Step 1: About Forges
There are a lot of different kind of forges, but there are some ways that they are all similar. For one, they all work via the principle of harnessing the power of the fire gods, and bending it to your will. No matter what kind of forge you build you will need a blower motor. All the forges that I built, I have used the blower motor out of the wood stove; the ones for blowing warm air into the room, but really any blower motor that produces about as much air as a hair dryer on high will work.(although using an actual hairdryer is frowned upon, for ethical reasons)
A forge fire differs from a normal fire in that the air is actively injected into the coals, rather than being passively absorbed. A literal hole in the ground with an air supply going through a pipe actually makes a great forge. I speak from experience.
Whether you are using coal, charcoal, gas, or oil, All forges work by this principal.
PS: legends exist of mystical artifacts known as "Induction forges" however, I dismiss these rumors on grounds of blasphemy.
Step 2: Coal Forges
Coal forges are my personal favorite, simply because they are easy to use, they produce a lot of heat, and they are easy to build and maintain, and can be built from things you probably have in your garage.
The downside is that, unless you have the right connections, Coal is unbelievably expensive, and in the area I live, it does not in fact exist. Now if you can buy it locally, it is dirt cheap, unfortunately the amount of places where It can be bought locally can be counted on your fingers. So unless you live in those places, be prepared either drive a couple hours or pay through the orifices for your coal.
As coal forges are what I use almost exclusively, they are the only kind I'm going to have explicit instructions on how to make. This is not the typical pot forge design, but rather more of a trough Forge, like into the sort used by the Vikings, and my personal favorite Sort of forge, as it is easier to heat longer lengths of metal in them.(swords. I'm talking about making swords.)
You will need as follows:
X2 27 1/4" lengths of electric fence post,
X8 9x4 1/2" fire bricks,
X6 Railroad spikes,
X1 5" long hunk'o steel
X2 27" steel flat
X2 4 1/2" steel flat
X2 8 3/4" rebar
X9 1 1/2" rebar
Squirrel cage fan
Approx-2^2' sheet steel
A metal container with the bottom cut out that has a metal lid(such as a quart paint can)
As for putting it together, I'll let the pictures do most of the talking, I just have a few tips first.
Firstly, as with all metalworking projects you should make a burnt offering to Thor for strength and guidance. Secondly, whenever you're making something that's going to hold a brick, weld it together with a brick actually in place, otherwise it will almost certainly be too small. Thirdly, when you attach the railroad spike side piece thingies, put them on at about a 40-45° angle. Lastly, in the gaps to either side of the vent can just be packed with Clay.
Once it is all put together, you can slap some rebar legs on it and call it done, or set it in a permanent stone base like I did with my other one, or you can add sockets for removable legs.
Step 3: Gas Forges
The main advantage of gas forges is that they are extremely easy to use. To light them you simply toss in a piece of lit paper, turn on your gas and you're ready to go. After that you don't really have to do anything to tend them, maybe finagle the gas flow occasionally. As if that weren't enough, natural gas and propane are almost universally available, and fairly cheap.
On the downside, good gas forges are Pretty difficult to build. They also do not get nearly as hot as coal forges, AND then you have the risks of gas fires and explosions (seems paranoid, but I've seen it happen)
Step 4: Oil Forges
An oil forge is almost more of an upgrade to a gas forge than a forge in its own right. I some ways, I have to to say oil is the best. for one, the waste oil is basically free, and you can put any kind of oil in there: used motor oil, fryer oil, bacon grease, it will all work. additionally, if you do right, it will get hot enough to melt iron. AND, unlike with gas forges, there is no risk of the fuel exploding, which is nice.
Unfortunately, the doing it right part is easier said then done. Oil forges are finicky, and you have to tweak the oil and air flows constantly to maintain a good hot, sustained flame. I've been trying to build a good, reliable oil forge for a while now, to mixed success.
Its fairly easy to build an oil forge, if you have the right kind of gas forge to start with, which is to say, one with a forced air supply.
To convert it, you have to build an oil reservoir that is watertight and elevated, then have an oil feed hose that steps down to an approximately 1/4" metal tube. Then you drill a hole about two inches up on your gas burner, and stick the line in. That's really all there is to it, then light your gas forge like normal, and when it's nice and hot, you start to gradually turn the gas feed off and the oil feed on.