DISCLAIMER: do @ your own risk, blah blah, more legalese and then some more. Use help of an electrician if you don't know anything about electricity. Wait... if you don't know anything about electricity, this article is not for you, don't attempt !
It all started with me needing to heat treat some metal - pieces too big for my trusty and very inexpensive HomeDeport Butane Torch.
Called couple of places in vicinity, cheapest was about $120 to heat treat what I needed (size of the lot doesn't matter that much to most industrial shops).
Not good - I wanted to have flexibility of doing it myself. Settled on making my own electrical oven (120V, no need for240 outlet).
Here's components and whole process.
- The oven is made out of refractory firebricks. MCM sells them, so do many local ceramics/pottery shops (look for kilns in YP etc).
I got 10 pcs of K23 type: this one is good to 2300F. Plenty for me, as highest I need is about 1800F (for A2, O and W tool steels are lower temps). Local shop sells 'em for $4.25 (very un-American price ... so used to NNNN.99). Please note this is about most expensive area in USA (miles from Manhattan) - you might be able to get them cheaper where you live. Bricks are 9 x 4.5 x 2.5. These are very light, off-white yellowish and are little bros of what they use on Shuttles to insulate.
Oven interior is 9 long x 4 wide x 4.5 tall. Plenty for my purposes. Should do for a knifemaker etc. To buid, you will need High temp mortar , caulk or cement. ~$3 HomeDepot Fire resistant mortar does the trick (look in Caulks, it is rated to 2000F). When squeezed, it is black and greasy, like a shoe polish almost.
Easiest way to get to this size: lay 2 bricks down (so that long and narrow side is facing down), parallel to eo on long sides, with 4" separation. Top and bottom off this construction with single brick atop and one @ bottom. Get the picture ? Set bricks apart.
Now, important ! We need to make grooves in the 4 middle bricks, to tuck in the heating element. 1/2 wide, 1/2 deep groove did the trick for me. I milled it on my mill, HAND GUIDING the bricks into 1/2" endmill @ very low RMP. Bricks are VERY soft and are easy to cut. You can scratch them with your nail.
Look inside of the middle section, with 4 bricks joined: 2 side bricks will have the groove going the length of the brick. Top and bottom bricks will have have grove not all the way (ending 2" BEFORE ends of the bricks). It is easy to see why. Height-wise, I milled my groves in the middle of the brick. No need for super accuracy.
Squeeze pinky-thick bands of mortar in the middles of surfaces-to-join. Join the bricks, apply some pressure to get the gaps as tight as possible. Some mortar might get squeezed outta gaps - be careful with it as it is nasty (does clean off hands rather easily).
YOU WANT TO KEEP INSIDE OF THE OVEN AS WHITE/CLEAN AS POSSIBLE, to reflect most of the heat back into the inner space.
Now it is bottom time :) Lay 3 bricks down, wide side facing down, join @ long narrow side. Easy. Same thing with mortar.
Now, let's join bottom and middle pieces. Lift middle and place on bottom part, center off. Easy. Separate, add mortar, join, squeeze.
Top - identical to bottom. DO NOT join the top with the rest :) It will remain a separate piece.
Make sure surfaces are fair and square, gaps will let valuable heat out and you aint got :) that much of it using only 120V
Now, let's make it look a bit better. 36"x36" decorative aluminum from HD will set you back $17 or so.
Make "boxes" or enclosures for middle and top pieces. Very easy to do - just like a school project. To bend near perfect 90DA, I used pieces of hardwood 2x4. To get nice edge : slam the tin in between 2 pcs of wood, using the very wood to do the slamming, sliding it on the floor.
I did overlapping lips, so that corners look nicer. No real need for it. SHould not take you more than 30m to do enclosure ( I am no tin knocker meself, but managed it pretty quickly). YOu're not gonna be eating off it, but would want to impress you friends.
As you know, duct tape hold 7/8 of known Universe together. But we are better than that so we will use self-adhesive alluminum foil from HomeDepot. $3 or so. Slap it around liberally, but tastefully.
Get some mortar applied to bottom and inside walls of middle enclosure, set the (joined) bricks in.
In top enclosure I drilled 4 holes, 1" or so diagonnaly from the corners and put some 0.08 wire loops through and bend the wires on the inside.
Idea is to have (support for) a handle for the top piece. Apply some mortar to inside of the top enclosure and put the top 3 (joined) bricks into the enclosure.
Now, what do we do about heating element and control for it ?
We need a coil of nichrome or Kanthal A1 wire. Pluses and minuses to both, but Kanthal is more durable and goes to higher temps. I used nichrome wire that I got in clothes dryer repair kit. Use ohmeter and cut 8 Omh worth of wire. YOu'd need to have it coiled with 1/2" OD. Mine came pre-coiled.
Let's drill 2 holes to let the wire outta oven. 1/16" drill bit should do. Have 1.5" horiz. separation between the holes. Up to you where the holes will end up. In the grove or above is probably better. Scrath thin and shallow channels leading from holes on the inside and into the groove.
Where holes exit on the outside: cut the tin around (go from upper edge, rectangular cut on 2 sides, bend the resulting pc down @ 90DA, we will use it as a support). Idea is to remove tin from where electrical wires will be located. I also got what looks to be 1" square ceramic blocks with 1/2" through hole @ the same ceramic craft store. Pennies per. I mortar-glued these atop of the 2 holes, on the outside, using the bent (see above) tin as support.
Stretch the coiled wire uniformly so that it covers the inner perimeter of the oven. It is very important to have 0 stress on the wires as they are tucked inside of the groove. Make sure coils don't touch ea with at least 1/16" separation throughout. Don't do any sharp bends either. When wire heats up it softens and any of the stress points will shorten it's life.
Tuck it in. Make sure all the wire goes inside of the grove, should not see much of it sticking out when looking from above.
Get the ends straight, 3" or about and run them out of the holes we created earlier. I crimped on high-temp contact pieces that came with my dryer repair kit. Heard about people using long (3" or so) crews or threaded rods to attach the wire. We need a nice solid contact.
Now, feed any excess wire back into the oven and coil it up there.
Measure resistance on across the 2 leads outside of the box. Should be8 Ohm. Don't go much below that as it will stress out the electrical system (wire will increase it's resistance as it heats up, but still you will have the initial surge).
Measure resistance between the leads and the enclosure. Should not be any contact between the two (I didn't bother grounding my enclosure, but it probably is a very good idea).
Now - to control portion of it. TO be able to control the temperature, we need to be able to measure it (thermocouple probe) and control the current flow through the coil to maintain the temp where we want it.
For the temperature we want what's known as "Type-K thermocouple". Omega.com, MCMaster, Ebay. I got a kit from Omega: enclosed and grounded type-K TC with some wire in teflon insulation. $24. Ebay will be much cheaper. We need the TC to be 6" or longer, to be nicely inside of the oven. TO run the TC: drill another hole, as close to the bottom surface of the oven and let the TC in, from outside. Can use a drop of mortar to secure TC, NOT AT THE TIP !, to the bottom of the oven. Make sure the TC doesnt touch the coils. You don't have to feed the whole length of TC into the oven. I don't know if just having the tip in will do the trick, I fed 1/2 of my 12" long TC into the oven.
Now, let's do a quick test. Hookup some 15A wires to the ends of the coil (I attached $7 9" 15A 3-wire/prong HomeDepot combo to my leads) CHeck for shorts etc. Insert TC in, hook up the leads to voltmeter, set up to measure millivolts.
Make sure the outlet you're about to plug-in is capable of 15A @ 120V. MAke sure it is wired properly and has a breaker @ the panel.
Have safety goggles on - just in case the wire blows up :)
Keep the top open and plug the wires into electrical outlet. In few seconds,you should see coils go pale yellow. The voltmeter will go crazy and climb to may be 20mv in a minute or so. Close the top and few minutes later (10 or so) you should get to 40mv area. 41 is where we want to be - which matches 1800F ballpark.
Unplug. Don't remove the top - give it 30m or so. It will cure all of the mortar we used.
Let's talk control. Clearly you don't want to plug/unplug the electrical wire and watch the voltmeter to regulate the temperature.
Enter Omega Fuzzy Logic controllers. These will read the output of TC and drive external relay as reguired to maintain the temperature pretty darn accurately. They will also use some interesting math to make sure not to overshoot by too much etc. Nice. Too bad they retail for $165 and above.
Here's where Ebay comes in handy. Same item will set you back $35-$45. Great. Look for Omega controller, with SSR output (see below) and straight 120V-250V device, meaning you won't need 12 or 24V supply to feed it.
Omega controllers are highly modular, have nice bright display and are The Name in this game.
The device that controls on-off condition for our heating coil is a RELAY. They could be old fashioned: electro-mechanical or Solid State. Solid state is the way to go. Again, new ones are $25 @ Omega (get 25A model, with 4-32VDC control, single phase) or Ebay: low single$. Also, heat sink might be a good idea. Don't waste $20 hard earned dollars on retail one - a piece of aluminum plate will work fine for our tiny oven. Drill matching hole and attach the SSR to the plate.
Again, Omega Controller can control both types, but SSR is better one. Most mechanical relays just won't last long with arc resulting from 15A current.
Read Omega's manual on how to wire, teach (it does learn thermodynamics of your coild and oven !!!) and operate the device. Again, be careful, follow the instructions, make sure not to mixup the Control and Load terminals on SSR. Make sure you control hot wire of the 120V circut (ground is always connected to the coil).
Having a 15A fuse in your oven is probably a good idea.
To operate: get the load loaded in, make sure it doesn't touch the coils (which should be neatly tucked in),put the lid on, program required temp @ controller and off you go.
There are controllers (used in ceramic kilns etc) that allow you to program multiple ramp/hold steps in a sequence. Unfortunately these are expensive and beside that, this is NOT A DEVICE THAT YOU FIRE AND FORGET. I advise you keep an eye on it throughout the process.
Things that are firing @ 1800F, inside one's place of residence, definitely require adult supervision. I like I promised in the subject: Merry Xmas !