The Shifted Rect To Circle Transition is the flat pattern (development) of a sheet-metal transition piece that joins a rectangular opening at the bottom to a round opening at the top, with the round opening moved sideways from the centre of the rectangle – an offset or eccentric "square to round". The two openings are parallel and horizontal. Cut the pattern from thin plate and fold it along the bend lines to make duct transitions, hopper outlets and chute pieces where the round outlet is not on the centre line of the rectangular inlet.
It appears in the shapes library under Thin Walled Developments, next to the Rect To Circle Transition (top circle centred over the rectangle) and the Arbitrary Rect To Circle Transition (top circle can also be tilted). The shape is defined in 3D; the property page shows the 3D part, and the part that is cut is the 2D flat pattern built from it.
Shifted Rect To Circle Transition in PrimeShape: the property page (left) and the shape as a wireframe with its dimensions (right).
How the transition 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 both offsets 0 the top circle would be centred over the centre of the rectangle; Top Offset X and Top Offset Y move it from there. The top circle is Center H above the base and always parallel to it.
The top circle is divided into equal steps, the number being set by Dev. Segs (rounded as described in the properties table), a quarter of them for each corner of the rectangle. Each corner of the rectangle is joined to its quarter of the circle by a fan of flat triangles, and each side of the rectangle is joined to the circle by one flat triangle. The triangles are unrolled one after another into a single flat outline.
The part has one seam, the straight line where the folded part closes. It runs from the middle of the rectangle side at +X (the side Base Height long) to the +X point of the top circle. That side is made from two half triangles, one at each end of the pattern, so the seam appears at both ends of the flat pattern. The outline is one closed path: the seam at one end, the developed top edge, the seam at the other end, and the developed base edge.
Bend lines
With Show Bends on, a bend line is added on every fold between two triangles: the lines that fan out from each corner of the rectangle to the top circle. 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 longer than 2.1 times 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 transition properties first, then the development properties, then the Transform group.
Property |
Description |
Default |
Top D |
Diameter of the top (round) opening. Length units, minimum 1. |
50 mm (2 in) |
Base Width |
Size of the base rectangle along X. Length units, minimum 1. |
100 mm (4 in) |
Base Height |
Size of the base rectangle along Y. Length units, minimum 1. |
150 mm (6 in) |
Center H |
Vertical height from the base rectangle to the centre of the top circle (the height of the part, not a slant length). Length units. No minimum is set on the property page. |
100 mm (4 in) |
Top Offset Y |
Distance the centre of the top circle is moved along Y from the centre of the base rectangle. Length units; negative values move it towards –Y. The default puts the edge of the top circle in line with the base edge at Y = 0 when seen from above. |
-50 mm (-2 in) |
Top Offset X |
Distance the centre of the top circle is moved along X from the centre of the base rectangle. Length units; negative values move it towards –X. With both offsets 0 the top circle is centred over the rectangle, as on the Rect To Circle Transition. |
0 mm (0 in) |
Dev. Segs |
Number of steps the top circle is divided into for the development. The value used is rounded down to a multiple of 4, with a minimum of 4 (for example 10 builds as 8, 3 builds as 4), so each corner gets the same number of triangles. More segments give a smoother top edge and more bend lines. Whole number, minimum 3. |
24 |
Show Bends |
On: add the bend lines between triangles to the pattern as marking. Off: the pattern is the outline only. |
On |
Bend Length (0 for Full) |
Length of the bend marks at each end of a bend line. 0 marks the full bend line from edge to edge. Length units, 0 or more. Used only when Show Bends is on. |
12.5 mm (0.5 in) |
The dimensions drawn on the 3D view are Center H, Base Width, Base Height, Top D, Top Offset X and Top Offset Y.
Notes and limits
•The top circle cannot be tilted on this shape; it is always parallel to the base. Use the Arbitrary Rect To Circle Transition for a tilted top.
•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 the mean sizes for a part folded from thick plate).
•The top edge of the pattern is made of straight segments, one per step of the circle, so a low Dev. Segs value gives a visibly faceted round opening.
•The offsets are not limited by the size of the base. The top circle can be moved so that it overhangs the rectangle when seen from above; check the 3D view to make sure the part is what you intend before cutting.
•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 shape has no aliases (no former names).
The Shifted Rect To Circle Transition also has the Transform properties (Base X, Base Y, Rotation, Scale, Mirroring X), listed after its own properties. Polar Array applies only to 2D shapes, so this 3D shape has no Polar Array properties.