Building a simple forge torch
Disclaimer: building a torch is DANGEROUS. I am not an authority on combustible gas and am not responsible for any errors or accidents. This is simply to document my build, for my burner, and not to be taken as an instructional guide. If you wish to undertake a torch build of your own, do your research, take precautions, check fittings, and understand the risks involved.
This is actually the second torch I've built, in the same style and model as my previous one, reusing some of the same parts in fact, but upgraded and replaced a few of the worn out components.
First off let's lay down a few distinctions. A solid fuel (coal/charcoal/coke powered) forge operates largely off the principles of correct air blowing and containment of heat to get the coals hot enough, whereas an efficient propane forge simply needs the most flame contained well enough to heat the workpiece.
So (among other variables), bigger, hotter flame = bigger, hotter forge. Pretty simple. For this build, I decided not to do a complicated burner build, but simply improve on my current model, which is very easy to make.
Gas torches require a combustible gas under high pressure to be mixed with air, which is then ignited. Under the propane torch umbrella are two main types; blown burners and venturi burners. Blown burners make use of gas as well as air pumped into the torch shaft, so the air, as well as the propane, is forced in. This is more efficient and economical than a venturi burner but is more complicated to build. I'll likely try a blown burner in the future but it's nowhere near a necessity now.
Note: another bladesmith messaged me to say that it's actually entirely possible to convert a venturi torch into a blown burner. I am interested in this but that upgrade will be for another time
The venturi burner's namesake is the venturi effect. The basic idea of the venturi effect is that when gas flows from a wider area of pipe, through a choke, or restricted section of a pipe (like a funnel), the pressure decreases, and velocity increases inside the restricted area. Not only increasing the speed of the gas down the shaft, it also sucks air into the intake.
So, a venturi burner is a torch that works by shooting a stream of propane down into a cone, into a tighter shaft, and then expelled into the forge where it is burned. Oxygen is sucked in and mixes while in the shaft, where they combust in the forge, producing the precious flame. The end of the torch is also a flare, slowing down the gasflow once more so that it ignites as it is expelled.
Basic construction is cone, pipe, then cone, with some way to expel propane down the first cone.
The method I used is known as the Ron Reil style burner (I actually didn't know the name before a follower filled me in, a huge help for more research), what I ended up piecing together after research on various forums and threads. I cherry-picked different principles and this is what I came up with.
I started with three black iron pieces; two reducing couplings on either end of an 8" X 3/4" nipple pipe, threaded at both ends. I drilled two holes through the intake coupling, 1/2" diameter.
The propane piping is set up like this: a regulator and hose goes from the propane tank and screwed onto an adaptor, which screws onto a brass nipple, which is inserted perpendicular to the intake coupling and capped on the end. The brass nipple has a #52 hole drilled in the center, this orifice is for ejecting the propane into the torch shaft.
Note: make sure you clean out the burrs after drilling the orifice in the brass nipple. If you don't, they will clog the hole and inhibit the propane flow. I did this carefully using some needle files and a wire brush.
The view down the brass nipple pipe: you can see the orifice drilled in the center. Check it inside and out for burrs.
The individual dimensions of the brass pieces aren't actually all that important: I just went to the local Ace, explained what I needed along the lines of the above, and they put the pieces together for me.
Just a word of warning for those of you who (like me) are not familiar at all with piping: make sure the thread sizes match. When I first built this torch, I went back and forth to the hardware store several times before I realized there wasn't one standard thread size. Who knew, right?
To summarize, the basic setup is like so: Three black iron componenets: two bell couplers on either end of a 3/4" nipple. One end expells the flame into the forge, the other is the intake and supports the brass piping.
The propane goes from the tank through a hose, through an adaptor, into a brass nipple piece which is inserted perpendicularly into the intake coupler and capped on the other end. The propane is expelled through a hole in the center of the brass nipple piece, into the black iron shaft, and expelled out the flare at the other end where it is burned.
Now, where the brass pipe is inserted into the intake coupler, you will notice that there isn't anything holding it still: the pipe will be relatively free to rotate, which you don't want happening. I marked the opposite side from the orifice with a sharpie so that I know when it is oriented correctly. It could be possible to redesign the torch so that the brass pipe is impossible to turn, but I kept it as is, and make sure to double check the orientation before I light the forge. The ability to rotate the pipe also enables me to help fine tune the burning, to get 100% combustion.
As time goes along I will be making upgrades, tweaks, and repairs to the burner, but this is a very solid, efficient, and working torch that is the perfect size for simple knife forging.
With that, I've completed (for now) the new forge build. The entire series can be found on my site here. If you have any questions or comments, please shoot me an email, I would love to hear them! I am not an expert, and there is loads that I have left to learn.
A very valuable resource that I found only after completing the torch can be found here. I believe it is written by Ron Reil himself, the pioneer of this style of burner. It is very clearly a go-to resource if you plan on building a torch of your own.