Mind the Gap

[Joe]

A recent discussion about fit when soldering raised some questions in my mind and niggled at some half-remembered advice.

Although it is true that in general it is difficult to achieve a fit too tight for successful soldering, I have certainly had some failures with tube in reamed holes so it is certainly possible.

Soldering (brazing) as a process requires a gap between the pieces being joined. I recall advice from long ago to aim for 1.5 thou (about 40 microns) for silver solder. My newly purchased Brepohl suggests 100 microns or 200 for hollowware. General advice from various places suggests a fit too small to see daylight (about 10 microns I suppose). I can't get anything to solder across 200 microns and fill gaps that large with fine fillets before soldering on both sides.

Conversely, as we all will know ruefully, solder is rubbish at filling large gaps... but how large?

I'm disappointed that I couldn't find any authoritative numbers, even websites that discussed the geometry of the meniscus seemed to reach wildly varying conclusions! Do you suppose that we could arrive at a few advices amongst the experts?

1) what is the smallest gap
2) what is the largest gap & is there solder more suited to large gaps
3) what can you use to bridge a gap too large

My own inexpert stab at which would be:

1) 40 microns ( a thin sheet of tissue, but definitely still see daylight )
2) 100 microns (2 pages of the Radio Times)
3) scrap it and try again

[Dennis]

Thank you Joe,

I have never yet had a gap too small to solder and it would be interesting to have others join in. For practical purposes I presume we are talking mainly about perfectly round objects in perfectly round holes. So, as most of us would not use reamers or broaches, except for the pins of hinges, it would be outside our experience.

I do know that if I can see a gap without holding it up to the light, I prefer to add some filler. For this I Keep some swarf (the curly bits of metal you get when drilling) in a small box with an anti-tarnish tab. This can be easily cut into pallions and packed in prior to soldering, to good effect.

[Green Beetle]

Thanks, Dennis. Good tip, I'll try that!

[Patstone]

Thats interesting information, I made a ring and made holes in it as a decorative feature and drilled one in the wrong place - whoops the drill slipped. Managed to fill it up with a snippet of silver wire the same size as the hole, soldered it and rubbed it down well and you cant see it. Must be doing something right for a change.

[Dennis]

Joe's thread has haunted me since last week because I feel that only another engineer can add something meaningful to this.