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| Silver Soldering/brazing Technique |
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| Chuck in E. TN:
I have a noob question on Silver soldering/brazing. I have only silver soldered 1 part in my life. I have a part that press fits together with finger pressure. It's a shaft for a crank, and the other part is a flange for the crank bearing. Is this type of fit 'loose' enough for silver solder to wick/wet between the parts? Should I flux the mating surface of both parts before pressing together? Chuck |
| Troutsqueezer:
Chuck, without actually seeing the parts it is just a best guess on my part. I would flux both parts before pressing them together. Many folks will bevel the part that the shaft fits into, on both sides. That gives a fillet around the shaft which adds strength. |
| picclock:
Hi Chuck I think the optimum gap for strength is 3-5 thou. If you have a fixture which will ensure correct alignment then make the gap correct and go for it. Without the fixture its trickier since you will have to leave enough material to ensure positional accuracy while brazing with enough of a gap in the other places to allow good penetration. Its quite amazing how much penetration will occur even with a smaller gap , however the info is that the strength is not there - never could figure out why - perhaps someone with greater knowledge on this board could enlighten us all. From the cupalloy site (http://cupalloys.co.uk/strength-of-silver-soldered-joints-c100.html) :- "Results have shown that the shear strength of a joint, made using a 40% silver quaternary brazing alloy, achieved maximum shear stress when the joint gap was 0.1mm. A variation in the joint gap produced lower figures. It is thought that the lower strength seen with smaller gaps is attributed to lower levels of joint soundness caused by flux deterioration, and or, poor metal flow" Probably the big tricks here are to ensure even heating of both parts by surrounding them with fire bricks or similar, using a long lived flux, and a torch(es) powerful enough to get the parts to temperature in a reasonably short time. To prevent unwanted braze material from getting onto other areas use soft pencil lead or tippex (allegedly fumes - but you should be doing this well ventilated anyway). Hope this helps Best Regards picclock |
| srm_92000:
I've had success with fairly close fitting parts (Crank webs for Stuart 10v). As has been suggested, leave a chamfer on the shaft or if possible leave a fillet. And CLEAN then CLEAN again and CLEAN a bit more just before fluxing don't even touch it after and solder straight away. Did I say make sure it's CLEAN first. :thumbup: |
| Chuck in E. TN:
I want to thank all who have commented and offered advise on the techniques of silver soldering/brazing. A lot of good info to think about and apply. A little background on me. I have soldered for electronics for many years. I actually taught high reliability soldering for the USAF for many years. I have also done some plumbing soldering over the years using propane and mapp gas, even an old kerosene blowlamp. I have only silver soldered/brazed once, and can't find that part now! So, I am familiar with the process and what solder flow and wicking look like. I have my Dad's Proto torch set, regulator and acetylene tank that I intend to use for silver soldering/brazing. It needs a new hose, as the original is looking dry rotted and I don't trust it. I'm looking for firebrick to make a hearth, and have found a local supplier for the solder and flux, and a replacement hose. I have uploaded a photo of the parts I need to silver solder/braze. The part, shown assembled at the top, and the 3 components at the bottom, is a connecting rod for a model steam engine. The clevis on the right end does not get soldered. The flat part on the left gets soldered to the end of the rod in the position shown at the top. The rod part is run of the mill 3/16” CRS rod, not modified other than faced in the lathe and threaded for the clevis. The flat piece is 1/16” crs plate, drilled and tapped 2-56 on the outaide holes and drilled 3/16” in the center. As made, the flat piece is a light press fit on the rod. I used a ball pien to tap the rod into the flat piece on the anvil part of my bench vise to get a flush fit. I can twist and pull the flat piece off the rod with my fingers. I used a 3/16” drill to make the mating hole. If I use a #12 drill the flat part is loose on the rod. Hope this clears up your questions and gives you a mental picture of what I'm trying to do. Chuck edit: Fixed link. Don |
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