Page 1 Drawings 12a + 12b - Boiler Detail
I have now split the boiler construction over two pages to allow faster loading on slower devices. Clicking the "next" button cycles between the two pages
1. Firebox Formers    
I have made the former for the outer wrapper of the firebox using a couple of lengths cut from a railway sleeper, screwed together and then hand-sawn to form the tapered shape. It's a big box and needed a few hours work to produce. Cardboard templates were made of the top of the firebox by tracing from the drawing, marked onto the former and then profiled with a plane and orbital sander. The angles for the backhead and the bottom of the box were marked onto the former next. A large piece of paper was then placed over the former and the lines transferred to the paper, giving me a flat layout, or development, of the wrapper. This came out at 608mm x 280mm, quite a big lump of copper.
The former for the firebox inner was made in a similar fashion but with a separate piece at the front for the combustion chamber. This was turned to 5.1/2" diameter and then screwed to the inner former, aligned with the boiler centre-line. Although I started with a HSS tool, one of the pictures shows a polished carbide tip in use once space permitted. These are super-sharp and work well turning wood.
This time, the development had to include the wrap-around section for the combustion chamber which is joined with a strap at the bottom. For this the copper needs to be 462mm x 280mm.
3. Flanging Plates    
The various flanging plates have been made from an offcut of 40mm laminated oak kitchen worktop. The shapes were marked out, allowing for any bends in the plates, then cut with a jigsaw. The backhead plate needed some extra shaping and this was done on the mill. Because of it's size different workholding methods were needed; the first angle was created by clamping direct to the table, the other two by screwing a piece of box-section mild steel to the reverse and holding in the tilt-and-turn vice.
The combustion chamber front plate and the firebox backplate were made in a similar manner to the backhead plate. The front tubeplate former has been made as a double-duty item with the rear tube plate on the rear. If this doesnt work, I will need to split them in two.
The front throat plate was the largest of the formers and the most challenging to make because the drawing section is staggered. Once finshed, I noticed that the milling cutter and collet in the mill had tarnished overnight where I failed to clean down properly. There must be something in the oak that affects the steel - I must be more thorough in cleaning up afterwards when machining hardwoods.
3. Rear and Front Tube Plates  
I chose to make the rear tube plate first because it is one of the smallest items and not too expensive to replace if I messed it up.The rear tube plate was cut to shape from a 129mm x 178mm piece of 1/8" copper sheet. After that, the plate was annealed and the forming began. I started with a rubber mallet but changed to a steel hammer and hardwood drift after a short while. Couldn't get enough movement with the rubber mallet. I found that a backing piece was essential to stop the flat area deforming.
It took me four further heat-ups to get the plate fully formed. Then I clamped it to the mill table and trimmed the flange height to half an inch. I'm not going to put the holes in until I have sourced the superheater flue tubes in case I get the hole size wrong. The next part I made was the front tube plate, marking out the 7" dia circle on the 178mm guillotined blank. Then I hacksawed away the waste and followed with a first heat-up to bright red and quenching in water to get the blank nice and malleable.
As before, the flange was slowly formed around the former, working until resistance was felt. The workpiece was then reheated and quenched before forming continued, this cycle of working and then annealing continuing until complete. This one took five reheats before I finished. The flange was trimmed to 7/16" long in the lathe using a polished carbide for the cut.
4. Boiler Barrel    
The drawing shows a develpment of the boiler shell and the first thing to say is that the dimensions are rubbish. The 19.9/16" dimension should be 19.1/4" and the 21.3/8" dimension should be 20.3/4". These dimensions were duly marked out on the blank sheet and the waste removed. I don't have a bandsaw but a good-quality blade in my jigsaw worked fine, brushing on some suds to help lubricate the cut and keep it cool. The edges were then filed to a good finish with particular care to the edges forming the bottom join.
Next, it was annealed in my temporary home-made kiln using a propane cyclone burner because it doesn't get starved of air as easily as normal burners. However, it's a big lump and took ages to get hot. I think I need the next size up. I had to let it cool naturally as I don't have a village pond nearby, then it was into the rollers and much heaving and grunting as the barrel was rolled.
Halfway through I removed the workpiece for another heat-up before finishing the roll. It looks easy but is actually quite hard work. Aside from the waiting time whilst cooling, I must have spent a good two to three hours on it. I then spent another hour or so teasing it to the final shape, the two flats at each end of the roll being hammered to shape. I made an anvil from an old acetylene gas bottle that I scrounged from the local gas stockist but I was hoping to find a length of old telegraph pole.
I also had to file out some more material from the join to get the sizes at the ends correct and I made a couple of plugs from the oak offcuts to fit each end, rather than using the tube plates. Just a little more to be filed out at the front for a really tight fit. After that, the boiler strap was drilled as per drawing and held in place on the boiler shell seam under the rachet straps.
With the straps pulled up really tight to keep the seam closed, the holes were drilled through in turn with a pistol drill and a 3/32" steel rivet dropped in. Once all the holes were drill and the rivets were keeping the seams together, each rivet was removed in turn, the hole deburred and a copper rivet inserted from the inside. The picture isn't very good but I'm half-way through swapping the rivets. Once all the copper rivets were in place, the barrel was once more placed on the acetylene cylinder anvil and the rivets trimmed and belted flat into the countersinks. The strap was eased to the shape of the boiler at the same time.
5. Tube Plate Holes    
The drawing only shows guide dimensions and I found it easier to measure the tube centres on the drawing and transfer these numbers to a larger sketch. CAD users will, of course, just be able to lay these out on the screen and print out the numbers. Because we only get one shot at this, I decided to drill a piece of MDF first and see how well spaced the tubes were. As you can see, a few of the holes are a bit too close together so I moved the various tubes to suit and entered the new dimensions to the sketch.
The rear tube plate was then clamped onto another piece of MDF, centred up and the holes drilled and reamed using the DRO for accurate positioning. The 1.1/4" superheater flue holes were step-drilled up to about an inch and the boring head used to bring them to final size. After a bit of a clean-up and deburr, this is the other side. The tubes are a tight fit at the moment, which is what I wanted, and I shall skim a few thou off the tubes to create a shoulder for a positive location prior to soldering. The front plate was made in a similar fashion.
5. Boiler tubes    
The boiler tubes were initially cut using a plumbers pipe cutter and then faced to length in the lathe using a backstop fitted to the rear of the spindle. They were then returned to the lathe and the ends turned down by a few thou for a sliding fit into the two tubeplates. My ER25 collet adapter came in useful because it allows the work to pass right through, unlike the 3MT lathe fitting. Here is a trial fitting of the tubes in the rear tube plate.
The two superheater tubes were faced to length in the lathe and then spigotted either end in a similar fashion to the boiler tubes. Because I don't have a fixed or travelling steady - I haven't done anything further with the big steady I made for the smokebox yet - I made a quick bearing and collar to fit in the bore of the tube. This supported the end whilst facing to length but a traditional centre was used to support the work for turning the spigots.
Thirty pegs of differing lengths were made from 14mm wooden dowel to assist with locating the tubes in the front tubeplate. The shouldered section to fit the tubes was turned first, follew by parting off to varying lengths. They were then set in a short length of pipe to support the back of the collet and the taper turned using the compound slide, changing the angle to suit the length. It worked but was a lot of faffing around.
Once all the tubes were assembled into the two tube plates, solder rings were placed over each of the tubes before smothering the whole lot in flux. I have some 38 percent medium temperature silver-solder for this first operation. This is to allow a lower-temperature silver solder to be used when joining the tubeplate to the combustion chamber. Only the rear tubeplate will be soldered at this stage, the front tubeplate is the wrong way round at present and just being used for support.
After smothering the plate in HT5 flux, the assembly was brought up to temperature by applying heat to the underside of the tubeplate and rotating the assembly by hand. I had it stood on a piece of steel inside an enclosure of Celcon-style blocks and had to get it to cherry red before the solder would flow. Once cool, I gave it a thorough clean in hot soapy water, then pickled in Citric Acid until bright pink copper showed everywhere. A few tubes near the centre hadn't taken the solder so the whole thing was fluxed up again and another heat-up undertaken to get the last few tubes secure.
6. Firebox Inner Wrapper    
The development outline of the inner firebox was marked out using a paper template that I created by wrapping it around the wooden former. The waste was then removed with a jigsaw, the plastic protection peeled off and the workpiece annealed. I used a bigger torch this time and got the copper up to red heat in about three or four minutes.
It needed two more reheats before I was able to reach this stage and then it was a case of easing the combustion chamber to final shape for a snug fit inside the tubeplate. The throatplate will be made next so that the firebox can be fitted to the shape. However, I have made a mistake here and it needed some additional work to achieve a satisfactory outcome.
Before I started building the boiler, I obviously did some research on the internet and one of the things I came across quite early was a set of pictures from Station Road Steam showing a set of formers and a collection of cut plates that they had for sale way back in 2007. I promptly downloaded these pictures and have used them as a casual reference since. In this instance, however, the person who cut the plates made a mistake with the firebox wrapper, specifically the cut line from the base up to the combustion chamber. Because the front of the firebox slopes up and forwards, the combustion chamber needs more material at the bottom than the top. The paper template that I made produced the effect shown on the left, revealing a gap between the throatplate and the lower part of the combustion chamber.
What I should have done is shown with these two sketches. Points A - A are created by projecting the angled front throatplate up to the centreline of the boiler. I fell into the same trap that the other person did but, in my case, because I chose to make a flanged throatplate this covered the gap that would have been left if I has made the butt-join throatplate instead.
The throatplate has to come forward at the top but it will still leave the angle a little too upright. This won't matter, however, it just increases the water space at the top of the plate, but it did require a strap around the underside of the combustion chamber. The soldering has been done in stages with 38 percent silver solder, the joining strap first, followed by the packing strap. Next, the throatplate was held in place with four copper rivets and the first side soldered.
These are the only two photos I've taken of this part of the build. The heat was applied to the inside of the box and the solder was rested on the outside of the box and covered with HT5 flux. My soldering won't win any beauty prizes but at least I can see that full penetration has occurred and I'm happy with the joint.
7. Inner Backplate    
The backplate for the firebox inner is one of the smallest pieces coming in at a mere 188mm x 160mm. It was duly marked out around the former like all the others. Same procedures as before and this is the finished backplate having a test fitting.
The firehole ring gets soldered into this before the backplate is assembled and that was made next. The drawing shows a piece of 14swg tube wrapped with a 10swg ring and, luckily, I had a suitable piece of 2" OD tube in stock for the inner. To create the outer, I calculated the circumference and milled up a piece of copper to suit. After bending, and finding it too short, a new piece was made 3/16" longer. I've obviously forgotten how to calculate.
This was annealed before wrapping around the tube. Then the ends were soldered together in the vice with medium-temperature (38%) silver solder. Finally, the two parts were lined up and a pair of 3/32" holes drilled and copper pins fitted to keep the rings in place. Then they were soldered together before turning both ends to length in the lathe. One tip to offer others: don't try and turn the thin-walled copper tube in a 3-jaw chuck without some additional support. I distorted my first piece of tube and had to bin it. After the ring was soldered on, it was much more rigid.
Finally, the firehole ring was squashed to shape in the vice and the shape marked onto the backplate. The hole was stitch-drilled freehand with a 3mm PCB drill. This time, I just chiselled out the waste piece and filed to size.
Before soldering the firehole ring into place, I secured the backplate to the firebox and drilled four rivet holes, two each side, for holding in place during soldering. Because of the angled back, the backplate wanted to keep falling inwards to the firebox and the packer clamped to the crown stopped this happening. Finally, the firehole ring was soldered into place..
With the help of a colleague we were able to get two burners working on the inner firebox, working one side each and letting the heat flow round to the back. The firebox is set up with its firehole resting on a couple of blocks to get the heat in from below and the solder has been preformed and rested on the edge of the backplate inside. Flux was laid around the pre-solderded firehole and also over the rivets, inside and outside. As many blocks as possible were laid around and over the top as we could get without impeding the torches. After a good clean-up, this is the final result.
8. Firebox Outer Wrapper    
The firebox outer wrapper is the second largest piece of copper, coming in at 608mm x 280mm and was duly marked out by drawing round a paper pattern. As before, the waste was removed using a jigsaw, leaving a little extra on the bottom edges, and then brought up to temperature for annealing. I've found that it is sufficient to reach dull red or a little brighter to soften the copper. Going higher into the cherry reds doesn't make the workpiece any softer, or remain maleable any longer, so is just a waste of gas. First bends were made by resting the workpiece on top of the former and just pressing down. This caused the crown to rise a little and subsequent forming was done under my press. I only used this because I don't have a clamp long or deep enough to keep the top in contact with the former.
The sides were eased down using a hide mallet and after two more reheats arrived at the point where I could hold the bottom edges in place with a pair of sash cramps. This can be left like this while I cut the front throatplate. Then the wrapper can be eased to the final shape.
9. Front Throatplate    
I decided to put the two bends in the front throat plate before cutting to shape, the lower section being held between two lumps of steel held in the vice. The upper, reverse bend was folded around the former to get the basic shape but the former is not quite right and an allowance has been made to compensate.
The bend needed to be higher and is now in the right place. The two bends were adjusted until the plate could be stood on the drawing and matched the draughtsman's lines. Then the outline of the former was marked on the plate, the flange allowance marked and the waste removed. After cutting, and before the flanging started, a sanity check was made by resting the firebox outer wrapper on the throat plate to make sure I was in the right ballpark.
The former was also double-checked to make sure that it fitted the outer wrapper correctly. the packing pieces are there to simulate the flanges. Now it's time to get on and form the flanges but I shall need to make some new formers. These fancy-shaped formers that I made are fine for getting a visual outline of the final shape but rubbish for clamping down on because of the angle. These are the two formers that I used to form the flanges around. This oak worktop has certainly come in useful.
I find this one of the easiest ways to hold the plates, and using a hardwood drift to form the shape. One thing I did find was that the plate was trying to straighten itself when I was working the area around the fold lines. I had to put the throatplate in the vice between two lumps of steel and get the angle back to thirty degrees. This, in turn, caused the edges to splay out again. I also re-annealed this plate about seven times before I was happy with the finished article.
Here it is, offered up to the firebox outer wrapper. It's slightly askew but will be simple to ease into the correct shape. And two views from the side and the inside.
Moving on, the barrel was offered up to the front throatplate and a pencil line drawn around to mark the cutaway. A felt-tip pen was then used to mark inside this line and was used as a drilling guide. The workpiece was mounted on the mill under timber packers and a 3mm PCB drill loaded up. Because of it's rigidity no centre-drilling was needed. I stitch-drilled the first half of the plate.
The upper section was drilled in two stages, swapping the single clamp to the other side half-way through. To clear the waste from the centre, I chose to use some old Abrafiles that I have and had to modify my hacksaw to take them as the original frame has been lost during various moves. The advantage of these is they can be used at any angle and don't break if the frame is twisted.
About five minutes work to cut the centre section away. Then it was onto the careful job of dressing to make a nice fit around the barrel. This took quite a time as I didn't want to go too far and have to try and fill any excessively large gaps. Emery bands in the Dremmel were used for this job. I managed to get the front throatplate filed up to a good fit today but, as expected, it took a lot of effort to get a really nice, tight join.
The front throat plate was clamped to the mill bed and the side flanges reduced to just over a half-inch high. Although the usual way to join these is to just solder the tube into the tubeplate, I didn't really feel this was good enough. I decided, therefore, to make some form of collar to butt the front plate up to and three pieces of copper were milled to 1/4" wide and shaped to fit around the barrel.
The front plate was placed on the barrel and adjusted until the various lengths agreed with the drawing and a pencil line drawn around the periphery. The strips were then lightly clamped to the barrel, pushed and shoved till they matched the drawn lines, then clamped tight. 3/32" holes were drilled at various positions and rivets temporarily fitted to hold the straps. Once happy with the position, the rivets were reversed (heads inside) and hammered flat into countersinks on the outside. There is loads of space under the cleading and these straps will not interfere with the fit. In fact, they will help.
The white dots on the straps are a series of 8BA holes that I have drilled through the straps and filled with correction fluid. They will be available for fitting studs to after soldering to fix the cleading to. Whether I use them or not remains to be seen but it's easy to make provision now rather than trying to drill and tap some blind holes later. Here is the barrel after soldering and the tippex has stopped the solder entering the holes. After cleaning up and filing down any surplus solder, the throat plate was fitted onto the barrel and gently tapped around until it butted nicely up against the strap.
Finally, the inside of the barrel was peened outward until it slightly flared over the throat plate and trapped the barrel in place. This assembly was now mechanically sound and has been soldered together. The greatest gain here is the fact that there are now two faces for the solder to join, the vertical and the horizontal, and results in a far stronger join. And I get somewhere to fix the cleading for free, if I choose to use it.
10. Inner Girder Stay    
I bent up a couple of lengths of 3/4" angle from the copper offcuts, shaped the ends and drilled the six half-inch clearance holes for the cross-stays, remembering they are left- and right-handed. Next they were clamped together, three 3/32" holes drilled and then riveted with copper rivets. These are entirely within the water space and need no special attention. I then stood the assembly in the upright "T" position, laid a length of high temperature silver solder in the gulley and did a cook-up. They don't really need soldering together but it did have the added advantage of annealing the copper again. The assembly was then placed on top of the firebox, clamped at the front and a pair of 3/32" rivet holes drilled right through, one front and one back, to keep the girder in position during soldering.
Rivets were loosely laid in the holes with a tiny ring of solder underneath and the girder melded to the top of the firebox. It was then set up in the hearth at about thirty degrees with a couple of lengths of solder up the gully for the lower leg and one on the top edge for the upper leg. Heating was done from inside the firebox and below the girder until we had a full melt. After the box had cooled for five minutes, the cinder blocks were cleared away to speed up cooling. As can be seen, the rivets are standing proud of the girder where the solder melted away, and these can be dressed back. Full penetration has occured with solder visible inside the firebox.
11. Outer Girder Stays    
The outer girders were marked out on the plate then cut to shape with a hacksaw. These will not be machined, plus or minus a sixteenth of an inch is fine for this.
A couple of sessions of annealing and bending got to here continuing until they matched the end view on the drawing. Then they were set up on the mill and the half-inch holes drilled as before. When fixed to the top of the firebox, these holes need to be aligned to allow the cross-stays through.
Three rivet holes were drilled in each girder, then these were clamped (with difficulty) to the top of the box and spotted through. 8BA nuts and bolts were used temporarily to hold them in place while the bottom lips were teased to the shape of the firebox. These will be removed and loose rivets were set in prior to soldering. If I were to do this again, I would get these girders affixed before putting the firebox backplate in, this would make it much easier to position and clamp the girders to the top of the box. As it was, I was faffing around a good while before I was happy with the job.
I set the firebox up in the hearth tipped over to about thirty degrees and laid solder on the the top of the join on the upper one and in the gulley of the lower one. Heat was applied from below each component until the solder melted and wicked down into the joint. However, I didn't bend the rivets over enough and the top one lifted a little at the end and I wasn't happy with the joint. The lower one was better but I should have used more solder and daylight could be seen in a couple of places under both girders. I decided to revisit both joints and started with a really good clean up and pickle to try and get everything scrupulously clean. After tapping down the wider gaps the assembly was fluxed up again but the existing solder got in the way of getting the ends down tighter.
During the second heat-up, I noticed that I was putting in more heat than before and that the solder under the girder was starting to re-melt but, more importantly, the solder around the top corner of the backplate was starting to go semi-liquid and I backed off straight away. After cooling, cleaning and pickling, I could see that the heat had also lifted the central girder slightly away from the top of the box at the back and, by now I was wary of another cook-up in case I made matters even worse. I decided to drill and tap four new holes, one in each girder at the firehole end, and some sloppy 4BA bronze screws were made. These were fluxed, then screwed down touch-tight and the whole assembly fully fluxed up again.
Rings of solder were placed around the screwheads and solder laid in the gulleys as before, including some in the centre girder and on top of it. Then my colleague and I applied heat from two torches, one of us concentrating on the top part and one on the lower. Once we were up to melt temperature, one torch was removed and the other used alone to draw the solder down into the joint. We were in better control of things this time compared to when I went it alone and we now have solder along all the joins with no daylight showing. This is the lower girder but the others are similar.
12. Dome Bush    
The drawings show that the dome bush is optional on Britannia because the regulator is in the smokebox. I was going to omit the dome bush but John the Pump suggested that it would be useful as a means of flushing out the boiler - nice, big hole - and it could do the additional duty of supporting the steam collector pipe instead of adding another small hole in the boiler shell further along. There are no dimensions given for the bush, just an outline on the drawing of the shell so I scaled from this instead, then modified to suit my needs. The bush and it's cover plate were made from bronze offcuts starting with the bush. A hole saw was used to remove the centre and a parting tool modified to trepan the outer size.
The turning of the bush was quite straightforward and the component was then set up on the mill to drill and tap eight 4BA blind holes for the cover plate. I chose a 2" PCD so the hole positions were simply "1.0, 0.0" and "0.707, 0.707" respectively. The cover plate was machined from a weirdly-shaped offcut of bronze using the four-jaw chuck. The old wheel bearing came in handy for extending the centre to provide support against the intermittent cut.
The packing pieces between the jaws and the workpiece were to create enough room for the turning tool to function without clouting the jaws. Second op. was undertaken using soft jaws, bringing the flange thickness to size and maching the spigot for a good fit in the the bore of the dome bush. Then it went to the mill to be drilled - same settings as the dome bush - and finally back to the lathe to be face to length. I've stopped at this point as I haven't yet decided whether to use a gasket or an O-ring for sealing, it will get pretty hot here.
13. Inner Assembly    
When the time came to join the tube assembly to the firebox inner, the front of the combustion chamber was manipulated until it was a snug fit inside the rear tube plate flange. This mostly involved flaring the edges of the chamber outwards, effectively stretching the metal a little, and letting the tube plate force the combustion chamber back to shape. Prior to setting up in the hearth, a strap was made to hold the two parts together and set the angle of the tubes correctly. On the drawing a line was projected forward from the underside of the girders to the front of the tubeplate and the distance from this line to the outside of the front tubeplate measured. A length of 8mm x 40mm flat steel was cut to length, a spacing piece drilled and then welded on. This was then lightly clamped to the inside of the girder stays and bolted to the front tube plate with a spacer between to set the correct distance, allowing for the thickness of the strap.
The overall length was checked and the girder clamps tightened up. The whole assembly was then set up in the hearth, well-fluxed and with 1.5mm dia 55% solder sitting on the rim. 2mm dia would have been better but I have none. Heat was applied to the underside edge of the combustion chamber tubeplate, chasing the melt around the top. Because we were close to the girder stays, we refluxed those joints as well, just in case they started to soften and also the tube ends nearest the edge.
Once the resting solder had melted, extra solder was added by hand until there was a good, filled joint all round. After cleaning and pickling, this is the result. The tubes are fine because high-temp 38% solder was used for those and the reheat has not disturbed them at all. It is more usual to solder the firebox together first and the tubes added after but, because of the depth of the combustion chamber, I thought it easier to do it this way. The last photo shows how difficult it would be to solder the tubes to the tubeplate after it is in situ.
14. Other Boiler Bushes    
I have now made all the bushes needed for the boiler, using either phosphor-bronze bar or offcuts of the unknown bronze plate that the smokebox door is made of. The clack, safety valve, blowdown and water gauge bushes were all straightforward turning with threads tapped to suit. The regulator bush and it's coupling nipple have a 7/8" x 32 tpi thread and these were turned and screwcut on the lathe. The bush was made first, turning the top-hat form and machining the bore to 0.835" diameter, the nominal depth of thread for a 32 tpi Whit form thread being 20 thou. The bush was reversed in the chuck and a small register machined in the bore to allow the coupling nipple to screw up tight. Then, using a Whitworth-form screwcutting insert, the threading tool was touched on the bore and the dial zeroed. Multiple passes were made until I was seeing 40 thou (diametric) on the dial and a few spring passes made to finish.
The thread should be slightly undersize at this setting and if I need to aquire a tap to clean out the thread at some point in the future, there should be enough material to clean up. I made the coupling nipple from hex bronze bar only because I have quite a lot of it. The end with the longer thread was turned first and the thread was screwcut right up to the shoulder, unpowered and pulling the chuck round by hand. The bush was used as a gauge, aiming for a free-running fit. The final size was marked on the cross-slide dial ready for making the other end. The component was drilled through and the 1/2" x 40 tpi thread for the steam collector pipe tapped at the same time.
To make the other end I used the bush as a mandrel to protect the original thread but that meant the gauge was no longer available, hence the marking of the dial. As it turned out the thread was a little oversize so it was held gently in the chuck with a turn of emery cloth for protection and the thread chase freehand until the correct size was obtained. Not so easy with a single-point tool and a machine with no reverse on it.
15. Boiler Bush Holes    
The various holes in the boiler shell for the bushes were next and I started by marking out the centreline along the top of the shell and measuring off the distances required. Because the drawing gives dimensions as a linear distance around the circumference a card template was made with the distances marked off. Using a template in this manner ensures that the holes are square to the centreline. Prior to drilling the holes, I fitted the front bung to prevent collapse of the tube when clamped to the mill. With all the handling recently it had gone a little out of shape and needed to be adjusted to fit the front tube plate anyway.
The dome bush hole was set up first and, as can be seen, it's a bit of a squeeze to get it on the mill. The boiler shell is resting on a pair of clamped steel angles, clamped with whatever was to hand and centred. This one is a big hole and, after piloting with a holesaw, was cut using a boring head. I had to change the tool position half-way through as the head adjustment ran out of travel.
The top feed bushes needed a pair of 5/8" diameter holes and these were formed with a step-drill. Standard twist drills are a poor choice for drilling copper because of their tendency to grab the work and pull themselves in. The two safety valve holes required 3/4" diameter holes and these were done in similar fashion but using the bench drill instead of the mill - not enough clearance on the mill to accomodate the throat plate. Again, with hindsight, it might have been smarter to make these holes before the throatplate was soldered on.
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