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  • heat treat oven

    I think in the near future I'm going to cast a heat treat oven using one of the formulae for the 'cement'. Probably going to go with perlite/furnace cement mixture, and am considering the method of supplying current to the element from the outside. What I'm thinking to do is cast in some mounting points on the outside for electrical connections, then running the element wire through small holes into the inside. The holes would be cast in place also, and be slightly larger than the wire, but not enough so that air currents would flow to any degree. I'm wondering what other methods might be useful to get the power through to the element from the outside. I don't want to cast in anything that might expand and crack the structure.

    I'm also thinking to cast in a serpentine pattern in the floor to lay the element coils into so that anything you put into the oven will sit above and have little chance of contacting the element directly. I've also wondered about having some kind of slide-out 'floor' that would act as a barrier, and also have a series of 'nodes' or whatever to act as supports for a workpiece. This would allow almost the entire surface of a workpiece to be exposed to the air inside for a more even heating.

    For the outside, I'm thinking of making a cage from steel mesh, which would come together at the top into a bit of a handle. I would use points on the mesh as hinge supports for the door section, which would be a secondary casting also supported in a wire mesh fabrication. My plan so far is to line the inside of the cage with old bedsheet or whatever, then layering in the refractory mixture to build up the structure. There would be an inner mold to form the inside shape and dimensions, which would come out after the refractory has cured. The bedsheet cage lining would allow moisture to dissipate, but keep the refractory in shape as it cures. Because it would form into the pockets between the mesh wires, a system is developed which keeps the mesh nested securely around the fabrication, without being molded into it in any way. This would keep the structure intact while eliminating any differing expansion effects which might tend to crack the structure.

    The casting would be done with the door side to the top- then subsequently the door would be cast over the top. I would probably mold a groove of sorts around the opening so the door casting would mold into the grooves. When the door is shut, it stays in place and seals about as well as can be expected. A latch of some sort would take advantage of the steel mesh wires to fasten to.

    I'm toying with the idea of molding a window area into the door so a piece of fireplace glass can be inserted. I don't know whether this could stand the temperature or not- I'd like to be able to melt aluminum and reach any annealing or heat-treating temperatures. It might be nice to have the window, but not required really.

    Something else that occurs to me- many parts that I might need to anneal or heat-treat would be small, so a small version of oven would be good enough. The smaller, the less power required to reach temperature, and the greater is the possibility of supplying that power through something like a microwave oven transformer. This would allow the element to be made from stainless steel sheet strips, and would be low voltage. It would be much safer because the voltage would be low and the transformer secondary would be completely isolated from line power. Anything larger than a microwave oven transformer would suddenly add an expense that I don't want to incur, so that sets my power limitation to about 600 watts or so. Obviously, this would only be suitable for a small internal volume oven, but maybe the shop would have this small oven, plus a much larger one for melting metals and treating much larger parts. I would be fine with that- run the small one from 110 and wherever you might want to put it, then the larger one might be the outside model.
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  • #2
    Just cast it and then drill holes for stainless steel all thread to pass through. Attach your electrical connection with stainless nuts and washer on both sides. Works very well and is easy to make and easy to repair if/when your element goes bad.
    Amount of experience is in direct proportion to the value of broken equipment.

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    • #3
      I could do that, thanks for the idea.
      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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      • #4
        Almost all of the castable refractory materials drill pretty easily. Don't be snorting the dust.

        I like the ss stud pass through idea as well.

        doug

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        • #5
          I originally thought a tiny hole for the wire to pass through would be good- it lets you make the connection outside the oven and repair it easily. But for a low voltage stainless strip heater, the pass-through would make more sense.

          I hadn't given any thought to how easily the castable material could be drilled, but it makes sense. A threaded rod through a hole isn't going to cause cracking either, since the hole wouldn't likely be that close-fitting, and with washers, etc, the hole would end up being quite well sealed. When the stainless rod expands lengthwise, it would only loosen a bit, not tighten, so that doesn't present a cracking issue either.

          I'm always happy when I can find a material or product of some kind locally that a project might require, so I was happy to see that I could get perlite easily and cheaply. Farm stores have often had things I need when they can't be found at hardware or builder supply stores. I had to laugh- I went to find furnace cement and first stopped at a fireplace center. No such thing sold here- but I did find it at one of the local builder stores. Everywhere else I asked, they did the usual head-scratching thing- what is that?- then one of the guys at Rona (where I didn't expect to find it either) said right off the top 'it's on the shelf in aisle x just past the brackets and above the bubble wrap'

          I have stainless sheet strips, and I have element coils, I have a few microwave oven transformers, stainless threaded rod, nuts and washers- I have some high temp electrical cords taken from heating appliances, and I see that wire rack shelving is priced within reason. If I go with the cage idea, there's nothing stopping this project except to allot the time to do it.
          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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          • #6
            On a Paragon furnace, another that I built and a purchased furnace I also use, the electric elements are doubled over at the end where they go through the insulation. The doubled over ends are tightly twisted to reduce the current in each wire by half. set screw type wire clamps are used outside the furnace to make electrical connections to the heater elements. Doubled over and twisted ends do not glow like the rest of the heater element and do not oxidize and lose connection in the wire clamp.

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            • #7
              Thanks, Jpfalt- I had considered that also. I've done that with my hot wire cutters, and it works.

              I've done a bit of reading up on this whole subject, and am surprised (though maybe I shouldn't be) that carbon has an extremely high melting point, and that carbon rods have been used as heating elements. Most of the references I read say silicon carbide rods- but then these were meant for industrial applications. I recall that we had a carbon rod heater at home in the fifties. Coiled wire would be easier to make and keep connections to, but it was interesting to see carbon rods in use. I think you'd be able to reliably get much higher temperatures that way, since the melting point of nichrome is close to what you'd need to melt steel. Of course you might use tungsten wire in those- and then again maybe tungsten wire is the way to go in the first place, instead of nichrome. As some have suggested, the temperature of the element is going to be higher than the temperature that the oven will reach, so you have to factor that in when deciding what the oven is going to be used for.

              It would seem that, barring industrial production of steel where current is passed directly through the material being melted, electricity is not used near as much as fuel gas for home shop metal melting.
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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              • #8
                If you wind your own elements, use Kanthal (A1) wire. It can withstand 1300 Celsius and is a lot more better than nichrome wire.

                Edit: Though I don't suggest winding your own unless you have lots of time and patience, but instead purchase already coiled elements from eBay. Coiling something like 1.2 mm diameter wire in the lathe gets very boring very quickly, the wire is stiff to wind and if you just happen to look elsewhere, the wire coils up on itself and it is a PITA to unmangle.
                Amount of experience is in direct proportion to the value of broken equipment.

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