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The Duct Lobster is the set of flat patterns (developments) for a lobster-back bend in square or rectangular duct: an elbow made from a number of short mitred duct sections joined end to end. You choose the duct size (Side and Width), the bend radius, the total bend angle and the number of sections (Parts), and the shape lays out one flat pattern per section. Cut the patterns from sheet, fold each one along its bend lines into a closed section, and join the sections to make the elbow.

It is the rectangular-duct version of the Arbitrary Lobster and works the same way, except that the section is a rectangle instead of a circle and the seam position is fixed. It appears in the shapes library under Thin Walled Developments. The shape is defined in 3D; the property page shows the 3D elbow, and the part that is cut is the 2D flat patterns built from it.

How the elbow is defined

The bend turns about a bend axis, which is the X axis through the shape origin. The duct section is a rectangle:

•Side is the size of the duct in the plane of the bend, from the inside (throat) face to the outside face.

•Width is the size of the duct across the bend, parallel to the bend axis. The two faces Width apart are the flat cheeks of the elbow.

The duct centreline is at Radius + 0.7071 × Side from the bend axis (Radius plus half the diagonal of a Side × Side square). The inside face of the duct is therefore Radius + 0.2071 × Side from the bend axis, not Radius. With the defaults (Radius 1, Side 50) the inside face is about 11.4 from the bend axis and the centreline about 36.4.

The total bend Angle is shared between the sections in the same way as for the Arbitrary Lobster:

•There are Parts sections and Parts − 1 mitre joints. Each joint is a plane through the bend axis.

•The section axes are spaced Angle / (Parts − 1) apart, and each joint lies halfway between two section axes. The middle sections each turn the full Angle / (Parts − 1); the first and last sections each turn half of that.

•The first section also has a straight length, Base Height, before the bend starts. The last section has a straight length, Top Length, after the bend ends. Both are measured along the duct centreline from the plane where the bend starts or ends. The free ends of the first and last sections are cut square.

For example, the defaults (Angle 90°, Parts 4) give two end sections that each turn 15° and two middle sections that each turn 30°, with mitre joints at 15°, 45° and 75°.

How the patterns are laid out

Each section is unrolled into one closed outline made of four flat faces side by side, so each pattern is 2 × (Side + Width) wide. The seam (where the folded section closes) is always on a corner of the duct. With Stacked off every seam is on the corner between the inside face and a cheek. With Stacked on, the seam of every second section moves to the diagonally opposite corner (see below). There is no Start Angle property on this shape.

The insert point (0, 0) is at the top of the first pattern. Each pattern runs from its seam to the right (+X), with the length of the section running downwards (−Y). The sections are laid out one below the other, first section (with Base Height) at the top and last section (with Top Length) at the bottom.

How the patterns sit against each other depends on Stacked:

•Stacked on – the seam of every second section is turned half a turn round the duct, to the diagonally opposite corner, so the mitre edge of one pattern follows the shape of the mitre edge of the next. The patterns nest into each other, each one placed Development Spacing below the previous one measured at the seam. This uses less sheet.

•Stacked off – every section has its seam on the same corner, and each pattern is placed below the lowest point of the previous one with a gap of Development Spacing, so the patterns do not overlap.

Bend lines

With Show Bends on, a bend line is added on each of the three folds between the four faces of every pattern (the fourth corner is the seam). They are marking entities, not cuts, and alternate lines run in opposite directions so the marking tool zig-zags across the pattern. If Bend Length is more than 0, each bend line longer than 2.1 × Bend Length is shortened to two marks of that length, one at each end, instead of a full line across the sheet.

Properties

The property page lists the shape's own properties in the order of the table below, then the Transform group. The shape is 3D, so there is no Polar Array group. Default lengths are shown for metric and (inch) settings.

The dimensions drawn on the 3D view are Base Height, Top Length, Side, Radius, Angle and Width. The Radius dimension is drawn from the bend axis with length Radius, so it stops short of the inside face of the duct by 0.2071 × Side.

Notes and limits

•The patterns are built on the duct surface given by Side and Width; there is no sheet thickness or bend allowance. Enter the sizes you want the patterns laid out on.

•Every mitre plane passes through the bend axis. Radius, Base Height, Top Length and Angle have no limits on the property page. Check the 3D view before cutting.

•Do not set Side to 0. Width is stored as a ratio to Side (Width divided by Side), so with Side at 0 there is no size to keep, and setting Width divides by zero.

•Because Stacked raises an odd Dev. Segs by 1, the value on the property page can change after the shape is built.

•The shape has no aliases (no former names).

•If the shape cannot be built from the values entered, the 3D view is cleared and the property page shows Warning: Cannot build shape from these parameters. Change the values until the warning goes away.

The Duct Lobster also has the Transform properties (Base X, Base Y, Rotation, Scale, Mirroring X), listed after its own properties. Like the other Thin Walled Developments it has no Polar Array properties.