Heat Treat Furnace Build

jbolt

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Dec 3, 2013
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I have come to a point with a couple of projects that have parts that will need to be heat treated so it is time to build a furnace. This will be my version of the many examples out there on the interweb.

Specifications:

Heat chamber is 7" x 7" x 16"
Dual heating elements wired in series, 240v, 3100w
PID / SSR controller.
k-23 firebrick heat chamber wrapped with three layers of 2300 degree 1 mm ceramic paper.
1" angle iron frame & door skinned with .060 sheet metal

Here is the concept in CAD.

Heat chamber.

Heat Chamber01.png

With frame and door.

Oven01.png


Before I can finalize the frame dimensions I need to get the heat chamber assembled. Today I cut all the bricks in preparation for assembling.

To cut the side and back bricks with the heating element grooves I made a jig out of 3/4" plywood. Time to put the CNC router to work.

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Bricks in the Jig ready to cut. The bricks are clamped with screws against plastic shims. I put several coats of shellac on the edges of the plywood to firm up the wood a bit. The jig only needs to last for 10 bricks so this should be fine.

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Cutting was done with a hacksaw with a carbide grit blade. It makes short work of the soft bricks. Unfortunately the length of the brick combined with the 3/4" plywood took almost all the blade length leaving a very short stroke. 1/2" or 3/8" ply would have been better.

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The jig is also setup to cut the rabbits. Here is the second op on the back bricks.

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The way the bricks are laid out there is a 1.5" x 1.5" filler on the corners. I need 16 at 9" long and 8 at 1.5" long. Fortunately I still have the tile saw from my construction business. The bricks cut like butter on the wet saw.

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Here are all the bricks cut.

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Last thing to do before assembly is drill the holes for the heating element and the thermocouple and make the recess pockets for the bolts that the elements terminate to.

I have to say this is a first. Machining bricks!. I used a damaged 1/2" 4 flute carbide end mill for the pockets. Holes wire drilled with regular drill bits.

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Hopefully tomorrow I will get the heat chamber assembled and mortared.
 
I'm enjoying this thread. I have a manual kiln that I'd like to put an automatic controller on to run my element. I'll be curious of he exact electrical pieces you use. Look forward to more posts.
 
I got thinking last night I should pre-fit the heating elements before assembling. I'm glad I did. I had to make some adjustments where the element turns back on itself and my calculated length was short.

Here are the heating elements from Budget Casting Supply. Un-stretched the coils are 31".

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I attached a board to my workbench and marked it at the length it needed to be stretched. The opposite end is clamped in a vise at the end of the bench. It takes some force for the initial stretch but after that you don't need to pull as hard to make it stretch more so it would be easy to overshoot the length if your not careful. The supplier says you can cut the pigtails but not the coils otherwise overheating may occur.

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Hear is the element pre-fitted. I used a small flat bladed screw driver and a 1/2" wide wood chisel to hand carve the cutouts where the coils turn. washed the brick last night and they soak up a tremendous amount of water. The firebrick is very easy to carve when damp as there is no dust. Having the bricks damp also helps with the mortar application and curing.

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For assembly is screwed a couple of boards to my workbench using a framing square. I also attached a couple of 8" squares the the table as a guide to keep the first three course square. The bricks were assembled from front to back so the back could be installed at the same time. I only got two pounds of refractory mortar which was exactly enough. I plan on picking up another pound to make a slurry and detail all the outside joints. I will let the assembly cure for a couple of weeks while I work on the frame. Once it has had some time to cure I will block sand the sides and back to address any high spots.

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Once the ceramic paper arrives next week. I can cut some samples and stack it with the sheet metal to finalize the frame dimensions. I'm still debating adding a thicker insulating layer. Not sure yet. I have seen a number of builds with just a sheet metal skin over a single layer of bricks and other builds with a second thick layer of ceramic board or insulation. I suppose the worst case is if the outer shell gets unacceptably hot I could add a second skin with a layer of insulation in between.
 
Hmmm... seems I missed a "R" in the thread title. Makes it sound like a medieval torture device....LOL
 
Very nice work, Jbolt! This should be an awesome addition to your capabilities.
 
Title issue fixed.

Nice project! Right up my alley also, so I will be following this one closely and hope to built a similar oven in the future.
 
I know a brick layer that use to wet all his bricks prior to use, Ask why and it was so the mortar would stick better. Quick question how long do you have to wait for the bricks to dry out before heating it fully.
 
This is an excellent build for us on this sight. I'm looking forward to doing one eventually. I enjoyed doing heat treatment and case coloring long ago.
Thanks
 
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