80²) = √(3600 + 6400) = 100 mm. The sector angle is 360 × r/L = 360 × 60/100 = 216°. The sector radius is the slant height, 100 mm, and the arc length is 2πr = 377 mm. Add seam allowance to the two straight edges. Reasoning: the cone surface opens as a sector whose arc equals the base circumference.
Traps that cost marks
Using diameter instead of radius in the sector formula; forgetting seam allowance; developing from a view that does not show true length; and ignoring sheet thickness on large-diameter work.