Soldering Bolsters

This is another little job that you're likely to do quite often. Attaching bolsters with solder applies whether you make fixed bladed knives or folders. If you've ever done any plumbing with copper pipe, the technique is similar.

For soldering bolsters (or other parts) you need to use silver solder, not the regular lead solder you use for plumbing. Silver solder (it really does contain silver --up to 40%) melts at a much higher temperature than lead solder (around 232C or 450F). That means you get a much stronger bond that's closer to brazing than soldering.

In this example, we're soldering brass bolsters on brass liners for a folder. You can also solder brass to steel or stainless to steel or brass. If you happen to use titanium for liners, like many folder makers do these days, you're out of luck. You can't solder titanium. You'll have to rely on mechanical bonds (pins).

Get your parts prepared for soldering. Make sure the bonding sufaces are clean and grease free. Sand paper is a good way to clean up the surfaces. Here, I'm going to leave the black marker line as a guide for where the bottom of the bolster goes.



Once the parts are clean, apply the flux to the surfaces you are bonding. Make sure you use flux designed for silver solder. Clamp the pieces together. I use locking pliers (Vice Grips) so I can adjust the pressure to hold the pieces firmly, but with a minimum of marking.
(Note: Consider making a set of "safe" locking pliers by grinding off the teeth on the grip faces.)

Here, the pieces are clamped together, ready for soldering. If you leave a bit of the liner long, so it sticks out from the front of the bolster piece, you can lay a piece of solder in place (as shown). That way, any leftover solder residue gets ground off later.



Now it's time to get out your trusty propane torch. Apply the heat to the back of the liner, just near the bottom of the bolster. That way you get a nice heat from from the bottom up towards where the solder is. When the solder melts, it has a nice hot path to run down between the pieces. Don't overdo the heat. You should see the solder melt and disappear inside. Depending on how tight the surfaces connect, you may see some appear at the bottom of the bolster.

(There's no picture of the heating step. I couldn't hold the torch and pliers and camera at the same time.:001_unsure

Let everything cool down and have a look. If it worked, the two pieces should be firmly fastened together.



Finally, here's the liner and bolster after a bit of clean-up.



One last note. Silver soldering forms a pretty solid bond. But if you're making a knife that's destined for some hard use, you should really back it up with some mechanical bonding like a small pin or two. Together, they ensure your knife lasts a lifetime or two.

 

It usually comes in a coil like regular lead solder. You can find it pretty much anywhere that sells plumbing supplies, usually in a package with some flux. Check the package for the silver content. Most of the time it's quite low (~4%). The higher the silver content the higher the melting temp.