I have some Radio Control ball links and I want to solder the balls to some piano wire (or possibly some silver steel I have hanging round) . AIUI the balls are stainless steel (don't know which one)and my attempts at soft soldering these have failed - basically the resin-cored lead-free solder doesn't want to stick. Can anyone tell me what is the best solder to use for this appliacation and what flux to use ? Pointers to a source of low volume supply would be useful too...
If you can get resin-cored solder to stick to even to mild steel then you are a better man than I am. For that you need acid-cored solder and/or a flux such as 'Bakers Fluid'. You probably wont find it at B&Q but a specialist plumbers' supplier (particularly if it is of the old fashioned kind) might be able to help.
Personally I think that all grades of steel are a silver brazing job and you will need a 'proper' propane torch set. By proper I mean interchangeable nozzles so that you can balance flame size to job size. So there is a substantial capital outlay for a start. Silver-solder ain't cheap. If you can find a couple of 1.5mm x 600mm sticks and a
250gm tub of flux for less than about UKP 10 you are doing well.
Silver brazing needs both practice and courage (the courage part is learning to get the work hot enough).
Even 'ordinary' silver solder grades are unsatisfactory with stainless steel but I have had reasonable success using 'Argobraze 56' to join stainless steel tubes to copper (at about three times the price of work-a-day grades of silver-solder). You will also need a specialist flux which goes, as far as I can remember under the trade name of 'Tenacity 4a'.
If you are still reading this then you can get both Argobraze and Tenacity from the 'Bruce Engineering' side of 'Polly Model Engineering'
Silver bearing solder ( about 5% silver, balance tin) and bakers fluid as a flux. I used to use the Canadian equivalent to repair restaurant equipment. This involved joining stainless steel to ferrous and non ferrous metals.
You can use a butane/propane mix torch for small jobs. One of the ones with disposable EN 417 gas cylinders used by plumbers as a replacement for the paraffin blowlamp will do for things this size - I assume it's normal r/c sized stuff, which is pretty small. About £10-£15 at B+Q/Wilko or the like.
A hearth can be useful, so the heat is reflected back on the work - you may not need one for very small work. A couple of firebricks will do - or at a pinch charcoaled wood is better than nothing. Use a four bits of scrap wood to form a box with one open side and no top, and it'll charcoal by itself once you get the torch on it.
I use some old cooker hobs for preheating parts, which makes it possible to do larger work with a b/p mix torch, but for large jobs you need a propane/air torch (or something with oxygen).
do a starter pack with four rods of ordinay silver solder and flux for £8.20. Ask for HT5 flux instead as you want to use it on stainless, it's the same price.
However I'd suggest using 456 solder (£8.40 for five 1.0mm rods) with HT5 flux (£3.10 for 50g). Does a better job on stainless, and it's cadmium-free too.
All soldering works by having molten solder wet the joint before it freezes. Just like water doesn't wet dirty surfaces (it wets the dirt, not the surface), molten solder won't wet dirty joints. When most metals are heated, they react with the oxygen in the air to create oxides and this oxide acts as dirt to prevent the solder from wetting the joint. The purpose of the flux is to act as a barrier to prevent the oxygen reaching the hot joint. Some fluxes (called active fluxes) contain substances that can remove oxide to some extent. The resin fluxes for soft solder don't do this.
Stainless steel is an alloy consisting mainly of iron and chromium and, when exposed to the air, the chromium oxidises almost instantly to form a microscopically thin coating that acts as a barrier to prevent the oxygen rusting the iron. The trouble is, this protective skin also stops the solder from wetting the joint, and the resin flux can't remove it. You can solve the dilemma in either of two ways: use an active flux (such as Baker's Fluid), or apply a resin flux and scratch the joint (using a stainless wire brush) through the flux to remove the chrome oxide. You need to do this to both surfaces before you bring them together, then make the joint without removing the flux. This latter method is not very convenient, but it does work. Most people say you need a special solder and flux to silver solder stainless. Whereas this is certainly true for joints that must meet a specified standard, if you just want a joint I've had no problems with using standard Easy Flo solder and flux.
Unfortunately I'm not really set up for silver soldering and also I don't want all the hassle of tempering the final article and removing the scale / putting a shine back on the balls. They have to exactly fit the link they run in so if the dimensions change as a result of the heating cycles then I'm stuffed.
I was hoping someone would know of some stuff akin to this used for soldering aluminium but if not then I will try to source some acid-cored solder from somewhere.
Silver soldering a stainless steel will produce a good strong joint but both the piano wire and the ball will be partially annealed and there will be damage to the ball surface finish.
Soft soldering is not as strong but preserves the condition of both the ball and the wire. With clean surfaces and the proper acid flux it's pretty easy. Multicore 96S cored solder is (or used be - my sample is a bit ancient!) available in small quantity packs and uses ARAX cored 96% Tin 4% Silver solder - pretty similar to modern leadfree solders.
Alternatively both 60/40 tin/lead and lead free solder work if first fluxed with Bakers No3 soldering fluid.
In either case the flux is viciously corrosive so thorough post washing of the joint is essential.
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You'll run the risk of any type of solder you use getting where you don't want it,especially on balls the size you're talking about. Maybe you could solder a short thread onto the end of the wire and use a nut either side,or one of the solder on thread adapters for piano wire.
In the application you describe it sounds as if mechanical strength of the soldered joint could be an issue. In which it might be a good idea to make more than one of your 'gadgets' and test a spare to destruction. If it does what you want then fine.
However, for the future bear in mind when silver brazing, the whole assembly is liberally covered in flux before starting. On heating this melts and covers the metal, its purpose is to keep out hot air and combustion gases. After the operation and during natural cooling, the flux solidifies and forms a crust thus continuing to protect the metal. Unless there has been really gross overheating, when the crust is washed off the joined metal parts the surface finish should be looking as bright and shiny as when the job was started. In the application that you describe, any change of the characteristics of the metal (temper or hardness) are probably irrelevant.