Canopy Hood: flat pattern

An extraction hood: a skirt round a rectangular opening, four sloping sides and a collar for the duct, in the middle or against a wall. An extraction hood: an upright skirt round a rectangular opening, four sloping sides and an upright collar for the duct, with the collar in the middle or against a wall.

Configurations

• Island hood. The collar is over the middle of the opening and all four sides slope.
• Wall hood. The back of the collar is in line with the back of the opening, so the back stands upright against a wall.

A worked example

The island hood configuration with the sizes SheetMorph opens with, in mm. 4 plates: front, back and two sides alike, each with its skirt and collar: 4 parts, 4 seams.

How it is developed

True-shape development of flat sides. This is an exact development: the surface unfolds without stretching, so the pattern closes exactly.

• Each side is laid out to its true shape: the skirt as high as entered, the sloping part as high as its slant height, and the collar as high as entered, one above the other.
• The collar is over the middle of the opening: the front and the back are alike, and so are the two sides.
• The plates are folded where the skirt meets the sloping part and where the sloping part meets the collar.
• Each side is a separate plate. The four corner edges of a duct are joints between plates, drawn as plain edges without allowance.

Checked before it can be exported

Every pattern SheetMorph draws is checked against the 3D part it comes from. For this example:

• Every station is a rectangle (passed): 4 stations: opposite sides and the two diagonals of each compared (max deviation 0).
• Plates flat, rolled or triangulated (passed): 0 flat, 4 folded or rolled from flat pieces (both develop exactly), 0 twisted and split along a diagonal.
• Surface connectivity (passed): 24 facets form one closed duct: every edge inside a part joins exactly two facets, every open edge one (V - E + F = 0).
• Developed edges equal true 3D lengths (passed): 72 facet edges compared between the 3D surface and the flat pattern (max deviation 9.09e-13).
• Shared edges and seam closure (passed): 20 internal edges are shared vertex-for-vertex between adjacent facets; 0 seam(s) have mating edges of equal true length (max deviation 0).
• Developed facets consistently oriented (passed): All developed facets are counter-clockwise (outside face up); none is mirrored or degenerate.
• Developed area equals surface area (passed): Facet area in 3D and in the flat pattern agree (difference 4.66e-10).
• Cut outlines closed and non-overlapping (passed): 4 closed outline(s); no outline crosses itself or another outline.

Assumptions

• The pattern is developed on the mid-thickness surface; inside or outside sizes are converted to it by one thickness across each opening.
• Folds are sharp lines; no bend allowance is applied.
• No seam, lap or weld allowance is added.

Limits of this module

• The collar is rectangular. For a round duct, finish the hood without a collar and set a rectangle-to-round transition on it.
• Each side is one plate from the opening to the top of the collar; grease channels, filters, hems and hanging lugs are not drawn.
• On a wall hood the back is one flat plate: its two fold lines are only marks where the skirt and the collar begin.
• No seam, lap, lock-seam, flange or weld allowance is added unless the module says so; corner joints are drawn as plain edges.
• Bend allowance and springback are not modelled; folds are sharp lines on the mid-thickness surface.
• No stiffening, tie rods, access doors or nesting onto stock sheets.

Conventions

Coordinate system. End A is at Z = 0, centred on the Z axis, with its width along X and its depth along Y. A duct that turns or is offset does so in the XZ plane, toward +X.

Corners and sides. Corners are numbered 0 (-X, -Y), 1 (+X, -Y), 2 (+X, +Y), 3 (-X, +Y), seen from +Z at End A. Rj_c is corner c of station j. Each of the four sides is its own plate; the plates are joined along the four corner edges.

Plates. Plates are drawn outside face up and stacked in the order of their numbers. The blank sizes listed are the bounding rectangle of each plate.

Material thickness. Say whether the sizes are inside, mid-thickness or outside sizes. The pattern is developed on the mid-thickness surface, one thickness larger or smaller across each opening; the views show the sizes as entered.

Hood. The hood hangs with its opening downward: End A is the opening, at Z = 0, and End B the top of the collar. The front is the -Y side and the back the +Y side. From the opening upward each side is the skirt, the sloping part and the collar, in one plate folded twice. On a wall hood the collar is moved toward +Y until its back is in line with the back of the opening.

Change the sizes to your own in SheetMorph, check the drawing, add seam and flange allowances, and export DXF, PDF, SVG, CSV or G-code. Check every pattern against your own standards before cutting.

Other hoods, plenums and cyclones

• Canopy, Round Collar: An extraction hood for a round duct: a skirt round a rectangular opening, a rectangle-to-round sloping part and a round collar, in the middle or against a wall.
• Plenum Box: An open box folded from one blank with a row of round spigots on its back or a side: the box with every hole marked, and the spigots unrolled.
• Inlet Box Scroll: The scroll of a fan inlet box laid flat: its straight sides and arc unrolled, each line across as deep as the box is there.
• Cyclone: A cyclone dust collector from its barrel diameter, to Stairmand or Lapple proportions or your own: barrel, cone, roof, inlet duct and gas outlet tube.