Heat Treat Oven
- Firebrick Box
- Metal Case
- Electrical Wiring
- Electronics Box and Final Assembly
- PID Controller Configuration
- First Heat Treatment
- Update 11/19/2014 -- Still Working
This documents my heat treat oven. It's not meaningfully different from several other DIY ovens on the net but I wanted to provide something simple that could be replicated easily and inexpensively.
WARNING AND DISCLAIMER: The voltages present in this project can kill you and the high temperatures it can generate can burn your house down. If you don't have basic electrical knowledge and common sense you should do something safer.
FURTHER WARNING: This project isn't done and hasn't been fully tested yet. Some people hold the opinion that it is too dangerous (no door safety switch, questionable wiring of the coils, etc). Approach this writeup with skepticism.
|Internal chamber||4.5" x 4.5" x 15"|
|Maximum temperature||1000 C (1832 F)||The thermocouple is the limiting factor here.|
My oven closely resembles D. Corneau's well-document oven. You should read thru that. Mine differs in a few ways:
- It does not require welding to assemble. (good)
- It lacks the door switch safety. (bad)
- Instead of a thermal cutoff safety in the control box I used a fan to keep the electronics cool.
I expect I'll add a door switch at some point. I omitted it mainly because I didn't know what switch to use.
I calculate the coils will draw about 12 amps, so I chose a fuse, fuse holder, switches, etc rated for at least 15 amps.
I found eBay to be the cheapeast source of firebricks. I got some K23-equivalent 2300F-rated firebricks in packs of 12 that worked out to around $4/brick. This box takes around 18 bricks to complete.
An important part of the brick layout is that it is self-supporting. You don't want to have to hold/clamp bricks in place for hours while the mortar dries.
I routed the slots with a screwdriver. Insulating firebrick is very soft and brittle. I originally bought hard firebrick which is an entirely different thing that won't work for this.
I used a mortar made of sand, fire clay, and portland cement in a 8:2:3 ratio mixed with enough water to be almost soup-like. The firebrick absorbs water at an amazing rate. I soaked the brick in water prior to give the mortar a chance at setting without drying out. From a structural point of view this mortar was hit-or-miss: one mixup seemed to hold great, another didn't. But the main purpose is to seal the cracks between the bricks.
Important note: The heating element coils that I used are intended to be stretched out before use so that the turns of the coils don't touch each other. I didn't do that. I'm running them at 20% of their power rating so I don't think they'll burn out. But they do move around a lot at temperature. This has proved challenging to work around. I've had to add a lot more staples to hold the coils in than I expected.
I don't want to get into welding. Instead I used a pop rivet gun and roofing flashing to construct a crude shell around the main box and door. I put many layers of flashing on the side that holds the hinges to make it stiff enough to not deform under the weight of the door.
The pop rivet method worked out great. Amazon has the rivet tools and rivets for cheap. I used 1/8"x1/8" rivets. The flashing sheet metal is easy to work. The only real downside is the edges can be sharp and need to be filed or hammered down.
Other basic sheet metal working tools are: sheet metal shears, small vise, big pliers, etc.
At the recommendation of someone on Bladeforums I used threaded rod and nuts to build bus bars for the coil wiring to attach to and high temperature wire (found some cheap on Amazon) to run between the bus bars and the electronics box. This seems to work. If I need to rewire this again I will add washers so that there is less chance of the wire working its way out of the gap.
The resulting heating circuit measures at 10 Ohms. This should give 120 VAC/10 = 12 amps. I am using a cord I hacked off an old circuit breaker that was rated at 15 amps.