Breeches Trunk (Fish Mouth) flat pattern

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 Diameter
• H Height
• ANG Angle
• DV Divisions
• THK Thickness

Worked example

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

Diameter	400 mm
Height	500 mm
Angle	60°
Divisions	32
Thickness	6 mm
The flat pattern fits a plate of	1238 × 500 mm

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.