Building a Large Gas Forge
by Steve Smith
Rocky Mountain Smiths
This is not an article with detailed plans. While I do go into some detail where things get tricky, the article does not include detailed drawings and a dimensioned parts list-you will need to adapt the details to the materials you have on hand, your shop equipment and your skills. I think that half the fun of building something is figuring out what the details are going to be like anyway. This process makes the final product much more yours and builds skills.
My first gas forge was pretty basic. It consisted of a 12" diameter by 18" long tube, lined with 2" Durablanket insulation and fired with a single, blower driven propane burner. I later added inadequate brick shelves at each end (too short, the bricks kept falling off). It has served me well for about seven years.
Recently, I (finally) widened the brick shelves and was considering some additional improvements. Instead, I decided to build a new, somewhat larger forge to incorporate many improvements. Many of the ideas in this forge come from Ron Reil. He describes a forge of this size on his web site (http://www.webpak.net/~rreil). He has some excellent information on forges and burners available at his web site. Heaps of thanks and credit go to Ron for posting this data as a service to the blacksmithing community.
I wanted a forge a little larger than my old one in both diameter and length. I decided to make the outside 14" diameter and 24" long (the decision was aided by finds in the scrap yard….). One burner had worked quite well in my old forge; I decided to put three in this new forge. The burners are naturally aspirated (no blower) per Ron's design.
I particularly appreciate two of Ron's ideas. He has individual shutoff valves on each burner, and a movable interior wall. This allows the forge to shrink to fit the work. The second idea I really like is adding an idle circuit to the gas line. Closing a single valve shuts off almost all the gas flow to all burners. The residual flow is set by a needle valve. This way the forge can be immediately dropped from full blast to idle when you take a piece out, then brought back up to full in an instant.
Building the Forge
On a trip to one of the local scrap yards I noticed that they had 15 gallon metal barrels (14"x24"!) for $2. I took two. At the yard, I dumped about a cup of extremely well perfumed stuff out of each (industrial cleaning solvent or some such). When I got them home, I washed them out well, but they still stink of perfume. I stood them in the sun full of water, baking soda and detergent for a week, rinsed, and they still stink. What finally toned the perfume down to just detectable levels was opening the ends of the barrels and letting them stand in the sun for another week. I opened the ends of the barrels with a Sawzall, truly a fine tool. I left about a 1" ring of the flat end in place to help reinforce the barrel edges.
I chose these barrels (made out of relatively thin steel) because they were cheap (recycling at its finest) and because I have a MIG welder. If your welder or your skills aren't up to metal this thin, you can have a skin rolled at a metalworking shop out of something thicker, such as 11 gauge. If you don't have a welder, I urge you to make friends with someone who does-it will really speed up construction over mechanical fasteners. If you do use nuts and bolts, use Loctite on them-some of them will be buried out of reach by the time you are done, and it would be awful to have a screw loosen with no means of tightening it. If you can't beg, borrow or buy a MIG welder, you are welcome to come by my shop.
The barrels need several things to turn them into forges. They need feet, brick ledges, burner ports and a floor. Having balanced entirely too many bricks on my old forge, I also wanted the new forge to have doors.
I felt that the barrel material was too thin to attach feet at single points, so I reinforced the mounting points. First I cold shaped a piece of 1/8" x 1" steel to match the curve of the barrel. I then welded four pieces of ¼" x ½" strap to the curved piece (see end view drawing). Place one of these assemblies at each end of the barrel and you have feet.
Set the barrel up on the feet (a helper might be useful here) on top of a piece of 3/8" or ½" plate, and carefully tack weld each assembly in place. If you've done this right, the forge won't rock on a flat surface (assuming your plate was flat….). If it rocks, you can grind a little off one foot, or weld a little more on.
I started my brick ledges by welding in a piece of 2" x 2" angle iron. On my barrels, a 9" length resting on the barrel walls (and projecting out of the end, like a narrow ledge) came to just the height I wanted for the floor (note that you need to know this height before doing this step!). Fit the angle iron carefully, making sure that both ends are the same height above your work surface. The easiest way to set the height is to leave the angle a little long, and then grind the corners a little until you have the desired height. Don't forget to allow ¼" for the actual brick ledge, which will go on top of the 2" angle iron. Tack weld the first one in place, clamp the second one in and eyeball both through the forge body to make sure they are parallel to each other. Then weld both down solid. Lay a piece of bar stock across both of them and see if the angle is sagging. If it is, bend it back into line.
My brick ledges are 18" of ¼" x 6" plate, mounted on top of the angle iron. I wanted the edge of this plate to end up just at the inside edge of the end of the barrel, which was a problem since the top and bottom of the barrel had a rolled lip at each end. Before you go any farther, measure the top and bottom of your barrel. Mine were different at each end; the bottom necked down to 13 ½" for some reason. Rather than chop out part of the rolled barrel end (and leave a weak spot to take all the stress), I chose to slot the ¼" plate to fit. I did this with a 4 ½" side grinder. Making a plunge cut with a side grinder is not a very safe operation. If you aren't comfortable with it, don't do it. You can avoid the plunge cuts by mounting the plate as close as possible, and then adding a filler piece.
I felt that plunging with a small grinder made the operation controllable. Keep your wits about you, and remember that the grinder is far more controllable if the wheel is pulling the grinder away from you, rather than pushing it towards you. Use a guard and a handle on the grinder. Don't do this with a larger grinder. There are two types of wheels for these grinders; one is thin (about 1/8") and has fiber reinforcing, the other is thick (about ¼") and much more rigid. I like the second type in this application. I roughly slotted the plate in two places, at an angle to match the barrel edge. When you find that they aren't in quite the right places, grind a bit more.
Before you install the ledges, round the outside corners. Take off all the sharp edges with a file or grinder, and weld the plates in place. Lay a piece of bar stock across both ledges at once to check for sag, which you should straighten out at this time. Now add a diagonal reinforcing bar as shown in the side view drawing.
Drill holes for the burner mounting tubes. This can be done in a thin bodied barrel with a hole saw (hold on tight to the drill!). Not great for the hole saw, but it works OK if you keep the drill speed slow. You will definitely want ear plugs. Cut a thicker body with a torch or plasma cutter. I put the holes at about 1:30 on the clock face.
Before you attach the burner tubes, you need to put some tapped holes in them for screws to hold the burners. I tapped ¼"x20 holes in them (three on top, three on the bottom). You could also drill oversized holes and weld nuts on the outside. Be sure to put the bottom holes at least 1" away from the bottom edge of the tube. My barrels had raised reinforcing areas that gave me a little trouble getting the screws in. I suggest that when you weld these tubes in place, you put screws in the bottom holes (all the way in) to keep weld splatter out.
Tack the burner mounting tubes at one spot on the edge, and eyeball for the correct angle. You can use a small square to check them side to side; point them right at the center of where the floor is going to be by eyeballing something lying in the center of the forge. Tack a couple of more places and eyeball everything one more time before welding in place. The tubes should be oversize, allowing some angular adjustment of the burners, so the alignment isn't too critical. Note that I am not aiming the burners tangent to the insulation-I haven't found it to make much difference.
Weld in a piece of metal plate to start your floor off flat. What size piece you use depends on what you decide to do with your floor, so read about that part below.
I welded several pieces of 1¼ x 1/8" flat bar to the side of the barrel to provide a surface for mounting the gas plumbing. Loosely assemble your plumbing and you can figure out a good arrangement of flat bars. After the forge was all done, I attached the plumbing with twists of fence wire. Crude but effective.
Two more items and the welding is done. These are the pivots for your door hinges. I machined them out of 1" round stock, facing off both ends of the stock on the lathe, and drilling a ½" hole through most of the length. The piece should be long enough so that when standing on the brick ledge, the top is half way up the side of the barrel. Drill a smaller, radial hole to intersect the bottom of the ½" hole so your hinge pin doesn't get locked in place by vacuum. Make sure that the ½" hole fits the piece of ½" cold rolled that you will be using for a hinge pin. When all is satisfactory, clamp the pivots firmly to your brick ledge and weld in place. I welded them around the base and at the upper edge where they touch the side of the barrel.
I fabricated my doors from 14 gauge steel. I cut out the door shape, and then welded on a 1 ½" edge strip. I cut an opening in the door of about 3"x6" (convenient to block with a single fire brick). This opening I edged with stainless steel, which will stand up to the high temperatures in the opening much better than mild steel. You can make the insulation fit the door nicely by pushing the door into the insulation to mark your cut lines. I found that I didn't need any means of attaching the insulation; it just stayed in place by itself.
Make hinge pins by drilling a ½" hole in a piece of the 1" round stock (faced off flat). Weld a piece of the ½" round cold rolled so that it sticks out the bottom about 1" (weld on the top only so your hinge will seat nicely on the flat 1" round). Bevel the end of the pin that sticks out. Apply a little grease to the pin and bearing surface, but don't get any grease on the outside (where you will be welding). Put the pin in its socket, and set the door in position. Now space the door up from the brick ledge with a piece of 14 gauge steel to provide clearance.
Here comes the kludge part. I welded the pin assembly to the door by building a bridge (read "glob") between the door and the 1" round with the MIG welder. This is not a great kind of weld. I reinforced it with a couple of pieces of 3/16" round running from the top of the pin assembly to a point further up the side of the door (these were about 4" long). These rods are what actually take the stress. While you weld, the grease will smoke and possibly even catch fire (if you put too much on), but it serves its purpose-the welding current will not weld the pins in the sockets. After everything has cooled off, take out the spacer plate and try your door out. You will need to regrease the pins.
Now is the ideal time to paint your forge. First, go over everything you can reach with a side grinder, using a wire brush in it. The knotted brushes last much longer than the straight bristles. I don't advise using a cup brush for this-you want the type of straight brush called a "stringer bead" brush. This is much more controllable than a cup brush and will get into much tighter corners. Go over the entire surface of the barrel. You don't need to remove the paint, just scratch up the surface so the new paint will stick. Scratch up any paint you couldn't reach with a small piece of sand paper. Remove all rust.
Wipe down everything with mineral spirits (to degrease the metal). Your liver will appreciate it if you wear chemical gloves and have plenty of ventilation while doing this. Mask off the top of the brick ledge with tape and paper. Paint with 1000 degree high temperature paint (you find this at auto parts stores or homeowner barbecue maintenance). I painted two forges with one can of spray paint.
Earlier, I gave my floor a start by welding a piece of 1/8" x 8" steel plate in the bottom of the forge. I happened to have a bunch of 1 ½" soft insulating brick (2 ½" is the normal size), which I laid on top of the steel plate. I then made the rest of the floor up out of castable refractory. Ask for a refractory that is resistant to flux. You want to mix refractory with a minimum of water, just like concrete. Put waxed paper over the top, and flatten with something hand sized (and flat). Leave the waxed paper on top overnight to slow drying.
You probably won't have a bunch of 1 ½" high temperature bricks lying around. Instead, you could just lay a floor of 2 ½" soft fire-brick. The soft bricks are easily cut with a hand saw (that you don't care about the teeth on). If you cut the bricks, it is very important that you do so with the brick wet. This will eliminate fire brick dust, which is a bad thing to breathe. You could also pour the entire thickness of the floor, but that will use a lot of castable refractory and make the forge very heavy. You could replace the 1 ½" bricks with high temperature insulating board material, or even use ceramic fiber wool with spacers to keep from crushing the wool (you will need some material to pour the floor against, not just the wool). Just using a 1" layer of castable on top of a metal plate would not be a good idea, as I don't think it will be insulating enough.
After the castable has set up overnight, you need to cure it. You can do a reasonable job of this by firing up the forge at a low pressure for a few minutes, then shut it off and block the door openings with fire-brick to trap the heat. Do this several times before heating up the forge for real. This slow heating will help keep the floor from cracking.
The burners are quite simple. Take an 8" pipe nipple and cut the threads off of one end, cleaning up any burrs. The gas jet assembly is attached to a 1 ½" to ¾" pipe reducer (see drawing). The 1/8" gas pipe is held in a hole in a short piece of steel with a set screw. The hole is sized to just fit the gas pipe, and is concentric with the rest of the burner. The jet is drilled in the center of an 1/8" cap.
The alignment of the jet with the burner axis is pretty important. I made a mandrel the same size as the 1/8" gas pipe, attached along the axis of a piece of ¾" pipe. Screw the reducer onto the mandrel, put the short piece of steel on the thin part of the mandrel, and weld a couple of 3/16" rods on to hold it in place on the reducer. This is real easy if you have a welder and a lathe; if you don't, there are other ways of doing this. One other way is to put the jet in the side of a 1/8" pipe, and then mount the jet crosswise on the reducer. You do this by filing notches on opposite edges to hold the pipe, and then tapping holes for straps to clamp the pipe in the notches. Ron's web page describes several options (or write me for more details). I used a #58 drill to make the jets.
Another way to make the jet is to use 1/8" OD soft copper tubing. Fill the end of the tubing with silver solder, then drill it out with a #58 drill. Before you mount the burner, eyeball through the soft copper tube to the end of the burner, and align the jet down the burner bore. I think this kind of jet works better at high pressures (it doesn't block the air flow as much as the larger rigid pipe).
The last piece of the burner will take a little forging. This is a larger diameter, flared tip to make the flame stable. Cut 3" of 1" pipe. Hot forge this over a solid round to make half of it flare 1/8" in diameter, so that one end is 1" diameter and the other is 1 1/8" diameter. Tap 2 or three holes in the 1" end and mount it on the ¾" pipe. The mounting position is set experimentally. Light the burner (in a vise), and slide the flared end back and forth. Once you find the extremes where the burner stops working, mount the flare in the center of the range. Not a critical adjustment. Mine ended up sticking out 1 ¼" past the end of the ¾" pipe.
Having done both, I really recommend that you buy burner flares from Larry Zoeller. He makes them out of heavy walled stainless steel in a press, and they come with set screws installed. Both Ron Reil and I have found that his flares allow idling at a significantly lower pressure than with hand forged flares. Larry can be written to at 4312 Lahnna Drive, Louisville, KY 40216.
If you have one burner that just doesn't work as well as the others (now or later), the first place I would look is inside the jet. If a little bit of fluff gets in the hole, it will really interfere with gas flow.
Buy 2" thick, 8 pounds per cubic foot, refractory ceramic fiber insulation. Most blacksmiths refer to it as Kaowool, a brand name. I buy Durablanket (a Carborundum product), the 2600F variety. Durablanket claims to work at higher temperatures than Kaowool, and I got a lot better price on it. I buy insulation products from Renler, in Englewood, Colorado.
Cut a piece of insulation the same length as the outside perimeter of the area you want to cover. This will be too big on the inside radius, but the wool will squish down just fine. Roll up the cut piece, insert it in the forge and unroll into its proper place. If you use 8 pound per cubic foot density, it will be self supporting.
Long term exposure to the fibers that the wool gives off aren't good for you-work outside, with the wind at your back. Better yet, wear a respirator.
Having used this forge for a month or so, I realize that the large interior volume chews up a lot of gas, about 5 pounds per hour at 8 psi if I'm remembering correctly. Size the forge for the size of your work. Write if you have any questions. Have fun!
6333 East Hwy. 402
Loveland, Co. 80537
23 Nov 07
©Golden Age Forge