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Step 2Build Furnace
Building the furnace is pretty straight forward. Mine is very closely related to this furnace, the only difference being that mine is smaller.
http://www.instructables.com/id/Ultimate-Guide-to-Making-a-Cheap-but-Effective-Alu/
The theory behind the furnace is basically just to trap the heat from the blowtorch so that you can heat up a crucible really hot. We do this by filling a paint can with concrete but leaving a cylindrical hollowed out portion in the center. By placing your metal in a crucible sealing it in the furnace we can obtain temperatures much higher than by just heating the crucible with a blowtorch. The furnace also gives a nice even heat.
Im not going to go into great detail on the construction of the furnace because the other guide does
a fantastic job of it, however i will give a brief overview of construction and a few things that are critical to success.
1. Cut 1/2" pipe down to length. This is the channel the blowtorch goes into. Its ok it its a little bit too long, just make sure that the air holes on the blowtorch aren't blocked, otherwise you will just get a very cold, useless yellow flame. one end should be cut with about a 30 degree angle so that it lies flush with the wall of the cylinder
2. Cut a hole in the can so that your 1/2" pipe will press fit into it. Its important that the pipe enters into the very edge of your hollowed out cylinder, thereby creating a heat spiral in the chamber, which helps prevent hot spots. (The other guide describes this much better than myself)
3. Somehow suspend your 1" pipe ~3/4" centered above the base of the paint can. I used a tent stake taped to the edge of the can. You can see that I wrapped my 1" diameter pipe in cardboard and covered the ends with masking tape, this was so that concrete wouldnt get into it, and to make it easier to remove once the concrete had cured.
*important* make sure that concrete cannot get into your small channel for the blowtorch and that after pouring the concrete that the pipe goes into the hollowed out champer (not blocked by concrete) the way i did this was to cut a circle in the cardboard surrounding my pipe which the smaller pipe fit into.
4. Let it cure for at least 2 hours then put it in the oven for an hour. I removed the cylinder after an hour then put it back in for 3 more.
*important* make sure that you dont use rubber tapes to secure the inner cylinder because they will melt all over your oven and smoke and possibly catch fire. I used masking tape which browned slightly but never caught fire. I think this was because of the prodigious amounts of moisture coming off the concrete so pay VERY close attention when you are baking it. If i were to do it again i would use wire to secure everything instead of tape. The masking tape and cardboard are pretty much safe because they are sorrounded by concrete and unable to burn.
5. Immediately test fire it for 30 minutes after taking it out of the oven. Since the concrete is already hot this will help prevent stress fractures from the rapid heating of the blowtorch. This fully cures it and i used mine immediately after this step. Be warned that it could fracture any time shooting out tiny thousand degree particles of concrete so eye protection is REQUIRED.
6. Build the crucible. My crucible was pretty simple, i just drilled two 1/16" holes on opposite sides of the 3/4" copper pipe caps and bent the wire into a sort of hook shape which went into those holes, this allows the crucible to rotate when you do your 'pour'. I put a 90 degree bend in the wire crucible holder so that the brick could sit atop it.
7. Melt down stuff in your furnace just for fun.
http://www.instructables.com/id/Ultimate-Guide-to-Making-a-Cheap-but-Effective-Alu/
The theory behind the furnace is basically just to trap the heat from the blowtorch so that you can heat up a crucible really hot. We do this by filling a paint can with concrete but leaving a cylindrical hollowed out portion in the center. By placing your metal in a crucible sealing it in the furnace we can obtain temperatures much higher than by just heating the crucible with a blowtorch. The furnace also gives a nice even heat.
Im not going to go into great detail on the construction of the furnace because the other guide does
a fantastic job of it, however i will give a brief overview of construction and a few things that are critical to success.
1. Cut 1/2" pipe down to length. This is the channel the blowtorch goes into. Its ok it its a little bit too long, just make sure that the air holes on the blowtorch aren't blocked, otherwise you will just get a very cold, useless yellow flame. one end should be cut with about a 30 degree angle so that it lies flush with the wall of the cylinder
2. Cut a hole in the can so that your 1/2" pipe will press fit into it. Its important that the pipe enters into the very edge of your hollowed out cylinder, thereby creating a heat spiral in the chamber, which helps prevent hot spots. (The other guide describes this much better than myself)
3. Somehow suspend your 1" pipe ~3/4" centered above the base of the paint can. I used a tent stake taped to the edge of the can. You can see that I wrapped my 1" diameter pipe in cardboard and covered the ends with masking tape, this was so that concrete wouldnt get into it, and to make it easier to remove once the concrete had cured.
*important* make sure that concrete cannot get into your small channel for the blowtorch and that after pouring the concrete that the pipe goes into the hollowed out champer (not blocked by concrete) the way i did this was to cut a circle in the cardboard surrounding my pipe which the smaller pipe fit into.
4. Let it cure for at least 2 hours then put it in the oven for an hour. I removed the cylinder after an hour then put it back in for 3 more.
*important* make sure that you dont use rubber tapes to secure the inner cylinder because they will melt all over your oven and smoke and possibly catch fire. I used masking tape which browned slightly but never caught fire. I think this was because of the prodigious amounts of moisture coming off the concrete so pay VERY close attention when you are baking it. If i were to do it again i would use wire to secure everything instead of tape. The masking tape and cardboard are pretty much safe because they are sorrounded by concrete and unable to burn.
5. Immediately test fire it for 30 minutes after taking it out of the oven. Since the concrete is already hot this will help prevent stress fractures from the rapid heating of the blowtorch. This fully cures it and i used mine immediately after this step. Be warned that it could fracture any time shooting out tiny thousand degree particles of concrete so eye protection is REQUIRED.
6. Build the crucible. My crucible was pretty simple, i just drilled two 1/16" holes on opposite sides of the 3/4" copper pipe caps and bent the wire into a sort of hook shape which went into those holes, this allows the crucible to rotate when you do your 'pour'. I put a 90 degree bend in the wire crucible holder so that the brick could sit atop it.
7. Melt down stuff in your furnace just for fun.
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In your materials you list a 1"x7" pipe. I assume that this is the pipe that goes inside the quart can to make it hollow.
But my quart cans are only 5" deep. Are you using can with another dimension?
And a 1" diameter hole seems way too small. From your last picture on Step 2, if that small copper cap is 3/4" in diameter, I'd estimate that your inner hollow is at least 2" in diameter. Would you say that sounds right?
Thanks!
(I'm setting this all up in my workshop right now, and keep getting confused by the various dimensions listed)
Also, I'm a little confused by the direction the bottom pipe is supposed to go in. In your pictures on step two, it looks like the pipe (and the blowtorch's flame) go straight towards the center of the furnace.
The pipe is supposed to be approximately tangent to the wall of the inner part of the furnace. The idea is to cause the heat to convect around the crucible rather than heat up a spot on the side of the crucible thats touching the blowtorch. My implementation of it sort of failed (e.g. I didn't get the pipe orented properly) and as a result I burned a hole through several crucibles because of the intense heating in one spot. Basically follow what I wrote and tried to explain, not what my picture showed ;)