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Flux question

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Tony Martyr27/11/2009 14:13:32
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226 forum posts
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I am construction my first boiler. One side of the firebox outer wrapper is formed by attaching an extension plate to part of the outer tube, the other side being unwrapped to form the opposite side..
This plate is butted to the edge of the outer wrapper and held in place with a wide copper strip which overlaps the butt joint and is riveted on both sides. I am currently riveting both the throat plate and this extension plate in place but it will be a couple of weeks before I am ready to silver solder. I have cleaned the surfaces before riveting.
My question is, should the interface between the riveted plates be thinly coated with flux so that it is trapped in the space where (one hopes) solder will run, or should the interface just be a clean void?
Clearly the solder will dry out before I heat it for soldering - does that matter?
Tony
KWIL27/11/2009 17:02:59
3681 forum posts
70 photos
The flux dries out, not the solder! 
 
Clearly flux dries when you heat it normally, before it then melts, so it must help. If the rivets are tight you may well not have a gap for the solder to flow through, albeit we are looking at a desireable gap of say 0.003".
mgj27/11/2009 18:42:30
1017 forum posts
14 photos
You will need to run a line of centre pops around the circumference, or one side of the plates to ensure there is a gap for the solder to run into. Whatever the kind of joint.
 
See the solder spec for max and minimum gaps and make sure you meet it. AG2 which is hte standard Easyflow needs about .002.ie .004 on diameter.
 
However since that joint has to take being soldered again later as you come round the backhead, you might want to use a solder in the 680deg range for the preliminary joints, and then come to AG2  which melts in the 620 deg range for the later ones. That way earlier seams don't fall to bits. 
 
BTW no gap, no joint. You if you rivet first without ensuring you have a gap  there is only one place for that bit - the trash can. Or drill out all the rivets and put the gap in.
 
CLECOS are very good for holding things together like that. They come with one plastic face, which one replaces with a steel button - they were designed for holding sheet metal together before riveting, and they do it rather well. (And for silver soldering too) 
 

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