This demo is actually 0.8 mig welding wire not the nichrome (I had already finished that when I did this video) Its just a simple bit of angle with a spring loaded tensioning plate. Once you have your wire you work out the threads per inch or thread pitch if your lucky enough to have a metric lathe and set the lathe to that. The setting for .8 mig wire is actually closest to 31tpi (always go next lowest or the wire will ride over your coil) but my lathe will only do 28tpi without changing gears and thats why there is a bit where the gaps are larger in the video.Anyway I ended up with my 12 metres of nichrome wire coiled into approx 700mm this was then stretched to give me a 2.1 metre element.
The element I made had a resistance of 12 ohm so with our power outlets supplying 240 volts ac thats:
240 / 12 = 20 amps so if you dont have anything other than 13 amp sockets then your going to have to alter it a bit to bring it down to what you can manage.
The wattage of this thing is: 20 x 240 = 4800 watts so its a power hungry beast but since it melts the metal so quickly it is not that bad.
There are many types of refractory mix just do a search and you will find many different recipes, I am not going to go into which is best as I havent tried them instead I will just tell you what I have done and once we get to the end we can find out how good or bad it is together.One thing that seems pretty consistent between all the homemade mixes is perlite this is a white lightweight material that does not tranfer heat very well. Cement too is quite obviously consistent as is sand the fourth one however seems at first to be quite hard to come by in any proper quantity is betonite clay. Its just cheap clumping cat litter which you may know but cant be bothered to crush it up well dont bother just measure out how much you need and chuck it in a bucket with as much water as needed, its more than you first think by the way.
Cat litter soup yummy, basically stir it until its a quite thick soup if you have a big clump that wont seem to go add a bit more water.
So anyway the mix I used is 3 parts cat litter, 3 parts sand, 3 parts perlite and 4 parts cement put the sand, perlite and cement into a dry bucket and mix it well then add the cat litter soup and mix, you will most likely need to add a bit more water but the difference between too dry to mix and sloppy is quite marginal.
For this project I decided to cast the furnace in 3 seperate sections there are a few reasons for this, mainly that if something is wrong I wont have to start again from scratch.
The inner form on my project is a piece of 200mm plastic tube with some 14mm tube wrapped around to form the channel that the element will sit in. After I took this picture I was doing some browsing and found a blog where the person in question said that removing this tube ruined the channels they where supposed to create so with this in mind I moved the screws to the inside of the plastic tube in the hope that I could remove the plastic tube but leave the rubber in place in the hope I could cut it in half to get it out easier.
EDIT: I am currently at the stage where I have taken this out and I dont reccomend this way, I would suggest maybe some foam in place of the rubber tubes this could then be burnt out when ready.
For those that dont recognise it this is a tig welding gas nozzle they are usually a couple of pound each and available from shops that sell welding equipment, as you can see this is a number 5 not for any other reason other than its what I had. Speak to the guy at the shop and ask to see the different types you may find one you like better for whatever reason. They have a thread molded on the inside m10 in this case so its perfect for creating a connection from the inside to the outside. Here I have sealed it off with modellers clay to stop the refractory mix from getting inside.
Here I am setting up the outer shell ready to fix the innerform into before adding the refractory mix. In my case I am using parts from a scrap space heater. Now you see where I got the name.
Putting it together.
The inner form is secured to the outer shell by two long screws through the ceramic tig nozzles and it is secured down to the board ready to pour the refractory mix into thus:
Actually I have to come clean here not long after a took this photo I suddenly realised I had forgot to add the sand to the mix so it all came out again to be remixed with the sand.
Its all about the base.. no stop it!
The base is a little more of the space heater which conveniently had 3 holes equally spaced around it so it was a easy job for me to add another 3 so I could tack 6 bolts to the inside which will have the legs attached to them. I had to break the connection as it had to be a bit larger to accept the main body, like so:
So after I had reattached the two ends I cut out a circle from a scrap bit of metal
and welded it to the bottom ready for filling with refractory mix. As I am a welder by trade I am not going to show you the welding I did on the base it might cause you to doubt my day job but it was the first bit of gas welding I had done since my college days.
Inner form removed.
Well I got the inner from out in pretty much one piece, I did it by drilling holes in it either side and levering it up until I needed to drill more holes to repeat the proceedure until it was free. There was a slight break at the top but nothing major to worry about there is also a few air bubbles which until it is completely dry could cause a problem by blowing out but I think as long as I dont raise the temperature too quickly is should be ok.
You can see the reason I did the edit on the “body” post when I tried to remove the rubber tube it broke the channels out quite badly meaning that now I need to use pins to hold the element in and so I believe that foam to form the channels is a much better idea as you can just burn it out with a blow torch when you have removed the inner form.
Installing the element.
Started by turning the terminals to fit into the tig gas nozzles the element will connect by a couple of m5 bolts on the inside and some terminal connectors on the outside.
Once I had put the terminals in place I packed modellers clay around them to hold them in place this was “baked” for 15 minutes with a heat gun which I tried to keep around 130°C by trapping the thermocouple on my multimeter between the washers shown below.
Finally the element in place and you will note the pins holding it in place.
The lid is yet another bit of the space heater or should I say two bits they where cut to 60mm strips which where then tacked to the right size.
I could have cut it down to weld it end to end but figured the extra thickness would give a good place the attach the hinge for lifting the lid. I then add some thick wire to form reinforcement, it ended up looking like an amateur attempt at a dream catcher.
The li’l Bertha as designed by Dave Gingery he puts a criss-cross pattern then puts kinks in each to tighten them I however tied the bars with twists of smaller wire. The center is a empty 1kg fire cement tub which was screwed into position in the center and the whole lot was clamped down to stop it moving before I poured the refractory mix. I changed the mix for the lid to make it a bit lighter, I replaced a quarter of the sand and clay with perlite time will tell if this was a good idea or not as it may be too weak a mix. The following 2 pictures show right after the pour and a little while later when I smoothed it out.
It looks a mess but there is logic to the mayhem.
The mains come in at the bottom of the box to the terminal block and split with one side going to the element, the live is switched with the solid state relay (SSR) and the other to the transformer via main switch at the top.
The transformer feeds 24 volt ac to the controller and to the power supply for the fan to operate (not shown on this picture)
The controller output is going to the SSR Via a switch and a limit switch which turns off the signal when the lid is opened and this turns the element on and off as needed.
Thats it really quite simple when you break it down.
The Li’l Scarlet
Finally it went together I made a hinge and handle for the lid and its now melting aluminium quite quickly too 😀
The lump of aluminium you see here took about 45 minutes from almost cold (90°C) and that includes adding extra to the pot after it had melted the origional charge. The other good thing is that just as I was about to pour it the heavens opened up and holding it to temperature was easy.
There are still one or two little jobs I want to do to get it complete the main thing being a stop for the lid and I may if I feel up to it pretty it up by painting it but thats just cosmetic’s.