Activities on these pages about foundry work are very dangerous. The chances of having a fire, causing injuries, or even dying are very good if you are not careful. I am not a professional foundryman, and will not attempt to disclose all hazards. If you choose to try this hobby, every precaution must be taken to be safe. Remember, a hobby is only fun until you're standing in the middle of the street with your family watching your house burn.
So far I've done all of my casting using my charcoal furnace. The problem with charcoal is that it's like raising children: It really smells bad in the beginning, and gets rank occasionally after that. That's why I decided to change to propane. I decided to go with propane instead of natural gas because I didn't want to dig up my backyard. The furnace pictured above is another one of Dave Gingery's creations.
Here are the forms that make up the body of the furnace. The one on the left is for the middle section. It has bolts and a reinforcing plate welded inside, with the bolts extending through the metal. The bolts are attached to lifting arms on the frame so that the section can be raised during use. The middle form is the bottom section, which has a plate welded to the bottom. The plate has a hole in it in case there is a spill inside the furnace. This form also has an oval hole layed out in one side. The hole is where the mixing tube from the blower enters the furnace. It also has small angles welded at the bottom to attach it to the frame. The form on the right is the lid. The wires inside help to support the refractory lining. It also has a handle on it. When finished, the lid can be raised using a foot pedal on the frame, and the handle is used to swing the lid out of the way to allow access to the crucible. The four forms in the front are used to make plinths. A plinth is basically a block for the crucible to sit on. The plinth has two semi-circular grooves in the bottom to allow any spilled metal to flow out of the hole in the bottom of the furnace.
Before the refractory lining can be installed, an inner form must be put in place. The inner form is just a piece of light gage sheet metal that is taped together. The tape allows the form to be removed after the lining has had time to set up. I made some channels at work that had a width of 2-1/2" to match the lining thickness. These channels are put in, and the outside form is put around the outside to keep the section round.
I used a refractory lining called Plicast LWI-24 to fill the forms. It tends to be a little runny. Going from left to right:
*The middle section has an 8" diameter hole through the entire height.
*The bottom form has a 2-1/2" thick layer of refractory in the bottom, with a 2" diameter hole in the center. The rest of the height has an 8" diameter hole to match the middle section, except for the hole for the mixing tube.
*And the lid has a 2-1/2" diameter hole for a vent.
Here is a blurry picture of the furnace parts after the inner forms were removed. It also shows that the mess was cleaned up . . . well, it pretty much shows that.
This is what a plinth looks like. It's upside-down now, but this view shows the grooves in the bottom.
This is the blower that delivers the air and gas mixture to the furnace. The blower is an 8" diameter centrifugal fan. It has a box on the front with a small plate to regulate the amount of air delivered. It is driven by a 110V 1725RPM 1/4HP motor. The motor pulley is a 4" diameter, and the impeller shaft has a 2" diameter pulley, which gives the fan a speed of 3450RPM. The piping in the picture is for the propane, of course. The gold colored part on the left is a solenoid valve that is normally closed. This means that it is closed until electricity is supplied to it. The solenoid valve is a safety measure, so the gas flow will be shut off in case of an electricity outage. It is wired with the motor so that they will both work off of one switch. The valve on the right is a regular manual valve that is used to regulate the gas flow. The piping then goes into the tube on the right, the mixing tube. This is where the gas and air are mixed and delivered to the furnace.
This picture shows the furnace running to cure the refractory lining. This step is necessary to dry out the lining completely. You can't tell but the four plinths I made are down in there too.
Using the Furnace
Everything is in place and ready to go. I wanted to try the furnace a couple of times to see how it was going to work. My goal was to make some ingots. The crucible is full of scrap and on the plinth. The middle section is raised, with the lid also, to allow access to the inside of the furnace. The white substance between the mixing tube and the furnace is called Kaowool. This stuff is amazing. It has a consistency like cotton candy that's been smashed a little. It can get red-hot and not be consumed.
Now I'm lighting the furnace. Using a small propane torch is easier than a piece of burning cloth. The blower is running and the solenoid valve is open. I'm opening the manual valve with my left hand. After the furnace lights I lower the middle section and adjust the gas and air flows.
Here I am adding aluminum to the crucible. New material should never be added to the crucible without being preheated to evaporate any moisture that may be in the scrap. What I do is to place the scrap in the vent hole for a short period before putting it in the crucible.
Ain't that cool? Or should I say "Ain't that hot?" This is about fifteen minutes after I started the melt. The aluminum is somewhere around 1500°F, which is superheated well enough. I have to judge the temperatures by the color of my crucible since I don't have a pyrometer to measure them.
This is a picture of me setting the crucible down on some bricks in the sand. The molten aluminum has a bunch of stuff called dross floating on it. Dross is the paint, dirt, burnt bug parts, etc. that is on the metal. Since this stuff has a lower density than the aluminum it floats. I skim the dross off and place it in a pan behind the bricks. After skimming the dross the metal is ready to pour.
Here I'm pouring the aluminum into my ingot mold. And that completes the tour.