Sketching and Reading of Sheet Metal Drawings
This domain tests freehand sketching and the special conventions of sheet metal flat-pattern drawings. You must recognise bend lines, bend direction, bend radius, seam allowances, notches and the difference between a finished view and a developed blank.
What to read on a sheet metal drawing
- The title block gives material, gauge or thickness, quantity, finish and part number. Check these before marking any sheet.
- A flat pattern shows the developed outline. Bend lines are usually chain or dashed lines; a note such as “BEND UP 90° R3” gives direction, angle and inside radius. The bend allowance is not always drawn, so you may have to calculate it.
- Sketch proportionately: outline first, centrelines next, then details. Use construction lines lightly and label dimensions clearly. Never sketch a bend line as a cut line.
Worked example
A bracket drawing shows two legs of 40 mm and 60 mm measured to the tangent points, with a 90° bend, inside radius 3 mm, thickness 1.5 mm and K-factor 0.33. The bend allowance is BA = (90/360) × 2π(R + Kt) = 0.25 × 2π(3 + 0.495) = 5.49 mm. The flat length is the sum of the straight legs plus one bend allowance: 40 + 60 + 5.49 = 105.49 mm. Reasoning: the neutral axis length is preserved during bending, so the blank must include the arc length at the neutral axis.
Traps that cost marks
Reading a bend line as a cutting line; using outside dimensions without subtracting bend deduction; ignoring the stated inside radius; and forgetting that a left-hand and right-hand part are mirror images.