Heat-Treat Furnace Controller

The furnace controller is complete, all the bits fit into an ATX PS case. It has two safety features to avoid electrocution, as the furnace heating elements, made from kanthal wire, are conductive and therefore pose some hazard.



1.) Since each leg of a 220V circuit is "hot", each pole is isolated by its own solid state relay. I also chose solid state relays because of reliability, and better heat regulation - with an SSR the furnace controller can be configured to switch on/off much more often which would otherwise cause excessive wear to a mechanical relay's contacts.



2.) The relays control terminals are interrupted by a microswitch which I plan to fit to the door latch as an interlock. Any time the door is open, the coils will be switched off, yet the controller will still be on.



I had originally built the furnace frame using galvanized angle iron and sheet. Given the higher temperatures this device will operate at, I didn't want to risk any of it being burned off; zinc is a toxic metal and the fumes will kill you. It probably wouldn't burn off, but I decided to go on the safe side and rebuild the frame after dissembling it and dipping the the various parts into concentrated hydrochloric acid. This dissolves the zinc coating and leaves the metal bare. Since bare steel will rust very quickly (especially if heated!) I coated it with Rustoleum high heat BBQ paint. This can supposedly stand up to 1200 F, so it should work.

The last order of business is the door construction. The door will contain several 2,300F firebricks of the same type as used in the oven, and for a gasket material I will be using ceramic fiber blanket cut to shape.

Electric etching pencil

Sorry about no posting, if anyone actually reads this blog. Which, I doubt.

After a day of frustrating work on my home-built plasma cutter, I decided to try a much smaller project to take my mind off of things. I got this idea from this article on vintageprojects.com - an electric arc etching pencil. I've been wanting to make this for years now so I can write my name on my tools or label control panels in projects.



I started with a small threaded bolt from my junk box and shrinkwrapped it to prevent the sharp threads from shorting the first layer of the coil as it has only enamel insulation.



The magnet wire is 20-something gauge salvaged from some old toroidal switching cores. I wound the thicker stuff on a larger bolt as an experiment. When I ran out of wire to wind I just scraped of the enamel, soldered on another piece, and kept on winding.



I wound two coils using this method and saturated them with some superglue to hold the windings in place.



The hardest part was finding some material to use for the spring. I ended up cutting some pieces out of a springy motherboard bezel (the part that mates your mobo to the PC case). I drilled a hole for the small bolt and bent the spring into a U shape. I tightened two screws on the core bolt to trap the other end, and attached a peice of 16 gauge copper wire to the small bolt to serve as the electrode.



I wired up the ground clamp and decided to give it a try. It took some fiddling with the gap between the solenoid and the small bolt but it worked! It definitly needed a handle though, as its too small to hold and gets hot. I used a 3/4" wooden dowel and drilled a hole in it lengthwise to press fit the coil.



I can write on pretty much anything conductive. The tip buzzes along as your write and it seems to work best on bare metal, stuff with mildly conductive coatings also works but its more difficult to get a smooth oscillation.