Here is a goodview of the turbine shaft still in the lathe You will notice turned seatings in the foreground the closest turned seating is the identical diameter of the outside of the wings, and slots will be cut into this turned seating for the wings to slide back into, and then be held in place with babbet and the wrought iron rings, a view of this I posted first.
Alittle further you will notice another turned surface this is for the 36" cast iron rings that have cups casted around its surface.
You will notice that this turned surface ends abruptly and it is this detail that is extremely important because if it is not correct the rings would bind on the side of the pressure box or they could bottom out too far from the box if the shouldering ended too soon
Keep in mind that these surfaces are tapered and again if the tapering is too tight on the interior of cast iron surfaces it could crack the casting when the wood expands as exposure to water eventually takes place
there is alot of ifs here--
-if the wood turning is too tight failure of the casting takes place -if the woodturning is too loose then the cast iron collars will spin around and destroy the axle
getting it just right is quite a feat, it was always in the back of my mind that should one of the cast collars crack, to have it replaced would cost thousands of dollars because a new pattern would have to be made by a pattern maker and then casted to allow for shrinkage in the casting process.