Angular Boot, Rectangle to Round flat pattern

What it is

The boot: a rectangular mouth below and a round pipe leaving SIDEWAYS, horizontal, at mid height. Its round mouth is not a circle — it is the angled cut of the pipe, an ellipse of axes d by d/cos(ang) — and that is what separates it from the other transitions here.

Where it is used

Floor and wall registers in air conditioning, exhaust collectors, side take-offs on fan boxes, suction mouths on round duct, any job where the pipe leaves sideways rather than upward.

Measurements the calculation needs

• D Diameter
• X Measurement X
• Y Measurement Y
• ANG Angle
• H Height
• V Outlet offset
• DV Divisions
• THK Thickness

Worked example

The values the form comes pre-filled with, run through the tool itself:

Diameter	250 mm
Measurement X	300 mm
Measurement Y	150 mm
Angle	30°
Height	200 mm
Outlet offset	200 mm
Divisions	16
Thickness	3 mm
The flat pattern fits a plate of	785 × 441 mm

Frequently asked questions

How is this different from the eccentric rectangle-to-round?

There the top mouth is a flat CIRCLE and the pipe goes up. Here the pipe runs horizontally and the mouth is its angled cut, which is an ellipse. Welding a circular mouth to a pipe cut at 30° leaves a gap that grows with the angle: on a 250 pipe at 30° the major axis of the cut measures 289, not 250.

Where do I measure h and v from?

From the plane of the rectangular mouth to the pipe's AXIS, and from the centre of the rectangle to where the cutting plane crosses that axis. That point is the centre of the ellipse, and it is the only datum that still means the same thing when the angle is zero.

Can I enter a zero angle?

You can, and it is a legitimate case — the pipe is then straight and the mouth is an upright circle. What disappears is the collar: with a square cut it becomes straight pipe and needs no pattern at all.

Why does the drawing have creases on the long sides?

Because on those two sides no point of the mouth has a tangent parallel to the side, and no choice of apex avoids it. On the two sides of length y the tangent is parallel and the meeting is smooth. The two creases are part of the piece, and the printed sheet says to mark them.

Can the round mouth be larger than the rectangle?

It can, and on both classic plates used as tests it is. This part deliberately does not carry the eccentric transition's guards — they would reject both pieces from both books.