Really comes down to "what size gap"

It is usual to just give boiler rivits a light tap, enough so that they hold the plates in place and give the "tightish" fit mentioned but not to fully hammer them down resulting in a tight joint.

If you do need to hole two parts together during soldering then you can also either place solder foil in the joint or add a few punch marks to the surface which just holds the plates a fraction apart so the solder can flow. With sheetwork as found in boilers you will also get some expansion as the metal is heated which tends to open up the joint as well so what may have been together when cold will have a suitable gap once hot.

The gaps Terrance is talking about are large ones that will need a lot of solder to bridge and that is if it will stay in the gap more than likely it will just run to another part of the job or onto the hearth

Edited By JasonB on 12/04/2018 10:04:24

interesting topic……..

In practice i have found that the smaller the 'gap' the better….it's normal when making clock parts to clamp the parts tightly together and I can't remember the last time, or any time come to that, when the tightness of the joint prevented the solder from flooding the joint. I normally use a couple of old engineers clamps, amongst other stuff, and use quite a lot of pressure but the solder still runs and I like the joints to be as inconspicuous as possible so usually put some effort into getting a very close fit before soldering. When I solder spring barrels together I use a warding file first while holding the tube closed against the file until no light coes thru the joint…then the soldered joint is such a fine silver line that it can be easily polished out to invisibility.

I've been trying to braze brass handles to brass plates using silver solder and it was a nightmare. I used 55% silver solder in thin wire format and the powdered flux. Then I spread the flux throughout the joint heated the brass up until glowing red and then applied the solder.

But all that happened was that it beaded and fell off – nothing crept into the joint. The heat caused the copper in the brass to leech out to the surface so it looked a real mess.

In the end I used JB Weld!!! What did I do wrong?

Best wishes,

Nick

The strength of a silver solder joint comes from the ability of the alloy to flow into a gap by capillary flow, No gap = no joint, simples!

It is true that the narrower the gap the better the capillary flow, but again there must be a gap.

shaun@cupalloys.co.uk

I suppose most of us do not section our joints after they are made, hard to tell if the solder has simply run all around the edge of the two parts or if it has flowed right into it without cutting in half and looking.

I probably get to see my joints more with teh fabrication work I do as they are often machined after soldering which can cut across a joint, not something you tend to do on a boiler.

There has to be some "gap" – albeit only a very small one.

I am lucky enough to have copies of Kozo Hiraoka's Shay/Climax/Heisler books and since reading them, I've used his recommendations on making good silver soldered joints and fabrications. He goes into the requirements for a good joint in great detail but it can be summarised under the following headings:

1) Use a suitable silver solder and flux

2) Select an appropriate heat source (i.e. torch)

3) Clean the joint surface(s) well

4) Provide appropriate clearance (by machining, by punching or by filing)

5) Fit the parts together securely (but do NOT use clamps!)

6) Silver solder only a few joints at a time (and clean everything between 'solders' )

7) Always flux before assembly

8) Set up an 'assembly' – such that gravity will assist the capillary action (rather than resist it)

9) Provide the part(s) with support where necessary

10) Apply heat from the back wherever possible

11) Apply the heat to the heavier part

12) Help the flow of solder with a 'scratch' rod

This is detailed in Kozo's 'Shay' book but referenced in the others too – but is the best all round description of how to make small soldered fabrications that I think I've seen.

As mentioned, he always makes a slight "gap" between the parts to be silver soldered. I've found his idea of using a centre punch to make a few 'dimples' on one part ( very small 'stand-offs' ) – to hold the two surfaces slightly apart (thereby enabling capillary action) very useful.

BTW – The Kozo books are still available from Village Press in the US (not that cheap I'm afraid given $/£ exchange rates but still much less than some eBay/Amazon sources would suggest). They are beautifully written and illustrated and contain many novel and useful ideas and techniques for any ME. They would certainly be on a short list of my favourite modelling/engineering books.

Regards,

IanT

Edited By IanT on 12/04/2018 13:27:56

It is worth the enormous cost of two half inch square pieces of metal and 1cm of solder wire to experiment to see just how thin and 'flowy' solder becomes when melted. Point to note is that it is not like the old plumbers solder that was specifically designed not to flow too easily so it could be used for gap filling ropey bits of pipework.

.

Predictably enough … Shaun seems to have summarised perfectly ^^^

MichaelG.

We really need a metallurgist to explain what really happens when you 'solder stuff' together.

Under all the smoke and flames I think you would find an awful lot of 'alloy' chemistry going on.

If it doesn't happen, you might as well use Araldite to stick it together.

You don’t really need to know the reactions at the atomic level. All you need to know is that if there is no possible gap for the solder to flow across the joint, there will only be a soldered line at the external or internal contact point. This is simply not good enough – the joint needs maximum surface area to be properly effective.

It would be like stick electric welding a butt joint without any weld depth – pretty useless under high load, and further, access to the other side of the joint may mean a single line of solder along one corner rather than both sides and the butt end being attached.

I think metal is generally rather stronger than plastic (epoxy resiins) and most certainly has a far higher temperature range of application!

Hi all

Can I ask what readers think about the need for cleaning the joint surfaces before silver soldering which has been mentioned in passing several times in this thread?

The writings of the late Tubal Cain emphasised the need for absolute cleanliness, and he quoted the relevant British Standard in his own support.

Conversely, the chap from Cup Alloys who delivers the excellent lectures at ME shows, and who is clearly very experienced, seems to feel that the flux will clean the metal at brazing temperatures so cleaning is not important.

Regards Rod

Remind me not to buy a boiler from Fizzy, cavalier attitude to say the least.

Edited By KWIL on 12/04/2018 16:59:16