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This entire DIY minimum cost/maximum potential workshop idea began with my backyard foundry. I found a good deal for an entry-level backyard foundry - A fairly small (maybe 12" tall, 6" diameter interior)foundry made of a refractory plaster with a metal shell painted orange with 10 clay crucibles for $40. I first added a venturi style burner that seems to be super popular online. It seemed as if it wasn't burning hot enough, as many metals didn't melt correctly. Rather than liquefying, they would burn onto the sides of the crucibles, ruining the metal as well as the crucible. So I changed it from the heat creating a vacuum to pull in air into a shop vac hooked up in reverse forcing air into it, using a router speed control knob to vary the amount of air being forced in. But I kept experiencing the same issues. As a result, I picked up an IR thermometer capable of 1085*C (just shy of 2000*F) which was about perfect for me, since it covers pretty much everything short of steel, and cost about $30. I think I was actually burning the metal. Not that it wasn't hot enough. After becoming capable of monitoring the temp and finely adjusting it via the air and gas input, along with my use of flux more regularly, my melts have gone much better. Though I'm going through propane quicker than I would like. Therefore, I am building a waste oil burner. For my actual fuel to come from oil that is otherwise an inconvenience to dispose of is like 180* different from that scenario. Currently I get my propane tanks filled at U-Haul, which is a significant savings over buying a tank exchange. Propane tank exchange here costs typically $25. But it's also regularly severely underfilled typically 75% capacity. Whereas the U-Haul refill costs $15, and they fill it completely each time. So a tank exchange costs $32-33 per full tank worth of propane compared to U-Haul's $15. So, U-Haul is less than half of the cost of a tank exchange. But even then, assuming I get 3 solid nights of smelting out of one tank of propane, I'm paying $5 per night just for the propane in order to melt metal. I can accomplish the same with waste oil without it costing me a cent in gas. On top of this, I also found an O2 concentrator that produces 5 liters per minute of 93% pure oxygen for $60 on Craigslist. I plan on running this in-line with the forced air on my torch for my foundry. I would prefer to be able to fill my own O2 tanks in order to use high-pressure O2 in the system instead of low-pressure O2 mixed with high pressure air. But even still, this method should still burn a good deal hotter. Though I haven't tried it yet, so I'm not positive.
The propane burner is made up of steel pipe on the outside, with a black steel 1-1/4" to 3/4" reducing coupler connected to a 12" long 3/4" pipe nipple into the foundry where it's finally connected to a 1-1/4" to 3/4" reducing coupler that has been mostly sheared off by the flame going completely around the inside of the foundry, coming right back to where it started, acting as a huge cutting torch. On the intake side, there are 8 tapped holes (one set of 4, 90* apart at the beginning of the pipe. And another set of 4, 90* apart about an inch further inside the pipe) with corresponding screws set in them that suspend the propane inlet. The propane inlet is made up of 1/4" brass pipe. It begins with a ball valve where the propane hose is connected that is connected to a 90* elbow and about a 6" nipple that is capped off. The cap is then drilled and tapped to fit copper welding tips that act as a nozzle to spray the propane in a fine mist that mixes with the airdown inside the pipe which doesn't actually combust until it gets inside the foundry. Originally, the flame created a vacuum effect pulling air through the steel pipe. But I added a tube that fit over the inlet pipe reducer and onto the hose from the shop vac on the other end. I cut 4 notches into this tube to fit around the screws that suspend the propane inlet inside the steel pipe and is held in place with a metal band clamp. And then I cut a hole in it to feed the propane pipe through. The router speed control knob is a neccessity if you don't have an adjustable shop vac. It's far too much air at maximum. I've gotten this foundry too hot for the thermometer to read (so 2000*+F) at night during winter, though it's AZ, so that was still only like 50*F. But still not a nearly 120*F headstart during summer days. I'm sure it's easier and takes less time and fuel to run during summer days, but it's so miserably hot that I don't want to be outside, let alone dealing with a massive torch and metals at thousands of degrees.
I'm starting with a metal trashcan that would have otherwise been thrown out. It's a good size that will fit a good layer of refractory inside while still leaving plenty of room for a large crucible. I'm considering making drop-in refractory sleeves that would shrink the volume for more precise melts of smaller volumes. Maybe making them with varying refractory materials to compare them against one another. Since the trashcan isn't exactly small, I'm probably going to make the bottom layer extra thick - and reinforced - to allow me to mount pneumatic wheels to it so I can roll it around the yard and into/out of a little storage shed to protect from the elements. Currently I'm still experimenting with refractory mortar recipes that I'm considering using.
I've broken this project into multiple parts based on different components of this foundry. There are different pages for the burner, castable refractory, tongs and crucible.
Since this foundry will use a trashcan as the shell, I want to put it on casters to wheel it into a shelter from the elements. As a result, I want a light filler refractory material to surround the firebricks. Perlite does well to lighten it, but it makes it very fragile to anything aside from heat. So in between the brick and the shell, it should fit the bill quite well. I also want to use a kind of hinged horizontally opening design to pull out my crucible. Doing it that way will let me skip the step of switching between tongs to pull the crucible from the foundry and tongs to pour the metal with. So I kind of want a 3-piece lid - the top vertically opening lid with small hole to drop metal into the crucible, and then two half-cylinders that open to either side on a swiveling bar. I'm sure my description sounds like a foreign language so I will try to get a schematic or prototype up ASAP so we're on the same page.