Silver Soldering/brazing Technique

[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.
 

[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

[Troutsqueezer]

Hi Chuck, I would flux both parts, press them together then put a ring of silver solder around the shaft (sitting upright, clevis up) so that it rests on the flat piece and braze. You should be left with a fillet from the flat piece to the shaft. I would think that should be strong enough.

Fire brick can be a pain to track down. Home Depot sells HardiBacker board which is used to set tile on for kitchen or bathroom surfaces. It is cheap, can be cut up easily into any size/shape and will stand up to MAPP temps. Been using it in my hearth for a few years, no probs.

-Dennis

[Chuck in E. TN]

 During my research for my silver soldering project, I cam across the .pdf linked below. It has good info including pictures of a reducing flame. I had run across several references to a reducing flame but this is the first illustration I've found.
 www.richard-whitehouse.co.uk/Soldering%20notes.pdf
 I also found fire brick at Lowe's.
Chuck

[picclock]

Hi Chuck
Thanks for the pictures. Those parts are really small compared to what I imagined. Should be very easy to silver solder using a thin pallion (piece of silver solder) bent into a ring. just flux the parts and go for it. I know that even the thinnest of my silver solder supplies will still be way in excess of what is needed for that job. Being so small it will not need a very powerful torch. Once it has been brazed but before it gets cold (say 2-300C) if you dunk it in a tub of water all the flux will come off leaving you with much less to clean up.

Hope you post pictures of the finished project.

Best Regards

picclock

[Chuck in E. TN]

picclock,
I've gotten so much help on this project here and on other forums, I would be foolish not to post pictures! Probably get drummed out of the forums...
I had a small accident with the flux in the kit I bought. The plastic bottle shattered in my hands when I tried to open it. Now waiting delivery of a replacement.
The first 7 or 8 rods that I made had to be tapped into the flat piece with a hammer! I cannot pull them apart with just my hands! I was thinking about all the advise to allow clearance for solder wicking and upped the drill bit 1 size for a couple. Definitly a slip fit on those. I'll try soldering the loose ones first.
 
Chuck

[Troutsqueezer]

I think in this case, a tight fit is fine. The fillet will add the needed strength.

[picclock]

Hi Chuck

If you need to get better alignment, even with a loose fit, you can either knurl a part of it, or if its two flat faces that need to be brazed and you need to ensure an even gap, just centre punch the faces and the metal raised will set it. When I first started brazing getting the gap set correctly was always the challenge, especially if the alignment of the parts had to be precise.

Looking forward to the pictures.

Best Regards

picclock