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 Side along the tilt
- Y Side across the tilt
- H Height of the low side
- ANG Cut angle from square
- THK Plate thickness
- R Bend radius
Calculate with your own measurements
No account needed: enter the measurements in millimetres or inches and get the figures and the drawing. The scaled PDF template, the DXF for the cutting table and the 3D model take a free account.
Diameters and sides are outside measurements. The flat pattern already allows for the plate thickness, working on the mean line.
Inches or millimetres? mm inches
Worked example
The values the form comes pre-filled with, run through the tool itself:
| Side along the tilt | 500 mm |
|---|---|
| Side across the tilt | 300 mm |
| Height of the low side | 400 mm |
| Cut angle from square | 30° |
| Plate thickness | 1.5 mm |
| Bend radius | 0 mm |
| The flat pattern fits a plate of | 1594 × 688 mm |
Result of the example
| Plate | 1594 × 688 mm |
|---|---|
| Area and weight | 0.87 m² · ≈ 10.2 kg in 1.5 mm steel |
Ordinate table
| # | Position on the plate | Element height |
|---|---|---|
| 1 | 0 mm | 400.4 mm |
| 2 | 149.3 mm | 400.4 mm |
| 3 | 647.8 mm | 688.2 mm |
| 4 | 946.3 mm | 688.2 mm |
| 5 | 1,444.8 mm | 400.4 mm |
| 6 | 1,594 mm | 400.4 mm |
Assembly notes
The pattern develops the SHARP CORNER, on the mean line — the theoretical layout, which is what gets delivered. If you fold it on a press brake with an inside radius, the blank comes out LONG: each fold needs the deduction 2*(r+t)*tan(a/2) − a*(r + k*t), with r the inside radius, a the bend angle in radians, and your tooling's k. On a 500x300 tube in 1.5 mm sheet with four 90° folds that is 3.5 mm at radius 0.75 and 9.0 mm at radius 4.5 — always to be taken off. A welded corner from separate plates needs no deduction at all.
Fold to an angle of 90° BETWEEN THE TWO FACES — what a protractor reads on the finished part, not the press stroke angle. Every crease on this part has the same angle.
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.
You can work out the figures right here, with no account. PDF, DXF, 3D and history take an account — free, with unlimited calculations.