Canopy Hood, Round Collar: flat pattern

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

Configurations

• Island hood. The round collar is over the middle of the opening and all four sides slope.
• Wall hood. The back of the round 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. 5 plates: front, back and two sides, each its skirt, a flat triangle and half of each corner cone, and the collar rolled from one plate: 5 parts, 7 seams.

How it is developed

Triangulation (true-length unfolding). This is a triangulated approximation: the curved surface is divided into small flat triangles, each laid out at its true size. The finer the division, the closer the pattern is to the surface; SheetMorph states the deviation it found.

• The sloping part is a rectangle-to-round transition. Each side of the opening is the base of a flat triangle whose point touches the circle; each corner is joined to the arc of the circle between two such points as part of a cone, divided into narrow triangles and unfolded from their true lengths.
• The plates are cut at the four corners of the opening, and each corner cone along the line to the middle of its arc, so that every plate is one side with half a corner cone on each hand.
• The collar is over the middle of the opening: the front and the back plates are alike, and so are the two sides.
• The collar is rolled from a separate plate, seamed on the back, and joined to the hood round the circle: a cone and a cylinder cannot be folded from one flat sheet along a curve.

Checked before it can be exported

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

• Surface connectivity (passed): 108 facets form one band from the opening to the top of the collar: every edge inside a part joins exactly two facets, every open edge one (V - E + F = 0).
• Developed edges equal true 3D lengths (passed): 324 facet edges compared between the 3D surface and the flat pattern (max deviation 1.47e-12).
• Shared edges and seam closure (passed): 103 internal edges are shared vertex-for-vertex between adjacent facets; 1 seam(s) have mating edges of equal true length (max deviation 1.14e-13).
• 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 4.19e-9).
• Cut outlines closed and non-overlapping (passed): 5 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 the thickness.
• Curves are chains of chords between division points.
• No seam, lap, weld-preparation or pressing allowance is added.

Limits of this module

• The collar is no wider than the opening is wide or deep.
• The collar is a separate plate joined round the circle: a cone and a cylinder cannot be folded from one flat sheet along a curve.
• Grease channels, filters, hems and hanging lugs are not drawn.
• Curves are chains of straight chords between division points.
• No seam, lap, weld-preparation or pressing allowance is added, and plates are not nested onto stock sheets.

Conventions

Coordinate system. The part stands on the Z axis with End A at Z = 0. Angles round the axis are measured from +X toward +Y, counter-clockwise seen from +Z.

Divisions and thickness. A circle is divided into 4N equal chords. Sizes are inside, mid-thickness or outside sizes; the pattern is developed on the mid-thickness surface and the views show the sizes as entered.

Hood. The hood hangs with its opening downward: End A is the rectangular opening, at Z = 0, and End B the top of the round collar. The front is the -Y side and the back the +Y side. On a wall hood the collar is moved toward +Y until its back is in line with the back of the opening.

Sloping part. The sloping part is a rectangle-to-round transition. Each side of the opening is the base of a flat triangle whose point is the division of the circle that faces the same way; each corner is joined to the arc between two such points as part of a cone, a fan of narrow triangles.

Plates. The hood is cut at the four corners of the opening and along each corner cone to the division half-way along its arc: each plate is one side, its skirt, its triangle and half of each corner cone beside it, drawn standing on its edge at the opening. The collar is rolled from a plate of its own, seamed on the back.

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: 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.
• 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.