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11-10-2010, 07:29 PM

Holescreek Senior Member

Join Date: May 2007Location: Centerville, OHPosts: 116

Home made heat treat oven

I make small parts that require some level of heat treatment when they are done and usually

just go the easy route with a torch and quenching medium. Lately I have been making someprecision parts that have enough time in them that I don't want to risk using the torch method.

I looked at the alternatives on the web as far as ovens available since local heat treat companies

have minimum batch charges of around $120. In the end I decided to make my own afterreading enough online tutorials and watching as many videos as I could find.

Everything is pretty easy to locate except the firebrick. The firebrick used in kilns/ovens is notthe thin hard brick that is used to line fireplaces, this stuff (IFB 2300/IFB 2600) is really light

and soft and measures 4.5 x 2.5 x 9". I started making calls on a Monday morning after puttinga list together of "refactory" suppliers in my area. About the 6th call I found a place across

across town that made ovens for the aluminum smelting industry that would sell me all I wantedfor $2.95 each. I ran over there and bought 20 bricks. While I was there the fellow in shipping

asked me what I was going to use for mortor and I told him that the internet video showed usinghigh temp caulk sold at HD. He laughed and suggested I buy 5# of the real high temp mortar

from them for about .59 cents a pound.

Home made heat treat oven - Machinist Web http://www.machinistweb.com/forum/showthread.php?t=1343

1 of 15 2/3/2014 11:35 PM

After locating the brick I hopped onto Ebay and started searching for PIDs, thermocouples, SSR's

(solid state relays) and Kanthal A1 elements. The PID (proportional–integral–derivativecontroller) is the digital unit where you set the desired temperature and reads the feedback

information from the thermocouple inside the oven. Until the oven reaches the desiredtemperature the PID sends a low voltage signal to the SSR to keep power going to the element.

Once the temp is reached, it turns the power to the element off. It samples the temp several

times a second to maintain the temp.

Here are my Ebay purchases. THe element is made by an extremely fine seller that customwinds the Kanthal A1 to the diameter and length required. I asked for 1/2" OD and 15A draw at

220V. The element was around $24 shipped. The SSR, 1300C thermocouple, and PID all camefrom another seller and were about $65 IIRC.


2 of 15 2/3/2014 11:35 PM

#2

#3

I'll finish this story up in another panel, stay tuned!

11-10-2010, 07:51 PM



A couple of good sources to share:

http://www.viddler.com/explore/rashid11/videos/3/

and Dudleys Element Paper.

I found the Dudleys paper near the end of my project and went back and corrected somemistakes I had made before firing the unit up. It is an extremely important bit of text for anyone

making an oven!

Attached Images

dudleys_element_paper.pdf (2.48 MB, 81 views)

11-10-2010, 08:14 PM


3 of 15 2/3/2014 11:35 PM



While I was waiting on my ebay purchases I played with my bricks to figure out the size I

wanted to end up with. I decided to make a front load oven so I wouldn' thave that rush of heat

shooting up when opening a top load oven. I finally decided on an internal chamber of 6.5 x 4.5x 15.5".

It is very tempting to keep making the chamber larger bt then you end up with longer operating

times to reach temp and consequently, more power usage.

The brick itself is extremely easy to work with. For all the full length cuts I used an old crosscut

wood saw to make the angled grooves. On the radiused ends I just dug the grooves out with aflat bladed screwdriver. Luckily I had already read somewhere about the importance of the

angled grooves but for a different reason. The grooves being angled lets the coiled element layinside them without falling out, PLUS it lets the heat flow out of the groove rather than building

up inside them, burning out the element sooner.


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5 of 15 2/3/2014 11:35 PM

#4

The mortar I used dries very quickly. I had just enough time to get the bricks buttered as it

started to dry.

I got all of the bricks in place and left everything to dry over night. While I was at it, I mortared2 bricks together to make the door.

11-10-2010, 08:24 PM



Here's a view of the interior before the elements were installed:


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After it set up I used some angle iron to make a frame for the bricks to sit in. The top comes offin case I should ever need to make repairs.


7 of 15 2/3/2014 11:35 PM

I ordered a few feet of high temperature 3/8" square door seal and a small tube of high temp

adhesive from Mcmaster Carr for the door. While I was ordering I also bought 3' of high temp

wire to use in connecting the element to power.


8 of 15 2/3/2014 11:35 PM

#5

I ended up using a router to cut a 3/8" groove in the door brick for the seal and glued it into the

groove. Once the door was made, I fit it onto the steel frame with a hinge on the bottom. Thehinge has adjustment slots so I can adjust the door as the seal changes over time.

The Kanthal A1 element has to be stretched to the length needed to fill the grooves in the brick.Mine ended up somewhere around 64" long. I used a piece of wire to punch a hole through the

brick for the ends to pass through.

11-10-2010, 08:50 PM


9 of 15 2/3/2014 11:35 PM



Based on the connection instruction in Dudleys paper I used a piece of old ceramic tile as an

insulator, and stainless steel screws to connect the 220V to the element. A special note here: Do

not untwist the ends of the element! Being twisted the ends have a higher electrical resistanceand do not heat up (glow red) like the rest of the element. This keeps the heat INSIDE the oven!

Once ned of the element is connected directly to one 110V leg of the 220V circuit. The other endof the element is conected to the switched line from the SSR.

I'm no electrician, here's what I came up with for a schematic:


10 of 15 2/3/2014 11:35 PM

I made a small steel box from 20 ga sheet metal to contain the PID, a 220V breaker switch (Igot for $4 from a local electrical surplus dealer) the SSR, and a terminal block for the 220V

connections. Later on I added a modified wall wart to supply the 12V for the PID.


11 of 15 2/3/2014 11:35 PM

A bit more about the PID. This thing works but the manual is done by one of the worst Chinglish

writers I have run into yet. It took me 3 days of playing while under power to figure out how toset a temp, and how to actually start the oven! I think it only cost around $35 but what a PITA! I

bought it because it looked simple to operate like the one in the vidler video. I wouldn't makethat mistake again. The PID did not come with any instructions, I had to search online for a PDF

and it was for a model that used 120V. Mine uses 12VDC, so I added the wall wart transformer.

Here's how the working side ended up looking:


12 of 15 2/3/2014 11:35 PM

#6

After it was all assembled I made a series of burns at increasing temperatures to let the Kanthol

properly oxidize as well as let me learn the method of programming the PID. I went to 150, 300,450, 1000, 1500 then 2000 degrees, letting the oven cool to approximately 200 degrees

between the higher temp runs. From 2000 degrees it took 10 hours for the temp to drop backdown to 200. When the oven first hit 2K I could leave my hand on the top of the brick outside. I

let the oven run for 15 minutes @ 2k then shut off the power. An hour later, I could no longer

comfortably (safely) touch the outside of the top bricks. The heat radiated out of them for manyhours. I've thought about adding insulation to the outside, but haven't gotten to it yet.

It took 39 minutes to reach 2000 degrees, then 10 hours to cool back down.

11-10-2010, 08:53 PM



I had to cut a photo to meet the 4 pic max so here it is:

Since I have room, here's Teddy the shop dog showing his typical disinterest in any project that

doesn't involve eating:


13 of 15 2/3/2014 11:35 PM

#7

#8

#9

Hope you enjoyed the thread! -Mike

11-11-2010, 12:08 AM

Ray C Senior Member

Join Date: Oct 2010Location: Near Annapolis, MDPosts: 1,093

Totally cool project! Great work!

Amazing only 15 Amps... Seems it would draw more than that.

Last edited by Ray C : 11-11-2010 at 12:11 AM.

11-11-2010, 06:39 PM

Paul Nicholson Senior Member

Join Date: Jul 2010Location: MAPosts: 1,427

Very well thoughout and done. I'm impressed... Good luck.

11-12-2010, 10:31 PM



Quote:


14 of 15 2/3/2014 11:35 PM

#10

Originally Posted by Ray C Totally cool project! Great work!

Amazing only 15 Amps... Seems it would draw more than that.

you determine the amount of current you draw based on the length of the kanthol element. It is

a small percentage of an amp per foot. Once you tell the ebay seller what amperage you wanthe calculates the diameter of wire and length required to give you what you ask for.

01-19-2014, 11:26 PM

Jagcat Junior Member

Join Date: Jan 2014Location: Cheshire UKPosts: 2

One is advised not to untwist the ends of the element when making the connections - this

REDUCES the resistance of the element at this location (not increases). It is the resistance ofthe element that generates the heat, the formula being I(amps)=V(voltage)/R(resistance). Any

rise in temperature at this location will increase the operating resistance and therefore the heat.If the element is heating up excessively at this location one could wind thick copper wire tightly

around each individual end of the element to both REDUCE the resistance and conduct any heataway.

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