j.w.s.

Tempering oven on a whim..

21 posts in this topic

I'm still battling with my back pain issue and I couldn't do much else around the shop today, but I've had this old toaster oven laying around for a while now that I used to temper in, It's just not deep enough for some of my blades so decided to rip it apart, upgrade a few things and rebuild it in a larger, insulated shell. For those of you that may have wondered, there is no insulation in a cheap toaster oven, it's a miracle anything can be cooked in them in the first place! My main motivation is to use it for a tempering oven during my bladesmithing classes which start next weekend. We'll be hardening in the forge, but the steels we're working with require more than just a little guesswork when it comes to tempering. I figured the elements from the toaster oven should lend themselves nicely to a low temperature oven build such as this, but I'm adding a thermocouple and pid which will be miles beyond the analogue components of the original. I've no idea how hot these elements can get, but I'm going to wire it up in the morning and give it a whirl.

 

-J

 

post-7730-0-33919000-1414794088_thumb.jppost-7730-0-43484600-1414794104_thumb.jppost-7730-0-07280500-1414794123_thumb.jppost-7730-0-35843500-1414794144_thumb.jppost-7730-0-78421700-1414794159_thumb.jp

Share this post


Link to post
Share on other sites

That's a pretty brilliant idea.  I almost bought a toaster oven the other day, but was disgusted by the complete lack of insulation in the thing. Not a single one on the store shelf had so much as a gasket around the glass door to prevent heat from escaping..... and the gap between the door and frame was a solid quarter-inch or more.

 

Using the guts from one, though, would definitely be "easier" than building from scratch.

Share this post


Link to post
Share on other sites

Alright, after changing the wiring around because running it in series (four elements in a row) gives too much resistance and wont give fast consistent heat, I rewired the unit so it was two separate series running in parallel and the resistance was a lot better at the sacrifice of the amps drawn. I managed to break 1100F run like that. So, it got me thinking, just how much heat can one of these elements take?! Well, I hooked 1 up by itself and set about to find the answer.. to the best of my ability to determine such things, I think it hit about 1900F before blowing the element.. it's ok, my wifes toaster oven is sacrificing itself today to repair it. I looked for a cheap one on the shelves last night at Walmart but they're all using these weird quartz elements now, not the incoloy elements we're used to. While i don't need to to get there, I think it might be possible to add resistance to the elements individually in order to limit the intensity at which it burns, thus providing suitable temperatures even for most common hardening (1450 to 1675F). I need to check this for resistance and then I can calculate the amp draw and potential heat production with the right resistor in line. :) It's not nichrome but I'm still really impressed what these things can do, particularly because we're talking 120vac and >10 amp draw for a decent tempering oven, and the fact that they are in some sort of ceramic insulator for protection makes it much more attractive than nichrome coils.

 

J

 

post-7730-0-61174600-1414932028_thumb.jp

Share this post


Link to post
Share on other sites

Great Idea! I am curious if you went with 220v which would drop amperage down could you pull more watts from the elements?

Share this post


Link to post
Share on other sites

Well I decided today to just run 3 elements in series since I burned the 4th one out and I'm not going for hardening temperatures. This configuration seems like a nice middle ground in overall heat output versus time to get to heat. I checked my wife's toaster oven today and it uses those quartz elements as well - tomorrow I'll check salvation army for a matched replacement for the 4th one but it runs great off of 3 for my classroom purposes. Also where I'm holding my knife making classes only has 120vac available. I'm thinking of trying a second 240vac build using broiler elements for our production shop though. While it would drop the amperage I'm not sure how capable these elements are with the wattage but it's worth the experimentation. Elements are cheap and durable, the pid, thermocouple and ssr were under $25 and the steel/kaowool were just left over scraps from other projects. I just used the toaster oven because it was laying around. If anyone cares to jump in with some technical input I'd be more than happy for it. :)

-J

Share this post


Link to post
Share on other sites

No technical input from me, just a bravo. I wish I had the know how to do the same. I use a toaster oven myself, aided only by an oven thermometer. This would be a huge improvement.

Share this post


Link to post
Share on other sites

I simply forge on. Have to. I have paying customers who take my classes. My back pain becomes so bad,I sometimes drop to my knees. Only cancelled one class due to cancer treatments. Just about topled over head first into the brine tank Sat. I told the students I was ok. Course....I wasn't.  But at work/onthe job, I'm a big baby. I take time off work. Hey, I have paid sick leave.

 

Forge-on. I wish you well. Find a good doctor.

Share this post


Link to post
Share on other sites

Great Idea! I am curious if you went with 220v which would drop amperage down could you pull more watts from the elements?

 

 ok I will bite.... how do you increase the wattage?

Share this post


Link to post
Share on other sites

 ok I will bite.... how do you increase the wattage?

 

Firstly I believe ironsmith had it correct but wrong at the same time. If we look at W, or wattage as is was put, as and expression of electrical power, the most common usage, we do indeed drop amperage draw when increasing voltage. However I think he's meaning Wt or Wth an expression of thermal power produced. Only the resistance of my elements will remain constant. So, if I'm drawing 120vac and my resistance is 15Ω I'm drawing about 8 amps and producing 960 Wt. If I increase my voltage to 240vac the resistance is still the same, but I'm now drawing about 16 amps and producing 3840 Wt. If I wanted to produced the same Wt with 120vac I'd need to lower the resistance to 3.75Ω and my amps drawn would be 32! Unfortunately the gauge incoloy I have at 3.75Ω which is about 12.5" worth would be decimated in a matter of seconds with that draw at 120vac. I dont know if that makes any sense, it's at the xxxxx  crack of dawn here and I haven't had any coffee yet.

 

J

Share this post


Link to post
Share on other sites

To a point you are correct. Power factor is voltage x amperage,  and at that simple level amperage doubles when voltage halves.   Anyway I feel the physics is not quite right.  Wattage is a statement of power used, and not all elements can handle an input of 240v.  If they are 120v elements that is what they should run, your average person should not go playing with changing voltages without first learning something about the subject.  The average poster does not have much knowledge of electricity, and I cite as an example the statement just made about using 220 volts,  I have no clue where in this planet that is a standard supplied voltage,  but it shows assumptions abound, and working from wikipedia like assumptions can kill someone. If the element is  made for 120 volts, and you exceed the ability of the element and it fails,  How will it fail? Perhaps it will just stop working, or it could ignite? maybe shunting voltage to the case (and the user in contact with that case)?

 

This is the point I want to make:  if you play around electricity, where will the power/heat go if there is a mistake ? we need to know before making any "adjustments" and anyone making suggestions, should be responsible enough to include warnings about failure to observe safety guidelines.

Share this post


Link to post
Share on other sites

I have to agree with Steve about the dangers of people who don't have the proper understanding of electricity modifying electrical devices. But, for someone who DOES have the necessary knowledge and such, it sounds like an interesting project.

Share this post


Link to post
Share on other sites

Well, I should have premised this with the statement that I'm a former high voltage electrician and while this isn't the 120kV lines or 34.5 transformers I used to get paid to monkey around with I still treat it with respect. Even 120vac in the wrong situation can be extremely dangerous. So far, it has been an interesting project and while I won't be trying to get higher heats out of this particular oven, I'm quite impressed with the 1100F it's capable of as configured, and it has prompted me to think about designing a larger box using commercially produced elements available from my local mom and pop store. In the past I used elements from electric heat pumps, but they're extremely delicate and prone to failure. Something shielded in some sort magnesium/ceramic is much more attractive. The elements themselves are designed to be in contact with metal, there is no current running through the outer shield, only through the center wire which is encased in the insulating material. The fact that these are designed to not short out by accident (like touching an element with a blade) makes them even more attractive and much safer when proper care is taken when wiring them up. Truth to tell, I would much rather have someone use these to learn from rather than unshielded nichrome or the like. That being said, any home brew electrical project can be dangerous and if you don't understand what you're working with you shouldn't attempt it in the first place.

J

Share this post


Link to post
Share on other sites

Its a good idea, i am just trying to keep people alive.  I also am an electrician.  I try to avoid getting into teaching people about electricial online anymore because there are so many self appointed experts, many get upset when told they are incorrect,  angry I have a license while they dont have a clue and blame me for correcting them rather than allowing someones shop to burn. 

Share this post


Link to post
Share on other sites

Well said JWS, and you hit the nail on the head at what i was thinking about with doubling the voltage, it's been a long while since i've cracked a book on basic electricity  so i  defaulted to the old standard p=VI, which is power, when dealing with heat the measurements are in Joules which is basically watts/sec.

 

Looking on the net, this answer was the best i could find about it.

 

The amount of heat produced by an electric current flowing in a conductor is given by:
Q = I2*R*t

 

( with 120v)   8amps * 15ohms * 10 sec= 1200joules

(with 240v )   16amps * 15ohms * 10 sec= 2400joules

where Q is heat; I is current; R is resistance of the conductor; t is time

or:

 one can say the the resulting work done by the electric current is heat. This gives rise to a second formula:
W = V*I*t
where V is volts and W is work.
 

120v * 8amps * 10sec = 9600joules

240v * 8amps * 10sec =    19200 joules

you do get more heat with 240 :)

if you can give me more info or if I am incorrectly understanding this please let me know !

 

Thanks for challenging me Steve ;)

Share this post


Link to post
Share on other sites

It's amazing how you guys can string a bunch of english words together and manage to create entire paragraphs I can't understand one jot of.

 

I thought this was going to be a simple plug-n-play operation, but I'm sensing that it's just not worth the hassle for a layman.

Share this post


Link to post
Share on other sites

Have you seen Storm crows' setup under axes, hawks, spears page 4 titled  "nose to the grindstone, well hawks to the belt"  (01 dec 2013). His setup looks very good for heat treating. I have never met him but my guess he would talk your ears off.

Share this post


Link to post
Share on other sites

It's amazing how you guys can string a bunch of english words together and manage to create entire paragraphs I can't understand one jot of.

 

I thought this was going to be a simple plug-n-play operation, but I'm sensing that it's just not worth the hassle for a layman.

 

lol.. It's never simple plug and play with electric.. the simplest form of plug and play in this operation would be a toaster oven, but unfortunately they aren't insulated on the inside and after some poking around with my laser thermometer and a few thermocouples, it's amazing how they can swing in temperature over such a short difference. In other words, you're just not going to get even or consistent heat from one. If the whole thing was insulated it wouldn't have the heat blead and it'd be great. This whole string of english that makes no sense is the theory mumbo-jumbo behind what is happeneing during my build. All I've really done is changed the chasis of a toaster oven, added some cool electronics like a PID and a thermocouple, some good insulation and reconfigured the way the elements fire. Simple plug and play for me, but while not as dangerous as taking apart a microwave, it could still be other than harmless for other people.. :)

Share this post


Link to post
Share on other sites

Been looking around on the great Google machine and found a few really detailed home built tempering ovens. thanks for getting me all fired up about them. 

 

now i'm gonna have to try it!

Share this post


Link to post
Share on other sites

wished I'd stayed and gotten that nuclear physics degree....maybe I could understand all this now...... :wacko:

Share this post


Link to post
Share on other sites

wished I'd stayed and gotten that nuclear physics degree....maybe I could understand all this now...... :wacko:

 

You only need a nuclear electrical degree... :unsure:  :D

Share this post


Link to post
Share on other sites

Nuclear Electrical degree? There was a bunch of Russians(with those very degrees) mucking about and upgrading grandma's toaster oven with some interesting tweaks near Chernobyl wasn't there? :)


Come to think of it , some of those self same chaps were giving it a go on three mile island too!

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now