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The Filleted Rect Lobster is a lobster-back (segmented, mitred) bend in a duct whose cross-section is a rectangle with fully rounded ends: two flat faces joined by two half-round sides. The shape builds the bend in 3D from a number of straight pieces mitred together, and produces one flat pattern (development) for each piece. Cut the patterns from thin plate, fold or roll each along its bend lines, and weld the pieces together to make the elbow. With Flat set to 0 the cross-section is a plain circle and the shape makes a round lobster-back pipe bend.

It appears in the shapes library under Thin Walled Developments. The shape is defined in 3D; the property page shows the 3D bend, and the part that is cut is the set of 2D flat patterns built from it.

How the bend is defined

The cross-section of the duct is Diameter deep and Flat + Diameter wide. The two flat faces are the inside (throat) and outside of the bend; the half-round sides, of diameter Diameter, are the cheeks.

The bend turns about a line through the shape origin, parallel to the flat faces. Radius is measured from that line to the inside flat face, so the centre line of the duct is at Radius + Diameter/2 and the outside face at Radius + Diameter. The bend starts on the plane through the origin square to the base piece, and turns through Angle.

The bend is made of Parts straight pieces. The angle is shared out evenly: each joint between two pieces is a mitre on the plane that halves the angle between them, and the step from one piece to the next is Angle / (Parts - 1). The middle pieces each cover one full step; the first and last pieces each cover half a step. The first piece is extended straight back by Base Height before the start of the bend, and the last piece is extended straight on by Top Length after the end of the bend, so the two ends of the elbow are square to the duct.

With Parts at 2 there are no middle pieces: the elbow is two pieces joined by a single mitre.

Flat patterns and insert point

Each piece is divided round its girth into flat strips and unrolled into its own closed outline, so a bend of Parts pieces gives Parts separate patterns. The strips are:

•each half-round side split into Dev. Segs / 2 strips, and

•when Flat is more than 0, the outside flat face as one strip and the inside flat face as two half-width strips.

The seam of every pattern (the line where the folded piece closes) is along the middle of the inside (throat) face. The pieces are laid out in order from the base piece to the top piece, one below the other in the -Y direction, with a gap of Development Spacing between one pattern and the next. Every pattern has its seam on the Y axis (X = 0) of the shape. The insert point is the shape origin (0, 0). The highest point of the first (base) pattern is at Y = 0. The outline of a mitred piece usually rises above its seam, so the top end of the first pattern's seam sits below the origin by that rise and is not necessarily at Y = 0.

Bend lines

With Show Bends on, a bend line is added on every fold between two strips of a pattern, including the folds where a flat face meets a half-round side. The seam, where the pattern closes, is not marked. 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 x Bend Length is shortened to two marks of that length, one at each end, instead of a full line across the plate; shorter bend lines are left full length.

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. Defaults are shown for metric and (in brackets) imperial units.

The dimensions drawn on the 3D view are Base Height, Top Length, Diameter, Radius, Angle and Flat.

Notes and limits

•The patterns are built on the surfaces given by Diameter, Flat and Radius; there is no plate thickness or neutral-axis allowance. Enter the sizes you want the patterns laid out on.

•The half-round sides are made of straight segments, one per strip, so a low Dev. Segs value gives a visibly faceted mitre edge on the pattern.

•Angle, Radius, Diameter, Flat, Top Length, Base Height and Development Spacing are not range-checked by the shape. Use an Angle greater than 0 and less than 180°, and a Diameter greater than 0, and check the 3D view before cutting.

•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 Filleted Rect 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.