Unfolded lateral surface of segment (sector) of pipe elbow
It is cut out on sheet metal with further circular rolling.

exhaust vent elbow
Sheet thickness (s)
mm
Outer diameter (D) of cylinder
mm
Height (h) along axis of cylinder
mm
Number (n) of split points
point
You can download a free program for working with files with the .dxf extension   here
You can download the DWGTrueView thumbnail viewer   here