|"Didcot" a GWR 14xx class loco.|
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0-6-2 GER 1003
0-4-2 GWR 14xx class
This is Part 2 of the Didcot: The fabrication of the boiler.
Here are new photos and text of the boiler progress of the Didcot. I started with the boiler on the first of April 1999 and it was tested on the first of November 1999. The drawing of the boiler was changed to metric and modified to suit the Dutch boiler inspector, and a ACAD14 file is available.
I presume these photo's will be helpful for new boiler builders; as I found there is not much information about model boiler building on the internet.
The valves were milled from cast iron and I hope this give a good service on the bronze port face of the cylinder block. On my 3.5" 0-6-2- ‘Mona’ I used a cast iron port face with bronze valves and this proofed to be excellent in service. The buckles are milled from a piece of 5 mm thick brass and are on the photo still attached to each other. Such a buckle allows the valve to set it self to the port face under the steam pressure.
By this locomotive there is one combined steam chest that sits between the two cylinders. Because this is an inside cylindered locomotive there is not much space for the steam chest (if one wants to place to reasonable sized cylinders also in-between the frames). The steam chest is only 20mm wide. The valves them self are 9 mm high.
This top-view shows how the valves will sit against the port face of the cylinder. Valve setting was done this way, which give a good visibility. The valve rods were made to length, so that afterwards (when the steam chest was put in position) there was no need to set the valves again.
I would like to take the opportunity
These are the boiler backhead and throat plate and their formers. Clearly visible on the left is the Tufnol disk with its locating bolt, that held the throat plate on its position during the flanging of the outer shape.
The complete formed boiler parts awaiting the next stage of manufacturing. A careful study of the drawing and design was now necessary, so the final boiler will be satisfactory. I only found later on that I still had made two mistakes! Although I used ACAD to design the boiler the boiler inspector found during his inspection of the parts that the distance between four of the fire tubes would be to small for good water circulation. He advised me to make a new firebox tube plate and arrange the fire tubes layout in such a manner, so that there was always a minimum of three millimetres of space between around each tube. So back to the computer and setting out a new tube layout. With some moving around and some puzzling this problem was solved, which took in fact more time than the fabrication of the new plate.
The second mistake was only a small one, but for me enough to correct. The top feed bush was located on the drawing in the wrong position. On the boiler drawing the dimension and location was different than on the general arrangement, and I only spotted this problem , when I put the drilled boiler barrel on the drawing. A disk of 3 mm copper was used to cover up the wrongly located hole and a new one was drilled in this one. Now on the correct position....... only 7 mm more to front of the boiler. I wonder if anyone had noticed if I didn’t change the location, but once you’re in studying the prototype that closely, you want to make your model as good as possible. Am I becoming a rivet counter?? I must beware!!
In this position the outer- and inner-firebox will be located once the boiler is finished. This is still the old firebox tube plate that was later replaced for a new one for the reasons stated above. The water space on the sides and back of the firebox is 8 mm (7/16”) and in front it is 10 mm (3/8”).
The inner firebox will stick out of the boiler for about 8 mm. This flange will than be used for fixing the ash-pan to it.
The drilling and boring of the holes for the fire- and superheating tubes was done on my milling machine. With the aid of the Digital Read Out is easy to locate the holes in the correct position, as determined with the CAD software.
The large superheater tube (25 mm) wasn’t drilled, but made with the boring head.
A test set-up of the tube bank. The holes are all a little oversized and not, as stated in most of the available construction notes in the ME press, reamed to a close fit. This would be incorrect for soldering, as the tubes will expand during the process and than there would be no gap for the solder the penetrate. To prevent leaks around tubes, a rough surfaced hole and a little over dimension of the hole are ideal, so that solder will always be able to get in the joint.
The circular layout of the tube bank is good visible in this picture. The one superheater tube will be equipped with a concentric stainless steel superheater element.
The 2.5 mm thick copper plate of the outer firebox shell was annealed first, and than bended by hand of a wooden former. The former was made of several plates of 15 mm thick MDF plate glued together. Each plate was first CNC milled in the contour of the firebox. A former like this takes some time in preparation, but once you have it, the copper plate is ready in minutes. Working with such a former block makes sure that everything will get the correct dimension and shape.
This was the set-up for drilling and boring the dome-bush hole and top feed hole in the boiler barrel. Alas, as described earlier, the top feed bush was in the wrong position, and had to be corrected later on.
The bronze dome bush was first turned in the lathe and in the milling machine the 12 holes were drilled and taped, that later on will be used hold the inner dome. Four small M3 bolts were used to clamp the dome bush to a steel plate, which in its turn was clamped in the vice. In this set-up it was easy to bore out the large radius on the underside of the bush, with which the bush will sit on the boiler barrel.
Here the inner and outer boiler are standing aside. The inner boiler looks that pink, because it was sand blasted after soldering. This gave the opportunity to check if all the tubes and other joints of the fire box were good. The small mistakes that occurred (not enough silver solder in one joints between crown sheet and fire box tube plate) could now easily resoldered. Better be safe than sorrow!
The outer boiler is now also soldered and the location is set for the hole for safety valve and the hole is made. The outer boiler is clamped to the milling table by use of a large T-bolt going through the firebox and boiler barrel. On top a steel plate with a hole in it, was used to take up the nut. An extra clamp is used on the throat plate to prevent the boiler shell from shifting during the process.
On the Swindon work drawings I found that the fire hole was conical shaped. I wanted to copy this on the model, because I experienced on my 3.5" tank locomotive, that you do not have a good view on the fire, once you are in the driving position behind the locomotive. A conical shaped fire hole would give a far more better view on the small fire box and it would give also the opportunity to control the fire better. For this purpose a special conical ring was turned from copper bar. On the outside of the boiler backhead it is only 36 mm in diameter, but on the inside it is almost 60 mm in diameter.
The fire hole ring is hardly visible lying on top of the fire box, but the good view inside the firebox is very clear.
Soldering was mostly done with two Sievert propane burners. The good summer we had in 1999 allowed me to work outside the workshop. With such a large boiler (although very small for 5" gauge) it is almost impossible to solder inside. The amount of heat necessary for soldering would melt the sealing in the work shop. The small parts I did solder in the work shop raised the temperature very quickly to 45° C.
When the parts were sufficient cooled down after soldering they were pickled in acid. The tank I used, normally functions as our chemical waste container, kindly provided by the city-counsel.
After about ten minutes in the acid, the copper comes out of the pickling tank with a nice pink clean surface. This removes the oxides and brazing flux residue from the copper in a fast and efficient way and also prepares the copper for the next stage of soldering.
Without the backhead in position the boiler was now fitted on the locomotive frame. Clearances between the firebox and the frame could now be checked and the height for the boiler supports could now be determined.
The crown stays in this boiler are of the so called ‘rod stays’ type and not the more widely used plate stays. They are turned from 5mm dia. copper rod and are screwed in the top plate of the inner firebox and silver soldered in the outer firebox. This type of stay is relatively easy to fit and solder in the boiler and is easy to check if all the individual stays did have a good fillet if silver solder all around. The water circulation on top of the firebox should be good with this kind of stays
The new locomotive 'Didcot' on 5" gauge (scale 1: 11.3) standing next to my 0-6-2 'Mona' on 3.5 " gauge (scale 1 : 16). Although the 14xx class loco was a diminutive locomotive, it's still a big engine to handle in 5" gauge and weighs already more now, than the 25 kilograms of the 'Mona'. The outer diameter of the boiler barrel of the 'Didcot' and 'Mona' are the same: 106 mm. Note also how good the view is inside the firebox, due to the conical shaped fire hole ring.
At an annual live steam meeting in Turnhout (Belgium) in May 1999 some model engineers of the 'Bromsgrove Society' from UK came over with a lot of British prototype locomotives. Ray Dodds GWR 'Metro' 2-4-0 is standing here next to my GWR 0-4-2 'Didcot'. Both locomotives are very much a like in over all size, wheel dimension, boiler and motion and steam engines. It gave me some indication in how my locomotive will perform on the track, once completed.
The only 14xx model I ever saw of an 14xx class locomotive is this Arfix 00-gauge 1420 locomotive. The small model is useful to see in three dimensions how my locomotive will become.
The 14xx class locomotive was also available in kit form from Winson Engineering, but this firm went bankrupt. Click here for details of the Winson locomotive owners. On that site you could see all the parts that go in to making the model, nicely laid out side by side. A new firm "Modelwork International ltd." is bringing the kit back on the market in March 2004
The foundation ring consists of four separate pieces of copper bar, that are riveted and soldered in position.
The photo shows the first part just after it was soldered, with the flux still on it. Where there isn't any flux, the copper will turn black after cooling down. Pickling the boiler in acid will remove this black oxidation layer.
A lot of heat is needed to solder the other pieces of the foundation ring. I did this alone holding the two Sievert burners and trying to hold the silver solder. This isn't the most comfortable way to proceed and some assistance would have been be more practical.
At our annual steam-up at Breda in 1999, the 'Didcot' stood on the small stand of the Ground Level 5" Gauge Mainline Association which Chris Boeckstyns and I set up for that occasion. This UK based model engineering society of which we both are members, is involved in running models of mainline prototypes (locomotives and rolling stock) to a schedule. Like playing with a 00-gauge train-set, only bigger and with real steam! Once the 'Didcot' is finished I want to build some goods wagons to go with it. From Doug Hewson I already ordered some nice scale drawings of a GWR brake van and a closed van. See http://www.the-hewsons.demon.co.uk/ for some nice pictures of 5" gauge rolling stock.
The stays are of the screwed and caulked type. At a spacing of 16 mm, M4 holes were drilled and tapped, to take up the 4mm copper threaded stays.
Drilling the holes in the throat plate called for an extended drill. Therefore the 3.4 mm drill was glued with Loctite in a 8 mm long steel rod. Drilling this way went fine, although the long drill needed some guidance by hand.
Tapping in copper plate isn't that difficult, as long the correct tap for the job is used. I can be a bit awkward when tapping close to the boiler barrel, but by removing one handle from the wrench it went fine.
The final silver soldering operation was the soldering of the backhead. For this soldering operation, the help of a colleague model engineer was needed, and Dick Summerfield (http://www.iae.nl/users/summer/16mmngm/ ) was willing to help. He kept the general boiler temperature high with the propane torch, when the soldering was now done with an Oxy-fuel torch. The greater heat applied made silver soldering a lot easier.
Whit this equipment the job was done within a few minutes and even soldering the thick fire door hole ring went relatively easy. Two 250 mm long and 1.5 mm diameter filler rods were used to solder the back head alone.
The boiler weighs now 9 kilograms and has a volume of 2.5 litres.
After the summer holidays (in which I went to the GW Society at Didcot England, for making photo's of the real locomotive which is still running there) we went for a steam-up to Breda with my 3.5 " locomotive.
I brought the 'Didcot' along, to see how it would negotiate the curves in the track, with the full boiler weight now resting in the wheels.
The model was pushed along by hand and all went fine. The 5" locomotive is rolling very smoothly on the track and is very stabile in the curves. Later we even coupled the 'Didcot' behind the 'Mona' and towed her along for several laps.
The trailing wheels are now equipped with working leaf springs. To keep them flexible, not all the leafs were made from steel spring strip (10 x 1 mm), but some of them were made from soft aluminium. If you look closely to the picture you can see the light grey ones, that are made of aluminium.
After all the tapping for the boiler stays was completed, the boiler was put in the dishwasher (together with an other boiler of an 3.5 " gauge GNR Large Atlantic locomotive 'Maisie', which a started a long time ago, but still didn't completed yet) .
After washing, the spotless clean boiler could now undergo the last stage of construction; the screwing in of all the boiler stays and placing of nuts on each end of the stay. This took up several evenings and resulted in some sore finger tips. Afterwards they were all caulked with a 'high melting point' soft solder rather than silver solder. The boiler of the 'Mona' is of the same construction (designed by 'LBSC') and this worked fine.
At our last steam-ups in Breda one of our club members found out that this method is not only easy during manufacturing, but is also a very good safety measure if the water level should drop to low. He had trouble and the water level fell dangerously low. The soft solder of the top row of the stays melted and water and steam extinguished the fire before any real harm was done. He dismantled the locomotive at home, cleaned the boiler, resoldered the stays and was back in business in no time!
The finished boiler was now ready for hydraulic testing. I had to make a small water pump for that, which was connected via a copper pipe to one of the blow down valves bushes. All the others were blanked off, except for the top feed bush, to which a pressure gauge (which I got for the Dutch Railway Workshops at Tilburg) was attached.
The boiler was completely filled
with water and pumping began. Within a few strokes of the pump the pressure
began to rise. Only to find that some stay were still leaking a few
droplets. So the water was let of, all the plugs were removed and the few
stays were re soldered.
When all was well, I contacted the Boiler Inspector and made an appointment for the official test. On the first of November 1999 the boiler was approved by the Inspector for the hydraulic test. A boiler certificate will be issued however after the steam test.
The unique number of the boiler stamped on the foundation ring. 9909 stands for the year 1999 and 09 means that is the ninth boiler that was tested and approved that year.
This wraps it up for now. I've now started with the tanks, cab and bunker and you'll see that in part 3.
There are two ways of doing things:
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GWR 14xx class, with the boiler fitted