How I am making an electric furnace

Discussion in 'Furnaces and their construction' started by Scott K., Sep 17, 2017.

  1. Scott K.

    Scott K. Copper

    So I recently posted pics of my electric forge (the one that started life as a bread maker, and now serves a much more important purpose), and I decided to do a 'how to'. But since that's already done, I don't have many build pics, and someone wanting to copy that may not use the same base, a step by step how to article may not work out very well.

    So, since I still need to build the burnout oven, I figured I could do a guide on building electric ovens in general. I will take plenty of pics along the way, and will discuss both the burnout oven and forge build. This will be a guide on building, and not a step by step. Simply because you may choose to do things differently, or throw in your own ideas. I basically just want to share what I believe are the important aspects of building the oven.

    So what makes me the authority on oven building? Nothing, really. But I did not see much online that I liked, and I like to think I'm fairly good and building stuf. I made some mistakes, and learned a lot. So I'm just sharing my experience with you. And it works.

    So I'll start off with the needed tools and parts. My next posting should be starting the build. Be patient with me. I'm a little slow with getting stuff done.

    Tools. Basic hand tools. A welder would be good, but if you don't have one, you could get away with rivets or screws. Or better yet, go buy a welder. Mine was a cheap lowes wire feed unit, and is fine for light work. Or at least make friends with someone who welds (if you don't mind being 'that guy'). You should also have a dremel tool, hand drill, rivet gun, and some way to cut sheet steel (bandsaw, jigsaw, angle grinder, something).

    As far as parts go; you need a box to become the oven. I used a bread maker for the forge, and will be using a microwave for the burn out oven. You could possibly also use a steel bucket, mini fridge?, make your own.
    You will also need a heating element, solid state relay (SSR), PID temp controller, cooling fan, ceramic wool or fire bricks, wire, switches, fuses and fuse holders, ceramic element holders/beads, ceramic wire feed throughs, electrical terminal ends and bus block, steel sheet, various hardware, and a bunch more stuff I'm forgetting. We'll figure it out as we go.

    So here are some pics of the gutted microwave and most of the other parts. If you are repurposing an old appliance, then the first step is to gut it. Basically, just remove all the screws until it falls apart. Save anything that looks useful (you can make a spot welder from the microwave transformer), and just get it down to an empty box. Like I said, in my next post, I'll be installing stuff into this empty box. image.jpg image.jpg
     
    Last edited: Sep 17, 2017
    Tobho Mott and DavidF like this.
  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Post away.....you've got plenty of company in that regard here.

    Best,
    Kelly
     
  3. Tobho Mott

    Tobho Mott Gold Banner Member

    Looking forward to the rest!

    Jeff
     
  4. Negativ3

    Negativ3 Silver

    Nice, exactly what I'm doing.
     
  5. PatJ

    PatJ Silver

    This is an interesting concept.
    I would like to see how it turns out.
     
  6. DavidF

    DavidF Administrator Staff Member Banner Member

    got me wanting to wind some coils :)
     
  7. Scott K.

    Scott K. Copper

    Me too:rolleyes:
     
  8. Scott K.

    Scott K. Copper

    So I didn't get much done today, but I won't get any time with it until Friday, so I'll go ahead and do a short post now. I'm basically working a double shift Thursday, so I get Friday off. Which means I'll get 8 uninterrupted hours of build time. Which if goes anything like last time, that's 4 hours building, 2 hour nap, and a movie. Then the kids come home...

    I'll take this time to discuss one major problem I had with the forge, how I fixed that, and how I plan to avoid that problem with this oven. The problem was that the area that contained the electrics was not propperly protected from the heat. The heat box was wrapped with about an inch of fiberglass insulation, and then two layers of solder shield/barrier stuff that lowes sells to protect your wall when sweating pipes near the wall. When I did a test fire, it heated up to 1300 quite nicely. So I bumped it up to 1500. Well apparently, the SSR overheated and failed,,,on. Before I new it, the box exceeded 2000 and was smoking quite badly. The fiberglass insulation completely melted. And that was outside the heatbox. This is why I cut that forge in half and basically made two separate boxes. One is the heater, the other is the electronics. So now there is two inches of ceramic wool around the heatbox, and a four inch air gap between that and the electrics. Much better now.
    image.jpg image.jpg image.jpg image.jpg image.jpg image.jpg

    For the burnout oven, I'm going to try to build in the air gap. So today, I made two steel walls to provide the air gap, and provide a good place to install ceramic wool.

    The two walls were cut from sheet steel, 22 or 26ga, and the sides bent to make flanges to screw into the oven. I cut the steel on my bandsaw and filed the edges smooth. Now I never claimed to be a good sheet metal man. But maybe if I had the correct tools, like a shear and a brake, then the finished product just might look a tiny bit better. But to bend the edges, I placed a piece of angle iron on the edge of a board, laid the sheet steel on that lining the bend line up with the angle iron, placed a board on top of that, then clamped it together. Then proceeded to beat the living dayligh...........gently tapped it into shape with a hammer. Not pretty, but it works. They test fit just fine. Now I can really see how much room I have to install the electrics. So as far as blocking the heat from the coils to the electrics, there will be 1/4in wool, steel wall, 2in wool, steel wall, 2in air gap, 1/4in wool, steel wall, then the electric parts. Should be good.
    image.jpg image.jpg image.jpg image.jpg image.jpg image.jpg image.jpg image.jpg image.jpg
     
    Last edited: Sep 18, 2017
  9. PatJ

    PatJ Silver

    I wonder if my wife would notice if some of her seldom-used appliances vanished?
    I need to try some of this stuff.
     
  10. DavidF

    DavidF Administrator Staff Member Banner Member

    I managed to get my wifes mixer for about 3 months before she noticed..
     
  11. Scott K.

    Scott K. Copper

    Just don't tell her it was my idea. I don't need more than one wife mad at me.
     
  12. Jason

    Jason Gold

    Man you move fast. Ceramic wool is the catsazz. I always have a chunk of it close at hand. I use it when soldering copper pipe. Nothing worse than seeing someones brand new equipment all black and torched to hell. Are you going to line the walls of that microwave with kaowool too like the bread maker?
     
  13. Scott K.

    Scott K. Copper

    The bread maker is lined with ceramic paper 'glued' in with water glass. The problem with ceramic paper is that once the starch is burned out, the ceramic easily flakes away if touched. I used it because it was thin and cheap, but I didn't know how fragile it would be. I'll line the microwave with 1/4 in wool, and eventually I'll reline the bread maker. The primary purpose of the liner is to electricity insulate the coils from the steel walls. they don't touch, but with heat expansion, I didn't want to chance it.
     
  14. Scott K.

    Scott K. Copper

    Well I didn't get as much done today as I had hoped, and I found a big problem with my bright idea of using a microwave enclosure. So here is my progress and problem.

    First off, I needed to get the element power wire feed throughs installed. I got these on eBay for about $6. They come with 6-32 hardware, but I used 8-32. The ones I got for the forge took the bigger screw ok, but these I had to bore out a tiny bit with a dremel tool and a diamond bit (harbor freight diamond).

    I needed to add a piece of steel to the area where I wanted to put the feed throughs. For one reason, the microwave wall had a bunch if holes in it I didn't like. But also, the ceramic parts have a little lip that go through the sheet metal hole to prevent the unit from sliding which would allow the screw to contact the edge of the hole. The microwave wall is too thin, so the two ceramic parts would hit each other causing the feed through to be loose. I hope this makes sense. The ceramic parts of the feed throughs are held in by a length of stainless 8-32 stainless threaded rod, with a nut on each side. I don't know how much torque these can handle, so take it easy. The side that takes the power wire got double nutted.

    image.jpg image.jpg image.jpg image.jpg image.jpg image.jpg

    With the steel sheet and feed through installed, I drilled all the holes needed to wire in the ceramic element holders. Then I laid in a piece of 1/4in ceramic wool. Next I laid the element in place and attached one end to the feed through.
    The attachment is large fender washer, heating element, large fender washer, and as always, double nutted for safety.
    You will notice that washers will have one side with a smooth edge, and one side with a sharp edge. Put the smooth edge against the element. Next comes the tedious task of wiring the ceramic bushings in place. I used .041in stainless safety wire. Work your way around until you get to the other end.

    image.jpg image.jpg image.jpg image.jpg image.jpg

    I then I installed the thermistor and heating element power wires. I used high temp oven wire I got from eBay.
    All that was left was to install the ceramic wool and previously made steel walls. The blue tubes I'm running the wires in are high temp fiberglass silicone coated wire protector tubes I got from eBay. It just seemed like the right thing to do.

    image.jpg image.jpg image.jpg image.jpg image.jpg image.jpg

    So now I'm ready for the rest of the electronics. However, my pid controller doesn't work, so I'm waiting for the replacement. It's coming from China, so it shouldn't take too long. But while I'm waiting, I plan on installing the other parts tomorrow.

    So, now, the problem. My thought process about using the microwave for the enclosure was flawed. I thought using something that already had a box in a box would save me a ton of time. However, this is not a box in a box. The back wall is the same for the heat box and the enclosure. Meaning, there is nowhere to put the wool. Meaning, the back of the oven will get very hot. Meaning, I have to fix it. My options are to either build an internal wall two inches in, or build an external wall two inches out. I don't want to lose internal space, so I guess I'm going to build a new back wall. I bought the steel sheet for it, but I'll build that part probably last.

    Hopefully my next post will have all of the electrical done, except for the controller.
     
  15. Negativ3

    Negativ3 Silver

    Scott, that looks to be coming together nicely. It's given me the inspiration to make my burnout furnace of two bent, sheet steel boxes, lined with kaowool.

    Not sure what I'd do about the door. may need a skeleton of angle iron or something like that...

    Keep the updates coming please.
     
  16. Scott K.

    Scott K. Copper

    Thanks. Yea, IF I build another, I think I would just buy a sheet metal brake and just do it from scratch. It would cost more though, and take longer to get done, but you would have more control over the exact size and shape.

    But, the door is pretty much figured out. Except for the latch.

    Make sure to posts pics when you build yours.
     
  17. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Scott, just a couple observations if I'm following your build properly.

    First, I like the approach of ceramic wool lining and hanging the elements in the ceramic isolators. It's very practical and allows for a lot of design flexibility and allows one to build a burn out oven to accommodate about any space. Repurposing anything suitable to aid construction is also great but as you have already surmised, packaging electronics adjacent to the oven is probably something you want to rethink on future builds.

    If you have only allowed for 1/4" of ceramic wool to insulate and thermally isolate the oven space, that is not nearly enough. I would say 1" minimum and better 2". Even with additional wool on the wall between the two the metallic enclosures, and especially if you are considering adding wool to the outside of the metallic oven enclosure, the metallic enclosure will still conduct heat and this will be a further aggravation to the electrical items in the adjacent enclosure.

    The only communication required between the controls and oven are power and temperature sensing, and perhaps a safety interlock of some kind so cabling to the oven is easily managed. Given where you are in the build you might want to consider a good sized cooling fan to move air through the control space. If the oven and controls are separated, the only price paid for the amount of insulation is the limit in max temperature achievable.

    On the door, I've used the ceramic paper as gasket with good result. I view it as expendable material. There is wool rope available with fiberglass or ceramic sleeving if you want a more durable door seal. -Many sources on eBay and web for sale by the foot.

    Best,
    Kelly
     
  18. Scott K.

    Scott K. Copper

    Thanks for the input. The 1/4 in wool is mainly to electrically insulate the coils from the wall. I do plan on having an additional 2in all around the hot box. There is going to be a rather large cooling fan for the electronic enclosure. As far as the heat migrating in the metal enclosure, I'm planning on adding heat-brakes between the hot side and elelectronics side. Basically thin slits or cuts in the walls to stop the heat migration.

    There will be 2 1/2in ceramic wool, 3in air space, and 3 steel walls separating the coils from the electrics. That should be enough. I hope.

    And I got some fiberglass oven door sealing rope from my local wood stove shop for the door seal.

    I do appreciate and listen to any feedback or advice, in case I missed or overlooked something. I'm pretty sure this one is ok heat/insulation wise. If it fails you're more than welcome to say 'I told you so'.

    Thanks again,
     
  19. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Sounds like you may get there just fine with the electrical components with the plan above.

    The following sort of depends on the operating temperature of your burn out oven but if your thermal circuit from inside to out of the oven, is 1/4" wool-sheet metal-and then a couple more inches of wool, you will be placing the carbon steel enclosure in a region requiring it to live at high temperature. At >1400F (actually below this temp too but more slowly), you'll of course lose the paint after the first fire but the sheet metal will slowly degrade beyond that with time.....at these temps the carbon precipitates out and the steel oxidizes more rapidly.....much more rapidly as you go beyond that temp.

    Best,
    Kelly
     
  20. Scott K.

    Scott K. Copper

    Yea that's a good point. One concern I had with the bread maker build was how long the hotbox walls would last. I'm kind of stuck with that for the moment. I just planned keeping an eye on it, and when/if the steel failed, rebuild it with stainless sheet. To be honest, when I first started the bread maker forge, I assumed the box to be stainless. But it's not.

    I also origally wanted at least an inch of wool between the coils and wall, but I didn't know the best way to install the coils. If I wired them like I did now, it would pull the ceramic bushings into, while crushing the wool. Maybe this would be ok? I don't know, it didn't seem right. Or if I wired it loose so it wouldn't crush into the wool, then the whole thing would have been dangly loose. Again, that may have been ok? With the 1/4in wool, I could wire it tight enough to not be loose, but not so tight to crush into the wool too much.

    There has to be a correct way to hang the coils on thick wool that I don't know about. I searched online quite a bit prior to starting the build, but couldn't find anything.

    I have just accepted the fact that one of these days I will have to rebuild the hot box portion out of stainless.

    So if any body else is copying my ideas here, just keep that in mind. This may not have been the best idea I ever had. But I will continue the build, and call this an experiment to find out how well it works, how long it lasts, and if it is a good method of building an oven.
     

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