This weekend I spent most of my time working on a boiler for Nigel. Well, two boilers as it happened. The first one was a total disaster! Below is how I did it properly and I will point out where I went wrong.
For a simple engine like Nigel I only need a simple boiler so again I followed the Mamod example and decided on a simple pot boiler, as they are known. This is just as it sounds. A simple, sealed metal container with a fire under it boiling the water turning it into steam. No fire tubes or super-heaters or any complications like that.
Mamod make their boilers from brass. There is a nice little film on YouTube showing how they do this. Unfortunately I don’t have a hydraulic press to turn a flat piece of brass into a cylinder in a few minutes so I needed to find an alternative method.
Brass tube is not so easy to come by but copper water pipe is easily available. I got a piece of 2 inch copper water pipe 1m long from the local plumbing place for about $50NZ. That gives me the body of the boiler. I was pondering for a while about how to make the ends. I have seen brass frost plugs used as ready made ends. That would work and be nice and easy but you shouldn’t really mix copper and brass in a boiler apparently due to issues with corrosion over time. Copper frost plugs are available apparently but I haven’t found any in NZ. I decided I would need to make my own end caps from copper. I then needed some copper sheet to form into the ends. I called around a bit and found the local sheet metal place has some I could buy. Then I had a bit of a ‘duh’ moment. I have 1m of copper pipe. The boiler itself would only be 100mm long leaving plenty of copper left over. I could make the end caps from that.
The boiler tube was cut by hand after wrapping a sheet of paper around the pipe to give me a line to trace around. Once cut I filed the ends smooth and square then finished them off by rubbing them on a sheet of sandpaper against a flat board.
To get a flat piece I simply cut off a length of the pipe and then cut it lengthwise. To hammer it flat you must first anneal the copper. This softens it so it can be worked. To do this you simple heat the copper to a dull red heat then cool it. I quenched it in water to save time. It’s the heating that’s the important part, not how it cools. The pipe could then easily be flattened out by hand and finished off by hammering it flat against a steel block.
Once I had the flat sheet I could start on the end caps. To do this you cut a disc of copper slightly larger than the diameter of the boiler. You then need to bend the ends of the disc through 90 degrees to form a cup shape. This cup is then pushed into the boilers ends and silver soldered in place. You start my making a former over which to shape the copper. I used a piece of the same pipe I used to make the flywheel from. It turns out by chance to be exactly the right diameter. The former needs to be the inside diameter of the boiler minus twice the thickness of the sheet, in my case 1.2mm.
This former has an end plate welded to it and an 8mm hole in the middle of it. The copper disc is cut to be 6mm larger diameter and with an 8mm hole in the middle. The plate is then bolted to the end of the former with an aluminium washer between the bolt and the copper so as not to mark it.
The former is then firmly clamped into the vice and you can start hammering. I hammer carefully all around the disc edge a little at a time. The idea is to slowly shape the metal, not get it all bent in one hit. So tap, tap, tap all around the edges is the way to go. The copper is still soft from the flattening operation so it bends easily. After a few minutes you end up with the shape forming.
All the hammering though ends up work hardening the copper. This incidentally is why you shouldn’t use copper for fuel or brake lines in cars. All the vibrations work harden the metal and eventually it might crack. You can tell when the metal is starting to get harder. It stops bending so easily and the sounds of the hammering gets more metallic. So you have to anneal again.
Now this is where I went wrong in the first boiler. I originally made the copper discs much bigger so the flange was larger. The means I needed to move a lot more metal with my hammering. It took me hours! I thought it would be necessary to bend the flange over then trim the surplus off. I eventually worked out that you only need to make the flange slightly larger than required. Keeping it short makes is much easier to fold it over. And instead of trimming all that is required is a quick file to get it nice and neat when you are done.
Since the former is all steel I was able to anneal the copper without unbolting it. I started using a hand-held Bernzomatic propane torch but then switched to a bigger Swedish torch running of an LPG cylinder. More heat makes thing much easier.
I found I only had to anneal the copper once with the smaller flange as opposed to at least 6 times to get to the stage above with the bigger flange. So as I say, don’t do that!
Once the flange is turned though 90 degrees the disc can be unbolted and removed from the former. I then filed the ends square and gave the edges a quick file also to remove hammer marks and make it smooth. I finished up by sanding the cap with a sanding sponge to get it nice and clean. It helps to file a little chamfer to the end of the flange to help start it in the end of the boiler tube. Notice which way around the cap fits into the end of the boiler. I made two caps of course.
Before fitting the end caps ready to solder you have to ensure the metal is very clean. I rubbed everything with my sanding sponge then cleaned it in acetone to ensure the metal was really clean. You then coat all surfaces in flux.
The flux is there to further keep the metal clean as you heat it up to solder it. Obviously this being a boiler it is going to get hot so ordinary solder can’t be used. Instead you must use a higher temperature silver solder. I used Prosilver 34T and Easyflo flux available from BOC. They have a handy silver soldering guide here in PDF format.
This is the second point where I went wrong with the first boiler. The Bernzomatic torch I was using couldn’t put enough heat into the boiler to be able to solder easily. When silver soldering you need to heat the entire area up to temperature so that the solder melts when you touch the metal. You don’t use the flame to melt the solder. My initial attempts at soldering were not very pretty. Using the larger torch made things much easier.
Once soldered the boiler has to be pickled! This simple involves removing all the flux residue left over after soldering. Traditionally this is done by soaking the boiler in sulphuric acid. There is an easier, safer and cheaper way though. Citric acid! This is available as a powder from the local supermarket. I simply dissolved a container of this in about 4L of water in a bucket and will leave the boiler in it over night.
With the boiler pickling away I started on the little exhaust valve to use as a throttle for Nigel. I won’t go into the details as I made them up as I went but basically it is a piece of brass as a body with a large hole through the centre. At 90 degrees to this larger hole is a smaller diameter one that forms the inlet and outlet for the throttle. A shaft with a hole drilled across it fits though the larger centre hole. By turning the shaft you can line up the hole through it with the inlet and outlet holes in the body. A lever is soldered to the shaft so I can turn it. And short pipes were soldered to the inlet and outlet holes. I can take more pictures of it apart once I clean everything up. This little valve on the exhaust works well to throttle the engine. Another idea borrowed from my Mamod traction engine!
Also, if you are interested in exactly how strong these little boilers are, I found this document online where they tested some very similar size and design boilers to destruction here: http://sgcox.site.net.au/ritg/boiler-tests.pdf
The boiler I am making will probably be run at around 15-20 PSI. The interesting thing in reading that document is I probably could have used plain discs for the end plates instead of flanged ones and saved myself a whole lot of work!