Truncated Rectangular Tube flat pattern

What it is

The rectangular-section duct that ends on a tilted plane — a roof, a sloping wall, the bottom of a funnel. It is to the rectangular section what the truncated cylinder is to the round shell, and the catalogue had one without the other. Rolled flat, the mouth becomes a line of four runs: straight on the low and high faces, sloping on the two sides.

Where it is used

Ducts dying into a roof or a sloping wall, intakes beside a graded floor, duct connections to equipment with a tilted face.

Measurements the calculation needs

• X Measurement X
• Y Measurement Y
• H Height
• ANG Angle
• THK Thickness

Worked example

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

Measurement X	500 mm
Measurement Y	300 mm
Height	400 mm
Angle	30°
Thickness	1.5 mm
The flat pattern fits a plate of	1594 × 688 mm

Frequently asked questions

Are all four faces cut the same?

No, and it is the same trap as the rectangular bend. The low and high faces come out RECTANGULAR, cut square: each lies wholly at one distance from the axis, so the cut does not vary along it. Only the two side faces come out trapezoidal.

Which side is the height given for?

The LOW side. The high side follows from the arithmetic — it is the given height plus the width times the tangent of the angle. Same convention as the truncated cylinder.

Why can't the angle reach 90 degrees?

Because at 90 the plane runs parallel to the tube's axis and cuts no mouth at all — it slides down the side instead of crossing. And at zero the part is a straight tube, with no cut.

Where does the seam go?

In the middle of the low face, which is the shortest generator and so the shortest weld. And on a straight line, which closes better than a corner.