Tuesday, June 12, 2007

Boiler 3

Some of my boiler making tools.

I formed the taper barrel using a set of bending rolls. I decieded that the chances of cutting the sheet to exactly the right dimensions was pretty small - the copper stretches when rolled - so I left the sheet oversize, and planned to cut it after forming. I know this makes for less convenient cutting, but at least it wont be short.
Just as well; after annealing and rolling, the taper ended up at the opposite end to what I'd planned - it didn't matter one little bit. So once it was almost to shape, I could work out exactly where to cut it.
I rechecked the dimensions once I'd got rid of the overlap, and found there was still a little to file off.

I drilled the strap and riveted it to one side, pulled the barrel up tight with a strap, and drilled and riveted the other side. Before doing so, all the surfaces were finally cleaned and covered with flux.
I laid two strips of (high temperature) silver solder at the centre joint, and added some more flux. I heated it mainly from underneath - with the strap at the bottom and once the solder flowed,added some to the rivet heads. Then I rotated the boiler so the strap was at the top and added more solder, again including the rivets. A few minutes in the pickle bath left me with a clean barrel, which I marked off and trimmed to size - in typical GWR fashion, all the taper is at the top, with the base of the barrel horizontal.

I could now do a trial assembly on most of the boiler; I'll use a few copper rivets to keep the components in place during soldering - but for the time being, I use 8ba bolts in these holes.

The trickiest bit for me was to drill for the cross stays needed on a Belpaire firebox. The problems are that everything slopes, and the reference dimensions are on the firebox inner, so not visible. In the end, I marked and drilled pilot holes on the girder stays, and put in a rod ground to a point at each end. Then I assembled the firebox and used a wedge to drive the rod into the outer walls, giving me centre pops. I transfered these to the outside with large springbow calipers - and a lot of checking. I drilled through the end holes, then could mark out the intermediate holes more sensibly. After a lot of checking, I had all the stay holes in place - as pilot holes. A couple needed 'adjusting' as the drill wandered on the inner surfaces -probably because the surfaces are not all horizontal. I had to enlarge some of the holes in the girder stay to get everything in line. Once I'd got them all lined up, I made up a small 'scribe' which mounted on the pilot rods, so that I could get the holes to their final diameter in the correct position. I hope the photos show what I did. The final diameter of the girder stay holes was 3/8 inch - I couldn't use a drill as it would just have grabbed and damaged things, so it was a matter of grinding with the Dremmel and filing.

I've left the holes in the outer wrapper just as pilot holes for the moment - in case I burn them; I would rather not have drilled them at all at this stage, but I couldn't see how to do them later!
Now I cut both the inner and outer wrappers to size - except for where the backhead joins the outer wrapper.

Its now just a matter of soldering things up. Its often recommended to do this in very few heats. I find it more reassuring to do more smaller ones - a very real problem is that once the job is up to temperature, the flux doesn't last long, and at that stage all you can do is stop.