Hardening oven saga

T Bredehoft said:
Just a thought, the steel in your assembly, when heated will expand. If an 8 inch piece of steel reaches 1500º, it will expand .08 inches. Twelve inches will expand 1/8". Who knows what rate the fire brick will expand at, but I suspect there will be a resulting disassembly of the mortar, at the very least. The furnaces I've examined used a fibrous mat as insulation between pieces.
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You're right, and I did consider this. This was the reason I applied the exterior mortar in individual sections rather than as all one covering. It still may fall apart, but the steel expansion should have enough room without causing catastrophic failure. The fire bricks are not physically attached to the steel, except in the door, so there is some slop there.
This oven is a bit of a blind alley for me, so I appreciate everyone's thoughts.
 
I got everything ready for a test run, opened the door, and noticed the element was hanging down. More WTF. Turns out that the water from all the mortar had seeped through the fire brick, got the tube mortar wet, which then reverted back to it's liquid state. Great.
There was no way to fix it at this point so I removed the rest of the element and as much of the mortar as I could. Remounting the element was now a problem. Trying to reglue all the wires in place and then thread the element through them was not an option as far as I was concerned.
As an alternative I decided to by a piece of 1/2" thick ceramic fiber board, mount the element on it, then attach it to the roof of the oven. So, I ordered a piece from Ebay, and a new element. I found that once the element heats up, it gets stiffer and hard to work with.
I cut the piece to size and used some baling wire to make the wire loops. This board is flimsy stuff and resembles styrofoam in its structural integrity. You can poke the wire right through it and I had to be careful not to tear it up.
Once I got it assembled, I mounted it up on the ceiling and connected it to the outfeed bolts. I fired everything up and the element seemed to be working fine. I got some initial smoke from the stuff that was on the wires (that was expected), but I noticed that the temperature reading from my thermocouple was going down, not up. I checked all the connections and noticed that I had an intermittent open somewhere along the line. I finally found a loose screw which tightened down on the element. Once that was fixed I tried it again. This time the controller relay wouldn't close properly. It would close on startup and the element would start warming, but as soon as the boot cycle ended the relay would open (even though the status light on the front panel indicated it was activated). I spent about a half an hour going through all the menu settings, power cycling it and trying everything I could think of to get it to work, but no luck. At that point I declared it officially FUBAR, and ordered another, higher quality controller (PID) which is specifically designed for kilns and ovens. It also claims to have clearly readable instructions, which the first one didn't.
The new controlled should arrive next week, at which time I will probably have to modify my control box as the new controller is slightly larger than the last. That will be a good time to take some pics and explain the innards.
Here are the UL approved connections on the back of the oven for the element. Magicniner will love these:

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This is the heating element removed to show the stellar workmanship involved:

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This detail shows the connection for the element which also doubles as a support for the board. The feed through bolt is some 1/4-20 threaded rod.

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Stay tuned for the next exciting episode where I try and cram 10 lbs of electronics into a 5 lb box.
 
I received the new PID controller. This one is larger so I had to construct a new control box. I built it out of 1/4" clear acrylic, which I have plenty of.
I got the PID mounted as well as the SSR, power switch etc. This is a shot looking down into the interior:

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The big box is the PID and the little square unit behind it is the SSR (Solid State Relay). I also have a 120v power bus for things like status lights, the PID and anything else I might decide to add (like a fan for instance). The element power takes a different route.
For those of you not familiar with SSR's, they are basically an on / off switch which is controlled by a DV voltage between 3-32 volts. As soon as the voltage is applied, the load terminals short together allowing current to pass through (just like a mechanical relay). The nice thing about these is that they can be switched on and off quite rapidly if you want a particular duty cycle on your load.
They typically have power ratings in amps (mine being a 40 amp unit). They also get hot since they are not perfect conductors. Therefore, you have the option to buy a heat sink which attaches to the metal underside to dissipate the heat build up. I highly recommend this since heat is a major cause of failure to solid state devices. You can also mount the heat sink outside of the enclosure (as I have done) to keep the inside from heating up.
This is the bottom of my enclosure:

IPLtyBK.jpg


I used a couple of standoffs I had to mount the unit. I also used some heat sink compound between the SSR and the sink to aid in the heat transfer. The bottom of the SSR isn't real flat and you want as much surface contact as possible, and the HS compound helps with this.
I hooked it up for a test today and it seems to work OK. This is a ramp and soak controller which makes controlling it a bit different than other similar controllers. It took me a while reading the manual to get the hang of how it operates. I hooked a lightbulb up as a load and tipped the thermocouple over to rest the tip above the bulb. Thermocouple works and I see it heating up in the PV (Process Value) display.

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Now I need to put a rear and top on, and modify the hangers on the side of the oven which held the smaller one.
Clear as mud?
 
Could the housing be made of aluminum instead of steel? Or steel sheet over aluminum frame?
 
EmilioG said:
Could the housing be made of aluminum instead of steel? Or steel sheet over aluminum frame?
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You can make the housing out of anything you want. I just made it out of acrylic because I had plenty of it, it machines well, and goes together easily. I pick up scraps of it at Tap Plastic. They sell roughly 12" x 12" pieces for a dollar each (all different thicknesses) at my local store. Great stuff for projects like this.
 
I got it pretty much done. I hooked it up and it seems to be working. It went up to 70C and held within a degree.

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I used a standard barrier strip on the back for the electrical connections. I still have to land the ground on the frame. I had a steel box before and I grounded the box, which made the SSR buzz when it wasn't activated (some weird loop back to neutral). I think it had to do with the way I had it mounted. Anyway, it doesn't do it anymore.

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You can see that I marked the polarity on the thermocouple. They produce a small DC voltage which varies with temperature. If you get the leads backwards your temp readout will go down with a rise in temperature. Then you know you have to swap the leads. It doesn't hurt the controller.
I'm going to have to figure out how long this oven takes to get up to 843C degrees so I can set a ramp time. I have a piece of scrap D2 to screw up as a guinea pig, so I figure it will take most of a day to get it dialed in.
 
Great project and a really interesting read. I can definitely relate to many of pitfalls with designing on the fly and working through the various modifications. One thing I am curious about now is how the acrylic enclosure will stand up to a fully heated oven. Hope things don't go soft at temperature!

Once again, great thread and really interesting to follow. :encourage:

-frank
 
Thanks Frank, I'm glad you're enjoying it. It's been a fun build overall, now I just got to figure out how to use it.
I think the acrylic will be fine. There is an air gap between the oven, and acrylic has to get pretty hot to start to melt. Nevertheless, I'll keep an eye on it.
One more thing I forgot to mention about thermocouples is that they come in different types. They are usually denoted by a letter, i.e. K or J. K type seem to be the most common, and that's the type I'm using. This controller has the K type as a default. They also come in different temperature ranges. I'm using a high temperature probe, good to around 2000F. Something to keep in mind if you ever want to build one of these things (which I recommend due to it's just being fun).
On a personal note... Like most of you guys, I like to build stuff. I watch a lot of videos of guys building things, and if it looks interesting, I build it. I don't care anymore if I really need it. My kids are all grown and almost gone, (my youngest is graduating from college in a month or so). I no longer have to scrimp and save to spend every last cent on tuition and housing (put 4 through college). I finally have the opportunity to waste some time and money on my projects. Life is short, build with reckless abandon. :)

Cheers,

Ted
 
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