Safety valve bonnet

[alanetm]

Interesting method to use tube to make the bonnet. Currently I am trying the method written up in EIM. I have done two so far the first a dismal failure and the second "very promising" but not good enough. Both were made using a wooden former with 22g annealed brass brazed to form the cone. In making the second I destroyed the wood former by the continual hammering. Now I have just finished making a Devcon liquid steel split mould (an expensive material as two 500g pots are required to get a mould of sufficient size to give it strength) and its back to start the forming process again. Watch this space as they say .....

[jma1009]

hi alan,
with a bit of care it isnt too difficult to make these things out of solid brass... assuming you can get brass bar of a large enough size.
i use clockmakers gravels to produce the top bevel. taff's method i havent tried and have always found annealing brass to be a wee bit unpredictable... but perhaps thats my lack of experience in annealing brass!
very ggod drawings and an eye for what makes the correct profile is top most IMHO! many have a go at producing GWR safety valve bonnets/covers but fail miserably to get the correct shape!!
cheers,
julian

[alanetm]

I agree annealing brass is fraught with difficulty. I annealed the flat to make the cone in my oven at 750 degrees and it wound round the former with finger pressure only. I used a torch to anneal locally whilst hammering out and that works OK too. But I find the brass splits at the brazed joint (Sif bronze No.1). That's OK for the bottom curves as the joint line is where one "ear" fits so the gap doesn't matter. However on the top it does matter and I am finding that a challange!

Alan

[taff]

Hello Alan, Rex and all,

I have at last managed to get into my workshop ( Garage ) but after five years of general household activity's its like trying to climb through a rubbish dump. Everything which could have been put in there has been put in there and its going to take the services of a skip to clear away enough for me to even get to the lathe let alone work on it. But don't dispair, I will find my formers soon and display them. I don't know what your drawings show in this design but I mention here that the full size safety valve and its base on the barrel are oval in shape set athwart ships as it were. On the model if this is made round it may obstruct the cover and should be allowed for. Some years ago I made patterns for lost wax castings of both the base and the body of the combined top feed and safety valve. I believe I passed these on to Pete Thomas when I passed over my Saint designs to him some years ago. It may be that he will do castings for them when he has my designs up and running.

Taff.

[Rex Hanman]

Thanks Taff.

[taff]

Hello Alan, Rex, and all others building GW models. At long last I have been able to get into my workshop and managed to locate the safety valve bonnet formers and have been abled to photograph them. One of the photos shows the brass tube I used to make mine. It is 1-1/2" OD and wall thickness is 16 gauge. You will see that thed tops of both formers are removable and when in possissio are held there with a nut and stud. The bottom of the formers are provided with a spiggot for holding it in the vice and are made up from castings which I cast in my back garden.One of the formers is for tapered boilers while the other is for parrallel round top boiler of the Dean type
The procedure I adopted was to well anneal the tube and then form the tube end walls outwards to form a skirt by beating from the inside of the tube with a small ball pein hammer which I made for the job.This was done while resting the side of the tube against the side of a round held in a vice and making sure that either a wood or steel insert was in place to make sure the tube didn't collapse too much while being worked. Once I was satisfied that the skirt was wide enough to fit the lower large part of the former the length was cut and the bonnet was then put on the former and the top of the former was put in place and pulled down with the nut and tightened. During working this tended to work loose and care had tobe taken to keep re-tightening it. At this point the bvrass tube could then be beaten down against the former with repeated annealings. Eventually the bonnet would be tight down on the former and it could then be finished by filing and emery cloth to get all hammer marks out.
Incidently the drawing over which the formers are displayed I my drawing for the crankshaft for the new full size Dean Single to be built by the GWS

Taff

[Rex Hanman]

Thanks Taff, really useful pictures. I had assumed the flat top of the bonnet would be a separate bit silver soldered on but it looks as though you may use the former for that bit too, am I correct? Never seen it done this way but it seems like a great method. I'll will certainly give it a go when I get that far.

[taff]

Hello Rex and all,
Yes you are correct, the tube is formed up against the underside of tyhe top of the former using a 7/32" dia steel rod which was breaten sideways against the bonnet to push the shape up to the top of the former. Some time ago I started to make the patterns for a lost wax casting for the top feed type of safety valve cover. I've done lost wax patterns for almost everything else including the cylinders so eventually I'll try to find it and finish both the pattern and rubber moulds. I'll need it for my "Saint's" when I get around to completing them.

Taff

[alanetm]

Well, the bonnet is made and is currently getting a coat of paint on the "ears" and then a final polish of the brass. I used the former method outlined in EIM (reference post earlier) Mind you the cost of the epoxy metal was horrendus and two 500 gram pots were required to make the former for the bonnet with enough left over to make a small former for the "ears".

I ended up with success on the third try of beating the bonnet out so I suppose that's not a bad result. I found the brass needed frequent annealing and the biggest danger was getting a "fold" and as soon as one started to appear it need immediate attention to beat it out.

So best ingredient to have ..... patience......it takes a while to beat outa little at at a time.

I will see if I can post some pictures of the result with formers once the paint is dry.

Alan

[alanetm]

So here are some pictures of my efforts.

The finished bonnet on the loco

The tool selection that were used and the main mould

The tool selection consists of starting at the right corner with the larger block of wood. This simulates the boiler cladding giving the taper and curve. I originally had the master of the bonnet mould mounted on this for making the epoxy metal mould but it was destroyed when the last half mould was taken apart. The finished split mould is in the centre. The two cones were originally one cone and used for wrapping the brass to form a cone shaped base from which to start. The brass being brazed along its joint with sif bronze. The wood cone was cut into two with the top part forming the inner clamp using a bar across the top of the mould to hold the brass cone firmly inside the mould whilst beating it out around the bottom. The tool used for the beating was a ball pein hammer and the rounded bar seen in the photo as the hammer cannot get into the cone easily. Top left is the mould for the ear and its wooden former for giving its initial shape. The final shape was beaten carefully with the same tool as the bonnet.The ear was made from copper.