Tube rolling or expansion is an art far beyond this blogspace. If you would like to read more about the technical aspects of it, click here. In short, it involves expanding the tube into the fixed space of the sheet. Engineers would say that we want to end up with the tube plastic and the sheet holes and ligaments elastic. After rolling in thousands of tubes in this brief life, there's still a certain thrill to it because it is one of those few areas left to artificers where the ideal balance between over-rolling or under-rolling is a matter of feel.
Here, we're looking at the combustion chamber or back end of the boiler, and the first eleven tubes are rolled in. The particular expander I'm using has a flaring feature, which I'll explain more about another day. The goop in the holes is Crisco and while attributing their fine product, I'm sure lubricating tube expanders is something they would not like to be associated with. But it does the job and doesn't interfere with the heat-transfer welding that comes later. In some of the holes, one can see the degree that the tube has expanded. The goop makes the transition look larger than it is - we want this transition to be gradual without sharp edges. (By the way, some measurements for you technical types: The holes were bored 1.531", the tubes are 1.50" with 0.095" wall thickness.)
I manipulated the expander by hand, using a T-handle. That way there wasn't a lot of racket and I could be in my quiet, happy place, feeling the moment of set. In addition to feel, one sign of a good tube set in sheets this thin (0.375") is just when the mill scale on the sheet right around the hole begins to split into a micro lattice. If you look at the photo, this lacing appears as a thin, lighter halo around some of the tubes. That can occur within a sixteenth of a revolution of the expander, so one has to be alert.
The next blog entry will treat flaring and beading the tubes on the front (smokebox) end, and heat-transfer welds on the back (combustion chamber) end.
