Heat Treatment Oven WIP - PEU

[PEU]

This thread is a repost of a thread I made at British Blades when I made my current heat treatment oven, tried my best to keep the substance of the original thread including some replies to my posts. In any case, feel free to ask new questions, but please here in this thread, not via PM.

ALL MEASUREMENTS ARE IN MILLIMETERS AND TEMPERATURES ARE IN CELCIUS, ALSO, I DID A LOT OF MISTAKES ALONG THE WAY, PLEASE READ THE FULL THREAD.

Not so long ago I started to draw/model a heat treatment oven, that its inspired... uhmmm... cloned...mmm... copied from the Paragon KM14D

later I decided to reroute the bottom so its not level with the floor bricks

I only need to draw the electronics housing which will be also 1.5mm bent sheet like all the other parts, at the moment I just left the thermostat floating there

Bought K28 bricks at a local company and purchased these parts in USA:

Thermocouple: Omega TJ36-CASS-14U-6-CC Type K Thermocouple Probes 1/4" 304 SS Sheath
Controller SL4896-VRE - TEMPERATURE CONTROLLER, 1/8 DIN, OUT1-PULSE, OUT2-SPDT, 100-240VAC
SSR: SOLID ST RELAY 25A SPST VDC INPUT N.O. 24-280 VAC PANEL MOUNT

These are my calculations and then my doubts, I hope some of you guys can help me:

According to my calculations, based again on what Paragon did, their oven has a surface of 2127cm² based on a chamber volume of 140x368x108mm

Mine is 1991cm² using the same calculations, almost the same.

Based on their specs the consumption is 1920 watts. And based on data from the Kanthal handbook, that translates to a wall load of 1920/2127= 0.9w/cm²

1920w at 220V translates to 1920/220v= 8.7A, now that I know the current it should be easy to know how much wire I need: 220v/8.7A=25.2ohm.

Again based on the Kanthal handbook, a wire of 1.5mm diameter is 0.82 ohm/meter so I will need: 25.2ohm / 0.82ohm/m = 30m which seems a lot at first sight.

CAD to the rescue, I sketched an helix of diameter 7.5mm, simulating a 1.5mm diameter wire spaced 1mm around turns wound around a 6mm bar, 7.5mm being the imaginary line that goes thru the center of such an helix. The result was: 1 meter of this helix equals 9.5m of kanthal wire. Also I calculated with the CAD model the total surface per meter, this end up at 445.3 cm²

My brick channel is 3.2meters long, so I calculated the kanthal total surface 445.3 x 3.2=1425 cm² . Having this value allows me now to calculate the channel surface load 1920w / 1425 cm² = 1.34w/Cm² value that is well within of the 3-4w/cm² of the Kanthal handbook for spirals inside channels (Table 4 / Page 10)

now to my doubts:

1) are the calculations OK?

2) Should I increase the current to say 12 or 13A to speed things up? for example bumping current to 13A (still below the 16A thermoswitches used here) I end up with 20m of wire and a channel surface load of around 2w/cm² still within limits.

3) The Paragon model does not have ceramic blanket at all, will the oven be touchable after a couple of hours of being turned on? or it will melt anything around it?

4) Is there a DIY cement I can use to glue the sides? I can find almost any raw material here

Thanks!!


Pablo

Hi Pablo,

your power calculation seems to be correct. The surface load of the element is based on the wire surface, its got nothing to do with the surface of the channel you put it in.
Also I would add a little more pitch to the coil. 1mm spacing is not enough. You can compensate the added length by winding it on a thicker rod. Kanthal recommends (s) :min 2x wire diameter

When the element ages over time its loosing power. To compensate for this you can make it a little more powerful from the beginning but if you have far too much power the PID will eventually have difficulties to compensate overshoot of temperature. How much is too much? Its difficult to answer as it depends on the type of PID and on thermal mass , insulation and thermal conductivity of your oven.

I can only tell you from my experience what I have done and what worked for me. There is always the possibility of running into unexpected problems. As you can read in another thread Brok has build an oven too and all the calculations are done right but still the oven does not work as intended and we have so far not been able to figure out what the problem is.

I avoided using mortar and used ceramic fiber stip in between the bricks.

would advise to use a Door switch and a relais in conjunction with the SSR for safety reasons. The elements should not carry any current when you open the door.

Thanks Ralph,

CAD to the rescue again, always using a 1.5mm wire and 3mm spacing I get these numbers with different diameter rods:

A good compromise would be using a 8 or 10 mm rod for creating the coil.

I do plan to use a switch so elements are not under power when the door is open.

What do you think about the outside surface of the oven, will it get hot? I mean too hot to touch after a couple of hours?

Thanks!

Pablo


And the choice for the PID and thermocouple was not random, I read thoughtfully posts made by you, Ralph, Tim and many others here. At this point I think Im ready to start shapping the bricks, then measure the final size and order the bent sheet parts.

For how long you usually keep the oven turned on? for 3 or 4 blades it should be around an hour or two right?


Pablo

 

[PEU]

Started!

Following IPU advise I made a guide, makes your life a lot simpler

the little aluminum piece is to adjust the just cutted parts

Im collecting the dust in that olive oil can, do you guys know if I can make some kind of glue with it to fill the voids?

Also you may had noticed that from the original CAD design I made a change, now the oven is bigger, chamber now is 200mm wide and 170mm tall, depth is 450mm

These are the new renderings:

Also received all the electronics/TC/SSR


Pablo

 

[PEU]

back and door finished:

also purchased aluminium angle to keep everything in place and leave space for the ceramic blanket and the perimeter bent sheets

will make steps to interlock the bricks, more white dust, I will dream of white dust, it goes everywhere!


Pablo

Fantastic stuff!!!

Its great to see your CAD drawing some alive. It will be interesting to see how closely the final product resembles your rendering.

BTW I like the bigger internal space - there's almost enough room to cook a pizza in there!

In regards to larger current - I say go for it. My oven pulls 15 amps and has a much smaller internal space than yours. As far as I can tell, it does not overshoot even at low temperatures - over the next week or two I might do some testing to see what happens when i add large chunks of steel to the oven - I'll record the results and let you know.

Great job on the build and looking forward to seeing it come together

The cad drawings were previous to my upgrade to ceramic blanket, and inner structure to keep it in place, but since this is just hidden structure on the outside it should look pretty the same. In my favor I made a belt grinder that its 99% accurate to the drawings, if you want to peek the full WIP photos: http://imgur.com/a/h7AHP#0 and some videos: http://www.youtube.com/user/pablopeu/videos?query=lijadora

You can mix the Brick dust with a little kaolin(pure Fireclay) and add enough water to the consistency you want , I would go for about 70/30 -80/20 Brickdust/kaolin. The more fireclay you add the more shrinkage you will get. Shrinkage will mean cracks. The mixture will need firing to get hard. Its not a "sticky glue" per se, you just add something to bind the brick dust together on firing.

Good to know, kaolin is cheap, will verify if the oven leaks too much heat, I doubt it will, but this would be a solution.


Pablo

I have dust in my soul.. my god, white dust everywhere! more work done:

only right wall and bottom left to go

Pablo

 

[PEU]

More...

This is the inner structure:

moved "the parthenon" to stands:

Instead of using a file I use leftovers, not as fast but you don't unsharp the file, these bricks are very abrasive

I don't have long enough clamps so I imagined this method to keep everything in place.

Drill and pop-rivetting

To the floor for the n-time to do the bottom perimeter

Doing it upside down

No more work today, I'm tired

I can't put the vertical angles because I need to route the bricks for the heating elements


Pablo

 

[PEU]

I'm not going to cut the channels on the CNC router, the dust is very abrasive and goes everywhere, so I made a stencil for the channels. Also purchased 1.6mm Sandvik Kanthal A1, cannot find 1.5mm but re did the calculations and 1.6mm was better than 1.8mm which was the other measure available. Using a lathe and a 10mm rod made the 2 spirals that I plan to use in series.

Only made the ends of the channels

To be continued!


Pablo


How easy is to work with a template! dremel (actually B&D) old dull endmill and making the channels was a piece of cake, I cannot imagine doing it manually any other way...

Pablo

 

[PEU]

the aluminum structure wasn't enough, so I ordered sheets for the sides, now its super compact, this is the inner structure, on the outside there is going to be a ceramic blanket and then the structure from the 3D renderings.

Pablo

more work done, all screws/washers/etc are stainless steel, still need to add pieces of kanthal wire all along the channels to keep the spiral in place.

Pablo

 

[PEU]

Inner work finished:

Pablo

Time to do some tests:

Put blanket underneath just in case, K thermomether and the attached thermocouple, thru a 25A switch I powered the oven:

ITS SLOW!

Did not measured current, but I measured 225v at the line, 20.3 ohms for the windings, it should be around 11A, here it is after 25 minutes were it reached 450C:

Not fully glowing yet.

Using my phone as a timer I measured how long it took to climb 10C, at the start of the test it was at 400C:

Now I have a couple of questions for you guys:

1) what is the total current you measure at your oven?
2) If you tested it, how long it takes to climb 10C ?
3) and how long to 800C?

I think I will need to lower the total resistance to increase at least to 13A

Ideas? Thanks!


Pablo

 

[PEU]

Did you run it without the door?
If yes this will make a big difference.
For reference, mine gets to about 850°C in about 25 mins with the door closed.

Vince, nonono, I just took off one of the bricks to take a photo. Did you ever measured total current?

Here is a photo when it was at 305C

Thanks!

Pablo

The JM28 bricks are not as insulating as the type"23" and have more thermal mass.
With 225V and 20,3O you get 2493,8W
Your chamber is 20x17x45 which makes for a surface of 4010cm²
So in the current configuration your oven has a power density (W/cm²) of only 0,62W/cm²
The paragon uses 0,8 and I dont kow what type of insulation. IPUs oven had a power ratio of 1,3 or even more if i remember correctly (you can look it up in the other thread).
So with the less insulating bricks and the low power density this is what makes it slow. Give it more power. Because of the bricks I would say more than 0,8.

According to paragon web data (http://www.paragonweb.com/files/wiringdiagrams/WKM14D.PDF) their power density is 0.9 W/cm²
To have the same current I should reduce the windings lenght by 1/4... its dissasembly time

Pablo

Paragon oven power density:
km14 = 0,88
km18= 0,76
km24=1,07
km32=0,74
km45=0,75
pmt18=0,78

so its not consistent but can give you a ballpark figure of what to aim for.
If you shorten the elements you will get more Watts but increase surface load of the elements at the same time. This shortens element life but I never found a graph or some figures of how much element life you really loose.

to get 0,9 you need ~3600W this translates with 230V to a 14,6O element drawing 15,7 Amps with a surface load of 3.77W/cm² (element surface), just sightly above the kanthal recomendation.

PS: I like your staples, nice idea!

I don't know whether you have seen this already, but this is a simple schematic that I know works for me:

http://forums.dfoggknives.com/index.php?showtopic=23468&pid=231485&st=20&#entry231485

Here are the results of my measurements tonight:

The measurement in Ohms across the elements

The voltage to the elements with them powered on

By my calculations: A=V/R 211.5/19.7 = 10.7A

W=AxV 10.7x211.5 = 2263W


Turned on the oven from cold (5°C), then took this pic a few seconds later

After 2 mins

After 4 mins

It seems to get faster every time I run it!

Maybe this is to do with the thermocouple now having a black oxide layer?

Also from your pic it seems your thermocouple is recessed into the bricks, or is that an illusion?

thanks for the measurements Vince.

Yes the thermocouple is slightly carved on the top brick, not sure if this influences the reading or not, I guess its just a case of the thermocouple not being calibrated to the thermometer.

What kanthal wire diameter did you used? I used 1.6mm.


Mine took almost 30 minutes to reach 700C so its still slow, and Im begining to think its the bricks, like Tim said, that do not isolate properly and heat is lost.

Regarding the schematic, I will rethink it so when the oven door is open and then closed it reenergizes automatically, but also be able to turn off the elements without powering down the full oven.


Pablo

 

[PEU]

What do you guys think about trying to fit the ceramic blanket around the oven INSIDE the metal sheets to compensate for the higher thermal conductivity of the jm28 bricks? or its a lost battle from the get go?...

Pablo

Pablo,

I mulled over the idea of using blanket outside the bricks. It didn't seem to offer me much benefit and looked like a fair bit of hassle to do (or at least, to do neatly).
There are 2 things that are likely to be going on with denser bricks to cause slow heating.
One is that the thermal conductivity is higher.
The second is that the thermal mass is greater.
Whilst adding blanket will help with the the thermal conductivity side of things, it won't help with the thermal mass issue.
I'd only consider adding blanket if it will not work as it stands and time is really tight. Bear in mind that you can use cheaper insulating materials on the outside, as they will not see the high temperatures. Mineral fibre or glass fibre temporarily wrapped around the outside should tell you whether or not it is worth pursuing. If so, you can design a tidy permanent system.
If it gets to temperature as it stands, but is slower than you'd like, I'd be inclined live with it for now and start collecting the parts to build a complete new oven (using Thermal Ceramics JM23 IFBs) and fit plugs and sockets on the control box. That way, you can Austenitize with the new oven and temper with the slow one, without having to wait for the first oven to cool.
If it will not reach temperature but you have the time to gather the bits, I'd again suggest a new oven build.
Regards

Tim

Whilst adding blanket will help with the the thermal conductivity side of things, it won't help with the thermal mass issue.

By this you mean that the bricks absorb heat that should remain in the oven? Im not quite familiar with thermodynamics thanks!

Pablo

Yes, the bricks absorb heat to get up to temperature.
Raising 2 kg of a material by X degrees will need twice the heat input that raising 1 kg of the same material by X degrees will need.
Even if the insulation value (thermal conductivity) was exactly the same for both brick types, the oven would heat slower with the denser bricks.
Tim

went to purchase 23 type bricks but the factory was sold out, yes, they are made here, hopefully the quality is equal or better than chinese, is there something I should look for? Thanks!

Pablo

I've had 3 noticeably different types of type 23 IFB.

The first oven I built was made from what I tend to view as "bubbly" bricks. There were marked as something along the lines of "GRD23LW".
The second was made from Morgan Thermal Ceramics jm23 bricks, as were the next 2 or 3. I tend to view these as "chalky."
The most recent was made from GRD23 bricks, which I tend to view as "cementy."
The Thermal Ceramics JM23 "chalky" are certainly the ones I'd choose for preference. They are the lightest and seem to be the best insulators by some margin. They are also the easiest to work, again by some margin. Their disadvantages are high cost and softness. The softness does not seem to be a serious problem in an HT oven. The cost, around twice that of the "cementy" ones, is not enough of a problem to stop me using them. They probably add under £50 to the cost of building an 18" (450mm) oven here in the UK. This probably works out at 10-15% of the total build cost and I feel they give a product that is a good 10-15% better for using them.
Next best, IMHO, are the "bubbly" ones. These have visible bubble holes in (I don't have one here to look at, but I'd guess the bubbles are around 2mm in diameter) and are denser than the "chalky" ones. They are pretty hard on tools, so shaping them with hand tools is a pain. They will rout easily though, so cutting the grooves with a router solves a lot of that problem. They are in the middle in terms of density and their insulation seems to be good, though not as good as the"chalky" ones. They are harder than either of the others and seem more resistant to general knocks and bangs. Sometime, I'll build an oven with bubbly bricks for the floor and chalky bricks for the rest, as I think it would give the benefits of both types.
The "cementy" ones are the cheapest, densest and seem to be the poorest insulators. The surface texture is fairly similar to a sand & cement mortar or the sort of rough-and-ready site-mixed concrete I used for my shed base. I tend to use these for gas forges now, and use JM23s for the ovens.
I appreciate this doesn't help you a lot 7,000-odd miles from said shed base, but it's about the best I can do; the differences are very obvious when the different types are side-by-side, but harder to describe.
If you can, try weighing the bricks before you buy them. I only recall weighing a "bubbly" one, which I think came to around 1.4 kg for a standard (230 x 114 x 76mm) brick. I have a vague feeling the JM 23 goes closer to 1.1 kg and the "Cementy" ones are probably nearer 2 kg.
http://www.morganthermalceramics.com/resources/datasheets/

The factory was sold out and they have to import more, so I began searching again, I got referred by a local kiln maker to the local distributor of Thermal Ceramics (http://www.morganthermalceramics.com/) and I had the chance to speak to their engineer.

He shed light on the different types of type-23 bricks, the JM bricks are made by Thermal Ceramics Europe, and the K bricks are made in the USA, they also carry a second brand made in Brazil, what is interesting is that the Europe bricks are heavier than their USA counterparts which make the USA bricks the best choice.
This may explain why Tim saw over the years different kind of bricks, the difference told me the engineer is in the raw materials
Prices were USD4.25+tax for the K23 bricks and USD2.70+tax for the brazilian brand.
Here is the specs page, http://www.refil.com.ar/sec_productos_refractarios_propiedades.asp, its in spanish but the relevant data its just numbers and units so no problems there.
I will go with the USA bricks

Pablo

 

[PEU]

got the K23 bricks:

They are beigeish compared to the plain white I had. Each one weights 850grams by the digital scale. Let the sawing beginnnn

Pablo

take two: CUT!

All done this afternoon, much more faster than the first one, mostly because of experience/confidence, the mark-cut-align process was much faster. These chalky bricks also cut faster and are less abrasive on the saw.
Since I had the blanket I added it to the perimeter, this and the bricks interlocking will make the oven more efficient (I hope!)
Tomorrow I will do the channels

Pablo

 

[PEU]

1st things 1st, celebration!:

Finished the oven chamber, the sides assembly is exactly the same as before so no need to repeat photos:

top part

Double blanket on top since heat rises I assumed it would be the hottest side

With a little effort I managed to close the top side, ideally the blanket should be uncompressed, will think if I add an extra blanket between the chamber and the outside sheets.

There is no doubt now that my problem was the bricks, even after reaching 1000C I was able to touch the front part of one of the side bricks, at that moment it was at room temp.

If you paid attention now I placed the TC a bit lower and more to the center of the chamber, so the reading is more accurate.

Lets start:

The grey TC being lower diameter leaded the readings until the oven reached around 650C at that point both TC (oven and door) were almost the same until the end.

Time elapsed every 100C:

When I was almost at 1000C I noticed an orange glow at the bottom of the door bricks, some heat must had escaped there, but hey, times can only improve if closed properly

After 30 min of being turned off, the chamber is still at 300C and the sides and top part are 55C, I guess if the oven were left powered it would be at least double that.

I'm happy!


Pablo

 

[PEU]

Sheets Bent !!

The box on the top goes to the right side

door and hinges

underneath view

chamber view

They did a pretty good job on the bending, only the base is around 1mm wider but it fits under presure.
Now that I have everything on hand the only thing I would chage would be the thin bents, these were 10mm but 20mm would had been better, nothing serious, just a detail for the plans, BTW I will make a PDF set of the cabinet just in case anyone wants to use them, will post it around next week.
This is the updated model:

Can't wait for monday to start making holes and screwing the parts together



Pablo

Ralph, Tony is right, it took 6 mins from 900 to 1000, for a total of about 20 minutes, I re did the test and it took 22 minutes, so that's going to be the average.

continuing:

just finished the door, I just need to add the hinges and cut the bricks to fit the chamber. Also the control box is assembled, now I need to start cutting holes, its over 30C here in Buenos Aires, I'm taking a break in a room with A/C

Hammer here and there and everything fits snugly, better snug than lose...


Pablo

 

[PEU]

High temp paint and assembly

Pablo

Almost there!

Ordered a RS485-USB adaptor from China on october 15, still did not arrive...


Pablo

 

[PEU]

its 99% done, I forgot to make the piece that pushes the safety button. Had to modify the schematic because with the light in series with the contactor coil, the coil did not energize. Moved the light to an unused NO switch of the contactor and problem solved.

Here is the final schematic

and photos

Tried to configure the upper limit to 1100C but the controller only allowed me enter a maximum of 999.9, I'm sure there is a way to go higher, still did not found it in the manual.

Pablo

Check out section 11.1 of the quick-start guide. You need to set the decimal place to zero, so it displays full degrees instead of tenths, then go back and set the range.

It's worth setting the bottom end of the range to zero, rather than -200 degrees or whatever it defaults to. You'll never use the negative range and it saves you overshooting every time you set it to zero in normal use.

I just found it and came to edit my post but you beated me to it ) Already set the lower limit to zero.
With a clamp I made a temporary push tab for the safety button, now I wonder what should be a proper autotune temperature, I use O2 D2 1070 5160 steels mostly, should I use 900C
What is your suggestion

Pablo

Bear in mind you can autotune at 4 different temperatures with your controller and let it select the closest set of PID terms to the target temperature.
I go something along the lines of 100 degC, 250 degC, 650 degC and 1000 degC.
Where the tuning accuracy seems most important, in terms of the finished product, is at tempering temperatures. Overshoot seems to be less of an issue at higher temperatures from my (limited) testing.
When I eventually get around to building a tempering-only oven to run off the same control box, I'll probably go 100 degC, 399 degC, 400 degC and 850 degC. The tempering oven will be used below 400 degC and the hot oven above 400 degC

Connected to the PC!!!

Tired of waiting for the adapter I purchased two months ago in China, I decided to make an adapter myself, starting from a USB- TTL converter I made a TTL - RS-485 adapter out of a DS75176 and a NE555, let me tell you that seeing it work for the 1st time I got a happiness rush exactly the same as seeing the oven work for the first time.
Now I can configure, monitor and set up the oven from the PC
Pics

heatshrinking to avoid shortcuts

And this is what you see on both sides when connected

Pablo

Finished!

made the safety switch tab and made a couple of rings to keep the door in place

The USB-485 adapter is working part time, its missing something, Im debugging it, maybe I'm lucky and the Chinese adapter shows anytime soon)

Pablo


Made the 4 autotunes to 250 500 850 1050 as suggested by Tony, and after finishing I wondered, what would be best, to autotune with the oven temperature stabilized or while its ramping to the target
Also I noticed the pliers I have are not appropriate for puttingremoving stuff from the oven with heat gloves on, what do you guys use
Also finally received the RS-485 adapter from china, but it connects for a short while and then disconnects the same way the one I made, so the suspect now is the PC, will look further into this, I really want to be able to connect it to the PC.

Pablo

I usually autotune from well below the setpoint. Some of the controllers I use at work have 2 different autotune algorithms. one is for use when the controller has already been stabilised at the setpoint in manual. The other is for use from a temperature some way from the setpoint, usually at startup. The at startup tune generally gives minimum overshoot, while the at setpoint generally gives the best damping. The difference between the two isn't great, but I prefer to have the minimum overshoot, especially for tempering temperatures.

I don't know whether the Solo has both algorithms, but it seems to give pretty good results, however it gets there.

As for the connection, have you tried a lower data rate On other controllers, I've found 9600 BPS tends to work better than 38 KBPS or 19 kBPS, so it might be worth a try.

As promised, sheet bending blueprints http://peu.net/mods/cuchillos/HornoTempladoPEU.pdf

Have a nice 2013!!!

Pablo

 

[PEU]

Today I had to fix the end connections of the oven, first on one end (orange) then on the other (red), I guess I pinched it with the straight pliers while I was assembling, now I used round pliers hoping it will not repeat.

(photo of previous bricks)
I hope the repair last longer, it was less than 10 firings after I finished it since I try to turn it on one time per week. If you guys can suggest a better way without major dissasembly I would like to read about it. Thanks!!

Pablo

its sorted out, hardened two blades today, I hope the fix last a long time

Pablo

It didn't last, so I changed how the elements are energized, heated the kanthal to straighten it, its brittle when cold, then routed it thru the holes and afixed it to a ceramic connector on the outside. To electrically isolate the wire added ceramic beads to each of them. The oven is normalizing a blade at 650C now, time will tell if this fix lasts.
The only inner connection is on the bottom joining each wall elements

Pablo


There was only weak point after I moved the end of the elements to the outside, the inner connection between the walls, today while at almost the temperature to harden stainless and D2 (1070C) it broke again... (
So I decided to make a new coil, I have kanthal leftover from the original build, this time I will build both walls with a single piece of wire and route it to the outside like I did a month ago, hopefully I will be able to use the oven without having to worry about the elements anymore...

Pablo

Yesterday fixed the oven with a single coil for the entire oven, did not twist the ends. Fired it to 1070C without problems. Now the current is 14.5A. I hope this time it will last more

Pablo


Been using it the last months without a single issue, I know posting this is an invitation for troubles but wanted to let you guys know.

END OF THE REPOST

Pablo

 

[Josh Rider]

Thanks for taking the time to do this!

 

[colu41]

Yes. This is awesome. I will be looking back at this thread quite often this week as I try to finish my build. Ordered 20 K23 bricks last night. Hope the 2300 will be fine, and I hope it's enough!

One question. You didn't use any type of high temp cement for the brick? Just framed it all together?

Just curious as I plan to cement them and then build a stainless box around it.

Thanks again.

 

[PEU]

Using cement may break the bricks if not using proper technique, since I interlocked all the bricks I found no need to also glue them. To properly match the bricks, just scrub them together and they will grind themselves to almost no gap.


Pablo

 

[colu41]

Interesting. Ok. I'll look into interlocking them, and use the angle for the corners.

 

[PEU]

Looking for something related to kanthal Google showed me this old thread of mine, my heating element broke after many years of service, an advantage of building your own oven is that repairing it is super easy, I had a replacement element ready, removed old one, new one in and in a few hours back to work...

Pablo