Pattern Development and Fabrication Layout
What this domain tests
This domain tests whether you can turn a three-dimensional fitting into a flat pattern that fits: choosing the development method, finding true lengths, and adding the correct allowances for seams, joints, and bends.
The three development methods
- Parallel line: prisms and cylinders such as straight duct and square elbows. Divide the end view, project to the elevation, and stretch out the perimeter.
- Radial line: cones and pyramids such as reducers and conical transitions. True lengths radiate from the apex.
- Triangulation: transitions where no two surfaces are parallel, such as square-to-round or offset transitions. Break the surface into triangles and build each one from its true length.
- The stretch-out of a round fitting is the circumference, pi x D, plus seam allowance.
Allowances and bend geometry
- Every pattern needs seam and joint allowance added to the developed size. A Pittsburgh lock takes roughly 25 mm (1 in.) total, drive-clear and S-cleat edges take their own allowance, and lap joints take one material thickness plus.
- Bends develop on the neutral axis: BA = (pi/180) x bend angle x (R + K x t), with K typically 0.33 to 0.50.
- Bend deduction converts outside dimensions to flat length: BD = 2(R + t) - BA.
- Provide bend relief or corner notching so the material does not tear at a corner.
Worked example: 90 degree bend, flat length
R = 3.000 in., t = 0.032 in. aluminum, K = 0.44. Neutral radius = 3.000 + (0.44 x 0.032) = 3.014 in. Bend allowance = (pi/180) x 90 x 3.014 = 4.734 in. Bend deduction = 2(3.000 + 0.032) - 4.734 = 6.064 - 4.734 = 1.330 in. Two outside legs of 5.000 in. each: flat length = 5.000 + 5.000 - 1.330 = 8.670 in., so cut 8 11/16 in.
Worked example: square-to-round check
A square-to-round is only right if each true length comes from the actual plan and elevation. Measure the diagonal in plan, use the elevation height to establish the true length of the slant, then triangulate. Seam allowance is added once, on the closing edge, not on every triangle.
Traps that cost marks
- Using outside dimensions of a bent part as the flat length.
- Forgetting seam allowance, or adding it to only one edge.
- Treating a square-to-round as a parallel-line development.
- Ignoring material thickness when finding true lengths on heavier gauge.
- Cutting an asymmetric pattern where the fitting is symmetrical; always check the centreline.