What about the two legs?

They are already in the catalogue: each leg is the double-cut cylinder. With the breeches angle at ANG, each leg takes ANG/4 at the joint with the trunk and ANG/2 at the top opening, if you want both openings parallel to the base. The trunk is the part that did not exist on its own.

Why is the mitre a quarter of the angle, not a half?

Because the angle given is between the TWO legs, and each leg makes half of it with the trunk. The mitre between the trunk and one leg is half of that again — a quarter of the total. It checks out on the case everyone knows: two legs at 180 degrees make a 90-degree elbow, and a quarter of 180 is the 45-degree cut every single-cut mitred elbow has.

Where does the seam go?

In a trough, which is the shortest generator and so the shortest weld. On a peak it would sit right on the point of the crotch, the worst possible place for a joint.

Why must the divisions be a multiple of four?

So the stations land exactly on the two peaks and the two troughs. The crotch is a point, not a smooth curve, and a station out of place cuts it off instead of marking it.

Y Piece Trunk (Breeches) development

What it is

The lower pipe of a Y branch, with its end cut as a fish mouth. A breeches piece is made of three parts, and this is the only one with geometry of its own: two mitre planes, one per leg, meeting along the crotch line. Rolled flat, the end comes out with two peaks — the two crotches, one either side of the plane of the Y — and two troughs, where each leg swings away.

Where it is used

Pipework bifurcations, extraction and flue breeches, headers splitting in two, air and dust distribution.

Measurements the calculation needs

D Outside diameter of the trunk
H Height to the crotch (the longest element)
ANG Angle between the two legs
DV Divisions
THK Plate thickness

Worked example

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

Outside diameter of the trunk	400 mm
Height to the crotch (the longest element)	500 mm
Angle between the two legs	60°
Divisions	32
Plate thickness	6 mm
The flat pattern fits a plate of	1238 × 500 mm

Result of the example

Plate	1238 × 500 mm

Y Piece Trunk (Breeches) flat pattern: drawing of the template with the worked example dimensions — Cut lines in purple, welds in green and reference lines in orange. Plate 1238 × 500 mm.

Assembly notes

Weld the sheets with the longitudinal seams 180° apart, not lined up. Lined up, the piece assembles into a different shape — and the longitudinal seam crosses the circumferential one.

Frequently asked questions

What about the two legs?: They are already in the catalogue: each leg is the double-cut cylinder. With the breeches angle at ANG, each leg takes ANG/4 at the joint with the trunk and ANG/2 at the top opening, if you want both openings parallel to the base. The trunk is the part that did not exist on its own.
Why is the mitre a quarter of the angle, not a half?: Because the angle given is between the TWO legs, and each leg makes half of it with the trunk. The mitre between the trunk and one leg is half of that again — a quarter of the total. It checks out on the case everyone knows: two legs at 180 degrees make a 90-degree elbow, and a quarter of 180 is the 45-degree cut every single-cut mitred elbow has.
Where does the seam go?: In a trough, which is the shortest generator and so the shortest weld. On a peak it would sit right on the point of the crotch, the worst possible place for a joint.
Why must the divisions be a multiple of four?: So the stations land exactly on the two peaks and the two troughs. The crotch is a point, not a smooth curve, and a station out of place cuts it off instead of marking it.

Y Piece Trunk (Breeches) dimension drawing: D (Outside diameter of the trunk), H (Height to the crotch (the longest element)), ANG (Angle between the two legs), DV (Divisions), THK (Plate thickness), marked on the part

Calculate a Y Piece Trunk (Breeches)

An account is required. The free tier covers 20 calculations a month.