Mind the Gap!

[Inflight]

I'm just getting started on a small steam project which calls for lots of silver-soldered joints. I spent today trying to solder about a dozen components together, with varying success. My problem is in trying to get the gap correct at fit-up. I brushed flux on the joint (surface area is about 1/4" sq) and screwed them together. The flux was sandwiched between the brass and created a thin gap I thought the solder could fill. Some of the joints were perfect but most had small voids. Any science to getting a nice fillet?

Ok, here are my questions:
If there is such a thing, what is the proper gap when silver soldering brass?

What is the correct thickness of the flux paste? I tried very dry which had less "boil off" during brazing. I also tried very thin, like ink. Didn't seem to work as well.

How can I estimate the correct amount of solder wire to use per joint?

 

[12bolts]

Hi Inflight,
Most of my silver soldering is done on stainless, but I have done some brass.
I use a commercially available thick paste flux.
My method, (Not saying its necessarily the right or best way, but it works for me)
Make sure you get everything nice and clean, put some flux between the surfaces, apply heat to the lower and preferably larger/thicker piece, as the flux boils away, use the silver solder stick to poke a bit more flux on, if the parts are thin/small to avoid overheating I transfer the heat to the tip of the solder stick and melt a small blob onto the work. Using a "weaving" action I heat the work until the solder melts and capillary action pulls it into the joint. Depending on the size of the work and the gap to be filled I just apply solder until the joint is neat and has a nice fillet. I have filled some pretty big gaps but you need something to support the solder. It wont build up like a weld fillet by laying bead on top of bead.
Are you using propane/mapp torch or oxy/acetylene. I find the oxy is a bit easier to control because you can get the heat right were you want it quickly, whereas the propane is a bit slower and everything gets a bit warm as the heat sinks away. I tend to use that on water pipe repairs so I dont have to roll out the oxy down the back paddock, and the cows dont mind if the joint looks a bit messy.

Cheers Phil

 

[Inflight]

Phil, so you are saying that you hand feed the silver solder like when tig welding with filler rod? I have been placing small lengths of silver solder wire next to the joint before heating; then letting it melt into the joint.

Seems like this silver solder is rather expensive stuff, so I'd rather not waste too much. :biggrin:

 

[12bolts]

Hi Inflight,
Yeah I just feed it in by hand.
I buy sticks from the local plumbing hardware store, NOT a handyman shop, they are about 2ft long. Comes in different grades, (silver/tin/copper ratios) and diameters. Honestly cant remember the price 'cos I just stick it on the account and SWMBO looks after the bookwork . But prolly only a coupla $ per stick. They go a fair way. Dont get fluxed rods, use uncoated. Anyway I would have thought that you would be wasting more solder trying to hope that you get small pieces to be in the right spot and melt in.
Just get the work hot enough, but dont overcook it, or you will need to stop, cool down and go abrasive back to shiny metal again.
If you get the heat in the right spot you just dab the stick in and it sucks right in to the joint. Even if you get it near correct temp the solder blob will adhere to the piece, but stand out as a ball, and if its near the right spot then gentle "weaving" with the torch will get the temp high enough and it will flow into the joint.

Cheers Phil

 

[churchjw]

Silver solder or any solder works best with no gap. The capillarity action between the very tight surfaces helps draw the solder into the joint. You can get the solder and/or base metal too hot. If the either oxidizes it will not solder correctly. As Phil said the best way is to heat the larger of the 2 pieces being joined and when the metal gets up to temp feed the solder in. Judging the right temp is a mater of trial and error and comes with practice.

Jeff

 

[Inflight]

Thanks for the advice, guys. I tried dabbing today and it worked very well. Just sucked right into the joint. )