Tempering Oven...Starting Over...

CDHumiston

CDHumiston

Well-Known Member
As many of you read in my prior thread on tempering ovens I went out and bought an expensive drying oven. Well, it failed miserably. The oven died on the second use with the on/off switch failing and getting stuck in the on position. Shortly after that the oven died completely. The return process was very slow and painful as the selling company was a US based Chinese firm. They finally agreed to take the oven back, but I had to drive nearly 100 miles to return it myself. The online company is Toolots. I'd suggest everyone stay away from them.

The good news is, my wife and I were going to yard sales yesterday and I found a commercial cookie making oven. It has nothing but an on/off switch and no other controls. The manual stated it was set for a very stable 325. However, when I brought it home and tested it the temp stabilized at 275. It was very accurate and never varied by more than 2 degrees.

I only paid $5 for the oven, and I found it multiple places online from $150 to $1000 in various used to new states. Mine looks barely used. I decided to take it apart and try to see if I could do anything to increase the temps. The first thing I found is the oven has a very well-made heat regulating system and lo and behold there was an adjustment screw! I adjusted it up, put it back together and ran another test. I went too far, and the new stabilized temp was 425. After about 5 times taking the front panel off and making small adjustments, I finally got it to 310 which is good for me. I use stainless almost exclusively and AEB-L and 440C more than anything else.

Needless to say, I am very happy with the $5 purchase and at some point, I may install a PID controller.
 
I will bet you got a great find and would easily be converted to a PID controlled oven at what ever temp you set the PID controller for. Plus or minus 2 degrees.
 
CDHumiston said:
Anyone know what an acceptable temperature fluctuation in a tempering oven is?
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±1°F or maybe even ±2°F is just fine. At 310°F You're keeping your blades pretty high Rc - 62 Rc or so? AEB-L sure doesn't handle a high Rc just fine. I've been impressed with how good 26C3 holds an edge at 63 to 64 Rc and I suspect AEB-L would be just as good there. I used to temper for 60Rc with AEB-L, but now with the knowledge shared here I'm moved to 62 Rc and it does do better.
 
KenH said:
±1°F or maybe even ±2°F is just fine. At 310°F You're keeping your blades pretty high Rc - 62 Rc or so? AEB-L sure doesn't handle a high Rc just fine. I've been impressed with how good 26C3 holds an edge at 63 to 64 Rc and I suspect AEB-L would be just as good there. I used to temper for 60Rc with AEB-L, but now with the knowledge shared here I'm moved to 62 Rc and it does do better.
Click to expand...

Thanks. The oven seems to be fluctuating about 2.5 degrees up and down. So, it's low is 310 and high is 315. It goes back and forth between those temps as the heating elements cycle on and off.
 
Finished my practice heat treat on a small blade. 5 minutes at 1550 ramp up and 15 minutes at 1920. Plate quench. Followed by 2 x 2-hour temper cycles at 310 - 315 and my final result is 62 HRC.

Not sure if I need to snap this one to look at grain structure...I hate destroying blades.
 
A PID controller should keep the temp +/- a degree or two. You probably want to add Kawool insulation to improve temperature stabilization. Those countertop ovens are often poorly insulated.
To attach the PID I mentioned in your previous post, just rip out the wiring to the timer and temperature dial and hook the heating coils directly to the controller. The original temperature control on these units is typically too inaccurate for tempering.
The PID will need programming to match the heat loss of your specific oven. A good PID will auto tune to the right settings.
Once you get into it, it's really very easy to modify the oven.
 
MTBob said:
A PID controller should keep the temp +/- a degree or two. You probably want to add Kawool insulation to improve temperature stabilization. Those countertop ovens are often poorly insulated.
To attach the PID I mentioned in your previous post, just rip out the wiring to the timer and temperature dial and hook the heating coils directly to the controller. The original temperature control on these units is typically too inaccurate for tempering.
The PID will need programming to match the heat loss of your specific oven. A good PID will auto tune to the right settings.
Once you get into it, it's really very easy to modify the oven.
Click to expand...

This oven has nothing but an on/off switch on the exterior. The adjustment pot I found is in the guts of the oven. It's a forced air convection oven. I'm not sure what I'll need to do for the PID, but I'll figure it out. Kawool is a great idea as well!
 
As close as the temp is holding now no real need for a PID controller - other than make it easy to change target temp. My toaster oven that I used Kawool to insulate and added a PID for control normally holds ±1°F at most any temp setting from 300 to 400 °F. The TC probe is barely visible over the rack.

Ooops, that got big, but only 272kb in size.
TemperingOven.jpg
 
KenH said:
As close as the temp is holding now no real need for a PID controller - other than make it easy to change target temp. My toaster oven that I used Kawool to insulate and added a PID for control normally holds ±1°F at most any temp setting from 300 to 400 °F. The TC probe is barely visible over the rack.

Ooops, that got big, but only 272kb in size.
View attachment 82444
Click to expand...

Big is good for my old eyes!

I'm going to go with what I got for a while. I'll eventually add a PID so I can change temps...
 
Toaster ovens are not all created equal. Cheap ones turn the coils or heating elements on and off to regulate the temperature. There are 2 or three top of the line brands that regulate the power that goes to the heating elements. These more expensive ovens are supposed to maintain a accurate consistent temperature. Sorry I don't remember the brands. It sounds like that is what a PID does.
 
Robert Nese said:
There are 2 or three top of the line brands that regulate the power that goes to the heating elements. These more expensive ovens are supposed to maintain a accurate consistent temperature.
Click to expand...
Do you have examples of these ovens?
 
Robert Nese said:
Toaster ovens are not all created equal. Cheap ones turn the coils or heating elements on and off to regulate the temperature. There are 2 or three top of the line brands that regulate the power that goes to the heating elements. These more expensive ovens are supposed to maintain a accurate consistent temperature. Sorry I don't remember the brands. It sounds like that is what a PID does.
Click to expand...
The PID control systems I used in industry did modulate the power to the coils. The PID controllers we use here simply turn the coils ON-OFF to regulate temperature. The PID controller operates a relay, normally a SSR (Solid State Relay) to turn power ON or OFF. That ON-OFF cycle can be very fast which gives the effect of modulating the heating coils.
 
Hi,
I have tempered over a hundred blades with a Cuisinart conviction toaster oven I found at a yard sale. I laid 1/8 inch steel on the rack and put in a oven thermometer, it holds a steady temperature. Because I use a propane forge and do not have accurate temperature control and only have test files, I am guessing my blades are ending up between 58 and 62 RC. So if I do not have a RC tester if my temper is off 1 or 2 degrees I won't know .
 
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