Layout development calculator
A frustum unrolls to a ring sector: a Ø500 to Ø250 mm hopper 300 mm high is a sector of radii 650 and 325 mm through 138.46°. Choose a cone, a fish-mouth or a lobster-back bend, enter its sizes, and the calculator draws the part and its flat pattern to scale, with the radii and angle to strike, or the template ordinates to mark at twelve stations round the pipe.
- Outer radius R
- Inner radius r
- Sector angle θ
- Slant length s
- Chord across the sector
How it’s calculated
s = √(H² + ((D − d)/2)²); R = (D/2)·s / ((D − d)/2); r = R − s; θ = 360° × (D/2) / R, so the outer arc is π·D. Use the plate’s mean diameters.
How do you mark out a pipe template?
Wrap a strip round the pipe and divide its length, the circumference, into twelve. Square a line round the pipe, set each station’s ordinate off it, and join the points in a smooth curve: that is the cut. For a fish-mouth the ordinates are the cut-back from the cut’s lowest points; for a lobster-back segment they are set off either side of the square line, longest at the heel (the outside of the bend). Lay rolled plate out on its mean diameter, and add seam laps and weld gaps yourself.