For reasons too tedious to go into I am trying to keep a pair of wire-framed specs functional for another month or so by soft soldering the fragile join between the bridge and the lens frame. Initially I used standard 60/40, which didn't work well – about 4 days MTBF. Then I found a reel of lead-free (95.5 Sn, 4.0 Ag, 0.5 Cu) in my soldering drawer, tried with that, and it's held up a treat.

It set me wondering if I could use the stuff for a project involving multiple 'mortice and tenon' joints in a pendulum system I'm making in brass. I had been planning to silver solder, but soft soldering would make the whole thing so much easier. Preliminary experiments make me think that the lead free stuff will be more than adequate – perhaps even 60/40 leaded would be OK. But it set me thinking.

I've tried, but failed, to find to find data on the mechanical strength (specifically shear modulus, which is what mainly concerns me) of soft solders. I have found vague statements suggesting that 'the more tin there is the stronger it is' then 'the silver is added to increase the strength'. Hmm. What does that mean? Is a 99.5 Sn/ 0.5 Cu 'stronger' than the one with the silver?

Most of the information I've found about the properties of these alloys is focused on the mass production of electronic components, where mechanical strength isn't a major consideration.

Any light that can be shed into this fog would be most welcome!

Robin

Edited By Robin Graham on 15/10/2020 23:57:06

Thanks for replies.

On the specs front, I am quite pleased with my repair:

as was my optician. He hadn't noticed it when he measured the optical properties of the lenses – he obviously needs better glasses! I suppose I could put a bar across the top, aviator style – seems a bit 70's, but it perhaps the style will come back, as styles do. And I'll be a trend setter.

It turns out that frames of this type are silver soldered, so had I gone along that route for the repair the specs might have fallen apart completely. I wonder what Roy's frames were made from? Aluminium?

Returning to my main question, I made a test join using the Sn/Ag/Cu alloy:

You can see the solder line, which would probably be invisible with Ag/Cu, but it's livable with. That's a 3/16" (4.76mm) 'tenon' in a 5mm 'mortice'. So about 0.12mm gap. Maybe it should be tighter. Anyhow, after destructive testing I'm now sure the joins will hold up. I still don't know what the silver does though!

Keith – what book? I'd be interested. If forum rules don't allow you to promote your work here, please PM me.

Robin.

Edited By Robin Graham on 17/10/2020 00:05:38