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DGentile

WIP: Large Electric Kiln

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Well - it's tool-making time again wink.gif

During the last sword-heat treatment I did, I noticed that a somewhat larger kiln would have been really handy, as the gas-fired one I've got barely was able to take the blade... a problem I ran into not for the first time... also as I've been teaching lots of group classes recently I needed something to fit at least 4-5 knives at the same time.

And last but not least: especially for annealing I wanted an electric kiln, as it could be set to run through the night without any supervision... something I would not even dare to think about with a gas fired unit.... (unless you add a ton of safety features).

Decided to document the process and share it with you....

It's by far not the first electric kiln I've built... I think it's no. 10 wink.gif (my first one for myself years ago, followed by an improved model, followed by some for customers / knife makers) and now the large one again for myself....

Interior dimensions: depth: 1m (40"), width: 200mm (8"), height: 124mm (4,9").
Outer Shell: Stainless steel...
On wheels wink.gif

I'd started out by prepping all the fire bricks:
p747317157-4.jpg


and in case you wonder how this is easily and quickly done, repetitively for all bricks:
p990177428-4.jpg

I've built this little contraption

Modified core drill at the tip (I've milled two 8mm slots into it's side, almost along all the length, so I can push the brick-pieces out with a screw-driver)...
welded on a longer shaft and a matching and piece to fit my drill.
on the left side, bolted down to the table surface is a steel bar, acting as a guide for the stones, so that the holes are parallel to each other and similarly spaced:

p1017194284-4.jpg


and here's how it's operated:
p848786915-4.jpg


the "waste"
p741412967-4.jpg


After drilling the holes, I used a wood saw, rough blade, hand held to cut the chanel open at an angle.
Though, first I used an edge scribe to mark all the lines....
sawing goes very rapidly... and is easy...

Total time to drill out all holes and cut those open: less than 30 minutes.
and the result is quite usable too wink.gif - all stones are "automatically" drilled with the same distance and depth - thus the channels are properly aligned without any problem:

p1027116179-4.jpg




The I made the heating coils from Kanthal A1 wire (1.1mm)...
p878599218-4.jpg

I produced this from a roll of wire (approx. 140meters of wire) by mounting a 2.2m long, 25mm (1") diameter wood-shaft on the lathe (3-jaw chuck) and using some jury-rigged (just a few pieces of steel & clamps) as a "tail-Stock"... my lathe's maximum work length is 800mm - so I had to take off the lathe's own tail-stock and rig something a bit over 2m away from the chuck.
the good thing is, as it's wood and VERY slowly turning as I hand-feed the wire, there's absolutely NO need for any precision, 10cm in any direction is easy to live with for that process.
Wrapping 2.2m of heating coil though is a very tedious process.... if you like obscenely slow meditative work, than this might be your new addiction - otherwise the first 2inch are "fun" and then it's pretty boring wink.gif... but after 30 or 40 minutes (it felt more like a fortnight though) the thing was made.
Custom heating coil.... easy to make to any size - you don't need a lathe a slow turning motor with some sort of chuck or a table-mounted hand-drill press will do the trick too... but the lathe of course is practical.
if you have the patience and can get the wire, this will be incredibly cheaper than buying manufactured coils.
After I had taken the thing off it's supporting wooden core I cut it into two equal sections as my kiln has one "double coil" on each side...



All boxed up.... the top sheet has just been spot welded in place... I'll have to fetch more argon before I can TIG the thing nicely.
p548493081-4.jpg




and here's a how the wires are fed through the back:
The most important aspect is to make absolutely sure the heating wire does NOT make any electrical contact with the box...
especially not with a kiln operated at 400V...
I used old tig nozzles for that purpose with one end protruding into the stones...
I'll have to turn "locking rings" from steel tomorrow which will be welded to the back plate and will secure the tig nozzles into place
(those will basically be just a section of drilled through round stock with a set-screw).
p918616339-4.jpg




Tomorrow: assembling all the electronics, making the wire-box to fit the back of the kiln and final welding of the box.
Possibly also tomorrow making the door with the safety switch (door-open: kiln off)


hope you enjoyed this so far...

Edited by DGentile

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Looks the business, Dan.

 

Nice use of the TIG shrouds. I just might have to steal that idea for a future build.

 

Any chance, as you write it all up, you can major on the control/electrical side of things?

 

Most of the oven/furnace write-ups I've seen have tended to gloss over all the boring-looking technical bits. Whereas most of the guys I've spoken to, who are considering building one, are happy enough about the physical build, but rather nervous about the electrics and control.

 

Hopefully, all the questions will be answered anyway during the WIP, but there are one or two that spring to mind already:

 

That looks like a lot of element length you have there. What sort of wattage are you going to be running, and what sort of temperature range are you intending to run it in? Carbon steels only, or stainless as well? Are you looking to use it for tempering too, or does the cool-down time rule that out?

 

Overnight annealing suggests a ramp-and-soak controller. Have you found one that's affordable and easy to use? If so, please share.

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Looks the business, Dan.

 

Nice use of the TIG shrouds. I just might have to steal that idea for a future build.

 

Any chance, as you write it all up, you can major on the control/electrical side of things?

 

Most of the oven/furnace write-ups I've seen have tended to gloss over all the boring-looking technical bits. Whereas most of the guys I've spoken to, who are considering building one, are happy enough about the physical build, but rather nervous about the electrics and control.

 

Hopefully, all the questions will be answered anyway during the WIP, but there are one or two that spring to mind already:

 

That looks like a lot of element length you have there. What sort of wattage are you going to be running, and what sort of temperature range are you intending to run it in? Carbon steels only, or stainless as well? Are you looking to use it for tempering too, or does the cool-down time rule that out?

 

Overnight annealing suggests a ramp-and-soak controller. Have you found one that's affordable and easy to use? If so, please share.

Tim,

 

Feel free to steal the Tig-Nozzle/shroud idea ;)

 

to answer a few of your questions:

Yes - the electric side will get extensive coverage .... so stay tuned.

 

Usage: Carbon & Stainless steels, Tempering, but only for the secondary run or for steels such as D2 or some stainless requiring rather high tempering temps.

for the first run I use a cheap baking oven... nothing fancy, but gets the job done and doesn't cost anything off a junk-yard.

 

Ramping: right now I'm installing a very simple PID controller without ramping - but in a project to come I'll use an old notebook with a specific software and an external (USB) controller to do ramping... I don't have the link at hand right now, but it's about 120$ if memory serves me well.

It's basically just a connector for a K-Type thermo-couple to provide a computer readable interface and an alarm output running 12V to switch the SSR.

The software does the rest and allows easy graphical control of the curves... this has absolutely no limits on ramp-times, curves and stuff... much better than any "proper" controller.

I yet have to get the stuff for myself but have copied the idea from a friend ;)

 

Power / Wattage - to be frank, whilst I usually make the whole math this time I just made a 2m coil with the reminder of A1 wire I had on the shelve I guessed it to be approx. 140m...

I'll post a writeup for the math later...

 

 

 

 

 

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Very nice, this is excellent and very informative. Thank you for documenting the process so well. I look forward to seeing the electrical side of it, as I would be totally lost trying to do something like this myself.

 

John

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Sounds like you've got it covered, so I'll just sit back and wait. I'm told patience is a virtue.

 

Any information you can provide on the control system you mentioned would be gratefully received. My last 4 builds have used the AutomationDirect Solo controllers (They seem to be the same as the Omega CN 7200, 7500, 7600, 7800 series controllers). It's a good controller, but doesn't seem very user-friendly when it comes to the profile programming, especially for non-geeks. I need to get something pretty soon for my next build and if there's something that may be better available, I'd like to give it a try.

 

My last build was a 42" sword-length oven with around 6 kW of input power. With hindsight, I could have gone a lot less. I basically just doubled up on the elements I had used on my earlier, 18", builds. This was partly because I only wanted to make one change at a time, and partly because I already had the elements, I'm basically lazy and, faced with a choice of 3 kW, 6 kW or winding fresh elements, it seemed like a no-brainer at the time. I really should try it on just one set of elements and see how hot it'll get. I've successfully taken the 18", 3 kW, version to 1176 degC/2150 degF, which was the highest HT temperature recommendation I could find at the time (S30V treated for maximum wear resistance) and it was still rising quite fast as it approached that temperature.

 

I find lots of power tends to make accurate control at low temperatures a lot harder.

 

Still on my to-do list is a tempering oven to go with the sword oven: a 4' long piece of 4" pipe, a fairly low-powered catering rod element (I'm guessing 1000W would do it) clamped to the outside of the pipe, along with a thermocouple and the whole thing insulated with Rockwool slab. The plan is to run it from the control box that runs the sword oven, once the blades are quenched and the sword oven is finished with.

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Sounds like you've got it covered, so I'll just sit back and wait. I'm told patience is a virtue.

 

Any information you can provide on the control system you mentioned would be gratefully received. My last 4 builds have used the AutomationDirect Solo controllers (They seem to be the same as the Omega CN 7200, 7500, 7600, 7800 series controllers). It's a good controller, but doesn't seem very user-friendly when it comes to the profile programming, especially for non-geeks. I need to get something pretty soon for my next build and if there's something that may be better available, I'd like to give it a try.

 

 

Tim,

 

I have found the info for the pc based controller but the website is down - I'll send the owner an Email (we had email contact while ago) and see if he still provides the stuff...

 

if not - don't fret ;)

 

In case the kit is no longer available I'll do it myself and share the info here:

This is how I'd do it:

- USB Thermocouple Interface (from http://www.robotshop.com/phidgets-1048-usb-thermocouple-interface-4.html)

- USB 12V Relay Board (http://denkovi.com/product/41/usb-eight-channel-relay-board-for-automation-12v.html)

 

The Software I would probably write in C++ on windows or maybe something else... I've done stuff like that years ago, guess I'm pretty rusty and not really up to date in that regard, but then again it's not something overly complex either.

I guess the whole thing would take me about a week to make a nice box, mount the parts and write a crude but working Software.

The software basically would read out the thermocouple interface a few times per second and then send a signal to the USB Relay board in case the kiln's SSR needs to be powered or shut down.

The algorithm would probably be the most complex part - the part where the Software decides WHEN to cut the power and when to re-enable it... I guess I would have to include heat-up speed and take that into account and start to shut the kiln off once it gets within a predefined margin of the desired temperature... then on again... and repeat this with an increasing frequency the closer to the set point it gets... basically same pricipal a modern "fuzzy-logic" pid controller is based on.

 

but again - if I can spare myself from that ordeal by buying something "affordable"... I gladly go that route ;)

Then again it's nice to have an option.

 

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After having done the math, and shortening of the coils completed:

Wiring of the kiln's main power:


!!!IMPORTANT: THIS SECTION HAS BEEN UPDATED - the solution with three SSR is problematic - perfect solution further down this thread!!!!


p144903929-4.jpg
This has been CHANGED... READ MORE FURTHER DOWN THE THREAD!!!!!!!
That probably only looks remotely familiar to those who have done that before... but yes, I'm using THRE solid state relais.
The SSR are rated (switch part: 480V max at 25Amps (so well over the kiln's max), control part: 3-12V DC)

as I'm using two individual coils (it is easier to get more power as with a single linked long coil)... I use the three-phase 400V system we have here to connect the coils like this:

Coil 1: One end to SSR1 linked to Phase 1 (400V), other end to SSR2 linked to Phase 2 (400V)
Coil 2: one end to SSR3 linked to Phase 3 (400V), other end to SSR2 linked to Phase 2 (400V)

The reason I use three SSR is for operator security only!
This way I can make sure, that when I switch the kiln off, open the door or whatever there is NO electricity at all runnign through the coils!
if I'd use only two SSR (or if I'd be using a single coil: only one SSR) with a 3-phase system, when I would "interrupt" the SSR (open the door) and would by accident short-cut the coil to the metal box/frame there would still be 163V "live" on the system...
only by securely "disconnecting" all live-wires I can make sure this does not happen.


The rest in this post is good - no mods to this.... so just make sure to read up on the change for the wiring a few posts down!

Also worth noting: the grounding of the thing to the box' case...

The red-black wires are the control-wires (12V DC operating the SSR)... those will be connected to the PID controller.

p36766519-4.jpg

Business end wink.gif ...
The wires are ultra-high temp rated (400+ °+C (750F)).
These will be connected to the coils


p501523525-4.jpg

Cable locks


p132850672-4.jpg

The box' back plate mounted to the oven with the wires passed through.



p288343318-4.jpg

Coiling the wire a few times will prevent massive heat up...



p57725535-4.jpg

here's how it's being connected



p262714877-4.jpg

Control (12V DC) wires attached



p489462314-4.jpg

Box closed and mounted


p381323686-4.jpg

p402384995-4.jpg

making sure the kiln is nicely grounded... that's important if you don't want to end up like a roasted chicken one day...


p22215654-4.jpg

Testing if the grounding has a connection from the plug to the case...
also testing if NONE of the other contacts has a contact with the case... (that would be really bad).
better to test before operating the thing...


p318945565-4.jpg

Preparing the kiln for spray painting wink.gif


AND FINALLY THE RESULT:

p224488743-4.jpg


p501796320-4.jpg


p38465078-4.jpg


p206738137-4.jpg




Initial test confirms good operation: 600°C (1100°F) in 10minutes, 15 minutes to 830°C (1520°F)... haven't tested any further yet... tomorrow.


hope you find all this useful wink.gif

cheers,

Daniel

Edited by DGentile

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Part two - as the system tells me that there are too many images...



Installing the door-switch:

p510132582-4.jpg



p96031316-4.jpg



switch finally mounted:
p260892658-4.jpg



adding a steel-square bar to the door-hinge to press the switch in the lowered position:
p135989892-4.jpg


p76592036-4.jpg




Installing the securing "clamps" for the TIG-nozzles:
p150840255-4.jpg





and that's it for today... more tomorrow...

Edited by DGentile

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Really good WIP so far. Thank you. I have plans to totally chop and reconfigure a large pottery kiln that I own into a sword length kiln at some point. Do you cut all of the brick with a hand saw, or do you use a masonry blade or something else for ripping and cross cutting bricks?

 

If you have to bond bricks together, do you find that you need any special product or will something like satanite or AP green work?

 

As far as being able to use the same kiln for hardening stainless steels and tempering carbon steels, would you say that's improbable? Accurate temp control from 400F to 2200F?

 

I'm just learning about electric kilns, so go easy on me. Thanks. Again, great work here.

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Salem,

 

thanks....

 

to answer your questions:

bonding the stones... if you have to, there's a specific type of "glue" for that purpose...... at least here any of the large pottery supply stores has got it for repairs... don't know how it's called though.

 

cutting: I have an old, rotten piece of sh*t vertical bandsaw I do hate with a good bit of passion and thus abuse for anything nasty such as cutting those stones ;)

 

temp-range : possible, but you have to somehow accomodate to start tempering (kitchen oven for example) until the kiln has cooled down, which will take a good bit of time.

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LAST PART of the WIP:

 

first for those who are interested in the math:

(I did the calculation today and had to shorten my coils quite a bit... bummer... but it was done quickly enough.)

 

if you calculate your heating coils you need to know a few things in advance:

 

Resistance per meter (or foot or whatever) of your heating wire...

-> I'm using Kanthal A1 with a diameter of 1.1mm - that has a specified resistance of Rs= 1.49 Ohms

 

Desired "Power" in Wats:

-> I wanted something close to 7600W

-> I use two separate coils running "independently" (not linked)

 

Voltage of your system:

-> mine's going to run off 400V

 

P (Power in Watts) = U (Voltage) * I (Amps)

-> I = 7600 / 400

-> I = 19 A

(each coil thus: 9.5Amps)

 

 

now to calculate the length of the coils...

 

(Voltage^2 / Power)* dW (wire diameter in mm) / Rs (Resitstance per meter of the wire):

(400^2 / 3800) * 1.1 / 1.49 = 31m for each coil

 

Total resitance of the coil: 31 *1.49 Ohms = 46 Ohm

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After having done the math, and shortening of the coils completed:

Wiring of the kiln's main power:


!!!IMPORTANT: THIS SECTION HAS BEEN UPDATED - the solution with three SSR is problematic - perfect solution further down this thread!!!!


p144903929-4.jpg
This has been CHANGED... READ MORE FURTHER DOWN THE THREAD!!!!!!!
That probably only looks remotely familiar to those who have done that before... but yes, I'm using THRE solid state relais.
The SSR are rated (switch part: 480V max at 25Amps (so well over the kiln's max), control part: 3-12V DC)

as I'm using two individual coils (it is easier to get more power as with a single linked long coil)... I use the three-phase 400V system we have here to connect the coils like this:

Coil 1: One end to SSR1 linked to Phase 1 (400V), other end to SSR2 linked to Phase 2 (400V)
Coil 2: one end to SSR3 linked to Phase 3 (400V), other end to SSR2 linked to Phase 2 (400V)

The reason I use three SSR is for operator security only!
This way I can make sure, that when I switch the kiln off, open the door or whatever there is NO electricity at all runnign through the coils!
if I'd use only two SSR (or if I'd be using a single coil: only one SSR) with a 3-phase system, when I would "interrupt" the SSR (open the door) and would by accident short-cut the coil to the metal box/frame there would still be 163V "live" on the system...
only by securely "disconnecting" all live-wires I can make sure this does not happen.


The rest in this post is good - no mods to this.... so just make sure to read up on the change for the wiring a few posts down!

Also worth noting: the grounding of the thing to the box' case...

The red-black wires are the control-wires (12V DC operating the SSR)... those will be connected to the PID controller.

p36766519-4.jpg

Business end wink.gif ...
The wires are ultra-high temp rated (400+ °+C (750F)).
These will be connected to the coils


p501523525-4.jpg

Cable locks


p132850672-4.jpg

The box' back plate mounted to the oven with the wires passed through.



p288343318-4.jpg

Coiling the wire a few times will prevent massive heat up...



p57725535-4.jpg

here's how it's being connected



p262714877-4.jpg

Control (12V DC) wires attached



p489462314-4.jpg

Box closed and mounted


p381323686-4.jpg

p402384995-4.jpg

making sure the kiln is nicely grounded... that's important if you don't want to end up like a roasted chicken one day...


p22215654-4.jpg

Testing if the grounding has a connection from the plug to the case...
also testing if NONE of the other contacts has a contact with the case... (that would be really bad).
better to test before operating the thing...


p318945565-4.jpg

Preparing the kiln for spray painting wink.gif


AND FINALLY THE RESULT:

p224488743-4.jpg


p501796320-4.jpg


p38465078-4.jpg


p206738137-4.jpg




Initial test confirms good operation: 600°C (1100°F) in 10minutes, 15 minutes to 830°C (1520°F)... haven't tested any further yet... tomorrow.


hope you find all this useful wink.gif

cheers,

Daniel

Edited by DGentile

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Salem

 

I'm no expert, but I've built 5 electric HT ovens so far and I've found out a few things on the way.

 

The soft bricks tend to suck all of the moisture out of whatever jointing compound you use, so it's quite easy to stick the bricks together with a thick layer of the compound, but it gets more difficult as the joint gets thinner. Because I want a close joint, I've tended to fit them dry. Where I've been concerned about gaps, I've just slathered the outside of the joint with watered-down fire cement (Satanite and AP Green seem to be unavailable here, so I can't comment on them). If you do the inside as well, you need to avoid getting the mix on the elements.

 

Cutting depends a lot on the actual bricks you use. JM23 branded bricks are the easiest and offer the best insulation. They cut with a cheap hardpoint tenon saw, no problem. They are quite chalky in texture. I've also used low-iron GD23LW bricks; bubbly in texture and horrible to cut; I killed a tenon saw in 3 cuts and used a new blade from a mechanical hacksaw for the rest of the oven. By the end, there was no set left on the saw blade. They are quite a bit harder than the JM23s and a little denser, so not such good insulators. I've also found unbranded 23-grade bricks. Denser still and even poorer insulators, the texture is more like a sand/cement mix and ease-of-cutting is somewhere between the other two types. The JM23s are more expensive here in the UK (they are only made in the US and in Italy) but I feel they are worth the extra cost.

 

For my first 2 ovens, I hacksawed either side of the element grooves, broke out the centre bit by tilting the saw blade, then filed the base of the grooves with a suitably-sized piece of studding (allthread).

 

For the last 3, I used a router, together with a shopvac and respirator. Infinitely easier and works for all 3 types of (soft) brick I've come across. I'd expected it to kill router cutters fast and bought several cheap ones, but I've probably done over 20 ft of 10mm wide groove and I'm still on the first one.

 

It is possible to use the same oven for Austenitizing stainless and tempering Carbon steels, but it's not as straightforward as it might initially appear.

 

I have found that there is a tendency for the oven temperature itself to overshoot the setpoint at tempering temperatures, as a result of the very high heat input needed to get the stainless Austenitizing temperatures. Added to this is the tendency to heat the workpiece by radiation fron the elements themselves, rather than from the oven as a whole.

 

I've got it all down to what I think is an acceptable standard of control by paying attention to detail: a fast-responding control thermocouple, a short cycle time on the controller output and a ramp/soak controller. I'll admit to being slightly obsessive about temperature control, and to having access to instrumentation that few others on here will have, as part of my day job.

 

As a good alternative to going high-end on the control, one very good knifemaker I know buries his hardened blades in dry sand in a fish-kettle for tempering and sticks the whole kaboodle in his HT oven . It gives a huge thermal mass and effectively eliminates the overshoot issue, but obviously adds many hours to the tempering time.

 

As Daniel says, the biggest problem with using the same oven for both really seems to be the cooldown time. It takes several hours to cool the oven from around 800 degC (1472 degF) to around 200 degC (392 degF) and it's a long time to leave fully-hardened blades. For swords, a spell at intermediate temperature in the kitchen oven isn't even an option.

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For the last 3, I used a router, together with a shopvac and respirator. Infinitely easier and works for all 3 types of (soft) brick I've come across. I'd expected it to kill router cutters fast and bought several cheap ones, but I've probably done over 20 ft of 10mm wide groove and I'm still on the first one.

 

Tim,

Question regarding your approach with a router...

 

I thought about using a router - but as mine can't tilt (and I have not seen many a router who can be tilted on it's base), I would have cut straight 90° to the stone grooves which would not give a good retention to the wire... so how did you do this?

 

thanks

 

Daniel

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I just cut the grooves straight in and pinned the elements fairly closely using little U-shaped staples pushed into the bricks. The staples were bent from 1mm Kanthal wire.

 

The outside diameter of the coiled elements was only just under 10mm, so they were a pretty good fit in the grooves. They did feel a bit springy as I was fitting them in the first one, and I wasn't sure the staples would be enough to hold them in use. Everything seemed to stress-relieve on the first firing and I've not worried about it since.

 

The element seems to sit quite happily in the groove. Not the best photo and the oven's upside-down in it; that's the thermocouple on the "floor". There seems to be an Oxide layer on both the element and thermocouple, so it's been up to temperature. This one's an 18" oven.

 

DSCF0035.jpg

 

The odd angle is because the oven was standing on end at the time; the plan was to allow it to be used with a salt-pot, should I one day feel the urge.

 

DSCF0040.jpg

 

The 42" oven hadn't been fired when this photo was taken

 

DSCF0046.jpg

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You guys are oven masters. Thanks for the replies, I am now that much more educated. I wish I could think if some way past that thermal mass issue, with kiln cool-down vs. the need to temper immediately. What a bugger to need both a sword length aus. oven and a tempering oven.

 

Maybe I'll do something like build a drum forge for sword HT, then use a PID controller that can be hooked to either the oven or the long forge. I could still use the oven to anneal prior to grinding and such.

 

Maybe I need to just finish building my forging press and forget about this until I have the time and resources. Thanks again for the info, I'll be back to read this many times I'm sure.

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Tim,

 

Thanks for the "insight" ;) ...

Simple but wonderful idea (the A1-clamps) ... I'll use this for the next I build!

 

also nice idea with the salt-pot option.

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UPDATE ON THE WIRING OF THE KILN - IMPORTANT


As mentioned a few posts above - I had to make some changes to the wiring system... but of course I documented everything - read on:

Story:
The solution with using a third SSR to disconnect all power when the kiln is switched off or the door opened does not work... when I designed this late night I did of course not consider the displacement of phase on three-phase 400V system when connected as described earlier...
This wreaked havoc on the third (center) SSR which had Phase 2 connected to both coils...
To make a long story very short:
- 50$ SSR
- 2 hours driving
+ working kiln - priceless wink.gif

But now to the solution of the problem...
Again, to remind you of the WHY for the third SSR in it's original design: I NEEDED to be able to automatically KILL EVERY ELECTRICAL CURRENT going through the wires when the door was opened... for operator safety... when you've got a rather low powered 230V (110V US) kiln with let me say no more than 1kW power than the Current would be 1000/230 = 4.3 Amps @ 230V ... if you get zapped - this hurts but most likely won't cause serious injury...
now with 400V, at 3.8kW each or a total of 7.6kW - the current would be 7600/400: 19A - that could potentially be lethal.
Most home built ovens I have seen only use a SINGLE SSR to cut off the power from passing all the way through the coil - thus it does no longer heat up... BUT the current is still "on line" and if you manage to touch one of the coils (remember we use steel tongs, blades and stuff in and out of the kiln) AND mange to be "grounded" the current will ZAP you bad ass!! electrical current tries to go the way of the least resistance - and that could be you.
Now this is not as problematic on a single phase system if the SSR "breaks" the phase and thus the coil is only left with the neutral connected... but even there usually with AC wiring this doesn't matter... so there's going to be a current.

As I wanted to be 100% on the safe side - especially considering class-students working the kiln - I needed a solution for this:
it's called a 3-Pole Circuit Breaker with an electric switch (230V)... or also called a "contactor"...

here's a link to the manufacturer (they're a global company): http://www.klocknermoeller.com/dilm/dil0m.htm

these things will really KILL the power on all three phases once the switch is powered off... they work by having a 230V electromagnetic switch installed, as long as it's powered the big 3-phase wires are connected - if the 230V power is disconnected then the large contacts are automatically disconnected too.

To get the 230V I used one of the 400V Phases "connected" with the Neutral... on a 3-Phase system any phases over the neutral has 230V.
I had to install an extra cable to the door switch (thus I could accomodate the door-open kiln-off solution) and use this switch to break the 230V circuit powering the 3-pole breaker.
Also I had to remove the third unnecessary SSR and connect all three input phases to the 3-pole breaker and from there run Phase one to SSR1, Phase 2 to SSR 2 and connect the third Phase directly to the other ends of the heating coil.

here's a diagram of the wiring:
p131433907-5.jpg



and here are the new photos:



door-switch - upgraded isolation to make sure the 230V are no problem (the switch is rated up to 250V)
p848076003-4.jpg



The 3-pole breaker wired (note: ceramic clamps re not required... but I had those in abundance and no other need for them):
p891998521-4.jpg



mounted
p555846928-4.jpg



additional cable for the door-switch:
p920229346-4.jpg




Everything mounted in place: worth noting extensive use of cable ties to make sure there are no wires "free floating"... neat and tidy:
also notable: added a heat-sink to the SSR... they can get warm.... and doing so improves their lifetime....
p673221389-4.jpg


p1052310861-4.jpg



THAT'S IT... a safe 3-phase 400V kiln... sword sized

the kiln is test firing now... 1100° seem to be no problem wink.gif



and I'm off to make some delicious chicken, jalapeños, cheddar stuffed tacos and will wash that down with a nice beer...
cheers.

Edited by DGentile

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Update - test firing complete ;)

 

reaches 1300°C (2372°F)...

 

 

Note: Probably would go higher.... but that would push the limit on the thermocouple, stones and stuff... besides I have no need for anything above 1200°C in a HT oven.

But it's nice to know.

 

 

Note 2: ahh, I can make Mokume again ;)

 

Note 3: I think with the remaining coils I'm going to build a Tempering Kiln...

though that will be a pretty basic set up - 230V, single phase... nothing fancy... .cheap ;)

Note 4: probably a bad idea, but I ponder to finally construct a single-unit pizza oven with a PID controller... something reaching 350°C ...

Note 5: nah, don't bother.

Edited by DGentile

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Superb work, clean as ever and very well explained ! Thanks to take the time and share your knowledgeS yes, with big S B)

Stephane

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Thanks indeed, Daniel! When I eventually get around to building one of these it will most likely be 230V single phase since 400V 3 phase is ridiculously expensive to get here (unless you live in an industrial zone), but it's still good to know how to wire things for it. B)

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Alan,

 

Glad this is helping...!

 

if you go single phase:

 

make sure to use a pole-breaker (contactor) (they're available for 2-poles as well... but you can use a 3-pole... doesn't make any difference).

you'd only need a single SSR if you go with a single coil....

but other than that, you can copy the schematics 1:1...

 

DO NOT USE JUST A SSR to break the power to the coil - THIS IS NOT SAFE and can still fry you pretty good.

There is still a current on the coil - it's just not a closed circuit anymore, but it's still the full load!

I've seen too many kilns without that safety...

Edited by DGentile

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Superb work, clean as ever and very well explained ! Thanks to take the time and share your knowledgeS yes, with big S B)

Stephane

 

Thanks mate!

 

You'll get an e-mail soon ;)

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I really like the simplicity of design in this kiln. I have been wondering how to HT blades, and this looks like a good solution. I am a tad nervous about self winding my own heating coils. Could you elaborate a bit more on how you make the coils themselves?

 

Can you buy ready made heating elements for this sort of thing?

 

Thanks

 

Julia

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