Fancy some stronger metal to metal joints? Then read on…
Im sure that every modeller will know how to solder using a normal soldering iron and a lead based soft solder. However I suspect that few know how to silver solder. Silver soldering is far stronger than soft solder and will not melt with the heat from a glow engine exhaust. I use silver solder for special exhaust systems, for attaching control horns to elevator joiners on control line models, or where I want to hide the horn inside the fuselage on an R/C model.
WHAT YOU’LL NEED
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Silver soldering is not hard to do and the equipment isnt expensive so theres no reason not to equip yourself to do the job.
The items needed – torches, silver rods, a tin of flux and a pot of paste type flux.The essentials are a gas torch or two, silver solder rods, flux and my recommended burn treatment! One can buy a small pencil torch which provideds a very delicate flame, this is fine for jobs such as joining brass tubes and small items but larger masses of metal will draw too much heat away from the work area so one then needs a bigger torch. I bought my pencil torch many years ago for about a fiver. The larger torch is available now in good DIY stores for about £12 including a gas cylinder.
Silver solder can be harder to obtain. You need a really good tool shop and these are not always easy to find. I buy mine from Squires Model and Craft Tools. Phone 01243 842424 for a free 680 page catalogue. Apart from a gigantic range of tools and materials, they have a large range of solders with different melting points and the fluxes to go with them. I prefer a silver solder with a low melting point, this is adequate for all normal model work and needs relatively less heat.
You can silver solder brass, copper and ferrous metals although you cant silver solder any aluminium alloy. You can silver solder piano wire but the temperature needed will soften the wire, so beware. I have seen perfectly good undercarriages ruined because someone thought that brazing them would be a good idea.
Remember that you are working with a flame that can be two or three inches long and which will spread heat far beyond the actual job. Work in an area free from flammable material and use a brick or tile under the work. A fire extinguisher standing by would also be a wise precaution.
The metals to be joined must be clean and to this end I file and sandpaper them to a bright finish. Have the flux and silver solder rod to hand. I prefer a powder type flux but there are paste types as well. If using a powder flux, warm the rod and dip it into the powder and then transfer this to the job while heating that with the torch.
A control horn being brought up to red heat
The metal has to be heated to a red heat. I find that a bright red is required to get the solder to flow properly but that depends on the actual melting point of the solder being used. The hottest point of the flame should be applied to the joint area, the rest of the flame heats the metal beyond the joint. With a small torch, this is not enough to prevent the heat being absorbed by the rest of the job and cooling the joint area, small pieces of metal being a possible exception.
The hottest part of the flame is at the tip of the bright cone of the flame. Take a piece of brass tube and apply the flame to it. You will soon see that when the tip of the cone is just touching the metal, the tube will glow red but as soon as you move it away the glow will vanish. A good joint will have a smooth finish and should flow or blend out onto the metal surface. Poor joints will be blobs that do not blend into the metal. Poor joints may be caused by dirt or insufficient heat.
The silver solder has started to flow. I am using a piece of slate as a hearth here.
If the joint has been moved before it has solidified it may look crystalline. Reheating the joint until the solder melts will cure this. Once the joint has cooled remove the remaining flux by washing or scraping the surface.
The different melting points of solder can be used to great advantage on complicated jobs. By using a high melting point, silver solder for the first joints, followed by a lower melting point solder, it is possible to make subsequent joints without melting the first one. Finally soft solder can be used for further joints without the risk of melting the earlier ones.
The finished silver solder joint on the control horn
The higher temperatures mean that metals remain hotter for much longer. It is all too easy to forget this and pick the job up long before it is really cool enough. Use pliers or forceps until you are sure it’s cool enough to handle.
I still burn myself in this way but I have found an excellent first aid treatment! It’s an ointment made by Weleda, called Combudoron Ointment. It removes the burning pain and prevents blistering. I always have a tube in the medicine cabinet. Weleda products are stocked wherever natural remedies are available and in many chemists too.
Thats it, now you know how to silver solder, it can add a new dimension to your build projects and provide a solution where metal parts are joined. Remember the precautions though.
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