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Heat Treating Oven Build Log

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  • Heat Treating Oven Build Log

    Due to several requests for more info, I created this thread to outline my heat treat oven project I finished recently.

    I'd like to say, I didn't do an excellent job on this project. I started it not knowing enough about what I was doing and so I'm going to outline what I learned as I went and what went wrong. PLEASE don't assume I am doing anything the best way possible. Please use common sense and your own judgement and be careful with electricity. Please don't attempt this without help if you are an electrical novice. Get some assistance from someone with more experience.

    This thread will take a little while to complete so bear with me.

    I drew heavily from this design including my brick layout as I found bricks of the same dimension from a local DIY pizza oven supplier.
    http://dcknives.blogspot.ca/p/electric-forge.html?m=1


    I'll start by posting some drawings of my design.

    Sent from my SM-G930W8 using Tapatalk

  • #2
    Additional photos

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    • #3
      The layout was pretty close to minimal brick cutting. I cut the bricks with a mitre saw and mitre box, which ruined the cheap saw. The dust is very abrasive. The bricks are brittle and delicate.

      I built the frame around the bricks and dry fit them all before marking and cutting the grooves for the coil.

      I numbered the bricks so they would go back in place the same when reassembling. They were slightly irregular so this was important. I marked them with sharpie.



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      Last edited by mattthegamer463; 05-10-2018, 02:22 PM.

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      • #4
        I initially tried to use a single length of nichrome wire but it was a quick failure. It melted pretty fast.

        I used bullet hinges and some angle iron to attach the door.



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        • #5
          The latch is just some hacked up bits welded together.

          I played with the top layer of bricks to find a layout that held itself alright with just a dry fit.

          After the first wire failure I realized I'd need more wire to improve the performance. I marked a route for my coil and I disassembled the oven. I used a cheap router bit in my drill press to mill slots in the brick. I used a shop vac to suck away the dust as it went. I used a piece of wood clamped to the drill table as a fence to help guide the blocks as I pushed them by hand.

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          • #6
            I made some brass bushings to attach the heater wire to the copper wires. I wound a coil on my lathe and put it in but it kept melting at the hole where it passed out of the brick. Turns out you need to twist two pieces of heater wire together for that hole otherwise the wire will overheat. Twisting it together will reduce that segments resistance and help it stay cooler.

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            • #7
              Unfortunately due to my foolish confidence, when I did the nichrome coil I glued every brick up with Dap 18854 high temp mortar. I now knew I needed to use Kanthal wire and make it about 2.5x longer to reduce the surface load. I went from 22 AWG nichrome up to 15 AWG kanthal, to improve the surface area by increasing diameter as well as reducing the resistance, thus allowing for more length.

              I didn't want to break it apart and scrape it all out in order to add more grooves, so I made due with what I had. I used abput 19.4 meters of wire on a 3/8" diameter rod to make a coil. The coil stretch is only about 2x which is not enough. It would be better to have at least a 5x stretch. At my local voltage of about 241V I have a surface load around 24.5 watts per square inch. It'd be better if it was more like 15, but I just couldnt do it with only 6 feet of groove in the walls.

              The coil is held in by kanthal staples jammed into holes drilled in the bottom of the grooves. Drill those before you assemble!

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              Last edited by mattthegamer463; 05-11-2018, 09:49 AM.

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              • #8
                The control box is relatively simple. Its a PID controller taking temperature from a stainless steel K type thermocouple that is installed in the top of the oven. The PID output drives two SSR relays which turn on and off the element power. I used two SSRs because I'm running it off split phase 220V. IMPORTANT NOTE: MY SCHEMATIC SHOWS ONLY L2 BEING SWITCHED BY THE POWER SWITCH, BUT IN FACT I DID USE A DOUBLE POLE SWITCH. IT IS IMPORTANT TO USE A DOUBLE POLE SWITCH TO CUT POWER FROM BOTH PHASE LINES IF YOU ARE USING 220V SPLIT PHASE AS FOUND IN NORTH AMERICA

                The timer module output is a switch which just connects or disconnects the PID from the SSR controls. I added a toggle switch to bypass the timer if desired.

                The same SSR control signal also runs through the door safety switch, which is a micro switch mounted on the door. I hope it won't melt in that spot but time will tell. This switch prevents you from killing yourself if you open the door and touch the wall coils with metal tongs while pulling out your part.

                The power meter uses a small coil to indirectly monitor the current through one of the wires that run to the element. It monitors the line voltage that the module itself runs off, and computes the instantaneous wattage and the kWh consumed since it was last reset. The kWh reset button is a small button to the right of the display that you press and hold with a paperclip. A neat feature to see what your oven is costing you to run.

                The lamps just let me see whats going on at a glance. One tells me if the oven is plugged in, one says if the door is open, one lights up when the element is on, one lights when the timer unit is finished.

                I used a few inches of bare solid copper wire to run from the brass terminals to the stranded copper wire, as the stranded wire insulation was melting. I made a shield out of some peforated aluminum sheet so it can't be touched but the heat can still escape. The plastic cable gland I used may melt so I may need to upgrade to metal later.

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                Last edited by mattthegamer463; 05-11-2018, 10:31 AM.

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                • #9
                  Machining the electrical box relay opening so the relays can mount on an external heatsink. The door switch has 220v on it for the 220v door open lamp so I made a shroud for it.

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                  • #10
                    I gave the outside a coat of paint and printed some labels for the control unit. Its held to a piece of angle iron thats welded to the oven side, with minimal contact to minimize heat conduction.

                    The outside of the oven gets to about 200 deg C in the middle of the top panel when the inside is approaching 1000 C. The paint may not last and might need recoating with something higher temp.

                    All done for now! Until it breaks down and needs repairs.




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                    Last edited by mattthegamer463; 05-10-2018, 03:06 PM.

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                    • #11
                      Love it. The nicrome wire info is gold. Most of this you need to fish out as you get on, and is not easily searched.
                      12x16" Delta 3d printer (Built from scratch)
                      Logan 825 - work in progress
                      My Blog - http://engineerd3d.ddns.net/
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                      Instagram - https://www.instagram.com/engineerd3d/?hl=en

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                      • #12
                        Wow, thanks for the great writeup. Lots to think about here, so I'm going to save it.

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                        • #13
                          Good job on the build and write up but ya know you can buy one new for $1000

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                          • #14
                            Originally posted by chipmaker4130
                            I get the idea some of these posts include photos, but I'm not seeing any...
                            They're posted through tapatalk, which usually seems to work. Not sure what the issue is but I think it may be on your end.

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                            • #15
                              well done Matt.....lots of good clear photos, good design......you should send it in to George
                              .

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