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The Arbitrary Filleted Rect To Filleted Rect Transition is the flat pattern (development) of a transition piece between two rectangular openings with rounded corners: a level filleted rectangle at the bottom and a second filleted rectangle at the top. The top can be a different size, can be moved sideways off the centre of the base, and can be tilted. Cut the pattern from thin plate and fold it along the bend lines to make duct transitions, hoppers, chutes and reducers between two rectangular or obround openings.

It appears in the shapes library under Thin Walled Developments. The shape is defined as a 3D part, drawn in the 3D view; the flat pattern, which is the part that is cut, is developed from it. If the top is level, use the Shifted Filleted Rect To Filleted Rect Transition, and if it is also centred over the base use the Filleted Rect To Filleted Rect Transition; both are built the same way with fewer properties.

How the part is defined and developed

The base rectangle lies in the XY plane with one corner on the shape origin: Base Width along +X and Base Height along +Y, with Bottom Fillet R on all four corners.

The top rectangle (Top Width along X, Top Height along Y, Top Fillet R on all four corners) is placed Center H above the base. While the top is level its centre is directly above the point Top Offset X, Top Offset Y from the centre of the base. Top Angle Y tilts the top, and Top Angle Z sets the direction of that tilt, measured in the XY plane. Top Angle Z on its own (Top Angle Y = 0) does not change the part. The tilt pivots about the corner of the top rectangle nearest the base origin (its -X, -Y corner): that corner stays at height Center H and the rest of the top moves, so with a tilt the centre of the top is no longer exactly over the offset point or at height Center H. Check the 3D view after setting a tilt.

The surface is made of two kinds of face:

•Sides – one face from each straight side of the base to the matching straight side of the top. A side whose straight part is zero length at both the base and the top (the fillet radius is half the side length at both ends) is left out.

•Corner strips – each rounded corner of the base is joined to the matching rounded corner of the top by a row of narrow strips. Each corner gets a quarter of the development segments (see Dev. Segs). If both fillet radii are 0 there are no strips and the corners are sharp folds.

The faces are unrolled one after another into a single flat outline. The pattern has one seam, the straight line where the folded part closes. The seam runs from the middle of the straight +X side of the base to the middle of the matching side of the top, so that side is split in two halves, one at each end of the pattern. In the flat pattern the seam is laid along the X axis with its top end on the insert point (0, 0) and its base end along +X. The outline is one closed path: one seam, the developed top edge, the other seam, and the developed base edge.

Bend lines

With Show Bends on, a bend line is added on every fold between two neighbouring faces – where a side meets a corner, and between the strips of each corner. With the default values there are 28 (29 faces, one bend between each pair of neighbouring faces). They are marking entities, not cuts, and alternate lines run in opposite directions so the marking tool zig-zags across the pattern instead of returning to the same side each time. If Bend Length is more than 0, each bend line is shortened to two marks of that length, one at each end, instead of a full line across the plate; a line shorter than 2.1 times Bend Length is left at full length.

Properties

The property page lists the transition properties first, then the development properties, then the Transform group.

The dimensions drawn on the 3D view are Center H, Base Width, Base Height, Top Angle Z, Top Angle Y, Top Offset X, Top Offset Y, Top Width, Top Height, Bottom Fillet R and Top Fillet R.

Notes and limits

•The shape cannot be built if Bottom Fillet R is more than half the smaller of Base Width and Base Height, or if Top Fillet R is more than half the smaller of Top Width and Top Height. A radius of exactly half gives a fully rounded (obround) end, as in the default values. The check is made on the values entered. A radius above that limit fails the build with the internal message "Bottom fillet radius exceeds rectangle dimensions" or "Top fillet radius exceeds rectangle dimensions"; that text is not shown on the property page, which shows only the warning bar described below.

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

•The pattern is built on the surface defined by the sizes entered; there is no plate thickness or neutral-axis allowance. Enter the sizes you want the pattern laid out on (for example mean sizes for a part made from thick plate).

•The rounded edges of the pattern are made of straight segments, one per corner strip, so a low Dev. Segs value gives visibly faceted corners.

•When the top is tilted, the four corners of a side or corner strip are in general not on one plane. Each face is laid out from the positions of its four corners, so the pattern is an approximation; the larger the tilt, the more it differs from the true surface. Check the fit on a test piece.

•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 Arbitrary Filleted Rect To Filleted Rect Transition also has the Transform properties (Base X, Base Y, Rotation, Scale, Mirroring X), listed after its own properties. Polar Array applies to 2D shapes only, so this 3D shape has none.