New HT oven build advice

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Maelstrom78

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Sep 21, 2013
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My Dad and Uncle volunteered to help me build a HT oven. Happy day since I am not confident in my electrician skills and they have good backgrounds in engineering and electrical things.

I found this informative blog with a HT build on 110v but I have a 30 Amp 220V service to my shop so I was wondering if anyone had a build/part list/ anything that would be helpful.

http://dcknives.blogspot.ca/p/electric-forge.html

They said if I could find good plans they would be happy to help me wire it.

Using the bladeforums search google thing I did find quite a few sites but the links to what elements or PIDs to but were defunct.

And maybe there is something that works better than the fine gentleman at dcknives blog nowadays?

Any help is appreciated. I know it is $ wise many times worth it more to send out for HT but I really do like the soup to nuts approach as my knives end up being gifts anyway.

Thanks!
 
I got my grinder plans from dcknives blog and its amazing.... Id follow his exact plan if you can. He's a smart guy, knows what hes doing.

I actually just bookmarked that page, I am planning on building an oven later this year or next year. Depending on if I don't end up buying an Evenheat :rolleyes:
 
If they are familiar with electric circuits and engineering, the switch to 220VAC will be simple for them. Let them handle all that.

Use heavier gauge Kanthal coils. They last longer. Your Dad and Uncle will compute the length and type you need

The enclosure should be robust,...and well insulated. It should be 18-24" deep. Study the specs on several commercial units to get the main idea about chamber size and heat output. Paragon and EvenHeat 24" knifemaker ovens are the most used.

You need a programmable controller, not just a regular PID. It needs Ramp/Soak ability with at least six steps. The ability to store multiple programs is very useful.

Use a 40amp SSR mounted on good heat sink fins. Mount the controller and SSR to the side of the oven...never on top.

Use two SSRs for the best safety setup when wiring 220VAC in the US. With only one SSR, you can get a nasty shock if you accidentally contact the coils and ground ( even with the PID/SSR turned off).

Put a door interlock ( called a limit switch to the engineer types) on the door. It breaks/opens the low voltage power line from the PID to the SSR when the door opens.
 
Personally I would build the controller as a stand alone item that was not attached to the kiln at all. Run a power cable from the kiln and plug it into the controller. This would keep the controller away from the heat and it can easily be moved from one piece of equipment to another.
 
The DC Knives write up is really solid.

I have a build thread here and you may learn from a few of my mistakes. But it also shows you can do a sheet metal case cheaply and without welding (important for a lot of us). I like how compact mine turned out but it's easier to run e elements in a big oven.

DC Knives shows mortar between the blocks. Hi temp mortar is expensive and doesn't stick to firebrick very well. Several people have made ovens without filling the gaps between bricks. I think I would go that route next time.
 
Thanks for your responses so far. There is a local pottery place that I can buy the soft firebrick from.

For the wire and electronics where would I order that stuff?

Thanks!
 
You might check this place http://www.auberins.com/index.php?main_page=product_info&cPath=1&products_id=4 out they have "PID's" and Thermocouples and enclosures for your controller and all the little accessory switches you will need to build a controller. The Ramp Soak feature is a must and they carry units that are capable of 30 programmable steps....They carry the heat sinks and SSR's which are a must to dissipate heat.
 
I'm with Glassguy on the separate control box. Auber have a ramp/soak controller that a lot of guys in the USA & Canada seem to like. Omega & AutomationDirect both have a ramp/soak controller that stores 4 diffent sets of PID terms. It uses the set from the tuning temperature closest to the setpoint. If you might build a tempering oven at some point, this can make it much less hassle when changing over.

Auber do not have a presence in the UK, so I have not tried them. Omega and AD both have very good support. I really would not recommend buying any controller unless you have seen the manual and understand it. If the manual is online, you can link to it if you get stuck and someone with experience of controllers may be able to help.

Electrics are going to need to be specific to your country, but everything else is universal. The British Blades forum has quite a lot of self-built HT oven information, largely because there are no European equivalents of Paragon and Evenheat. I came across some useful information from Australian and South African builders when I was researching to build my first oven.

If you are intending to treat batches, a door-latch with fast, one-handed operation is a good idea.

RCD or GFCI protection seems wise. I use a contactor to interrupt the main power. Having had SSRs fail "closed", I am not prepared to risk just breaking the low--voltage DC trigger circuit.

Thicker Kanthal wire is better for element life expectancy, but hard to fit into a small oven.
 
Of the half-dozen HT ovens I've built, two have had element failures on 16AWG kanthal A1 elements within the first year of use. They were both in use by full-time professional bladesmiths and the others have been OK in hobby or semi-pro use. The oldest is about 4 years old.

I stepped up to 1.6mm diameter (14 awg) Kanthal A1 for my most recent build, for a guy who has just retired early and is now making knives as a paying concern.
 
Ok, I have a preliminary plan and parts list that I would like advice on. Here is the list!

Interior Chamber Size: 9”w x 5 ½”h x 22 ½”d.
Frame: Welded Steel Angle 1/8 x 1 ½.
Exterior: 1/8 alum diamond plate, place 10-24NC sstl pan head screws to miss upsets.
Bricks: K23 soft firebrick 9” x 4 ½ x 2 ½ wt= 2 ¼# ea.
Door: Angle frame with adjustable hinge and welded inside Gr8 bolts (nuts to outside).
Latch: Cam locking adjustable type with refractory rope door gasket glued into groove.
Feet: Extended side vertical angles with foot levelers.
Elements: BCS Kanthal #7101 x 2 $49 120vac each x 13A.
Connection wire: BCS High Temp AWG12 x 6ft $24.99.
Connection: Micarta block located on bottom of oven with SStl 10-24NC screws. Use extension handibox with blank cover (no bottom on handibox).
PID Controller: Auber #SYL-2352P $78.95 for SSR output and soak feature
SSR: (2) Auber #MGR-1D4825 25A capacity with exterior heat sink.
Thermocouple: Auber #TC-K-KLN $27.75 2300deg F.
Finish: Black heat paint on steel angles.
Cooling: Possible cooling fan on PID and SSR’s heat sinks.



Thoughts? Red flags?
 
If you can find it in your budget to get some Kaowool (sp) to help insulate the brick from the exterior plate this will do two things keep the heat inside and keep the aluminum cool in case you bump it with your arm. If you plan to attach the PID to the side of the unit use angle iron and allow a little air flow between the exterior of both units. I don't think you will need the cooling fan you make the call. I have been using my DIY Furnace for 10 years without one and seems to be fine (I did tear down the exterior of the chamber to wrap the bricks with the Kaowool blanket). Get a tube of High Temperature Fireplace Caulking to seal your brick joints. One note I made my unit so I could easily remove the Top Bricks to access the interior to replace the Kanthal elements should they fail. Were you wiring this 120v or 220v....If you can access 220v Do It cuts the Ramp time to near nothing!!!!

Good Luck....:thumbup:
 
Thanks Berto. Planning on a removable top and an wired for 220v. I'll have to think a bit on how the kaowool and bricks would work together.

One thing I was thinking was I read that it's recommended to keep your blades away from the elements. Anyone try a thin slice of refractory brick by the elements? Would block the direct "blast" at the blades.
 
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I don't skin mine in metal.

When I built the first one, I wasn't sure how hot the surface would get, so built the frame, fitted the bricks and fully intended to clad the outside with either plastic-coated steel, aluminium sheet or stainless steel sheet, once I'd measured the surface temperature and knew what would be most appropriate. Using a type K surface thermocouple, I measured 129 degC (264 degF) at the outer surface of the bricks after several hours at 1177 degC (2150 degF).

It seemed pretty clear that cladding would look nicer, but would probably be less safe than leaving the brick surface exposed.

I borrowed an infra-red camera from work and used it to measure the surface temperature at 136 degC with the oven in use. I could press my hand against the brick surface for a reasonable period of time (tens of seconds) without harm or serious pain. When I took my hand off, the cooler handprint was clearly visible with the IR camera for some tens of seconds.

The surface temperature was obviously too high to allow the cheap option of plastic-coated steel. Aluminium looked like it would be fine, but it was pretty clear that brief contact with a metallic surface at something in the region of 250 degF would be significantly more dangerous than contact with the IFB at a similar temperature,

I actually did the math for putting a thin layer of kaowool between the bricks and a skin, and there was very little change in the calculated surface temperature. Certainly not enough to justify the extra work.

Your plan is for a 9" wide chamber using 9" bricks?

It gets a lot easier if you reduce the width so that a 9" brick across the top is supported both ends. I usually go 7" wide, though you could probably get away with 8".

When I built a sword-length HT oven with a 42" long chamber, I went 9" wide and 6" high (9" x 4.5" x 3" IFBs) and used a layer of 1" ceramic fiber board (1260 degC/2300 degF rating) and 3" of Calcium Silicate board (1000 degC/1832 degF rating) for the roof. It made for a quick and easy build and I used the same construction for the door as I used for the roof.



I have always used 3" bricks, rather than 2 1/2". Once you've cut the element grooves, a 3" brick still gives maybe 2 1/2" of remaining wall thickness. With 2 1/2" bricks, it would be down to around 2".

I don't like the allthread-through-the-wall method of connecting the elements. I now use twisted Kanthal through the wall and connect to the elements on the outside.
 
Great info! The ceiling info with the larger board is genius.

Latest question from my Dad.
"Any discussions about running power wiring in the same BX cable as the low voltage thermocouple wire? Seems to me we may experience inducing voltage into that wire if it is not shielded. Could cause erratic control issues.
What about a hinged side control box that folds flat against the top of the oven when not in use?
Thermocouple should have 12” 11GA wires. You can trim them to length."
 
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I have never read about issues with "noise" in low voltage TC wiring. I don't think it is an issue on builds like ours.

The TC wires should be heavy gauge, as you said, and can be cut to the needed length. Use a TC terminal block to connect them to the TC wires. NOTE: TC wires are not regular wires, They are two different metals and are polarized. You have to use type K wires to connect a type K thermocouple to the controller.

A hinged control box that flips up to store sounds good as long as the heat isn't being radiated or conducted over to the box. Make it so it stands away from the oven a good bit. I think 12" separation would be a good distance.
 
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