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Metalwork: How to solder a joint close to an existing joint without melting the first one? Answered

I'm trying to make a sculpture by soldering copper/brass rods together, but when I try to make one joint, it melts all the surrounding joints. I tried putting on some heatsink tweezers but they weren't effective. It's going to be quite complex, so clamping the whole thing and soldering all the joints at once won't be practical. Any tips for making a better heatsink? Should I just solder faster? At the moment I'm letting the metal get really really hot because I guessed it would make a better joint, but maybe I'm overdoing it?



Best Answer 9 years ago

If you are using tin or lead based solder, you don't have to get the metal really hot. The solder is more of a glue that needs the metal to be hot to stick. If you are using jeweler's silver solder (technically brazing), then you have to use different temperature ranges of solder, but again, overheating the metal doesn't work. In this case, because the solder boils or breaks down and the joint isn't as strong.

Anytime you are sweating pipe,you want to get everything cleaned ,fluxed, put together,and sweat it all at once,never do say a half of a joint or you wll have problems.When Im installing something like a set of tube and shower valves I do it as I said above,and when you get through sweating all of your joints,dont touch nothing,if you dont give it time to cool,you could have a leak.

Also when sweating ,keep the heat on the joint to where just the tip of the flame is touching,testing it every few seconds buy touching the soldier to the joint,when its ready you will see the soldier being drawn up into the joint.

It will take a combination of QUICKNESS... and the RIGHT amount of heat.
If you are trying to solder with a propane torch, you might have to turn the flame WAY down... or switch to a large solder iron. Solder is not a good method to ATTACH things to each other for physical strength. Solder will loose its connection if the objects have any movement on them (over time). Brazing might be a better choice. Another trick to use is to wrap the rods with very THIN wires at the joints you want to solder. The thin wires will not be seen because the solder covers them, but it gives extra strength and will HOLD the joints even if adjacent joints heat up too much.

If your'e using silver solder, like with jewelry, you can use a paste-mixture of yellow ochre and water to prevent the solder from flowing. Just mix it up and put it on the solder joints that you don't want to melt. However if you're cleaning the piece with pickle, the same as you would with jewelry, you'll want to scrub all the yellow ochre off the piece before putting it in the pickle because yellow ochre is basically iron oxide and the iron will contaminate the pickle causing it to copper plate everything. Otherwise I'd suggest submerging parts of it in buckets of water or sand. Or wet sand.

What you need is a high-powered soldering torch, and to "braze" instead of simply solder. The torch will heat very quickly, and then you immediately cool with water. The idea is to come in fast, hit hard, and leave before the heat that is flowing through has a chance to react. Using a standard soldering iron in such an instance simply will not do. With a torch, you should be able to make a joint and have it already cooling within a second, if you have good technique. By the time the heat reaches anything nearby, it will not be enough to reach a melting point. Do not cure solder with water though, as the solder will cool too fast and lose it's bond with the metal you are attempting to bond. Drench the surrounding area instead. The process takes practice, but is relatively easy once you get the hang of it.

Depending on the size of the item you could use some pliers or vice grips between the two joints to "sink" the heat away from the second joint while you heat the other. The problem is that if you clamp too close it can prevent the joint you are working on to not heat.

It's a question of heat transfer, If you can apply a lot of heat to the spot you want to solder, and quickly, you can do that bit before the rest gets too hot. However, copper is a very good conductor of heat, so you may find this difficult. Is there a way that you can clamp or tie things in position while you're doing this? L

Can you wrap a big ol wet rag on the other joint? Works with plumbing sometimes. You can also get pastes that help, I know they use them for gun smithing.