Wouldn't the flat out pattern be a trapezoid with one base = 5" x Pi and the other base = 12" x Pi and a length ( height ) = whatever the length of the desired reducer ? The "bases" would be parallel but not "concentric" ? Can't you make one out of paper to see if this makes sense ?
Your answer is ambiguous. If my way did not work out right, were you able to solve the problem by building a simple paper model ? Seems like you should be able to.
Oh, I mean does anybody know the formula to determine the flat out pattern of an eccentric reducer by knowing the small OD, big OD and the height of a reducer? Building the paper model is tedious as we have many different sizes of eccentric reducers.
I know the way to calculate the flat out pattern of a CONCENTRIC reducer, but not for ECCENTRIC reducer.
Personally, I'd download the free version of SketchUp (which is a surface modeller), create the reducer, use the un-fold plug-in to create a flat pattern and export this as a DXF.