Tapered Breeches flat pattern
What it is
The Y-branch whose two outlets TAPER. The catalogue already had the cylindrical breeches, split into a trunk plus two double-cut legs, and this one does not come from those: the leg of a tapered breeches is a cone cut at its LARGE end, by two different planes in one lap, and no part does that.
Where it is used
Reducing pipe bifurcations, air distribution in round duct, twin outlets from a cyclone or silo, extraction breeches.
Measurements the calculation needs
- D Diameter
- D1 Diameter 1
- H Height
- C Branch length
- ANG Angle
- DV Divisions
- THK Thickness
Worked example
The values the form comes pre-filled with, run through the tool itself:
| Diameter | 400 mm |
|---|---|
| Diameter 1 | 250 mm |
| Height | 400 mm |
| Branch length | 700 mm |
| Angle | 60° |
| Divisions | 32 |
| Thickness | 6 mm |
| The flat pattern fits a plate of | 1238 × 731 mm |
| Flat patterns produced | 2 |
Frequently asked questions
- Can the plain breeches trunk be used with two tapered legs?
- The leg is the larger problem, and it does not exist ready-made -- it is a cone cut at the large end by two planes. As for the trunk, the difference is real but smaller than it looks, and the measured figure is worth having: with the crotches aligned, the depth of the fish-mouth differs by 0.17 mm at the default values, 1.5 mm at a strong taper, and 5.4 mm with the legs at 120 degrees. It grows with the taper and with the angle, and vanishes as the leg thickens to the trunk diameter, which is where the part becomes the cylindrical breeches.
- Why can the leg taper not be chosen?
- Because it is not free. All three surfaces must be tangent to one common sphere for the joints to close, and that condition ties the taper to the trunk diameter, the outlet diameter and the leg length. Entering those three already fixes the taper; asking for it separately would accept input that does not close.
- How many plates come out?
- Two different ones, three plates. The trunk is cut once, as a strip, and the leg is cut TWICE from the same template, handed one each way. That is what gets the whole breeches out of two layouts rather than three.
- Where are the length and the height measured from?
- From the crossing of the axes. All three axes of this part meet at a single point, and both the trunk height and each leg length are measured from it -- centre to face, the way any catalogue Y-fitting is dimensioned. The crotch peak sits a little above that point, and it is the crotch you see when the part stands up.
- Where does the leg seam go?
- On the heel, the outer side of the Y. At the crotch the weld would be shorter -- the leg is shallower there -- but that is exactly where one leg welds to the other, and crossed welds are what the assembly note warns against. The heel is smooth, reachable and clear of both joints.
- Is the large end of the leg a compass arc?
- No, and it is not even one curve. It changes plane halfway round: near the heel the trunk does the cutting, near the crotch it is the crotch plane. Two elliptical arcs spliced, marked point by point. What is a compass arc is the free outlet, on the smaller radius of the sector.
