My electric forge build -- I've got pre-build questions

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zhyla

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Warning: several smart members have safety concerns about this oven. Do not blindly follow my design, it might get you killed!

I've got most of the parts to build something similar to D. Corneau's electric forge for the purposes of heat treating my blades. Er, well, my blade, since that's all I've made so far. I've got a few questions...

1. I don't have the means to encase the brick assembly in the metal case that Mr. Corneau built. My plan is to route all the wiring thru the ceiling and have it encased in an aluminum project box I have laying around which will house the PID controller, relay, switches, and indicators. I have no experience with firebrick, how hot will the outside of this thing get when the inside is 1800 F? If, for instance, I encased it in plywood, would it catch fire? I think a crack where heat escapes could make that a fire hazard. Better materials that don't involve welding?

2. The hinged door sure is nice:

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But I see a lot of gas forges just have a hole or an open mouth instead of a door. Obviously they need oxygen to do their job. If I built a removable face out of firebrick with a, say, 6" hole in it, would that work? I'm not sure if all my heat will be escaping or what. I'm not planning on getting into air-quenched steels any time soon, but if I did my hazy understanding is you can heat them up and then turn the forge off and let them cool down slowly overnight. Would a hole hamper that?

3. The firebrick I have is the insulating (soft) type. Do I want any kind of non-insulating thermal mass inside the forge to give it some temperature stability? I've got some hard (non-insulating) firebricks lying around, I could just set a few of those inside.

4. What do I want to rest my blade on while it's being heated? Does it need to be aligned so it heats evenly from all sides or can I just toss it in there laying on the brick and not worry about it?

5. I have some portland cement, silica sand, and fire clay left over from an embarrassing attempt at casting the forge as a single piece. I think the right mix of that will make a mortar suitable for bonding the firebrick together (portland cement to hold it together until the fire clay cures at high temperature). Advice?

6. Mr. Corneau did something interesting with the thermocouple, he put it in a tube of some sort and ran it up in the middle of the chamber. Does placement matter much? Does it need to be protected?

I'm sure I'll have more questions but these should get me going. Thanks guys! And if you're wondering why I didn't just do a blowtorch + coffee can forge, well, so am I, but fire sort of scares me :)
 
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Forges are gas or coal, electric is a kiln/oven. You'd need 240V at about 125A to get the 100k BTU equivalent of a medium size forge.

Ovens are usually as close to airtight as possible.

Now, we need to ask if you are building a forge or an oven?
 
zaph1 said:
Forges are gas or coal, electric is a kiln/oven. You'd need 240V at about 125A to get the 100k BTU equivalent of a medium size forge.

Ovens are usually as close to airtight as possible.

Now, we need to ask if you are building a forge or an oven?
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Well, I guess I'm building an oven? Thanks for clearing up my terminology. My google searches are turning up a lot more examples than previously, so that alone will probably get me where I need to go.
 
I found an example using expanded metal for the outside. That would be considerably easier for me to deal with.
 
Try talking to an HVAC company. They bend sheet metal all the time. Take them your dimensions and it'll probably be pretty cheap for the box.
 
The casing is really just there to hold the bricks in place. I have built several ovens just using welded angle-iron frames. They work. They are ugly. I get the impression most knifemakers have a strong aversion to ugly, but if you can get past that, fancy sheet metal is not essential.



I built the first one and wanted to find out what the maximum surface temperature would be so that I could choose a suitable skin material (I could get plastic-coated steel sheet for free). When it worked without a skin, laziness kicked in.

The maximum surface temperature I measured was 135 degC (275 degF) after a 2-hour soak at 1177 degC (2150 degF).
 
1/ see above.

2/ with an oven, you need to have something to close the hole. Whilst a hinge is not absolutely necessary, it does mean that you know exactly where the 2000-degree face, of whatever you use to block the hole, is when you are occupied trying to deal with hot steel against the (pearlite nose) clock.

3/ Soft firebrick is good.

4/ It's really up to you. I use a roughly-bent-up rack made from stainless TIG rod. Some guys have veritable works of art. It's nice to be able to shield the blade(s) from directly seeing the elements, particularly if you are tempering in the oven.

5/ I use dry joints. Again, it's probably lazyness and impatience. It's hard to get a thin joint because the soft brick is so porous it sucks all the water out of the jointing compound. If you are going to use wet joints, it's best to use something intended for the purpose. If you really must go for a DIY compound, I'd suggest just very wet clay.

6/ The most important things are that the thermocouple be placed so that it sees the same temperature as the workpiece and that it heats up at least as fast as the workpiece. It's worth pointing out that controlling tightly at tempering temperatures is harder than controlling tightly at Austenitizing temperatures.
 
Awesome thanks for the replies guys. I think I'm going to attempt using some galvanized sheet metal to make a frame and go for the hinge. If Tim's datapoint of 275 F on the outside holds true then I can solder the sheet metal together with my the torch I use for plumbing stuff. Roofing flashing is cheap, galvanized (so I can solder instead of braze), and comes in convenient shapes.

I have zero aversion to ugly stuff. I just want it to work and not burn my house down or my face off. :)
 
Finally got this thing all wired up and started testing it out. I've only taken it up to 200 F so far. No issues but I had soaked the firebrick before I mortared it together and thought it was a good idea to vent all the water out at a lower temp before going for it.

3x 40 ohm Kanthal A-1 coils in parallel at 110 V = ~12 A draw. Heats up to 200 F pretty quick, I didn't time it because I was busy watching for something to go wrong. My only concern so far is the top of the oven gets hotter than the sides. Possibly because all the steam was collecting up in there.

I tried soldering sheet metal, it's a PITA and takes too long. I used pop rivets. Wish I knew about these sooner, such a great DIY tool. This is all roofing flashing from Home Depot.

The mortar is a 10:2:3 sand/fire clay/portland cement, or something like that. It doesn't really matter much (I think) since the metal cage will keep it together.

Thanks for all the advice... if it breaks down at high temps I'll let you guys know :).

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shane_o_ca said:
Nice work. You should post over in the shoptalk section. Not many people visit this subforum.
Click to expand...

Shane,
I am not calling you out specifically, but using your post to show where being a nice guy and saying "good job" could get someone killed.

The oven build is not "Nice Work". It is very dangerously made and wired. See the thread in Shop Talk for more about why.

When a new maker makes a knife that is so-so, "nice work" is OK to spur him on. When he builds a piece of equipment that has major dangers, it is not a good forum thing to compliment that. If you are not experienced in the things that go into an oven build, don't comment at all.
 
shane_o_ca said:
Nice work. You should post over in the shoptalk section. Not many people visit this subforum.
Click to expand...

Shane,
I am not calling you out specifically, but using your post to show where being a nice guy and saying "good job" could get someone killed.

The oven build is not "Nice Work". It is very dangerously made and wired. See the thread in Shop Talk for more about why.

When a new maker makes a knife that is so-so, "nice work" is OK to spur him on. When he builds a piece of equipment that has major dangers, it is not a good forum thing to compliment that. If you are not experienced in the things that go into an oven build, don't comment at all.
 
I've added a big warning at the top of this thread to warn people there are safety concerns. In the Shop Talk thread hopefully we can work thru the issues so I can make this safe. Thanks.
 
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