I have a coal stove that works great for melting a lot of aluminum, but it takes a good 45 minutes to start the wood fire and then get enough coal burning for a good melt and is a pain to just do a small pour. Small electric furnaces start at around $250, but i'm a tight-ass so i figured i'd make one.
My $15 walmart space heater bit the dust so i opened it up. The heating coils were still intact, so i connected the coils directly to power, eliminating everything but the tip-over switch so i could turn it on and off. Without having a fan to blow air across them, the coils glowed red.
I have several fire bricks from an old wood stove that i use to braze stuff on and such. I laid two of them flat on a steel table and sat the coils in the middle and then placed 4 bricks on their edge to form a box and put 2 bricks on the top as a lid. Done, almost.
I cut the bottom off of an aerosol can to use as a crucible and put some aluminum scrap in it and plugged it in. After 15 minutes my temp gun was reading about 850 degees on the aluminum. After 25 minutes it still said 850 degrees. Shit. Need more power. v R = ^ I = ^ P = :)
The way the coils were constucted, it made it easy to bypass a few wraps of the coil with a female spade connector. First i tried bypassing about 1/4 of the windings. It lit up a bright red. Then i tried bypassing 1/2 the coil. It lit up quick and bright yellow, headed towards white and the wraps started to sag just as i cut the power. Two more seconds and i'm sure it would have burnt open. So then i tried powering 2/3rd's of the coil. Perfect. A nice bright orange and no sagging wraps.
After counting the wraps i cut off exactly 1/3rd of each coil and put my box back together. About 20-25 minutes later the aluminum was ready to pour. Success.
In it's original configuration, the heater used one coil for the low setting at 750 watts, and both on high at 1500watts. There's no amperage data on the heater but basing things off of 110v, Mr. Watt says this thing should pull about 13.6A on high. With my alterations, i should be around 18A-2000w. Pushing the limit of a 110v circuit on a 20A breaker.
Then i start thinking, if i splice together the two pieces of coil that i cut off and added them to the furnace, i could increase my power by 50%. 3000 watts of kick ass. Oh wait, that would be 27amps. Ehh... What's the worst that could happen? Trip a breaker? That's what they're for, right? So i added the 3rd coil and turned it on.
I was amazed the breaker didn't instantly trip, so then i had a brief moment of concern. I felt the cord and was surprized that it wasn't even warm. I used the cord that came with the heater. Since it wasn't even warm, i wasn't concerned about the 20ft of 12-2 between the outlet and my breaker panel in my garage. Ready to pour in about 15 minutes. Hell yeah.
I wanted to bring my good meter home and actually measure how much it is pulling but i forgot it. At this point i'm guessing that the manufacures claim of 1500 watts may have been over-rated. However tonight on the first cold start it did trip the breaker. Worked fine after resetting it. The resistance of the coils goes up as they heat up, so current draw drops back as they get hotter. Self-regulating to a certain extent.
Now that i've seen how stupid simple it is, i want to build a larger more permanent 220v version. Hoping to get into the 1800 degree range to do silver and brass. I may go with refractory cement in a bucket and suspend the coils from screws that would be screwed into the cement. Or lining a bucket with fire brick and backfill it with sand/paster of paris. You can easily cut fire brick with a wood saw and drill holes in it with a regular drill bit.
Disclaimer: Hot stuff is hot. Electricity can be dangerous. Blah blah blah....
Here you can see the glow of the lower coil that is wired as original and the coil on top is much hotter with about a quarter of the coil being bypassed.
These are the thinner fire bricks. 1.25"? I'm sure thicker bricks and sealing the gaps would make it heat up even faster.