What quantity of silver solder needed

Start to look at silver soldering boilers in copper.

From CUP there seems to be basically two types, 455 and 430, differing in melt temperature. Have all the Easyflow types mentioned in the boiler books now gone due to cadmium?

But what doesn't seem to get a mention anywhere is how much is needed?

Consider a Minnie boiler. Seems I will need some 0.5mm 455 for the flue tubes. Then 1.5mm 455 for most everything else. And some 1.5mm 430 for the first braze up of bushes and similar. What would be the suggestion? 1m 0.5mm, 5 rods of 455 and 1 rod of 430?

Then what about a BB boiler, somewhat larger. Boiler weight must be 10 times the Minnie, so do I need 10 times the amount of the solders?

I have assumed most soldering will use the 1.5mm rods, pretty much like the 1/16" steel rods with TIG or gas, but should they be thinner? Thicker? Rectangular?

Thanks

I'm no expert but 455 is the 'new' Easiflo No 2 and the 1,5 or 1,6 size is ideal for most boiler making.

I think that CUP would recommend the 430 for the initial heats but, personally, I only use the 455 with a Propane torch and the appropriately sized nozzle.

For a large boiler, I would get a friend (or even my wife) to add background heat with another torch and the whole thing should be well surrounded by thermal bricks but NOT the ones that came out of a storage radiator as these are designed to absorb heat and that is not a good idea.

I always use Tenacity No5 flux as this will still do its job even after prolonged heating.

It is important that you can heat the item quickly to the correct heat and maintain it throughout the soldering operation.

Quantity of solder is a difficult one because it depends on the number and size of the joints and the amount of gaps in them.

Brian

Edited By Brian H on 14/08/2020 19:12:05

I have never soldered a boiler but I alway calculate how much silver solder I need when I make a joint. I calculate the volume of the joint (capillary thickness, 0.004”, times joint area) and then multiply it by three for the fillets. I can then get the required length of solder rod. It is alway much more than expected.

JA

I should add I always place the solder, or most of it, in position before heating.

Edited By JA on 14/08/2020 19:47:50

It depends how neat you are. I have seen boilers that have so much silver solder where it should not be, such that there is very little copper showing.

I use a chalk solution (suspension) in meths and paint this around the joint. so the solder is confined to the area that I want it to be in.

Andrew.

Why do you want a fillet?

If your joint design is right, your heating technique is correct, you achieve the necessary capillary flow, you will produce a sound joint that carries the stresses in shear.

That is the strongest joint possible.

Why do you want a fillet?

It's a shareholders bonus. Colman's made more money from the mustard left on the plate than that put on the meat.

Silver solder is expensive enough without wasting it.

For more information, read my book or attend our lectures.

Keith

Keith

I agree in principle with your comments but you do not want any undercutting of the solder in the joint as this creates a nasty stress concentration which could result in a long term fatigue failure as the joints in a boiler are never in pure shear and always have some element of peel in them which will try to cleave the joint open. So a 'small' fillet is going to be preferable and I know our club boiler inspectors will insist on it. Just my opinion from experience of vacumn brazing aerospace parts.

Dave

If simulating castings then the solder fillet is necessary to avoid having to produce them with a filler.

Brian

I'll buy the need for a fillet on cosmetic grounds. Keep it small by using a narrow melting range silver solder. Just as well because these alloys are the most expensive.

A boiler inspector normally wants to see a "witness" of silver solder on a joint that implies total penetration of the silver solder through the joint and a sound strong joint. That is not the same as wanting to see a "fillet". Personally I'd question the the integrity of any joint with a manufactured fillet at both ends.

Undercutting in the joint area is normally caused by the filler metal dissolving the parent materials. This in turn is caused by a poor temperature/time profile in the furnace that leaves the molten filler in contact with the parent metals for extended periods.

Undercutting is a potential problem with all vacuum and controlled atmosphere furnaces.

The number of model engineers using a furnace for brazing is very small.

This is not a problem that the vast majority of modellers will encounter.

Keith

I estimate I used three times as much silver solder as an expert would have on my first boiler. Mostly due to too big joint gaps. Like many beginners I got too hung up on the joint gaps you read about, exemplified by a recent thread on here. The solder (Easyflow type) ran straight through the gaps and out the other side. Rather than trying to calculate how much you need, just get plenty. Always good to have plenty in stock. It never gets cheaper.