4
The wrapper for the firebox is quite complicated as the front section has to be folded all the way round to form the shell of the combustion chamber. I drew the outline out as best I could using CAD, leaving a bit extra on the edges just in case! I then printed the outline out, laminated it and used it as a template for cutting out the wrapper from 16swg copper sheet.
Firebox/combustion chamber wrapper
The bottom edges of the wrapper have to be joined at the bottom of the combustion chamber and the drawings suggest either a strap joint on the outside or a 'joggle' type joint. I didn't like the idea of a strap on the outside as the clearance between the bottom of the combustion chamber and the inside of the barrel is quite small and reducing it further by fitting the strap would not help the water circulation in this area. The strap could be fitted on the inside I suppose. Some years ago I had bought a special tool for fitting body panels on cars. This was a tool to put a joggle in the edge of a panel about 1/32" deep to accept a second panel on top. When fitted, the two panels would be flush on the outer surface. So I decided to use this tool to put a joggle in the ends of the wrapper to produce an overlapping joint. As the tool only produced a 1/32" joggle, the ends were 'joggled' in opposite directions where they overlapped. This meant that the joint only protruded 1/32" either side of the wrapper itself.
Folding the wrapper caused some problems and I eventually made a simple wooden former to shape the curved top of the wrapper and the combustion chamber was formed around the tubeplate. I then found that the combustion chamber part of the wrapper was too long so I had to trim one edge and redo the joggle on that edge.
Simple wooden former for top of wrapper
In the articles it suggests soldering all the firebox up in one go complete wth crown stays and water tubes but I did not fancy that at all as it seems too much to do at once so I decided to do it in easy stages. I would first solder the overlapping joint on the combustion chamber which would give me a nice solid tube to fit the throatplate and tubeplate to. Then fit the throatplate, tubeplate and backplate, all with Silverflo 24. I would then fit the stays and water tubes with 440 and the firetubes with easyflo.
29/08/2006
The wrapper joint was rivetted together with a few 1/16" copper rivets and the joint soldered with Silverflo 24. Then the throatplate was carefully fitted to the wrapper and again rivetted in place. There were a couple of gaps at the top of the curved flange where I had cut the slits in the sides of the wrapper slightly too long so these were plugged with small pieces of copper. The throatplate was then soldered in place, again with Silverflo 24. Next to be fitted was the tubeplate but first I had to drill the holes for the tubes. I used the tubeplate former as a template by drilling the holes in the former first and then using this as a jig to drill the tubeplate itself. This makes drilling the soft copper very easy and you also finish up with round holes rather than triangular shaped ones which tends to happen if you drill the thin copper on it's own. All the holes were drilled 23/64" diameter so that they could be finished with a 3/8" reamer but the two holes for the superheater flues were bored out in the lathe as the micro mill that I use as a drilling machine will only take 3/8" drills. The tubeplate was then also fitted in place and soldered.
Wrapper after soldering the 'joggle' joint
Throatplate and tubeplate in position
Before the firebox backplate could be fitted I needed the firehole ring and this was turned from a length of 1-3/8" o.d. by 1/8" thick copper tube. The actual shape of the firehole is elliptical so after turning the steps on each end of the ring to fit through the backplates, the tube was squashed in the vice until the inside height was 1". This made the width about 1-1/4". The drawings call for the hole to be 1" x 1-3/8" so perhaps I should have used 1-1/2" tube instead. However, no indication of the size to use is given in the write up so I had guessed at 1-3/8". The hole's plenty big enough as it is anyway so no problem. The ring was then held in position on the firebox backplate and its outline marked with a felt tip pen. A hole was then drilled in the centre, the plate mounted in the lathe and the hole bored out to nearly the required height of the ring. The hole was then filed out at the sides to give the proper elliptical shape and the firehole ring rivetted in. The ring was then soldered on the inside of the plate before the backplate was fitted into the firebox wrapper. The outside of the ring was soldered at the same time as the backplate was soldered to the wrapper. I pre-soldered the inside of the ring with Helen's boiler as I found that the solder would not flow through the joint between the ring and the backplate if only soldered on the outside because rivetting the ring over gives too small a joint gap. I was going to try filing some small nicks around the hole in the backplate before I fitted the ring to see if that would make any difference but I forgot!
Backplate fitted complete with firehole ring
It was when I tried fitting the firebox into the boiler shell that I realised I had made a cock up with the template for the wrapper. I had got the angle of the throatplate wrong and it was not parallel to the barrel throatplate! When I looked at my drawing I had taken a wrong dimension for the top of the angled slit in the wrapper sides with the result that the throatplate is not angled forward enough. Fortunately this is not a disaster. What it means is that the gap between the firebox throatplate and the barrel throatplate increases towards the top instead of being 1/4" all the way up and the bottom of the combustion chamber is 1/4" longer than it should be. This shouldn't make any difference at all, in fact it will help the water circulation in the throatplate area which is always a problem. Some boilers are actually designed like this for that very reason. If the error had been in the other direction i.e. the firebox throatplate sloped too far forward, I would have had to scrap the firebox and start again. I would not have been a happy bunny!
31/08/2006
The firebox crown stays are simple foldings from 16swg copper and consist of two full height girder stays and a central inverted 'T' shaped one which runs the full length of the firebox and combustion chamber. These were quickly made, rivetted in place and soldered with 440.
The last job on the firebox apart from the firetubes are the four 1/2" diameter water tubes which run vertically through the combustion chamber. These act as stays as well as increasing the heating surface and circulation. The only problem with the water tubes is that they are directly in line with the superheater flues so it's not possible to fit full length radiant superheaters to make the most of the steam generated. The ordinary flue length type only really act as steam dryers rather than superheaters.
Drilling the holes for these is not an easy task as both top and bottom of the chamber are curved surfaces. Just drilling through from top to bottom would be a very difficult thing to do so I marked the holes out on the top and bottom surfaces, drilled 5/16" holes with a centre drill and then filed the holes out slightly undersize using a template drawn with CAD. The top holes are near enough round but the bottom ones are quite elliptical due to the small radius of the bottom of the combustion chamber. I have found centre drills to be ideal for drilling thin copper as they do not snatch like an ordinary drill and give a truly round hole. The holes were then finished with a 1/2" reamer put through both sets of holes which ensured they were in line.
Template for water tube holes
Lengths of 1/2" copper tube were inserted through the holes and of such a length that the ends of the tubes stood proud of the wrapper by about 1/16". The ends will be shaped to match the wrapper after soldering. The tubes were then soldered in with 440 and the worst was over (I hope!). I've probably been a bit generous with the solder in places but I don't want to run the risk of any pinholes in the finished job.
Firebox with stays and water tubes fitted
I can understand now why people shy away from combustion chambers! I was reading Don Young's construction series in LLAS for 'Doncaster', the 5" gauge A3, and he deliberately avoided fitting a combustion chamber to Doncaster's boiler!
05/09/2006
Progress has been a bit slow over the last few days due to the fact that I gashed my leg about a week ago taking an old vice to bits to clean it and the injury decided to take a turn for the worse so I've been hobbling about trying to take it easy!
The boiler has two 5/8" dia. superheater flues and eight 3/8" firetubes so these were cut to length, the ends faced off in the lathe and one end turned down a few thou to fit the firebox tubeplate. The step formed stops the tubes falling right through the holes in the tubeplate. The holes in the firebox tubeplate were then opened out with a reamer until the tube ends were an easy fit thus leaving a few thou gap in the joint for the solder to penetrate.
The smokebox tubeplate was then drilled and reamed for the other end of the fire tubes and the holes for the superheater flues bored out in the lathe as done for the firebox tubeplate. All these holes were made an easy fit otherwise it's a devil of a job to get the tubeplate over the tubes! The tubes were assembled into the firebox tubeplate and each one fitted with a ring of Easyflo pushed against the plate. This is much easier than trying to feed the solder inbetween the tubes. The smokebox tubeplate was then fitted to the tubes to help keep the tubes in line. I did the same as for Helen's boiler i.e. made a simple jig to hold the tube assembly rigid. This was again just a piece of angle iron clamped to one of the firebox crown stays and to the smokebox tubeplate to stop any movement. A bit of time was spent making sure that the tubes were parallel to the centreline of the firebox and all perfectly horizontal and then the firebox joints were well fluxed and the lot soldered up. I made the mistake (I think) of leaving the piece of angle iron attached to the assembly when I put it in the pickle bath (because it was easier to handle) and the copper came out a rather dirty brown and all the solder came out copper plated! This was probably due to the presence of the steel in the bath so I won't make that mistake again.
Next job were all the bushes and these were simple turning jobs from phosphor bronze bar. As for Helen, I only started the threads with a taper tap rather than tap them fully straight way. This is to avoid damaging the threads during soldering and they will be threaded all the way afterwards. The backhead bushes are a bit of a pig because they have to go in at an angle due to the slope of the backhead otherwise the stay and water gauge bushes will not be horizontal. The bushes for the blowdown valve and boiler feed clacks don't really matter but I decided to do them all the same.The regulater bush is also angled but not the same as the others! I got round this problem by drilling all the holes undersize and then reaming them with the backplate mounted in the micro-mill with the bottom end packed up to slope the backplate at the right angle. The actual bush holes therefore came out at the right angle to the backplate and so long as the bushes were a good fit they would (should!) align themselves correctly. The bush for the blower stay is different to the rest as I decided to pinch an idea for the blower valve steam feed from Don Young's Horwich Crab. Instead of an external steam pipe taken off the turret, the pipe is fitted inside the boiler and connected to an annulus inside the bush. The blower valve has corresponding holes in the body to pick up the steam. It's just one less pipe to clutter up the backhead! I have also fitted an extra bush at the bottom of the backhead for an injector feed. The original drawings show only one feed clack for the whole boiler which I thought was asking for trouble if the clack decided to play up! Another change I have made is to fit a bush for a proper steam turret instead of the combined whistle valve/pressure gauge fitment on the original. I'm going to need at least one steam valve for the injector so this will make it easier to fit. The original drawings are for an A3 with the low cab so there would be no room for a turret anyway. As I am building the A1 version I've got more height to play with. This also means the water gauge can be vertical instead of slanting.
Bushes fitted to backhead and tubeplate with Silverflo 24
Blower valve bush with internal steam pipe
The dome bush, safety valve bushes,turret bush, and water gauge top fitting bush were fitted to the boiler shell and soldered with 440. Care was taken to ensure that the tops of the bushes (except the water gauge one!) were horizontal so the safety valves etc would be vertical when fitted!
Shell and firebox/tubes ready for inspection and final assembly
Apart from a bit of tidying up on some of the joints there's not much more I can do until the work so far has been passed so I'll probably get back to working on Helen again. I've had enough of boilers for the time being anyway!
05/10/2006
Steve Eaton has approved the work done on the boiler so far following the trip to Chesterfield but it will be some time before any further work is carried out as I want to concentrate on Helen again.