I know several model engineers who wouldn't touch boiler making - it's not for everyone. You need several sources of heat - a lot of it, somewhere safe to work, and you need to be aware that - unlike most aspects of model engineering, it is possible to get into a situation where all your work - and expensive materials - are irrecoverably scrapped.
I've got to say, I like the challenge, and satisfaction that its 'all my own work'. Then there's the lack of waiting time. Whilst its also true that there's a significant cost saving, this is definately not a good reason for doing it.
I bought a kit of materials from Reeves- this comprises all the copper sheet, bar and tubes. You really do need to have the correct materials, all in good condition, and, these days, of traceable quality.
Copper is interesting to work with. After annealing, its amazingly soft, and can easily be shaped - until it reaches a point where it work hardens. Its then time to stop and re-anneal before continuing. This is how the flanged plates are formed. It requires patience, and some very substantial formers . The formers are a lot of work to make unless you have a serious bandsaw, and are of course only required during the making of the components.
The firebox front and rear plates (Inner and outer), and the front tube plate were bought already flanged.
The boiler barrel is tapered - 3/8 inch difference in diameter over 10.5 inches, which works out as a 1 degree taper. As is typical of GWR practice, all the taper is all on the top of the boiler, with the underside lying horizontal. The drawings (and materials supplied ) require the taper section to be made up from flat plate. But there are alternatives - it could be made up from extruded tube, slightly streatched over most of its length. I've never done this, so can't comment on how difficult it is. I did seriously think about abandoning the taper altogether, and using seamless tubing of the right diameter. The small amount of taper could be built into the cladding, and no one would be any wiser - and I doubt if it would affect the operation or performance of the loco. (Can anyone comment?)
To make the tapered barrel, I needed some reference dimensions. I started with the front tube-plate, which I chucked on a 3 jaw chuck, from the inside. I also made use of a tailstock centre to stabilise it. I then took extremely light cuts - like 1 -2 thou. to create a smooth surface ready for silver soldering. Turning copper like this can be tricky - its material properties are such that it can grab, pull out of the chuck, and tear. This is mad much worse by the fact that it cannot be held very securely in the chuck without being damaged. My solution is to turn the chuck by hand. Also, to use a suitable cutting lubricant. Paraffin is suitable - and so is WD40! It makes a big difference to how freely the metal comes off. just take off enough material to produce a smooth, round surface.There was a significant hollow in the plate, so I removed it and tapped it to a better shape before continuing, as I wanted to remove a minimum of metal.