A while back I posted about my experience building a smallish heat treating oven ( read it @
Upgraded the same days ago to a larger size (2x increase), about 18" long, 4.5" wide, 4" tall. Welded (brazed) a cage, the rest of construction is the same: firebricks, HD mortar, tin. Also used ceramic insulating blanket this time around for better insulation.
Wired my breaker panel for a dedicated 240V circuit. 120V won't get you to reqd temperatures in this size oven in any reasonable amt of time. Resonable for heat treating of metals that is, it will be just fine for ceramics. For example, after pre-heating A2 to 1400F, you want to go to 1750F in minutes, not 1/2 of hour (heavy decarb, scale will result if you do).
Kept all of the old electronics: K-couple, Omega PID controller, SSR.
Went with Kanthal A1 (resistancewire.com). 17 gage wire, .432 Ohm/foot. They do have a min order of $50, but this much wire will last you a lifetime (or, if you can wind spirals, you can try Ebay'ing the excess to pay for the whole thing ).
Really like the results - the oven goes to 1200F in about a minute ! I guess larger surface area of wire pays off here.
Will see if I can flood the interior with argon to eliminate need for wrapping up steel in foil ( 1095 and likes, that harden @ 1450F, wont need it much, but A2 and other tool steel that need to go close to
1800F and above, will loose carbon like crazy at this temp). I exchanged emails with a well-known knife maker that uses this particular method with great success.You'd want to bring the oven to 1850F on first firing and hold it there for 30m to have Kanthal build nice'n'strong layer of Aluminum Oxide - it will protect the element for quite some time. Be aware that the wire will become very brittle past that firing, so make sure the spiral is placed into the groove the right way before you fire it up.
Like the previous time, I used my mini lathe to make the spiral, total of 12.5 Ohm, so it stays a bit under 20A load.
Easy does it, slowest RPM, work gloves, go 20-30 turns at a time, release/unwind more wire, repeat. The 17 gage Kanthal is real easy to work with, but gloves are a must, so is the eye protection.
Measure the perimiter of the GROOVE and stretch the coil to be that exact length. Easiest way - crimp one end of spiral in a vise, pull the other end, while having tape measure extended alongside. This way you get an even stretch - as opposed to stretching a small section at a time with your hands.
As you tuck the spiral into the groove, double check to make sure there are no shorts between coils. Also, there should be no stress in the spiral - especially compression-type. As the spiral softens when it heats up, such stress can cause shorts.
Have the groove made at an angle, so that the spiral doesn't fall out. I hand-guided firebricks into the end mill at slowest rpm, while tilting one side of the brick to get that angle. I put a piece of 1/2" thich piece wood under one edge of the brick to accomplish that. Mini mill of course.