As far as I know, standard plumbing copper tube hasn't the correct wall thickness for boiler making, it is much too thin.
Normally you have to buy specially drawn material to get the thicker wall thicknesses. The end flanges are usually made from the same thickness, or thicker material as used for the boiler shell tube. The end caps are then formed over hardwood or metal forms to give the correct radii to the corners and correct surface areas for the silver solder to bond to, ensuring the joint is at least as strong, if not stronger than the parent metal.
You will also required phos bronze stays to stop the end caps bowing out and deforming, and ALL boiler fittings in the boiler should be of phos bronze. Brass is a nono for correct use in a boiler, the zinc leeches out of the brass over time and it becomes a very weak and porous material.
It also has to be subjected to pressure testing to obtain it's certificate, and I think at the moment that has to be renewed and tested each second year. I am not positive as to the timeline but it does work like that.
Welded steels are subject to even more rigorous regulations.
How do I know all this.
I only gave up making model boilers a few years ago, but some things have been relaxed slightly since then.
Because of all the regulations and insurance requirements needed nowadays, it would be a lot easier for me to go out and buy a commercially produced one.
A lot of people do still make their own boilers, mainly because of cost, or the self satisfaction of making one. But again they have to conform to rigid standards to be acceptable for use.
If a high pressure boiler is to be used in a public place, it must have all the necessary certificates and insurance in place before it can be steamed. A public place is an area where anyone other than yourself is operating the boiler, so that includes your kitchen or anywhere else in your home when family members are about.
People scream and say 'I will do what I want in my own home'. Wrong, the law will just make sure that whoever is hurt, screws you for as much cash as they can get, whether that is your parents, kids, wife or just a visitor.
Making a boiler brings out the worst in people when it is discussed on a forum. The above is only the basics, you will still get people saying that such and such is OK for making a boiler, because I have never had one blow up. Maybe they are right, or just plain lucky. To me, if it doesn't conform to national and local, plus insurance regulations, you shouldn't do it.
BTW, there are different regulations concerning 'toy' boilers, Mamod, Wilesco etc., I don't know what they are, but I am sure someone will soon let us know. Maybe Steve (Cedge) can give us an insight into that, as he deals a lot with a model steam engine company.
Bogs