heat treat oven - mikescncshop

8/12/2019 Heat Treat Oven - MikesCNCShop

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Heat Treatment>

Heat Treat Oven3-26-11As I began to venture into plane making, it occurred to me that I should be making all of theparts myself. At least as many as I can. Since the blade and cap iron I bought for myKrenov style plane cost $65, I felt I could save some money by making these myself. I likethe A2 blades and although I won't be able to do the cryogenic treatment at this time, Ishould 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 offsetthe cost of the oven. I estimate that I will have between $250 and $300 invested when all issaid and done.

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

Another friend made the metal boxes for meout of 14 gauge steel which is stiff enoughthat it won't need any additional framing. Hedid such a nice job on them that I had to step

back and think of a better method ofmounting the electronics so it would look asgood as the box.

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

Treat Oven - MikesCNCShop http://www.mikescncshop.com/heat-treat

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

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

Yet another friend cut and bent the22 gauge pieces for the controlpanel. (Good thing I have friends).

Here I have the control panelmounted to the standoffs as wellas a box to shelter the connectorfor the thermal couple. I alsoadded some lifting handles.

The K2300 firebrick needed to begrooved to accept the heating coil.Since it is very soft, I was able touse a not-very-sharp 1/2-inchchisel to cut it.


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Idrilledholesfor the

thermal couple using a Forstner bit and a twist bit turning them with my fingers. Yes, thisbrick is very soft. The counter bore is about 1/2-inch deep to accept the ceramic tube thathouses the thermal couple. The through hole is just large enough to allow the thermalcouple 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 lengthbecause I didn't have any way to cut it withoutthe risk of shattering it.


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

3-27-11

A little more time in the shop todayand I was able to get the controlpanel pretty much laid out. Inaddition to mounting componentsunderneath, I cut a hole and

mounted a computer fan cover andmounted the proportionalintegralderivative controller (PIDcontroller).

Under the fan grille is a heat sinkfor the solid state relay. No fanyet, I will wait and see if it needsone. This is a 40-amp relay butwill be running at about 15-ampsso it should be fine.


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This image shows the l id in place.The lid doesn't have firebrickinstalled at this point and I stillneed to come up with a liftinghandle.

You can also see a white ceramicterminal block and rubbergrommet. I am still working out thelocation for the heating elementwires to come through the side ofthe oven. So far this is lookinggood.

4-2-11

Last night was fairly productive. Ilocated and drilled the holes forthe heating element wire to exit theoven. I also cut an opening in theside of the box that can be seen inthe next photo.

I finished fabrication of a heatshield and the control panel butforgot to take pictures.

This afternoon I used high-heatmortar to glue the bottom bricks tothe bottom of the steel box and toeach other.

I added ceramic fiber insulation tofill the gaps around the edges.

On the left side is the rectangularhole where the heating elementwires will exit the oven. It is sizedto allow a ceramic terminal block tofit through the steel wall to insulatethe wires from the metal sides ofthe box.


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After mortaring in all of the bricks, Iinserted the heating element intothe slots I chiseled previously.

This image shows the element inthe background and the terminalblock installed.

The completed base. Well,almost complete. I need toadd some ceramicinsulation at the corners.

The control panel is readyfor paint but it was toowindy to spray outsidetoday, hopefully I will get achance tomorrow.

4-3-11

With the wind gusting like crazy, I couldn't paint parts today. I started to work on a handlefor the lid but decided that I didn't like my initial design ideas. With the hardware storeclosed 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.


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

The control panel with wiring.


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

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

Now for the moment of truth.


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Here is the PID display showing room temperature. By turning on one breaker I can powerjust the control ler while I set the temperature. Once set, I can flip the second breaker toengage the heating element.

It's ALIVE and glowing!

As the element fired up, the thermocouple immediately sensed the change and begansending its signal to the PID. This is when I realized that the thermocouple wires werereversed as the temperature readout started dropping. After a quick wiring change all waswell.

I only ran the oven long enough to take the photo but it didn't b low the breaker so I must

have the resistance of the element about right or at least on the low side. I will have to finishthe lid before I can test at elevated temperatures.

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


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Above is a piece of poplar that I flattened and fitted with 100-grit self adhesive sandpaper. Iwill 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 bothpieces. Care had to be taken since the brick sands very easily and you can remove morematerial than necessary.


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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 andscrewed 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 enoughthat it retains this arched shape even with the weight of the lid pul ling down on it.


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Another view of the bail.

It was time to do some testing. I set the target temp. for 900-degrees. Six minutes later thetarget was reached and promptly overshot by nearly 100-degrees. I suspected that with therapid heating, the thermocouple could not send data to the control quickly enough. I resetthe 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. Itwent 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 minutesbefore shutting it down. The outside surface of the furnace didn't begin to heat up until Ishut 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 l ifting the l id 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 aless windy day. I am officially calling this project complete and a success!


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heat treat oven - mikescncshop

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