A Heat Treatment furnace
The little forge described elsewhere is good for heating things rapidly for forging but not ideal for soaking things for hours to normalise or temper. It also can't really get a large item to a uniform temperature.
I had been asked to make some kick-starter gears for the Ner-a-Car chaps, and those seemed like something that ought to be heat treated.
EN8 needs to be heated to 850°C for 30 mins or so, then quenched, then soaked for 1 hour per inch at 630°C or thereabouts. So I decide to make the little electric muffle furnace I have been buying parts for for a while.
I was lucky enough to find a big reel of Brightray Alloy C in a skip back when I was working at Sheffield University, so that was the element sorted out.
I bought some Alumina tubes from eBay, along with some Alumina blanket, fishspine beads, a couple of high-temperature connector blocks and a solid state relay and controller.
I used to work as a metallurgist, and learned a neat way to wind a muffle then. The trick is to make lead-ins of the resistance wire that are triple thickness so don't get as hot, and twist them tightly round the muffle to anchor the first and last windings. Take a loop round the back, pull the loose end into the loop (away from the tube) and twist with cordless drill or pliers. (not too tight, breaking the element is annoying)
This photo shows the tube set up on the lathe with an improvised wire guide and the anchor turns twisted tight.
The CNC lathe makes the next stage easy. I knew I wanted 10m or element for a 2kW power, which was 190mm at 4.1mm pitch. So M3 S30 G33 Z-190 K4.1 from the MDI window was all it took.
Here is the winding, and the end-anchor twist.
The element will expand when hot and come loose on the tube, so needs some support to keep the windings apart. There are some specialist compounds, but B&Q don't sell them, and fire cement seems to work fine.
The muffle was then wrapped in alumina blanket quite tightly, with (in this case) the lead-wires coming out the front. These were insulated with the fishspine beads through holes in the front plate of a case that I made:
I don't know how hot it would go, but 820C seems easy and the temperature is still rising at about 0.5 degrees per second.
Be aware that the supplied thermocouples from eBay are only good to 400°C. I am using better ones salvaged from scrapped instrumented parts at work.