Another silver solder question

Hi all, as part of my deliberations on the building of Emma Victoria, (I have started, all the 'easy bits' so far) I am reviewing silver soldering practices in readiness for the day I get to the boiler. After my traumas with a GLR vertical boiler I look forward to this with some trepidation I have to admit as I am sure a loco boiler is several steps up from a simple verical boiler although hopefully I have learnt a thing or two now.

My question is, looking back through many issues of ME I have come across reference to people hot rodding, these people seem to be experienced model engineers, however on the Cup Alloys website for instance they advise that hot rodding is not a good idea.

Another issue I struggle with is one of direct heat, avoid direct heat is often quoted yet I have seen in several photos and UTube videos the flame being applied directly to the joint.

Any guidance will be appreciated from you silver solder gurus.

Regards

Bill D.

Can't advise you in this instance but all I can say is what ever cup alloys advise i would take as gospel. They have given me loads of advise over the last few years and all of it has been brilliant, so give them a call and do as they say! Good luck Matt.

Bill

I think the "avoid direct heat" is probably more to do with oxy-acetylene or oxy-propane, where the flame is particularly hot and fierce. It can easily over-cook the flux or melt the metal locally! With a straight propane torch, you can't really avoid "direct heat"! I've built a few copper boilers in my time using only propane, and the sensible advice is to use the correct flux for the grade of solder you're using, heat things up quickly (so the flux doesn't go off), and make sure the surfaces are clean and well-fitting (with 0,1mm/0.004" clearances or so) before you start.

It would probably be a good idea to get somebody experienced to be with you when you start cooking up the boiler seriously - even if he (or she!) merely nods their head quietly in the background. If you're in a club, a likely choice is one of the boiler inspectors.

Hi Bill,

Use "hot rodding" as a means of adding extra flux to a joint already fluxed.

Home made flux coated rods are not recommended for the model engineer because

• the amount of flux is limited,
• the flux is not as active as a "factory made fluxed rod, and
• it invariably pushes the engineer to using a heating technique more akin to welding.
• Result lack of penetration onto the joint.

Heat the joint and let the silver solder get its heat from the joint - not the torch.

Use the thinnest rod practicable

More advice on www.cupalloys.co.uk/best_practise/

Keith

PS Thanks Matt for your comments

Indeed, beware a very hot flame - and in particular the blast pressure of the oxy flame. I avoid direct heat wherever possible but its not normally a problem with propane. Flux choice also becomes critical with heat type and boiler size.

Thanks guys and particularly Keith who helped me in the past with my GLR boiler. I understand better now the distinction between hot rodding as an addition to normal joint fluxing rather than trying to using it as the only means of fluxing.

Had another read of the Cup alloys guidance which is excellent.

I think I will have a few practice runs on different joints to try to brush up my technique, within the realms of affordability of the silver solder of course. Problem is that I have found that a small joint with very little mass of metal is easy to heat up and get a successful joint, very diifferent to heating up a whole boiler., would another gas cylinder and torch benefit in this respect?

Incidentally has solder come down in price, read the other day that silver prices have dropped sharply, maybe a good time to stock up a bit.

Regards

Bill D

Two torches do help greatly, a mate to hold one of them helps even more, I know, I'v tried doing it on my own, interesting to say the least. Ian S C

Hi BIll,

you are correct the price of silver has dropped recently and this is reflected in our prices on the web.

www.cupalloys.co.uk

Thanks for advice guys, one final (for the moment) question, I am never sure what size of rod to use, is there a 'formula' or rule of thumb for this. I assume it depends on type of joint / mass of metal etc.

Regards

Bill D.

Thanks for that Mike. To be more specific then, if you were making a boiler such as that found on a 5" gauge loco like Emma Victoria. what size rods would you use, ie what size for say the tubes and would that be different for the shell etc. I appreciate there is a lot of experience gained from trial and error, I would just like to benefit from other peoples trial & error as much as possible!!!

Hopefully when the series on Emma reaches the boiler construction it will have the same excellent level of detail that Henry has given so far.

Bill D.

I don't know if others find this, but with my propane torch I find that, even if the tip of the cone is the hottest bit, the most heat goes into big jobs if the tip of the cone is 1/2" - 1" away from the job. I wonder if too close interferes with full combustion of the gas.

Neil

rods - 1.5mm max for a 5" with propane.