Furnace Thermocouple Installation

The furnace is complete except for the thermocouple, which makes the controller into a very advanced thermostat. Without it the controller is blind and cannot regulate the temperature. What makes this sensor special is that it can measure temperature up to 2,300 degrees F, or the so-called cone-6 temp for kilns.

Most other devices would have long since melted or burned to a crisp.



The thermocouple works on a principal called the Seeback effect. The junction of two different metals creates a temperature-variable voltage. Each rod is a different alloy of some metal , and are welded together at the tip. A set of double-holed ceramic spacers ensures a seal on the kiln as well as keeps the rods separated.

I had to shorten the thermocouple to correctly fit my furnace. Here it is attached to the ceramic mounting junction.



I drilled a 1/2" hole between the furnace element studs. The studs and related hardware are all stainless steel. I have a rudimentary but cheap method of isolatingthe intense heat from the terminals from the feeder wire to the controller - I used about 3 feet of 14-gauge solid copper wire from a length of romex I found outside on the ground, and made radiator spirals.

I attached the thermocouple wire provided to the mounting block and ran it back to the controller.



The tip protuding into the oven interior by 2". Minimum according to directions is 1.5".



With the controller feed hooked up, and the protective cover in place. When energized these coils will be hot and live with 250V AC.



The completed furnace.



With the door open. I have run it to reach a temperature of 100 degrees. It works well, and regulates. The controller can be set into learning mode to figure out the system parameters and be quicker to ramp up and prevent overshoot.



Now just some things that need to be heat treated :)

Shaping the Handle

After I let the epoxy dry for a day or so I set to shaping the handle with my new hand-held drum sander.



First I squared it off with sander. I'm glad I decided to use the brass retaining pin. It it much easier to sand away than steel would have been.



I found that using a chisel to carve the wood worked faster, then I used the sander to smooth the lines.



I drew lines on the block to chisel the wood accurately. This wood is red oak, definitely not a forgiving wood for carving. I had to get it really sharp to work at all. I used newspaper and duct tape to both protect the blade from the vise and me from the blade.



The handle is still too thick for a comfortable grip. I removed 1/8" more width after this shot. I sued the 60 grit to get it shape and then smoothed it with the 100 grit.



Shaping done and ready for finish. I am still debating stain or not. It will get a poly finish for maximum durability, either way.

Khukri updates

The khukri handle modification had some progress over the last week.

I tried to silver-solder the guard on but with no success. Its difficult to get the heat up and even without overheating the flux. I tried several times with no success. If I had a induction heater it might work. So instead I used plumbers solder, which melts at half the temperature. That worked well, and seems to be strong. Then I cleaned up the blade and soaked it in a baking soda solution for 15 minutes to neutralize any residual flux and oxides.

While getting the tang to fit the handle I mulled over the methods I could use to sand and shape it. I don't have much in the way of power sanding tools, at least not in terms of something aggressive. The official knife-makers tool is a stationary belt sander with a 1" or so belt. These take up room and aren't cheap, and most importantly I don't have one. I did however have a few variety packs of aluminum oxide sanding sheets. And some PVC pipe scraps...hmmm.

The Edison bulb buzzed into incandescence: I could make a drum sander!

I cut a length of 1.5" PVC that was two endcaps longer than the sanding sheets. I wrapped a sheet around and marked it for the circumference, then cut it to size with scissors. I then used the sheet to knock the shine off the pipe and give some tooth for the glue to bite. I first thought of using wood glue/gorilla glue/3M 77 spray adhesive..aha! Sprayed both the pipe and the sheet, let them get tacky, and they went together well. Guess I got onto kind of a kick because I made another pipe in 60 grit and two more in 100 grit.



The endcaps where chucked int the lathe and center drilled for 1/4" bolts. I can use the same endcaps for each drum! I wll probably use a peice of wire or a bolt to secure it. I am thinking of either making a "live" handle on the far side and chucking the whole assembly in my power drill, or a mount and chucking it in my drill press.

Once the tang fit the handle inserted the two fiber spaces I was planning on using (felt), clamped it all up, and drilled throuhg the handle and tang. I had a 1/4" steel to use for the pin but steel sands like crap. I needed to use brass. All I have though is 1/2" brass rod, so I chucked it up and turned it to 1/4", chamfering the ends to make it easy to put in.



Then I used a file to make "bites" in the tang, and wrapped the tang in some kevlar fiber to help tighten the tang/handle fit. I then mixed up some 5-minute epoxy, coating the tang, filling the handle, and saturating the felt/fiber washers with it. I clamped again and drove the brass pin through. I left it inside overnight because its a bit cold out for the expoy to cure right.

See you next time, same bat time, same bat channel...

Furnace Door Attached

Using the frame as a template I built up a box of sorts to contain the firebricks. Some firebrick wedges and extra ceramic insulation keeps everything from jostling around in there, which is good because firebrick is fragile as all hell and likes to fracture. Also the firebricks will fall out the front if not positioned correctly.



I put a hinge on the outer sheetmetal on the door and used metal screws so I could get in to service/replace the firebrick should the need arise. Knowing my luck it will :) I also drilled holes in the oven frame and attached the door hinges using metal screws.



The next step was to cut some of the ceramic blanket to act as a gasket. I cut a piece the correct size, but had to peel it off in sheets thin enough to where I could close the door. It takes light pressure to fully close the door as the gasket compresses slightly, which should help with the seal. It is held in place with nails pushed at an angle into the firebrick.



Check out the impression, thats the seal! I left the middle open for the temperature sensor but decided it shouldn't go there anyways.



I am using a peice of mild steel as a latch for the door, which also activates the interlock switch when closed, allowing the heater elements to energize.



The latch is not heavy enough to push the switch by itself when the door is open.



Last thing I did was mount the controller above the furnace, well away from the door. I've had my face singed by hot air from a household oven, this thing can operate at four times the temperature!

The final two pieces of the puzzle are the element connectors and the kiln thermocouple.