A change of mind
My original intention was to make the boiler and firebox as a single unit that would be secured to the inner frame assembly by two screws through the frame spacers going into the bottom of the boiler, that would lift away for access to the motor and gearbox. As more and more boiler fittings,steam pipes, and various other bits and pieces were added, that became increasingly impractical.
First there were the boiler saddle pieces outside the frames, that you can see in the left hand photograph. These were not part of the original frame assembly (in fact they were an oversight), but were fabricated from pieces of nickel silver sheet and soldered in place. The problem was that they are a very tight fit between the splashers over the first two sets of wheels.
There are also very prominent exhaust steam pipes from the front ends of the cylinders to the smokebox. These are soldered to the smokebox, and bayonet into the cylinders. That means the boiler has to move forward to release the steam pipes before it can be lifted away, but the splashers prevent that and mean it can only be lifted vertically. To compound the problem, I added the pipes from the feedwater pumps to the boilder that can be seen behind the exhaust steam pipes in the photograph. These are soldered to the clack valves on the boiler, and fit loosely into the pumps. Getting the boiler on and off with that lot in place was becoming a fiddle, requiring various parts to be stressed, and I was concerned that before too long something would break.
So the original plan had to be abandoned. The boiler and firebox were split into two separate assemblies. The boiler normally stays in place (it can be removed, just about, for finishing, painting, and lining), and the firebox alone lifts off for access to the works. That meant devising another way to secure the firebox. There was getting to be precious little room underneath, so in the end, one of the screws holding the ashpan was given the double duty of securing the firebox in place as well. There is actually only one fitting bridging the gap between the boiler and firebox, and that is the regulator rod. That ended up attached to the firebox and fitting loosely into the regulator body on top of the boiler. However, that is getting ahead of myself. To finish this little saga ...
Either, I should have spent more time with the drawings, thinking about these problems before I started,
Or, that's just part of the fun of scratchbuilding!
Boiler fittings
The chimney was turned in two pieces (LH photo), just because it was easier that way. There is a little recess in the top of the stem of the chimney, into which the top fits. The base was flycut in the milling machine to match the boiler (RH photo). The scraps of card are to prevent the jaws of the vice from marking the chimney. The clever bit was shaping the flare around the base. Once upon a time I did this by hand using files, but using the neat device shown below in the milling machine, I can take the bulk of the material off using a ball-end cutter.
The base of the chimney is threaded and screwed on to an arbor that supports it. The arbor goes through a hole in the big block of aluminium, and is free to rotate and move in and out without wobble. In operation, the base of the chimney is pushed into contact with the guide you can see just in front of the chimney. I rotate the arbor with one hand and keep it in contact with the guide with the other. The cutter, applied as shown, then cuts a path for the flare that follows the shape of the seat. The radius of the cutter sets the radius of the flare, but fortunately ball-end cutters are available in a wide range of sizes. I originally made this device to mount on the cross slide of the Myford lathe, the milling cutter being held in the chuck, and it was intended for flares somewhat larger than this one (despite the height of the chimney, the flare at the base is actually very small). The setup I used here isn't quite optimal, but it proved managable. Final finishing was done by hand, and for this I find a rifling file is a very useful tool. This is like a conventional Swiss file, but instead of being flat along its length, it is curved. I guess they were originally used to form the helical grooves that spin the bullets inside rifle barrels – hence the name. I'm not sure where mine came from, having had them for years. This is the only use I have found for them, but for this, they are invaluable.
Such a tall chimney must be set exactly vertical, otherwise it looks awful. Cutting the seat correctly ensures it is vertical in side elevation, but the end elevation required some care. I cut a cardboard template that fitted snugly over the frames, with a cutout just the right size to hold the chimney in place so that it could be tack-soldered. Then the template was removed, and the soldering was finished and cleaned up.
The regulator is located in a little box on top of the boiler, and from there the steam is fed to the cylinders through pipes covered with cladding for insulation. The box itself was a straight forward piece of machining, again with a flycut base to match the boiler. One big advantage of using a milling machine is the ability to make components like this from solid, which is so much easier than cutting the walls and top individually from sheet material, and soldering everything up, hoping it all stays square. The steam pipes, however, were another of those challenges.
The cladding is roughly semicircular in section, but while the width is constant, the height is not, it tapers from the regular box all the way down to the cylinders. After some thought, I turned a disk of the necessary diameter and width, and using a variety of tools and files, formed the semicircular outer shape. The disk was then offset in the 4-jaw chuck and the centre bored out to boiler diameter, thus forming a ring with the variable height required.
I then removed a section of the ring at the largest radius and soldered the regulator box in place. The lower part of the ring was then sawn away so that the upper part, when sitting in place on the boiler, just touched the tops of the cylinders (I started off too big and carefully cut it back until it fitted). I was afraid that if I tried to solder it in place, the heat would melt the existing joints to the regulator box and I would never get everything back again just so, so I stuck it to the boiler with epoxy. It just rests on the cylinders so that the boiler and cylinder assemblies can be separated. If I were to do it again, I would make the top of the regulator box removable and put a screw through the body into the boiler, but I did not think of that at the time.
The finished boiler assembly