< Outer Dome >
The picture shows profiles of a typical outer dome. The red lines indicate boundaries between straight part and round part. Upper part of the dome can be turned in the lathe, while the base should be finished with files. The left-hand picture shows a dome finished in such a way. Specific angle of lighting makes the red lines distinctive. So how is a full size article? It may be seen as the right-hand picture. It has warped boundary along the warped base. Furthermore the upper boundary of full size dome is not clear, because real dome has 'transition curve' which is not specified in a design. Maybe it is better to finish only with files rather than machined in the lathe.
The left-hand dome casting is of 5" RAILMOTOR which I employed for my WILLIAM. The right-hand one is the original. It shows a difference of size between 1/12 scale and 1/16 scale. If two designs of dome are for the same diameter boiler, each can be exchanged like this.
I removed casting gate, burr and almost of casting skin with a file, and carefully checked profile of the job. Then I found it was not symmetric enough, so I decided to finish the job in the lathe.
Fly-cutting of the bottom in Keats vee block. The radius should correspond to a diameter of the boiler with lagging and cleading.
The whole job was moved to face plate in the lathe and centered by DTI. It needs a heavy balance weight to turn the job without vibration.
Dome inside was bored for following operation. Also O.D. of the base was finished temporary.
After turning a cherry wood mandrel to a hard push fit of the dome, the dome was hammerd into the mandrel. First, the middle straight part and the top round part were finished. I consciously filed 'transition curve' between them. Then the base was partly turned with a coin-shape tool, same as the chimney base.
The dome will be secured onto the inner dome with a screw. Top hole for that is opened here. I made a stepped hole to hide the screw head.
The whole job was moved to a vise, then the base was finished with files. Here I pay attention to 'boundary' which I specified at first, extending the straight part of both sides toward the bottom with a flat file, then finished with a half round file and emery cloth. Incidentally, you can obtain smoothly curved surface with swinging and twisting movement of the file.
Again in the four-jaw, O.D. of the base was finally finished. The mandrel was then hammered out with a round bar through the top hole.