The Enigmetallic "Hell Boy" Foundry Furnace
Here we have my workhorse foundry furnace. This furnace design has evolved over several years and incorporates several useful features that make home metal casting projects much easier. It interesting that the easier you make your work flow with superior tools, the more creative and adventurous your mind becomes toward other concepts.
Like many hobby foundrymen, I started by building the Dave Gingery charcoal fired furnace to whet my early casting appetite. While inexpensive, a charcoal foundry was slow, somewhat costly and cumbersome to use with the requisite auxiliary blower required. This kept my experiments small and ultimately rather infrequent.
From there, I decided to build an electric furnace similar to Gingery's "Big Bertha" electric furnace with the idea that I could melt in my shop no matter what the weather with little fumes or smoke to worry about. Indeed, this was true, but a single 12# melt of aluminum took several hours and again, the inconvenience of the process limited my pursuits.
Then I decided to move to a propane-fired furnace and my casting life will never be the same. If you have the means, I would recommend you skip all other furnace options and start with a propane furnace. The convenience, relatively low cost and ease of use make this combination tough to beat. I chose a naturally aspirated burner design that is based on the burner designs described in Michael Porter's Book "Gas Burners for Forges, Furnaces & Kilns". For the hobby foundry, there really isn't a better book on the subject of burners than this. Get a copy before spending a single cent on burner equipment. You won't be sorry.
Anyway, the following are some pictures taken during the construction of my latest furnace. This is definitely NOT a how-to. Instead, these pictures and descriptions are intended to spark ideas that might come in handy when building your own foundry furnace.
I chose an old well tank for my furnace body. I cut my furnace shell out of the middle of the tank using the center tank divider as the bottom of my furnace. Here I have cut a hole just large enough to insert my PVC guide shaft that will help center my chamber form. I have glued a PVC cap on the bottom of the shaft to prevent the shaft from pulling through the hole.
I made my chamber form from a common cement form that I found at Home Depot. These things come in all kinds of sizes, so measure every one and get the one that works for you. In this picture, you can see that I have made a form for the chamber floor that has four channels that will act as drains if I spill a charge of metal in the furnace. With the hole in the bottom of the furnace formed by the guide shaft above, any spilled material should drain out the bottom of the furnace without ruining it.
Here you can get an idea of how the PVC pipe acts as a guide for the chamber form. Once the top and bottom of my chamber form were attached to the concrete tube form, I simply slid the whole works over the PVC pipe until the top frame was resting firmly on the furnace body. Worked great.
Here, the chamber form is fully seated on the furnace body. This leaves plenty of room to fill the cavity with a castable refractory that forms the walls of the furnace.
A look inside the furnace body with the chamber form in place.
To allow the furnace body to be filled completely with refractory, I made small shims that kept the top frame slightly above the furnace body so the refractory could be smoothed to a nice level surface during casting.
Here is the burner tube (tweer) pattern and finished tweer tube. I used a process called parallel line development to make the pattern. This process allows you to mark the tube and cut it so that it perfectly matches the chamber wall.
Here you can see the nicely curved joint surface of the tweer tube.
I cut a slightly oval hole in the rear wall of my furnace body to accept the tweer. The fit on this hole should be very neat as you want to be able to weld it securely.
The fit of the tweer was excellent and you can get an idea of what it should look like when the fit is just right.
Here you can see just how nicely the tweer fits the chamber form. You will notice the tweer doesn't meet the chamber head on. Instead, the tweer meets the chamber tangentially in order to induce swirl and prevent crucible damage that results from the burner flame directly impinging on the crucible.
Once the furnace body was cast, the tweer matched the I.D. of the chamber perfectly. It is definitely worth the effort to get this part right.
I chose to weld the tweer tube into the furnace body.
The lid was made from the top of the well tank. I welded several 1" angle brackets to the inside of the lid to act as wire anchors. This proved to be an excellent choice and made for a very nice, stiff lid structure when completed.
After the wire anchors were welded in and a quick trip through the bead blaster. the lid is ready to be "wired" to add strength to the refractory.
Here, the lid has been wired and the exhaust vent tube form is in place. I used a 3" heavy cardboard shipping roll as my vent form and it worked very well.
The lid was quite heavy and required these adjustable support tabs to keep it stiff and properly oriented to the furnace body.
I made a cam-operated lid lifting mechanism that does away with the need to lift the (very heavy) lid every time you need access to the chamber. I used a section of 1" steel pipe and machined a bung that I welded into one end. I drilled and tapped the bung and used a common carriage bolt as the follower. The cam is a piece of 3" cold rolled steel that is drilled offset and welded to the foot-operated pivot shaft. This setup works beautifully.
Here is a view (although not a very good one) of the cam assembly from the rear of the furnace.
I welded a guide tube to the back of the furnace for the lid lifting assembly. The lift tube didn't quite fit inside the square guide tubing so I slit the square tubing along it's fill length with my cutoff tool. This allowed the tube to spring open slightly and made for a perfect fir to the lift tube. I simply went back and welded the tube back together every few inches and life was good. I left small sections of the cut unwelded so I could spray lube down the shaft to keep things working smoothly. You can see the unwelded sections as small lines down the back of the square guide tube.
Here is a shot of the pivot shaft bushings. The lift lever is welded to the shaft at the left of this image.
These images show how the lid lifting mechanism works. The left photo shows the lid in the closed position with the pedal in the "up" position. The middle photo shows the pedal in the "down" position and the photo on the right shows the lid in the lifted position. You can just see the 3/16" to 1/4" gap that exists when the lid is lifted to it's full height. This mechanism makes moving the lid out of the way a truly effortless operation.
Here is a shot of the wheels I made for the furnace. In true DIY fashion, these are lost foam castings from patterns made on my CNC foam cutter, with material reclaimed form busted engine parts and melted in my homemade electric foundry furnace. Gotta love that!
Here is a shot showing the finished (and used) chamber of my propane furnace. You can see the drain channels below the plinth block and the tweer tube orientation. The drain hole is directly under the plinth block at the center of the chamber.
A closer view of the chamber floor.
Here are some views of my burner mounted in the tweer. I used three thumb
screws to secure it in place and used 1/2' furnace rope as packing around it to
prevent air leakage past the burner into the furnace. The burner uses a
slide choke with a built-in ignition port for easy lighting. So far, this has
been an excellent combination.
A note about refractory...
Perhaps one of the more frustrating things about building a furnace is choosing what refractory you will use to line your furnace. In my opinion, the choice of refractory is the single biggest factor determining the success of a furnace.......period. You will find a ton of recommendations for DIY refractory mixes on the 'net and in many hobby foundry books and almost all tout the inexpensive nature of them. However, I will break with tradition and convention and suggest that mixing your own refractory is excessively laborious and, in my experience, false economy. Commercial castable refractories are exceedingly superior to any homemade refractory mix in every aspect. Commercial refractory is easier to mix, more durable, more thermally efficient and more reliable to work with. I have tried numerous homemade refractory mixes in my foundry furnace iterations over the years and I will NEVER use a homemade refractory again. I have had excellent results using Mizzou Plus castable refractory from Harbison-Walker (AP Green). This product is a fiber reinforced 3000˚ castable refractory that is very easy to mix and it makes for an absolutely beautiful furnace lining that is incredibly durable. It costs roughly $50 per 55 lb bag (out the door) as of this writing and the Hell Boy furnace required 5 bags. It was without a doubt the best money ever spent during the project.