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Hardening oven saga

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Ferrous Turner

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#1
Didn't know anything about these types of ovens except what I gleaned from watching a few YouTube videos. How hard could it be, it looked so easy in the videos! Actually it wasn't too bad, but like every other venture into the unknown there were a few screw ups.

I thought the ceramic fiber bricks were ridiculously expensive, so I picked up some regular fire bricks instead. Cutting these turned out to be quite time consuming (they are very hard). The regular masonry disc didn't do any better than standard metal cutoff discs, so I used those.

I also bought some high temperature mortar which came in a tube (much like silicon seal). I used it to glue the bricks together and it worked OK. It’s water based and cures by drying out. This burned me later.

This is how it started out:







I traced out a groove in the top in which I was going to put the 1500 watt heating element. After hours of grinding out the groove, I found out that one must stretch the heating element coil out in order for it to work properly. So much for the groove.

I stretched out the coil and ran it through wire hangers that I “glued” into little slits I cut in the brick (F#@k the groove). I used the tube mortar for glue and it seemed to work OK.



I then tested the coil and got very little heat from it. I measured the resistance and got about 26 ohms across the coil. This gives about 500 watts @120 Volts. Gee, I guess this element was meant for 220 Volts.

I didn’t want to run on 220V because my control box was all wired for 120. So, I cut the element in half and ran the two pieces in parallel. It lit right up to a nice glowing orange in that configuration. Nevertheless, I had to rerun my coils again. The top of my oven is starting to look ugly, but who cares as long as it works.



I mounted the thermocouple in the bottom rear of the oven. I enclosed it in a steel pipe and welded a piece on the end for it to screw it into.







It sticks out a ways because I’ll be adding regular mortar to the outside for more insulation.
(continued in next post)
 

Ferrous Turner

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#5
I got the element ($7), thermocouple, SSR and controller on Ebay. I'll get to that other stuff in the next post.
 

Ferrous Turner

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#6
Next thing was to build a frame. I got a few lengths of angle iron and got to work with the welder.



I wanted the top element work all done before I buttoned it up. At this point I just cemented the top on and slid it in the frame. I left a few inches on the vertical pieces to act as legs.
Once I got that done, I cut part of a brick off to fit with another one for the door. I built a frame similar to the other parts which fit around the outside of the door. I mortared the bricks to the frame and welded a couple of hinges between the frames. I also welded on a bracket to hold the control box.



I already had the control box built by that point, but I'll go over the details of that later.
I used another hinge for the latch. I welded a length of 5/8" threaded rod to the inside and used a butterfly nut for the hold down.





I had to seal the door, so I used some more of the tube mortar. I spread it around the opening, put some wax paper over it and closed the door down on it. I let it set for a day, but then you have to remember that this stuff dries out to harden and it wasn't doing a lot of drying all covered over with wax paper. So, I opened the door and carefully peeled the wax paper off. Of course, the wax paper had wrinkled and the surface was a little goofy, but I had the right shape anyway.



As you can see in the picture, I already had a couple of the sides mortared up. To do this I framed the sides with a combination of 1" angle iron and 1" flat stock. I just tack welded the pieces in place to make a mold. I mixed the mortar real soupy so I could easily pour it in the mold and smooth it out. I've done some casting this way and it works good. Doesn't crack either. I also mix my own mortar using just sand and portland cement. Nothing complicated or precise about it, I use about 2 thirds sand and a third mortar (and there's a lot of fudge factor there). The hardest part is estimating just how much you need per batch. You also need at least a day between sides to let it set, so 6 days minimum to do all 6 sides.
After the outer mortar is all done the thing weighs about 100 lbs (feels like it anyway), but it's pretty well insulated.

(continued next post)
 

magicniner

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#7
Do you have a safety switch on the door to cut power feed to the SSR so there are no live parts accessible with the door open?
 

T Bredehoft

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#8
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.
 

Ferrous Turner

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#9
A safety switch would probably be a good idea, especially if the element were running on 220V. However, I have an on/off switch right next to the door latch, which if I fail to turn off before sticking anything inside, makes me deserve a reminder. I also have bright red lights to remind me that the unit is on. The kids are all grown and gone, so I don't have to worry about curious fingers either.
 

magicniner

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#10
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.
The steel seems to be on the Outside?
It should stay within a temperature range which doesn't cause 3rd degree burns on brief contact ;-)
 

Ferrous Turner

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#11
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.
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.
 

Ferrous Turner

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#12
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:



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



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.




Stay tuned for the next exciting episode where I try and cram 10 lbs of electronics into a 5 lb box.
 

Ferrous Turner

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#13
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:



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:



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.



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?
 

Ferrous Turner

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#17
Could the housing be made of aluminum instead of steel? Or steel sheet over aluminum frame?
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.
 

Ferrous Turner

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#18
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.



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.



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.
 

francist

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#19
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
 

Ferrous Turner

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#20
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
 

Ferrous Turner

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#21
I gave it a test run last night. Initially it worked well, but then started struggling to maintain the increase in internal temperature at a reasonable pace. It was losing too much heat through lack of sufficient insulation. The exterior was hotter than I expected at that point in time.
I ordered some 2" ceramic fiber exterior insulation to cover the thing. I'll have to remove the control box and have it as a stand alone unit.
I'm also getting a small fan to cool the SSR heat sink. It got hot quick.
I'm getting an education in thermodynamics.
 

Ferrous Turner

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#23
I grossly underestimated the thermal conductivity of the masonry. As well as the outside, I'm also going to line the inside of the oven with fiber board to further slow the egress of the heat.
The SSR is another issue. That thing eats a lot of power. For my purposes I think a simple mechanical relay would suffice, especially during warm up which can take an hour or so. An SSR is good if you have a lot of rapid switching of the power to the element i.e. during a long ramp up where the element is heating faster than you want it to. My oven heats slowly, so I could use a plain switch to power the element until it gets up to temperature.
Once I get it all re-insulated we'll have to see how fast it heats up. I still think it will be slow enough to dispense with the SSR during warm up.
 

markba633csi

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#24
This is why 220v would have made more sense- half the current for the same amount of watts to the element, less voltage drop
across the SSR, less loss there, but even so the heatsink you have is woefully too small even for 220v
Yes a mechanical relay would be better for 110v
 

Ferrous Turner

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#25
That's the heat sink that comes with the unit, although a small fan seems like it would keep things cool enough. It's a 40 amp SSR and running 13.5 amps, so well within it's power rating. As far as 220 being a lot more efficient... It's not going to be any more efficient across the element since it is a purely resistive load (voltage times current). That's also assuming you use the original element as one piece, and not as two halves wired in parallel.
The SSR would be more efficient since I'm pretty sure there is a set voltage drop across it, so half the current would mean half the power loss. I'll have to measure that to see what it is.
A simple switch or mechanical relay has no loss, so that seems like an obvious solution for my purposes. I just need to find a relay which activates at the voltage set by the controller.
 

FOMOGO

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#27
I bought some insulation for the outside of the oven. Think I got enough?
I used that type insulation when I was in the heating business. It works well, but you don't want to breath the dust from it. Use a mask when working with it, and enclose it in something. Maybe some thin sheet alum. I've been enjoying your journey, and I believe in the end your persistence will carry the day. Cheers, Mike
 

Ray C

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#28
I made my kiln with that kind of insulation. It's great stuff! As Mike says, use a respirator and I will add, be careful handling it. It has shards of glass glass in it. I got a sliver of glass in my thumb that took 3 months to finally work itself out.

Ray
 

Ray C

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#30
You used this for the inside correct Ray? How did you attach the Kanthal wire?
Yes, on the inside. The bottom of the chamber was lined with insulating brick. The coil is spaced-out (zig-zag) on the brick and pinned-down with some SS wire that can take the heat. Heat from the coils rises up. I have a frame of SS that sits above the coils on which to place the workpieces.

Hope that makes sense.

Ray
 
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