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Old 03-19-2021, 06:05 PM
mburtis mburtis is offline
Join Date: Mar 2021
Location: Wyoming
Posts: 13
Default Want to learn to solder joints

So spent some time today playing around with soldering a sheetmetal lock seam. This was something I had never messed with. I've gas welded and rather enjoy brazing and have soldered plenty of copper pipe and wiring but never sheetmetal seams. Just a skill that I think would be neat to know for building tanks and such since I know people with antique cars with tanks constructed like this.

Did a practice today out of 20 gauge using what I think is called a flat lock seam. Just rough formed it with a hammer, wiped it with scotch brite, brushed a little of our normal hardware store flux on it, then put the joint together and beat it tight with a hammer. Heated it with the propane torch. Only 60/40 solder I had was really small stuff so it took a lot of solder to fill the joint. I didn't do any tinning of the joint or really flux it that well which is probably why I didn't get great solder past the first fold of the joint. I also overheated it pretty bad before I realized it. All that being said I cut it apart afterward and pried it open with a screwdriver and I think it probably would have held.

I know there are some knowledgable folks on here for this particular forgotten skill and i wanted to ask some questions.
1. Torch vs heavy copper iron, seems that the tradition method uses a big chunk of fire heated copper rather than a torch. I'm assuming the iron would give a little more control and precision in the heat? What about size and shape of the iron, are there rules of thumb for how big the iron should be based on material and joint size? What about torch setups to do it, tip type, tip size, torch set up etc. Is it possible to do a first rate job with an OA torch or plumber's acetylene torch with enough skill?

2. I've heard of various fluxes, killed spirits, rosin, etc. Pros or cons of the different types and where you might use one over the other ? I just used regular nokorrode paste flux, can you even find others very easily anymore?

3. Strength and usage of the different solder mixes, obviously traditional lead solder is pretty hard to get. The roll I used was marked 60/40 but most of our other rolls were 95/5 tin antimony. I would be very interested in a discussion on what solders are better from a strength or fatigue aspect and what solder might be best used in what application, or a solder that you might not want to use.

Links or pointers to resources on this information would be appreciated to or recommendations for a video showing how to properly tin a sheet and solder the joint. What about this corking tool, that's for driving down the seam in a grooved lock seam?

Picture of my rather sloppy joint pre solder.
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Old 03-20-2021, 02:44 PM
dwmh dwmh is offline
MetalShaper of the Month May 2018, July 2022
Join Date: Aug 2015
Location: Nr Oxford UK
Posts: 655

Matt for the flux for steel I use a liquid tinning flux, it is flux with solder incorporated. it is what was used for lead loading before Bondo came along. If you search for body solder/ lead loading requisits you should find it. this gives a perfect tinned surface to which you then add your solder.
David Hamer
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Old 03-23-2021, 07:40 AM
fciron fciron is offline
MetalShaper of the Month May 2022'
Join Date: Oct 2010
Location: Louisville, KY, USA
Posts: 126

I make a lot of copper lanterns. I use Sta-bright flux from the welding store, it's a killed acid flux, a liquid I apply with a half inch brush. I usually use an copper (about 1" square) I scored a bucket full of them at an auction years ago.

Roofer's Suppliers are another source for solder supplies. That's where you might be able to find a bar of Sal Ammoniac, for cleaning your soldering coppe and lead based solders.

I like the liquid fluxes because they will penetrate a closed seam, you can dab a little more on a spot that's giving trouble and get penetration. Ruby Fluid used by roofers is less aggressive than Sta-bright, a trade off for easier clean up. A quick rinse or wipe with a wet rag will do for Ruby Fluid, Sta-bright needs a good scrubbing or it will hang around and corrode things.

Roofers and I prefer a soldering copper to a torch because it is more precise and does not set the wood under the metal on fire. (I build quick wooden jigs for my lanterns.) I also like being able to tack things together with the iron, the solder solidifies quickly because it's not being kept warm by the surrounding metal which gets heated by the torch. If you're working with copper it's also very noticeable if the solder flows onto the the metal around the joint which can happen more with a torch. I use a lot of lap joints and the ability to press the joint closed with the copper is nice too.

For a lock seam like you have I would apply flux to the joint and then press my iron on top while feeding solder into the joint and the heat will pull both flux and solder into the joint. Takes a sizeable iron and plenty of heat.
Lewis Meyer
Falls City Ironworks
Louisville, KY
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