Heat Treatment Oven Project

 Heat Treatment Oven Project


After making my 3rd knife and finding it difficult to maintain 1000°C temperatures, sustained and accurately, I decided that an electric heat treating oven is the way to go. I have no intention (at this time) of spending $1000 on an off-the-shelf unit for my "hobby", so I set out to design and build one myself. Thank you MacGyver!

WARNING: I am offering this information for entertainment purposes only. I will not be liable for any use or misuse of this information. This project deals with hazardous voltages, temperatures and substances that you must understand and work with at your own risk. If you are not knowledgeable about electricity consult with a qualified electrician.

Design Considerations and Procurement

After reviewing the data sheets from Crucible Industries LLC., I am finding the heat treatment detail for stainless steels such as 154CM, CPM154, CPM S30 and S35VN and such require almost 1100°C (2000°F) for an extended period of time to convert Austenite into Martensite within the steel. CPM154 requires 1065°C (1950°F) for up to 1 hour, then quenched. This is effectively getting the steel "yellow hot" and a small gas forge doesn't do this without some extra effort.

One side effect of using an electric oven is that oxygen is sufficiently present to cause scale on the surface of the steel which has to be cleaned off, thus causing more work to be needed. To thwart the effects of oxygen, a gas blanket, say nitrogen can be used to purge the oxygen from the oven or a simple foil pouch of stainless steel can be used. The foil acts as a hermetic seal that keeps oxygen away from the surface of the steel while it is at carburization temperatures.

Bottom line, just under 1100°C for a half-hour with a SS foil wrap.

Insulation

I randomly happened upon an ad on Kijij for "pottery kiln bricks." The seller had multiple boxes of K23 insulating fire bricks from Babcock & Wilcox. I handily picked up a box of 25 and a few loosies. I love it when this happens!

These bricks are soft, lightweight and super easy to cut, shape, drill, scrape or however we want to work with them. The dust is a hazard, so always wear some safety glasses, gloves and particle respirator when cutting, sanding or drilling. Keep a vacuum handy.


People keep asking where to buy insulating fire brick. A type very similar to what used can be found here on eBay. Insulating Fire Bricks

Size

I wanted to build a front door kiln with a fairly small volume. I didn't want to build/heat a massive pottery kiln to do a few knives. I decided based in the bricks that a interior size of 13" (long) x 9" (wide) x 6.5" (high) would suffice for almost any knife I would be heat treating. This shape would accommodate a blade 15 inches long is place in the oven diagonally. I wanted to be able to reach in with a glove on and with tongs, so 6.5" seemed high enough. The total volume is just under 0.44 cubic feet, so small and relatively easy to heat up with the power I will be using.
 

Electrical

The overall circuit consists of a 120VAC 20A supplied electric heating element. My target total resistance is 6.6 ohms and thus the current draw from the heater itself is around 18A at 120VAC.  This works out to about 2160W, which is ideal for a small space like the one I am going for. My survey of off the shelf heat treating ovens from Evenheat range are typically 3500 to 7200 watts per cubic foot.

 
Model W/Cu. Ft.
KO 18 6051
KH 414 7228
KH 418 5421
Artisan 688 6480
KF 18 4608
KF 13.5 4608
KF 27 3840


Average 5462

My design should come in about 5000 W per cubic foot, which should provide similar performance to the Evenheat models of similar ratings and capacity.


Here in Canada, 20A circuits are quite common, especially in kitchens and is now code to have two in every kitchen for new construction. Not that I want to put my kiln in my kitchen, but that possibility exists. My garage has several dedicated 20A receptacles. I could have designed for 240VAC, but that would mean a 2 pole breaker and extra wiring and a new plug where I intend to use the oven. Your mileage may vary, so if you decide to go with a 240V version, you will need to calculate and adjust your over-current protection, wire sizes and heating element(s) and plug to match.

For heating wire I chose Kanthal A1 which is good to 1400°C. I bought some ready-made 3kW 220V heating elements (eBay). These are 31.5" long and 0.25" in diameter and work out to be 30 ohms each. I will arrange them in parallel, three at 20 ohms each. The formula for resistors in parallel is Rt = 1/1/R1+1/R2+1/R3... so we end up with a calculated 6.33 ohms, with my elements and multimeter reads as 6.6 ohms.  120 Volts divided by 6.6 Ohms = 18.1 Amperes. 18.1 A times 120 V = 2181 Watts. Just about right! The great thing about Kanthal is that the resistance changes very little over the temperature range of the wire. At 1100°C it increases only by  4% which in my circuit amounts to 6.8 ohms.


For temperature control I bought a small PID temperature controller and a Solid State Relay (SSR) that can handle 25 amperes and 380 VAC. This is Mypin TA4-SSR. The 400°C probe that came with it cannot be used in this application so I bought a K type 1250°C thermocouple on eBay. NOTE: Some photos show my initial choice for the temperature controller at CXTG-3000, which crapped the bed after 1/2 hour of use.

BE WARNED! Your PID controller must be suitable for driving a Solid State Relay (SSR). The Mypin part number would be like TA4-Sxx. The S means that it was meant to drive an SSR. If there was an R in that place, it was meant to drive a Relay. 

If you are unfamiliar with PID, it means Proportional Integral Derivative, which is essentially "lots of gas when you are getting up to speed and less gas when you get are at the desired speed." In this case, the heating power will be increased when warming up and then reduced as the kiln interior gets closer to the target temperature. One benefit of a PID controller is that as you get close to the target temperature, it doesn't "overshoot" and push the temperature too high; which can be the case with a conventional open/close heating circuit that is slow to respond.

Recommended hookup wire for the heating wire hookup is type SEW /SF2 or similar high temperature, silicone insulated wire. All I could find in my scrap pile was 12 AWG, so I went with that. The 20A power wiring will be 12 AWG and the rest of the control wiring will be 18 to 20 AWG or whatever I can muster from the scrap pile.



The rest of the electrical are miscellaneous breakers, fuses, wire, crimp connectors, nuts bolts,  are generally easy to find in the industrialized world, aka eBay, Home Depot, Lowes etc, and so on that I will document when I put this all together. Essentially everything I required could be had from off of eBay. I will publish a Bill of Materials in the PDF instructions.


Construction

I started by placing the fire bricks in a variety of different ways to create a suitable interior dimensions for heating small parts. Once I have the arrangement, I planned where the heating elements are going to be routed on the inside.

By using a pencil attached to various blocks of wood, I could draw parallel lines for the heating element grooves. A compass can help with the bends.
This will be the inside of the ceiling blocks with the power coming in on the right side.
I went over the pencil lines with a permanent marker.
I mortared the pieces together with high temperature fireplace mortar. Put a good bead down then smear it smooth. Both of the mating faces get this treatment.
Once the walls are stuck together, I start routing. Same goes for back and roof.








The magic is in this 7/32" drill bit and this piece of an old bed knob. I drilled a 1/4" hole right through the bed knob and inserted the 7/32" bit into the chuck so that about 1/4" is sticking out.







With a straight edge, I walked the bit along the lines to dig the groove. Periodically applying the shop vac to see how things are going. 

The net result is the grooving process worked out pretty well. On the right side wall I drilled a place for the connections to go through the fire brick.

Note: We cannot connect copper wire inside the oven as the wires would melt (1085°C melting point).


Now to cement the bricks together with high temperature mortar.


This is the view looking in from the front. Now for the electrical connections.

 

 

 

 

 

 

 Heating Electrical Connections

The three heating elements in this design are in parallel, which means I need to have six leads coming from inside to the outside. These will then be connected three N (Neutral) sides together and three L (Line) sides together.

Element Wiring

The three elements have holes that feed them from the right side of the oven and the control box will be on the right side as well. I will be passing the Kanthal through the firebrick and making a connection to the SEW wire on the outside of the oven. These connections will be covered and the SEW will be fed into the control box via a strain relief.

The process of installing the elements is quite simple. Twist about 3" of lead on each end of the element and feed the lead through the hole and staple the element into place with U shaped staples I made from carbon steel welding wire. My Kanthal comes wound pretty consistent and I made the groove size to fit, so not too many staples are required. Say 1 staple every 4 to 6 inches. More may be needed if your heating wire doesn't friction fit as tight. Use some needlenose pliers and push the staples in at about a 45 degree angle to secure the heating elements.


 Twisted ends about 3" long. These will pass through the holes so the electrical connection can be made outside the oven.
About 5/8" of folded-back wire should stick out of the fire brick.

 Vise-grips hold the end outside the oven while I run the heating wire in the groove on the inside.






This is how I am connecting to the elements from outside the oven. These are the brass wire nut from Marettes, the heavy duty screw clamp kind.










Checking the resistance of the ceiling element. Okay!

Mechanical Assembly

After the heating circuits are wired in, we need to put the brick panels all togther and mechanically secure them so everything fits tight. There also needs to be a hinge and a frame to hold the door on. An idea for a corner frame made with flat bar, ready-rod, a few pieces of angle iron and a little bit of welding came to mind.

 I cut the 1-1/2" angle to 18" (the length of the oven, less the door).
 Two 11 1/4" cross-members from 1/8" x 1 1/4" flat bar.
 Clamp and weld. Check with a square before welding.
I found these legs at a garage sale. They are from IKEA. A fancy touch for a $1.
I drilled the 12 holes in the frame to 3/16" and counter sank them so they are ready to attach the legs with flat head machine screws and nuts. We'll get everything done and painted first before screwing the legs on.







 Door frame. Marking for 45°.
 Door frame cuts. Three of these required at 14", 25 1/2" and 39 1/2"
Break it back on itself and tack weld the joints. I don't have a brake at home so I used a machinist's vise and hammer. Check for square and continue welding.








Oven Frame

The oven frame is probably way over complicated, but I was planning on using scrap that I had in my shop. I wanted to be able to adjust the door at the hinge to allow a precise fit. A simple way to do this is to slip a tube over a vertical piece of ready rod that is clamping the roof panel down.
 

On the left front the hole for the ready rod is drilled in a piece of angle and welded to the bottom frame.









Additional angles are welded to the rear sides. The front right is a single piece of 1/8" flat bar that is bent on a 90°. This acts as a clamp when tightened down on the left. 







A quick shot of paint as a primer and attached the legs.
Fitting the IFB panels in. I had some scrap 0.02 aluminum that I cut with tin snips and rubbed with a scouring pad to give a brushed look. The angles are corner supports from a suspended ceiling system that I painted to match the frame.









 Miscellaneous fabricated parts. Switch brackets and door hinge. The hold downs as 5/16" ready-rod with miscellaneous nuts and washers.
 Door installed. I used some 80 grit sand paper on a block to match the surfaces to close and make a good seal. Now adding the latch on the right side.
Neodymium magnet for holding door tightly closed. (Note: I changed this in 2015. See Updates at the bottom of this page.)


















Control Panel



The control system is enclosed in a suitable box, in my case I made something from material I had handy in my shop. 

The general layout is shown on the left.








Some 1/8" aluminum would be rigid enough to mount the controls in and act as a heat sink for the SSR.

I marked out the front and rear and drilled holes.
 The PID controller needs a square so I drilled the corner holes so the jig saw blade could get started. Clamped down to the bench I cut the square out rough.
 Filed to the square opening to the line.
 Test fit. 45mm is about right. Note: I had to replace this temperature controller as this one died after about an hour. The hole size should be the same for any 1/16 DIN device, 45mm x 45mm.


As much as possible I prewired the switches and other components to make the wiring in  the enclosure easier.




 Rear panel. This needs holes for the following: Main power cable, Fuse 1 (main power) and F2 (control power), Thermocouple, door switch and output to the heating circuit.







 The is the front panel layout.Clockwise from PID controller: 1: Oven On LED, 2: Door Open LED, 3 - Elements Off LED, 5: Elements Switch, 6: Main Power Switch, 7: Main Power On LED.
 On the underside of the oven I have a momentary NO/NC switch installed. It will shut the elements off when the door is opened. The NC contact is wired to the Door Open LED on the front panel. The 1/4" bolt allows for adjustment of the actuator.








The thermocouple is fitted to a piece of Glastic and cemented in with high temperature mortar from the bottom. The probe extends up into the centre of the oven. This is away from the elements and where the steel is going to be heating.








I will make a firebrick stand for the blades that will have a hole in the middle to accommodate the thermocouple.







The heating circuit wiring is connected with 12 AWG type SEW (SF2), a 200°C silicone/fibreglass insulated wire. The perimeter is an insulating board called Glastic which is a fibreglass composite. In hindsight and after reviewing some other designs, I could have brought the element connections out the back.





 Another few pieces of Glastic will make a protective cover for the electrical connections. Glastic is rated for up to 210°C (410°F). I am hoping the exterior will never get close to this temperature. Only testing will determine this.


This is a look at the wiring before the controller and oven connections are made. The LEDs are wired for 120 VAC.

On the left side is the main power switch along with the LEDs and Element Off switch.  The right side (back of the enclosure) contains the SSR, power fuses and I/O strain reliefs. Note that it's a good idea to put some thermal compound on the SSR when mounting it. This helps dissipate the heat.
 

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DO NOT CONNECT ANY LED DIRECTLY TO 120 VOLTS!


I wired the LEDs for 120 VAC with a single series resistor of 68k Ohms 1/2 Watt and a 1N4007 diode. This will light the LED and protect it from over current and excessive reverse voltage.

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PID controller is wired in and system check. Note I've bonded the enclosure case to the green ground wire with a stud on the rear panel. There is a wire that runs down to the metal frame to bond that as well.





One last check on the heating circuit. 6.6 ohms. Should be good!

20A receptacle with unit plugged in.










Let there be heat! Initially the temperature is climbing quickly, the slows to about 0.5 degree per second around 500°C. I will plot the temperature over time and see if I can improve the performance.














This is the SSR during firing. It has an LED in the control side that shows when it's on. SSRs are essentially triacs with optically coupled gates. The DC side can be from 3 to 32 volts. Putting voltage on the control side turns the triac on and the AC is allowed to flow.
















A brief video showing the PID controller pulsing the heating circuit. As the internal temperature measured by the thermocouple (PV or Process Value) gets close to the SV (Setpoint Value) of 100.0°C, the controller reduces the heat to slow down the approach and avoid overshooting the setpoint.

video



I will start some tests and report back my findings shortly.


Schematic

 






I've had so many requests for a simple 240 V version I made this schematic. Note the 240 V version uses some but NOT ALL the same parts listed on the HT2100 Bill of Materials.





 

Performance Graph


OBSERVATIONS:
Firebricks expanded/shifted leaving gap on door around 650°C, decreasing overall efficiency. These will have to be re-seated. Exterior warm to touch after 60 minutes (700°C).

Overall, I am thinking I can improve on the heating curve by increasing the insulation in certain areas e.g. gaps in door seal, slightly reducing the interior volume by adding some more IFB on the floor. Because it is cold in my garage (below freezing) I expected some influence of  the ambient temperature. The target is 1050°C (1922°F) but unfortunately, I ran out of time to prove this, but it looks a linear rise. Close but no cigar! Later.


Looking at the economics of owning your own HT oven, I am computing it's a big plus! My heat treatment service charges $17.50 for an 8 to 12" blade, plus I have to mail it there and wait. Express envelope is $10.92, so to send out for a single 9" blade is CA$28.42. Based on my last power bill, this oven would cost me $0.28 per hour. So running the HT2100 for 4 hours would only be $1.12. I am happy about that, even if I double this value for using my tempering oven time. Plus, I don't think I'd run this for 4 hours unless I had at least 2 or 3 blades in there. So savings all around.

video

 

 

 

 

 

 

After the heat run, I powered the oven off. Almost exactly two hours later I turned the power back on again. It was 386°C in there still! Talk about amazing insulation of fire brick!

 

Success!

I was able to get to suitable soak temperature for almost all the knife steels I could want. I did have to set the target "SV" temperature higher than what I wanted. Because of this I am thinking the oven was approaching its maximum temperature limit. Works for me!



 

 

 

 

 

Handy Temperature Conversion Chart


 

















UPDATES:
January 2015



I changed the magnet for a more positive closure system. This is a bolt welded on a piece of pipe. The female part at the oven in threaded. The wood is only a handle to crank the door closed.





Bill of Materials (draft) is here! HT2100 Bill of Materials and Schematic.


May 2015
I replaced the thermocouple. I was doing some work inside tweaking the elements and the tip of the probe broke off. Luckily, I had a replacement on hand as I was preparing to do a sanity check with two probes/controllers.

New Elements
When I did the first element change I added some support to the ceiling. It wasn't exactly falling apart, but I would imagine that successive element changes would leave a pile of rubble. I used 1/8" steel rod and cut it into piece slightly less than the width of the bricks. I ground the tip of the rod to a chisel. I then "drilled" it through the bricks across the ceiling. This creates an internal wire frame that holds the bricks better. I also touched up the mortar in a few places.

June 2016
Added schematic for simplified 240 Volt, 3000 Watt oven.

New Heat Run Data


82 comments:

  1. nice projeck
    n thank's, it's very help me..

    ReplyDelete
  2. Great Project, did you ever get the materials list complete?

    ReplyDelete
    Replies
    1. Bill of Materials is linked on from this page.

      Delete
  3. Great Products and specification, blog is creative and readable... Heat Treatment Furnace , Electric Furnaces, Industrial Ovens Manufacturer in India | AFECO Heating System

    ReplyDelete
  4. Nice job. Can you provide some information on programing/setting up the PID. I am rebuilding an old kiln, reduced in size and with new elements which I am coiling myself. I have the same controller and a similar SSR, except I am running at 240V. Most of the kiln work is streight forward, but the Mypin instructions are vague and most of the online information is for much lower temperatures.
    Thanks,
    John

    ReplyDelete
    Replies
    1. Hi John,

      The only things I did to the Mypin TA4 was set the Input Type to 'K' which automatically has a range of 0-1200°C. (Of course I had to buy a separate 1200°C probe as the one that came with the temperature controller was rated to 400°C probe.) Then I adjusted the set variable to 1080°C. I left the PID settings as default. I haven't found a use for the alarms as of yet so these are not set. The alarm 1 LED comes on at 100 degrees, but that's not a big deal.

      I did find that winding my own elements from 16 AWG Kanthal was a large improvement on heating times. The new elements heat up fast. I am getting to 1000°C in less than an hour now, which is much less waiting than before.

      If there are any specifics I can help with, please let me know. I followed the section in the Mypin instructions called "Parameter Setting". Input type was the important one. Good luck!

      Dan

      Delete
  5. Hi Dan,

    Can you let me know what dimensions your bricks were? I'm trying to order the correct amount to be cost efficient and I've seen quite a few different options.

    ReplyDelete
  6. Hi,

    The bricks that I found are 9" x 4-1/2" x 2-1/2". Essentially the walls are 2-1/2" thick. I used 23 bricks in total, but they come from Babcock & Wilcox come in boxes of 25.

    Cheers!

    Dan

    ReplyDelete
  7. Hi Dan,

    Thank you for such a detailed and methodical documentation of the process. It is very refreshing to have such detailed instructions!

    Were you able to get around to doing a bill of materials? I am from Australia and sometimes find it hard to source the same things that are listed in North American articles.

    I would also love a bit more detail into the electronics and wiring details. I really liked how you did it.

    Cheers,
    John

    ReplyDelete
  8. Great work! Execellent information. Which model led did you purchase from digi key?

    ReplyDelete
  9. Any 5mm domed LED will work. I had a little drawer full of LEDs that I picked through. Some were from the dollar store (look in the toys there for LEDs). Often you can get a pile of them for $1 and re-purpose them. Digikey has many to choose from, look around here: http://www.digikey.com/product-search/en?mpart=C512A-WNN-CZ0B0151&vendor=90

    Dan

    ReplyDelete
  10. This comment has been removed by a blog administrator.

    ReplyDelete
  11. Hi there . Would like to know what the highest temperature was that you have reached with this kiln?

    Regards Willem

    ReplyDelete
    Replies
    1. Hi Willem,

      I have gone up to 1100°C (2012°F). This is the maximum typically needed for heat treatment. The controller goes to 1200°C and the Kanthal wire is good for 1400°C. Most of the stainless steel I heat treat requires 1050 to 1080°C (1922 to 1976°F) .

      Delete
  12. Can you make a detailed material list.

    ReplyDelete
    Replies
    1. Yes there is a Bill of Materials link above. Or download the Excel file here:
      here

      Delete
  13. I was wondering about the support for the middle bricks in the ceiling. I heard some people say the fire-proof mortar isn't very strong. It seems like this mortar is all that is holding up these bricks; am I missing something? If not, in hindsight, are you happy with the ceiling?

    ReplyDelete
    Replies
    1. Hi Bart,

      I did reinforce the ceiling when I did the element change in 2014. I did this steel rod and sort of drilled them through the bricks. I touched up the mortar a bit at that time.

      In hindsight, I would have made the ceiling bricks rest on the walls, although this would have made for a narrower chamber. Also, in hindsight, I would have ran all the elements only in the walls. The element wire expands significantly when hot and droops if you don't staple it down every few inches.

      It's still working well for me, so I will let sleeping dogs lie. One day I will take what I learned and start on a new one.

      Dan

      Delete
  14. very nice. I think I might have a bit of a shortcut maybe. Was on craigslist and found about 6 ceramic kilns cheap (kind of). Great presentation

    ReplyDelete
    Replies
    1. Thanks! Getting some insulating fire bricks of any kind is the place to start.

      Dan

      Delete
  15. Hello.
    friend, you could help me.
    I'm designing an electric kiln for fusing glass.
    I intend to reach 1250 ° C.

    The idea of the kiln would be cylindrical.
    with a voltage of 220V.
    5000watts calculated; I do not know if I am right in this calculation.

    Oven dimensions that I think is:
    63.50cm in diameter
    35 cm

    Would I like an idea on the issue of resistances that will use
    If in series or parallel.
    Size of these resistances
    Wire Thickness

    If anyone can help me, I thank you.

    ReplyDelete
  16. I'm going to build your kilm thanks for all your pictures and information.

    ReplyDelete
  17. Can you give an over all price of the build ?

    ReplyDelete
    Replies
    1. Based on what I scrounged up and the deal I got on the firebrick around $250.

      Delete
  18. Did you put anything or staples to hold the element in the bricks? Or it's just by pression. And did it hold during all those firing?

    ReplyDelete
    Replies
    1. Yes, staples I made from Kanthal A1 wire. They are about 2" (50mm) long and shaped like a V, I used pliers to push them into the soft brick. I probably used around 50 of these to hold the elements in place.

      Delete
  19. Is there a reason to run multiple elements other than to set up the appropriate resistance? In other words; can one element (60' of 14ga A1 wire) in a single element for 18.8 amps @ 240v be used?

    ReplyDelete
    Replies
    1. That's perfectly fine. My original design used off-the-shelf elements that were 19 ohms each. As long as your getting even heating. In hind sight, I'd drop the ceiling element and run only element(s) in the walls.

      Dan

      Delete
  20. Nice job. Now just FYI a well made 2kw 220VAC laboratory kiln reaches 1000C for 20min and outside is NEVER hot. So it seems if you have invested in refractory wool for the outside you would have achieved this. Also the lab kiln i used, used wool for the perfect closure of the gap between the door and the kiln.
    Just my 2c for the would be builders
    Anyway, you did a great job, especially documenting all that. Unfortunately at the time i sold the kiln and now with the combined knowledge i see spending more money for the wool is unavoidable for me.

    ReplyDelete
  21. hello,
    I am making my own oven
    and I am interesting my some questions/precisions about your electrical sketch (I am not an electrician).
    1-"Oven on lamp" what is for ? and what is the difference with "element off lamp".
    2- What is the difference between S3 and S2 ?
    3- I understand position of S2 and S3 directly connected to plug 3 of PID but I don't understand the 2 del and their switch (it is switch ??) in the most bottom left part.
    sorry for my ignorance (other way to contact you ? I am not comfortable with social network...)
    Laurent

    ReplyDelete
    Replies
    1. Hi Laurent,

      The main power to the oven can remain on while the controller dictates whether the element is on or off. Also, when the door is open the element is powered off despite what the controller wants.

      The LEDs are showing status only. One is main power, another is Door Open (door switch is open), another is the status of the element power.

      The reasons for this configuration is than when annealing, you want to keep the controller powered up while turning the elements off, thus a long cool down can occur with the door closed. You can keep monitoring the oven temperature and the controller is a monitor only as it cannot turn the heating elements on.

      Delete
  22. Thanks !
    and I understood you use a momentary pushbutton with 2 throw (for S3) and S2 is a DPST switch ! right ?
    Thanks

    ReplyDelete
  23. Hello Dan. I am building an oven and really like/appreciate your build and sharing of information. I am using your schematic and am curious about the thermal cut out switch. Is the to prevent the electronics from melting if it gets too hot?

    ReplyDelete
    Replies
    1. Yes, the cutout button is made to kill the power if the insides of the control box exceed 105°C (221°F) This around the temperature where most wire insulation will be getting pretty runny. ;-) I did find the 90°C (194°F) button easier to come by.

      Dan

      Delete
  24. Just curious, would you be able to use all the same parts if you used 220vac instead of 110vac? I just think going 220 might be more efficient. Just curious what you think.

    ReplyDelete
    Replies
    1. The schematic above will not work for 240 VAC as there were some very specific things that I did for 120 V operation. Increasing the voltage will increase the power and make the unit heat faster, but the only way to improve efficiency would be to increase the thickness of the bricks or seal them better from heat loss. I would recommend going to 3000 Watts at 240 VAC. This will require a different plug, additional fuse, different LED resistors and so on. I would love to put up a schematic for 240 V oven, but haven't the time right now. Coming soon though.

      Thanks for your comment!

      Dan

      Delete
  25. Dan, I am near completion of my build. I have copied yours except I am only using 1 element. I am no electrician and am struggling with the wiring of the "Door open" and "element off" switches. I am confused by the markings on the schematic. I notice these 2 switches are noted twice. I have looked at the pictures for hours trying to see how the wires are routed. Can't see the bottom of the SSR. Any guidance would be helpful.

    Thank you!
    Clint

    ReplyDelete
    Replies
    1. Hi Clint,

      One contact of the door open and elements off switch are wired in series, eg first through one switch then out through the other. Opening either stops the SSR.

      I am using double pole double throw switches. One pole of a switch controls the SSR, the other pole does the LED. When the SSR pole of a switch is open, the LED pole is closed. That is why the LED are labelled as "Door Open" (SSR will be off) and "Elements Off" (also SSR off).

      I hope this helps.

      Dan

      Delete
  26. Dan, in your bill of materials you state that the two fuses "Must be 125v or 250v rated". I am assuming that you used the 125v fuses in your 125v oven and listed the 250v fuses for those building a 250v oven correct?

    ReplyDelete
  27. Most common glass fuses are 250 V rated. Some fuse models come in 125 V. Either will work safely at 120 V.

    If you wired this oven for 240 V operation, you would need to fuse each line (two main fuses instead of one) as each line could potentially fault to ground.

    ReplyDelete
  28. Great blog, TONS of info! Thank you

    ReplyDelete
  29. Dan, curious if the controller/element combo willdo tempering temps? 600F?
    Am looking to build an oven for heat treating swords and would be excellent to also be able to temper in it.
    Thanks again and excellent site!

    ReplyDelete
    Replies
    1. Thanks Thomas. Yes this setup makes an excellent, stable tempering oven.

      Delete
    2. DC
      A quick question,
      To wire a oven 220, the two hot leads go across the elements as opposed to hot/neutral for 120? And the supply side has to be adjusted accordingly as well, element calcs to make sure wattage is correct,
      What limits the current to the elements? Other than the breaker, just a little rusty on my electrical theory, im thinking the draw is constant based on the resistance of the elements??

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    3. The resistance (Ohms) in the heating elements limits the current. Current = Volts / Ohms.

      It's not quite as easy as doubling the voltage. There are a bunch of things in the circuit that need consideration for working at 240 V. The overcurrent protection is foremost. You will need two power fuses one for each hot. You'll also need to check that the controller will handle 240 V directly. The model I used is good from 85 to 265 V, but you need to be sure. The LEDs as shown will likely be very bright at 240 V, may want to change their resistors to 100kOhm.

      Dan

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    4. Dan,
      Thanks for your reply, i have been doing some digging and reading and actually put my thinking cap on, im a bit rusty, used to know this stuff in a past life but when you dont use it you loose it!
      I found some schematics on the Evenheat website, they confirm the design, theirs dont show dual relays controlling 2 banks of 2 elements,
      Heres a stupid question, cant find the answer on amazon or wherever but,
      What is the current draw of those SSRs?
      The controller shows 3a output, was looking at using some heavier SSR relays, they have 40A that are still inexpensive, thinking go oversize, 2 40A relays, to run elements on either side of the oven. Got to calc the draw etc, not 100% on direction im going on elements, ie wind my own or buy pre made,,
      Pre made are easy,
      In theory, the control circuit for the controller could be 110v and a separate feed circuit for the heat elements,, can even be two separate circuits, the two pole for elements and a separate single pole for the controller/lights etc
      ???
      Used to do motor controls in an industrial setting, but that was 35 years ago, so its coming back slowly, we didnt use a lot of SS hardware, mostly old school magnetic contactors, did do some dabbling with modicon programmables though so understand the theory behind it.

      Anyway, thanks and wishing you well,
      Tom
      kulariceball@gmail.com

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    5. Heres a link i found that gives draw of the relays
      https://cdn.sparkfun.com/datasheets/Components/General/SSR40DA.pdf
      Super low draw,

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    6. Hi Thomas,

      I hope I understand the question. The SSRs are rated to switch 40 A @ 380 V or less. Like an electromagnetic relay, they take a typically small voltage or current and switch a larger voltage or current on or off.

      The controller, if it has a relay output can switch 3A. What you want is a controller that has an output for SSRs. That will put out a small DC voltage and is wired directly to the input (think of it like a solid state coil) side of the SSR. These SSR outputs are usually low current mA DC. The polarity has to be right or the SSR won't fire.

      As for the 240/120 V setup, you certainly can do that. It means you'd need a NEMA 14 receptacle that has a neutral. Your controller power is from Line to N (120 V) and the heating element is switched via the SSR from L to L (240 V). You will also have to provide 2 fuses or a 2 pole circuit breaker to protect the oven itself.

      Regards,

      Dan

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  30. HI Dan,
    First, I'd like to thank you for this site. As a relative newbie I think you probably saved me hundreds of hours and a lot of money! So, Thanks!
    Now, two questions re. the heat treat oven. 1. Can I get by with a single 100 ft piece of Kanthal A1 18 ga if I don't do the ceiling? And, 2. I'm having a hard time finding a TA4 SSR but there are a lot of TA4 SNRs around. They're sol with the same 25amp SSR, so will it work the same? Will I still be able to use your wiring schematics?
    Thanks for the help!

    ReplyDelete
  31. Hi,

    If you make the total resistance the same you'll be okay for a 120 Volt oven. A smaller oven is good with around 2200 Watts @ 120 V
    16 AWG Kanthal A1

    2 parallel runs
    13 Ohms per run
    6.5 total Ohms
    0.34 Ohms per foot
    9.2 Amperes per run
    18.5 total Amperes
    2215.4 Watts
    38.5 feet per run

    Yes you can use the TA4-SNR. Any unit with an S in the first position after the hyphen means the main output 1 is ready for Solid State Relay. (N=none and R=relay.) So that SNR unit is Out 1= SSR, Out 2=none, Alarm 1=Relay.

    25A SSR is fine. Make sure you have a good heat sink (aluminum or copper) and use thermal conductive paste. That is what I use.

    If the controller set comes with a thermocouple it's probably a 400°C unit. You'll need to replace that with a 1250°C one.

    Dan

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  32. DC
    What do you use for supporting your knives in this oven?

    ReplyDelete
    Replies
    1. Hi Thomas,

      I use a piece of Insulated Fire Brick with some slots cut into it. Proper furniture can be bought, but I tend make use of what I have handy. :-)

      Dan

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    2. Thank you Dan, was figuring on doing something like that, for me the shipping is the killer, usually get fragile stuff in little pieces by the time it finds me

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    3. Thank you so much for putting this up here. I just finished my own oven and although I followed entirely different plans I found myself referring to your blog more than the plans. I especially appreciate the detail you go into on the electronics (and in general).

      Delete
  33. Hi Dan. I'm looking to build your 240V system, however I'm not reaching your numbers. Isn't a 2 parallel circuit with 38 ohms come out to 19 total ohms and 4500 wattage. 3 circuits would be 3000 wattage. Or am I doing the math consistently wrong?

    ReplyDelete
    Replies
    1. Hi Chip,

      The formula is based on Ohm's law. Start by taking the Volts (240) divided by Ohms (19). This will give you the Amperes. 240/19 = 12.6A Then multiply Amperes and Volts to get Watts. 12.6 x 240 = 3031 Watts. Perhaps you were multiplying V x Ohms?

      Good luck!

      Delete
  34. Hello Dan,
    Well, excellent work. I have been looking at your kiln for some while now, and am interested in breaking ground soon on my own. I am planning on using a PLC with PID capabilities, to gain some control over other processes. (Inert gas purge, HMI with graphs/ data collection, cue lights, buzzer, etc.)
    I have a question about SSRs. Is the output voltage scaled proportionally to the input voltage? If, for example, I input 24VDC and output 120VAC - would the SSR still output 120VAC with 5VDC input, or would it be scaled down?
    Thanks again bud. I was surprised to find a live thread. Most everything I find is years dead.

    ReplyDelete
    Replies
    1. Hi,

      The SSR is a solid state relay. It it either on or off. When the input voltage (DC) is applied it turns the output side on.

      In order to linearly control the output you'd need an amplifier or PWM or SCR based control. All we need really for the elements is on or off. Then we modulate the "on time" to get the oven temperature correct.

      I have a mate in Canada that is building an Arduino based PID temperature controller with all the bells and whistles. I will check with him and see if he's made any progress.

      Good luck and please let me know how it goes!

      Dan

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  35. Hi Dan,

    Thx a lot for all the info. I made a 240v version and just tested it today. It works really well and climb to 1100 in about 30 min. But one of the elements failed... Do you ever had problem with kanthal element?? One of mine looks like if it had a very hot spot on 2 inch and failed there. Except for this 2 inch the element seem fine.. I wonder what caused this... I have 2 more here so i will change it next week, but if i can find an answer on what caused this to not have to change it again i would appreciate. Maybe i pushed it too high at 25 Ohm?? I have a dual 25 Ohm setup for 13,5Ohm parralel, so 18 amps total for 9081Watt/ cubic feet. But i dont think i pushed it too high... What do you think?

    ReplyDelete
    Replies
    1. Hi, was your element pre-wound from the factory or did you wind it yourself? My "store bought" elements experienced one failure after about a year and a half; say 20 firings to over 1000°C. It's possible you have a defective element. Were the coils nice and uniformly stretched? (even spacing) If the coil is relaxed it may be too tight and cause a hot spot.

      I've over a year on my 18 gauge hand wound Kanthal A1 elements and they appear to be going strong.

      Hopefully it was just a bad element. I know it's a pain to change them out, but plan on doing that every few years. Once fired, Kanthal becomes extremely brittle.

      Dan

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    2. Hi Dan,

      After changing the dead element, i fired it again and this time it was working perfectly. I got cheap chinese already wound elements, and got a bad one. I already have some on hands to change them if they fail, but it take 3h each, so i hope i won't need to change them too often.
      I want to take the occasion to say a big thanks for the time you put on your website. You truly are an inspiration to me. I'm exactly the kind of guy like you that do all its stuff himself, and play with steel electricy and electronics. There is nothing like the feeling of having exactly what you need and having built it yourself. It's nice sometime to not start a project from scratch and have an input of someone who already did it, way faster and lot less design mistakes.
      I've ordered all that was missing to start making my knive, will start grinding my first knives soon out of the CPMS35VN blank i ordered!
      So thanks again, and keep DIY!!

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    3. Hi again,

      I'm currently designing a small cryogenic system for my shallow cryogenic treatment for my blade (-86C). When i'll have the system done and working, i'll make you the electronic controller for free, and give you the plan so you can build one for yourself. It will be my thanks for the time you saved me on some project. I wanted to do a system that not require any consumable like liquid nitrogen, so it will use thermoelectric plate (Peltier) to reach -86C. I will need to water cool the hot side of the plate to reach that temp, so i will mill an aluminum heatsink and use tap water so it wont need any kind of pump and chilling system.
      Don't expect the system to be ready soon, i need a good 6 month more to design the system and the controller electronics.
      I will keep you informed of the process!

      Mike

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    4. Mike, I would seriously be interested in a Peltier cooling system for cryo treatment. My favorite steel is AEB-L and cryo for that is spec'd at -70°C, so that would work perfectly.

      Let me know how it goes!

      Dan

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    5. Hi Dan,

      I tried to find your mail to send you some news but wasn't able to find it so i will post it here. The design is going well, the hardest part was to find a suitable sensor that dont cost an arm and a leg, as almost all cryo sensor are platinum... But i need to know if youre resady to invest a little bit in the project before i put time and money to build you the controller. The 4 stage peltier module that are the heart of the project cost 56$ each, and depending on the size of the box you want to build you will need 2-4 of them. You will also need 2 .062 copper plate that suits the size of the box you wanna build. So like 15 x 4-6 inch. They will be the main contact with the knife, and will be used to transfert the heat as the unit head need to be purged to be free of moisture. This lead me to the next item, do you have argon or nitrogen in you shop? Because you will need to purge the system head once assembled. Other than that you'll need a camping cooler, some old mattress springs, spare woods and polystyrene foam insulation panel.
      If youre ready to spend this money on the project then i'm ok to make you the electronic controller and send you the plan once finished so you can build one for yourself.
      If not let me know so i wont make 2 controller.

      Mike

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    6. So do i take your silence as a no?

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    7. Hi Mike,

      Sorry I missed this post. At this time I am not interested in a controller. I am interested in your project. Kindly keep us up with details.

      Regards,

      Dan

      Delete
  36. Hello Sir,

    I'm in the process of bumbling my way through a 240V oven build. I've butchered a cheap used pottery kiln for fire brick. Unfortunately every brick has two very deep channels cut through it for the long, spiral heating element that used to be present. Trimming the bricks square rendered them much thinner than yours (around 2"). I plan to make up for the thinness by casting the entire oven in 3 - 4" of refractory cement. I am working on a plywood enclosure right now for when I am ready to pour. Wish me luck, and/or chime in if you think it's a foolish idea.

    I have a few questions on the electrical side though:

    On the 240V schematic, I'm assuming the two main switches, following the fuses, are of the SPDT variety, both rated for 20A 250V?

    I'm assuming the DOOR and ELEMENT OFF switches needn't be rated as highly as the mains, since they are working the low voltage side of the controller?

    LED power will be limited by 100K or higher resistors, is is still OK to use the same diode afterward (1N4007)?

    Do you have any affordable suggestions for building inlet plugs into my enclosure? It will sit away from the oven, as opposed to being attached to it, and I envision the ability to unplug everything, stow the enclosure away from dust and children, and wheel the oven around by itself. I'm assuming I will need a plug for the TC and one for main.

    I'm sure I'll have more questions as I get farther into the wiring.

    Thanks for the quality write up and your constructive feedback!

    v/r

    Alex

    ReplyDelete
  37. Hi Alex,

    The main power switch needs to handle the current of the heating elements and control power so it needs to be ar least 10 A and rated for the voltage. Use a double pole switch for the main power as you want to switch both hots with the main switch.


    The control switches can be lower current, 6 A being common.

    The 1N4007 will work fine at 240 V as they are rated for 1000 V reverse.

    As for a plug, you could use a NEMA 6 plug and receptacle. For the thermocouple you can use a type K plug.

    It would be awesome to see your build.

    Dan

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  38. I've noticed most PIDs list k thermocouple then an input range k03 which indicates 0-600 in one example.

    If you buy a higher temp thermocouple that is a k type but it's range is greater say 0-1300 does this require a change on the PID to support?

    ReplyDelete
    Replies
    1. Yes. With this particular controller I set the Input UP limit to 1200 and left the Input LOW limit at 0. This allows the display to show the full range of temperatures with the high temperature thermocouple.

      Delete
    2. Thanks for the write up as I am building one similar to yours. I am still confused about the Mypin TA4-SSR PID you used. Can it be used with a K type thermocouple to control temps up to 2,300F or something less?

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    3. Hi Adam,

      The TA4 will display up to 1999 F or C. In Fahrenheit it will not go to 2300°. In Celsius it will go to the equivalent temperature (1260°C) if you can find a suitable thermocouple. For most heat treatment the high 1950°F is suffice. I do not recommend using an electric setup for forge welding.

      Dan

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  39. This is an amazing article ! THANKS for taking the time to document and post this excellent information.

    ReplyDelete
  40. I'm trying to make a smaller oven, 4"x4.5"x13.5" or .14-ish cubic feet. If I did the math right, I would only need 1 coil, similar to yours, to get just a hair under 5000W per Cuft. Can someone check my work? Each coil is about 20.6 ohms so...

    120V/20.6 ohms= 5.8 amps
    5.8 ampsx120V=699W
    699W/.14 cubic ft= 4992 W/cubit ft

    I haven't taken any measurements myself on the coils because I haven't bought them yet, but I figured that if I could do the math ahead of time, I could save myself some trouble by getting a close estimate. The coils are 40ft 18gauge Kanthal A1, pre-wound, at .515 ohm per foot.

    Thanks

    ReplyDelete
    Replies
    1. The numbers look good. One 20 ohm coil is about right as your chamber is smaller and less wattage is required. Just make sure to get the heat distributed as best as you can by placement of the elements inside the chamber.

      Dan

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  41. Hello from Calgary. It looks like we were going on the same learning path at about the same time. If only I had known. I would have been asking questions here constantly.

    If you'd like to see the projects ever, come to Protospace some time. It's in the north end of Calgary. Tuesdays evenings are the open house.

    ReplyDelete
  42. Thanks so much for this! I'm slowly acquiring parts to build one. One clarification from your BOM. For the elements off switch, you have "Switch, panel mount DPSP, 6A 250V" from Digikey. Unsure what a "DPSP" is. The closest on-off toggle I can find at Digikey is 679-1266-ND for a SPST and 679-1276-ND for a DPST. It appears to my eye in the circuit that you're calling for SPST, but wanted to verify before getting too far. I'm no one's electrician. Thanks!

    ReplyDelete
    Replies
    1. Hi,

      Yes the switch for "elements off" needs to have two poles and two throws. One pole is controlling the signal to the low voltage side of the SSR, while the other pole in the opposite throw is used to turn the "elements off" LED on.

      DigiKey's EG2398-ND would work fine for 120V applications. For 240 ovens, you will need to go to a suitably 250 V switch.

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    2. Dumb question #2: glastic. Sourcing this is tough in the states, somehow. Is GPO3 a reasonable substitute? It's only good up to 140°C, so not as heat resistant as yours. I just can't seem to find something that fits the properties.

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    3. Hi,

      Glastic was something I had handy. A piece of utility grade aluminum would work great. It's going to have to get pretty hot to melt the aluminum.

      Dan

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