By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by Allendale Jan 24th

Silver soldering a Minnie traction engine boiler

All Topics | Latest Posts

Search for:  in Thread Title in  
Mark B19/07/2020 20:39:42
56 forum posts
29 photos

I've just unearthed an abandoned project - a 1" Minnie Traction Engine.

It last saw the light of day 7 years ago when I was preparing for silver soldering my prepared copper parts for the boiler.

Before tackling the boiler I decided to practice on some scrap copper to make sure that I didn't mess up the job.

Originally my plan was to do the initial soldering with SF24 then subsequent operations with SF55 so I can safely add to the build. Having got my supplies from Cup Alloys (who were really helpful) which included Tenacy 5 Power flux I tried out my skills.

The results were mixed. SF55 + Tenacy 5 soldering is easy, it just work fine and I've used the stuff for years now for lots of things big and small.

SF24 and Tenacy 5 for some reason is a disaster. The solder simply doesn't flow and stays on the work on blobs. I've tried again recently and still can't get it to work even with appropriate heat.

I posted on this and some suggested SF24 soldering isn't for beginners... this put me off and somehow 7 years past, but I really want to make this boiler having found it again.

So that given that I'm successful with most efforts using SF55 silver solder would it be feasible to do the entire job with this? My worry is the multiple stages of heating making the boiler and melting previously made joints when adding new joints.

Can I do multiple heating operations with 55% silver solder or should I get my 24% soldering skills sorted out for the initial operations.

Brian H19/07/2020 22:31:42
1803 forum posts
105 photos

Using a 55 silver solder for all joints is perfectly possible although care is needed on subsequent heatings. The silver solder alloys with the copper to produce an alloy that has a higher melting point than pure silver solder. The trick is to only heat subsequent joints just enough to allow the solder to run, there is no point in getting the joints any hotter.

The only real problems come if the joints are VERY close together and the, wet rags or similar can keep joints already made, in a safe condition.

Absolute cleanliness is essential and the flux should be applied in sufficient quantity to last throughout the soldering session.

I trick I learnt whilst working at a company doing work for Rolls-Royce was to mix the flux with a couple of drops of washing up liquid in a spirit solution. Don't use water, it just boils and causes the flux to fall off. With spirit the flame can be used to ignite the spirit and this bakes on the flux so that it does not flow off.


Hopper20/07/2020 08:16:41
4817 forum posts
105 photos

You might want to check with the experts at CuPalloys Ltd. They seem to be pretty in tune with the model engineer's silver brazing/soldering needs and have a good book on the topic available. They post on this forum on related topics quite often and always seem very helfpful.

fizzy20/07/2020 09:14:44
1734 forum posts
116 photos

The problem your having with the sf24 is that the copper is not getting hot enough. I often have parts of a boiler going dull red when using sf55 and this can melt as much as 200 degrees lower than 24, so if I were to need to up the temperature by another 200 degrees I doubt there would be any flux left (it takes quite a while to get a boiler to that temperature and the flux doesnt last forever). I only ever use one temp of solder regardless of the size and type of boiler and have yet to have a single joint fail due to this. Brian is correct re getting the solder to stay in place - exactly what I do. Just beware that there is a lot of very bad advice as well as good out there.

Mark B20/07/2020 22:43:20
56 forum posts
29 photos

Thanks for the feedback - very true about there been bad advice and good advice out there.

The termperature difference between 55 and 24 very likely explains my difficulties with the 24.

I contacted Cup Alloys today and explained my difficulties. They have suggested that I try a 38% silver solder for my initial operations then go with 55. The 38% should be within the capabilities of my torch.

I'll of course be trying this out with test pieces before I take the plunge and start on my Minnie boiler.

Will report back...

Mark B24/07/2020 21:45:46
56 forum posts
29 photos

I received a delivery from Cup Alloys today. 38% and 55% silver solder.

Given my previous hopeless results with 25% I decided to practice by making one of the simple boilers in Stan Bray's book on Simple Model Steam engines. It's a very simple boiler with end plates and a single chimney in the centre.

To practice a multi stage soldering operation I soldered the end plates with 38% and the chimney and boiler fittings with 55% afterwards. I was using Tenacity 5 Flux. The operation simply worked - it was very easy!




I appreciate I applied too much solder in places, but the flow into the joints was just as I wanted.

I'll probably finally have a go at my prepared Minnie copper boiler parts tomorrow.

Jeff Dayman24/07/2020 22:27:57
1896 forum posts
45 photos

Your soldering on the small boiler looks good Mark. Smart to get some practice with that, and now you have a nice boiler for powering small model engines. That extra solder can be removed, it is not a big deal. Much better to have a bit extra than not enough. Good luck with the Minnie boiler! Clean things up often while soldering, don't skimp on adding flux if things are not flowing as you want.

Mark B26/07/2020 20:19:01
56 forum posts
29 photos

I have taken the plunge and starting silver soldering my Minnie 1' boiler.

Pictures tell the story which I'm pleased with although its not perfect. Starting with the barrel, wrapper and throat plate:





And them moving onto the first stages of the fire box:





To me it feels okay so far, but I'm open to comments.

I'm not a member of a ME club but aware of getting this inspected properly with the view to getting a boiler certificate. I live in East Lothian where there doesn't appear to be a club but I believe my nearest club is Esk Valley MES . I'm going to hold fire an any further construction until I've heard back from them and what they want to see.

Brian H27/07/2020 08:35:23
1803 forum posts
105 photos

That's excellent news Mark and the silver soldering looks really good so you should have no problems with the rest of the build.

I agree that you are best to stop now until you can get a boiler inspector to view it but I would not expect you to have any problems with the boiler inspection.

There appears to be an Edinburgh Society of Model Engineers, is that more local to you?


Bob Worsley08/08/2020 12:41:41
59 forum posts

I hate to criticize something that I have never done, but working my way towards.

I actually came across this thread trying to find one I started about boiler soldering.

The boiler joint between throat plate and barrel is just a line contact, 1.6mm or so? I really don't think this is good enough, particularly on a traction engine with all the driving stresses pulling that joint apart.

I have been working my way through a huge pile of MEs I have just been given. I see that Martin Evans on the Jubilee boiler does talk about using a thick throat plate without flanges, 5mm or 3/16" in that case. There are also a couple of articles by Martin Evans about boilers and strength in the early 60's. Issues 3107 and 3119

These are, of course, my opinions, and without ever having soldered up a boiler so with no practical experience. But, I am not going to start soldering any boiler until I know why you get weeps, I think poor fitting between flange and shell so solder can't flow.

Brian H08/08/2020 12:51:09
1803 forum posts
105 photos

Bob, the throatplate joint looks to be using a flanged throatplate to the barrel so there will be 1/4" to 5/16" of overlap.

Perhaps Mark will confirm and report on further progress.


JasonB08/08/2020 13:20:18
18915 forum posts
2082 photos
1 articles

It's just a butt joint on the Minnie drawings as mark has done and hundreds of other builders before him

Mark B08/08/2020 20:13:14
56 forum posts
29 photos

I can confirm that the joint between the lower part of the boiler and the throat plate is a butt joint, and this is the design by L C Mason, who feels like a respected model engineer and I hope plenty of approved boilers are made to his design.

I do however welcome and respect thoughts on the construction.

In terms of progress, I've actually not done anything since my last post I want to leave it so it can be checked out properly before I hide anything questionable. I have identified a "local" club to where I live who also do boiler inspections. I'm only in email contact with their boiler inspector so far who has requested to inspect the work before any further soldering is done... It's Esk Valley Model Engineering Society who are still a long way from where I live, but I'm going to get it properly checked out and I'm sure the design will be looked at.

I'll certainly post updates from my initial inspection and further pictures once I start building on my work so far.

Brian H09/08/2020 08:01:12
1803 forum posts
105 photos

My apologies for putting in incorrect information, I can now see that the throatplate to barrel joint is a butt.


Bob Worsley09/08/2020 16:08:16
59 forum posts

I did emphasise that it was MY OPINION.

You would think that Microsoft would be a professional outfit, but look at the mistakes they have made over the years!

Easy enough, now, to curve a piece of 90 degree copper to solder over the top, inside or out.

I have been trying to find out what the tearing strength of copper is. If the joint started to go, say a 3mm gap started weeping, then at what boiler pressure does it tear and leak or explode depending on the speed it tears. Way outside my area of expertise, which is why a read books.

Apologies to Mark if this is causing sleepless nights.

modeng200009/08/2020 17:07:49
232 forum posts
1 photos

There is an interesting article on joint testing called 'Joint Design Tests Subminiature Boilers AWS' that an internet search should find..


Jeff Dayman09/08/2020 19:52:44
1896 forum posts
45 photos

Bob Worsley- as others have mentioned Minnie's boiler is a proven design which many people have built and steamed with success.

A flanged joint is more usual practice for the throat plate joint in many boilers but this does NOT mean a butt joint will not be acceptable for Minnie in combination with the substantial side flanges of the throat sheet, the firebox stays, and material thickness / joint design/ physical dimensions of plates that were used by Mr Mason and all since.

What IS unacceptable is changing or editing any published and proven boiler design with offhand suggestions like your statement "easy enough now, to curve a piece of 90 degree copper to solder over the top, inside or out". If you did such unproven mods to a boiler you may unwittingly change the stress distribution in the system and CAUSE an unforeseen failure.

Such suggestions for changes to proven boiler designs like Minnie's need to be accompanied by calculations, tests, and several competed boilers steamed successfully, or public safety is at risk.

Bob Worsley15/08/2020 10:34:29
59 forum posts

I started another thread about how much solder to buy.

CUP responded and I would point everyone to their Best Practise Joint Design section in their helpful 'how to' section where they clearly state that a butt joint really isn't on.

I don't understand JD's assertion that modifying a published and proven design is not allowed. If in the progress of time better methods appear then it is wrong not to follow them. Changing stress distribution? That was what I was trying to do.

Reading more of my pile of MEs and again and again I read that if you have a little gap in a joint, or a leak in a joint, then fill it with copper chips and solder/resolder. I wonder if this is the cause of so many boiler failures. Hundreds of Minnie boilers have been made, what isn't known is how many could never be made watertight and the builder gave up in disgust. Soldering chips of copper does nothing for getting gap widths or even the length of the joint. Copper is rather soft to bang into anything. Perhaps this is one of those old practises that now needs to be pensioned off, joining caulking with soft solder?

Again, these are MY OPINIONS.

fizzy15/08/2020 10:50:14
1734 forum posts
116 photos

"I don't understand JD's assertion that modifying a published and proven design is not allowed".....thats because you are most likely not aware of the UK Boiler Regulations. You can do what you like in your own garden but if you want to join a club/get insurance/take it out in public then the rules have to be followed.

Phil H115/08/2020 11:05:34
315 forum posts
32 photos

In Bobs defence for a moment.... I have seen about 3 or 4 model locomotive boiler designs e.g., Rob Roy, Simplex, Betty and Jubilee all by Martin Evans or LBSC. Every single one of them have newer recommendations and or required modifications. So to simply say that you need to follow a published design is complete nonsense.

What I think is meant - surely - is that you should start with a published design, speak to your inspector and incorporate a few of the more up to date modifications/ recommended changes and you then stand a chance of success.

Phil H

All Topics | Latest Posts

Please login to post a reply.

Magazine Locator

Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!

Find Model Engineer & Model Engineers' Workshop

Latest Forum Posts
Support Our Partners
Eccentric July 5 2018
Eccentric Engineering
Subscription Offer

Latest "For Sale" Ads
Latest "Wanted" Ads
Get In Touch!

Do you want to contact the Model Engineer and Model Engineers' Workshop team?

You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.

Click THIS LINK for full contact details.

For subscription issues please see THIS LINK.

Digital Back Issues

Social Media online

'Like' us on Facebook
Follow us on Facebook

Follow us on Twitter
 Twitter Logo

Pin us on Pinterest