Posted by Valerie on December 05, 1997 at 17:51:10:

A friend of mine asked this question on email and I thought I would share it with you here:

Q: "Can I braze an extension on a brass threaded rod to lengthen it?"

Yes, you can. Of course, you want to do an especially good job because threaded joints are in tension.

Q: "Is brazing what I'd have to do to attach something to brass? and does the extension also have to be brass?"

The neat thing about braze welding or silver brazing (some people call silver solder) is that it does not melt the base metals. Because of this, dissimilar metals can be joined by these processes. The main consideration is how well each metal conducts heat. Brass will conduct heat away from the joining area faster than steel so more heat needs to be put to the brass than the steel. The size of mass of the metal is important too. The larger piece of metal obviously needs more heat than a smaller one.

There is braze welding and silver brazing. Braze welding uses a bronze or brass rod and has either flux added to the rod commercially on the outside or you dip the heated end of the bare rod into flux. If you have no flux, Mule Team borax (the laundry stuff you buy at store) will work. Dip the heated rod into the powder and it sticks. The bronze or brass rods are very viscous (thick like honey) when molten. This means that the joint between the two parts to be welded need a bit of a bevel on the surfaces and also need a little gap between them. The best way to hold these parts in alignment is to have a piece of angle iron sitting like a vee shape and then prop it up with bricks or weld little flat foot across the bottom point so it sits like a trough. Then you set your round rods in here and it always holds them in line.

Silver brazing is what would be best here. Silver is very low viscosity and so flows easily into tight joints. The surfaces of the joint (the touching parts) are coated with flux. For this it is best to buy some commercially. The parts are then placed in contact with each other in a way that they will not be bumped or moved. Then heat them up to an almost dull red but mainly watch the flux. When the flux gets watery looking, touch a thin rod of the silver braze alloy to the side of the joint. Capillary action pulls the molten filler material into the joint, just like water is pulled into a paper towel. This filler material will be pulled all the way through the joint if you were carefull to heat it up well.

For these rods, I would play the flame over the piece, not aiming it at the joint at first. This way, the brass will be able to hold that heat a bit. Otherwise, the heat is sucked out immediately. (Copper alloys are great heat conductors). Keep in mind that the brass will not have much range between being red-hot and becoming mushy. Also, the threads on the brass will melt easier than the main body so watch that heat. Maybe spin the rod to the other side to make sure that both sides get equally hot. Use a slightly reducing (excess acetylene) flame. That is the kind where you haven't opened up the acetylene at the torch enough to totally make the little feather at the end of the blue cone disappear.

For this silver braze, some people use a thin shim of silver alloy to pre-place in the joint along with the flux. This is okay, but you will find the thin rod a bit easier to use and make sure that it flows through whole seam of joint. Hmm. Keep in mind that the angle iron holder will be a bit of a heat sink. If there are little rods that you are using, maybe play the flame a bit on the underneathe side of the angle iron.

Make sure to let the joint cool down in still air. The brass and the silver have different shrinkage rates. if you plunge it in water right after the job, the brass and silver shrink at different rates, sometimes creating micro or macro cracks in the joint.

Oh, and one more thing: It is essential that your metal is clean and clear of oil and rust before doing this. If the threads have oil or grease in them, make sure to clean it back about 1" on each side of joint. Practice first and good luck!

Follow Ups:

• Re: Brazing Robert O'Neill 2/18/100 (0)

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