17 years ago
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
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 for
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
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
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
Where holes exit on the outside: cut the tin around (go from upper
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
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 be
8 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
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
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
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 !