By Steve Gosling
Readers of ‘The Oily Rag’ may recall that in issue 88, I recounted how I was talked into having a go at the safety valve cover for an Isle of Man 2-4-0 tank on the grounds that I ‘had done a bit of spinning’. Unfortunately the builder failed to point out that the cover was not round thus making spinning out of the question! During our annual holiday week on the railway at Hemsby this year, Father, Brian, the IoM tank builder and I were seated in the station reminiscing about this when Brian pointed out that the safety valve bonnet for his current project, a GW ‘Saint’ class 4-6-0, was definitely circular so perhaps I would like to try to spin that up? Always up for a challenge, I agreed to have a go and this is the result.

Now, the Great Western Saint bonnet, in the period in which Brian is modelling, is basically conical with the narrow end flared out and a rolled bead around the top. The bottom is flared out to meet the top of the boiler barrel which is itself conical with an axis angled down towards the smokebox. Just to make things interesting, there are beaten covers on both sides so arranged as to cover the clack valves. The whole lot is finished off with a bead around the bottom and is polished brass so there is no opportunity for any filler!

In issue 91, Tony Newberry described how he makes his from a silver soldered cone of brass, hand beaten to generate the flares at both ends. I didn’t feel that I had the skills to repeat the annealing process without upsetting the joint. I therefore resolved to have a go at producing the cone and top flare by spinning which would remove the need for a joint and leave only the bottom flare to be beaten.
I turned up a ‘chuck’ onto which to spin the blank. This was in two parts, split at the waist in order to be able to remove the job once it was complete. The main part was a piece of fence post built up with MDF but you can see that the top part was of steel. I did this so that once it was trapped but before spinning was complete, I could remove the brass and anneal it again without damaging the chuck.
The picture opposite and the two below show the progress of the spinning process. Once completely down, I trimmed the brass skirt off and cut the hole in the top. I then removed the steel part of the chuck and skimmed the remaining brass upstand until it was only a few thou thick. This, I rolled over using the spinning tool to form the bead. It worked better than I had hoped as I fully expected to have to solder the bead on afterwards.
Once it had had its first polish, I removed it from the lathe and formed the flare by beating it gently over a steel bar in the vice. The secret here is to hit it softly a lot of times and gently stretch it around with regular annealing. I had previously made up a dummy boiler top, again from MDF, on which to try it and I found it invaluable. (Below right.)
Now I came to the clack covers and, much to my surprise, these proved far more difficult to generate than the bonnet itself. I flanged them from copper over an MDF block again with lots of annealing in between (pictured right). After trimming, I silver soldered a flange on the back into which I had previously pressed a ‘hump’ to clear the water feed pipe. After cutting slots in the sides of the bonnet using a piercing saw, I simply pulled the clack cover flanges around with a pair of pliers until they fitted snugly against the brass. They were then secured with five 12BA screws. These should be rivets but I have left them for Brian to fit as he will want to adjust the covers to suit his boiler.

The last job was to fit the beading around the bottom edge. This was made from half-round 1/16” brass which I curved in my fingers before clamping it to the edge, sweating it into place and dressing off the ends at the cut-outs.
Completed Bonnet
I can honestly say that this is one of the most difficult things I have ever made and was quite a puzzle to work out how to approach it. Cost to me was 36 ¾ hours labour and a lot of bad language but it has given me immense satisfaction in the end and I am currently enjoying looking at it. I have sent Brian some photographs but I am under strict instructions not to risk it in the post!