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Heat Treat Oven

3-26-11
As I began to venture into plane making, it occurred to me that I should be making all of the parts myself.  At least as many as I can.  Since the blade and cap iron I bought for my Krenov style plane cost $65, I felt I could save some money by making these myself.  I like the A2 blades and although I won't be able to do the cryogenic treatment at this time, I should still be able to produce a pretty good part. 

First I would need an oven to heat treat the steel.  It shouldn't take too many blades to offset the cost of the oven.  I estimate that I will have between $250 and $300 invested when all is said and done.

After some online research and consulting with my friend who is a knife maker, I decided to go with an electric oven.  I found a video of a home made oven that gave pretty good overall instructions and decided to go with this design. 







Another friend made the metal boxes for me out of 14 gauge steel which is stiff enough that it won't need any additional framing.  He did such a nice job on them that I had to step back and think of a better method of mounting the electronics so it would look as good as the box.




















I added three feet to act as a tripod so it would be stable on an uneven floor.















I also added some square tubing to act as standoffs to mount the controls. 

This image shows the electrical layout with one of the spacers at the far right.










Yet another friend cut and bent the 22 gauge pieces for the control panel.  (Good thing I have friends).


Here I have the control panel mounted to the standoffs as well as a box to shelter the connector for the thermal couple.  I also added some lifting handles.













The K2300 firebrick needed to be grooved to accept the heating coil.  Since it is very soft, I was able to use a not-very-sharp 1/2-inch chisel to cut it.





















I drilled holes for the thermal couple using a Forstner bit and a twist bit turning them with my fingers.  Yes, this brick is very soft.  The counter bore is about 1/2-inch deep to accept the ceramic tube that houses the thermal couple.  The through hole is just large enough to allow the thermal couple to pass to the outside of the oven.

You can also see the completed groove for the heating element.













In this image I have the ceramic tube inserted.  The tip of the thermal couple stops just past the midpoint of the oven.  I left the tube full length because I didn't have any way to cut it without the risk of shattering it.




















Here is the thermal couple with a ceramic terminal connector attached.  It is a type-K with ceramic segments to separate the two wires.










3-27-11





A little more time in the shop today and I was able to get the control panel pretty much laid out.  In addition to mounting components underneath, I cut a hole and mounted a computer fan cover and mounted the proportional–integral–derivative controller (PID controller)
.












Under the fan grille is a heat sink for the solid state relay.  No fan yet, I will wait and see if it needs one.  This is a 40-amp relay but will be running at about 15-amps so it should be fine.







This image shows the lid in place. The lid doesn't have firebrick installed at this point and I still need to come up with a lifting handle.

You can also see a white ceramic terminal block and rubber grommet.  I am still working out the location for the heating element wires to come through the side of the oven.  So far this is looking good.



4-2-11




Last night was fairly productive.  I located and drilled the holes for the heating element wire to exit the oven.  I also cut an opening in the side of the box that can be seen in the next photo.


I finished fabrication of a heat shield and the control panel but forgot to take pictures.





This afternoon I used high-heat mortar to glue the bottom bricks to the bottom of the steel box and to each other.

I added ceramic fiber insulation to fill the gaps around the edges.

On the left side is the rectangular hole where the heating element wires will exit the oven.  It is sized to allow a ceramic terminal block to fit through the steel wall to insulate the wires from the metal sides of the box.









After mortaring in all of the bricks, I inserted the heating element into the slots I chiseled previously.













This image shows the element in the background and the
terminal block installed.









The completed base.  Well, almost complete.  I need to add some ceramic insulation at the corners.

The control panel is ready for paint but it was too windy to spray outside today, hopefully I will get a chance tomorrow.



4-3-11

With the wind gusting like crazy, I couldn't paint parts today.  I started to work on a handle for the lid but decided that I didn't like my initial design ideas.  With the hardware store closed today, I will have to wait to make changes.

I decided that paint or not, I would assemble and wire the thing to see if it was going to work.

I used 120 volts from one leg of the circuit to power the PID.  All of the wire is rated for high temperatures.  I used 12-gauge to connect the heating element and 20-gauge to connect the PID, thermocouple, and low voltage to the relay.  Ceramic terminal blocks were used  for the main connections.


The control panel with wiring.


This view from above shows how the terminal block sits between the box and the heat shield.  It actually protrudes slightly through both making sure that the wires can't contact metal parts.

The sides and bottom of the panel are open for airflow but a front cover should keep me from getting into the wiring by accident.  The shiny metal looks nice but I will have to paint it to avoid rust. 
Too bad I didn't have stainless steel for this part.


Now for the moment of truth.

Here is the PID display showing room temperature. By turning on one breaker I can power just the controller while I set the temperature.  Once set, I can flip the second breaker to engage the heating element.

It's ALIVE and glowing!

As the element fired up, the thermocouple immediately sensed the change and began sending its signal to the PID.  This is when I realized that the thermocouple wires were reversed as the temperature readout started dropping.  After a quick wiring change all was well. 

I only ran the oven long enough to take the photo but it didn't blow the breaker so I must have the resistance of the element about right or at least on the low side.  I will have to finish the lid before I can test at elevated temperatures.

4-10-11
Today I flattened the firebricks to get a good seal between the body and the lid.  Since there is no gasket, I will depend on the two flat surfaces creating a leak free fit.

Above is a piece of poplar that I flattened and fitted with 100-grit self adhesive sandpaper.  I will use this to make sure the top surface is flat and in a single plane.

Plenty of daylight showing under the straightedge at the top of the photo.

After a fair amount of sanding, checking, and more sanding I have a good surface on both pieces.  Care had to be taken since the brick sands very easily and you can remove more material than necessary.

This photo shows the tight fit between the bricks of the base and lid.

Next, I needed a bail handle for lifting the lid.  I had welded 1/4-20 nuts inside the lid and screwed eye-bolts into them.  I used galvanized clothes line wire for the bail.

I ran the wire through the eyes and twisted it together at one end.  The wire is stiff enough that it retains this arched shape even with the weight of the lid pulling down on it.

Another view of the bail.

It was time to do some testing.  I set the target temp. for 900-degrees.  Six minutes later the target was reached and promptly overshot by nearly 100-degrees.  I suspected that with the rapid heating, the thermocouple could not send data to the control quickly enough.  I reset the target temp. to 1500.  As it approached 1400 I reduced the target to 1415.  As expected, it overshot that temp and got up to about 1490.  I simply increased the target to 1500.  It went slightly above 1500 but eventually leveled off and held at the correct temp.

Here is the finished oven holding temperature.  I left it at 1500-degrees for about 15 minutes before shutting it down.  The outside surface of the furnace didn't begin to heat up until I shut it off and was waiting for it to cool down.  It eventually got fairly hot.  You could touch it but probably wouldn't want to leave you hand there too long.

Although I did my testing with the oven sitting on my table saw, in use, it will sit on the floor.  I don't care to be lifting the lid off of a 1700-degree oven that close to face level.

Eventually I will have to disassemble the control panel and paint it.  But that's a job for a less windy day. 
I am officially calling this project complete and a success!


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