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One of the things needed would be some green sand for making a mold. I did some research on the web, and found a simple mix of fine silica sand and fire clay would work. No one in town carried fire clay, and I had read where bentonite clay may work. No one had any bentonite clay, well, except the grocery store. Cat litter is rumored to be bentonite clay. So I bought a 10 pound bag of plain cat litter, and a bag of silica sand from the local lumber yard.
The cat litter was then ground to a fine powder in my coffee grinder.
Bottom left is the handle I used as a mold form, next to the right is the ground cat litter (clay), next is the silica sand and lastly some cast aluminum scraps to melt.
I wanted to test the green sand, to see if would work or if I needed to order some real fire clay or casting sand. I measured by volume, eight scoops of silica sand and two scoops of clay into the flat tray you see above. The mixture was then blended together by hand. Once mixed I used a spray bottle filled with water and started spritzing water on the surface, then mixing by hand, spritzing again; until the mix was somewhat damp. It was apparent the sand wasn't holding together very well. So I added one more full measure of clay and mixed it in, then added more water. It seemed about right. It would form a nice cylinder with the outline of my hand and fingers when a handful was squeezed together.
So the green sand mix was 8 parts silica sand to 3 parts clay by volume. One thing I could see right away is that I should have screened the clay. I started screening the silica sand, but it was so fine nothing at all was being screened out. I thought the clay would be fine enough after going through the grinder, but there are some small rocks and debris that do not grind up. Too late for this small test batch, but next time I'll screen the clay first.
I decided to try a small casting, to see how the green sand would hold up, and how fine the casting surface would be. So I drug out my old boat lower unit, using a hammer I broke off a few chunks to melt. To hold the sand for the mold I used a short 4" PVC pipe section. I laid my original, which was a handle off my lathe, on a piece of 1/4" steel plate. Then put the PVC pipe over it, and filled the pipe with sand. Tamping it down every inch or so with a wood 1x2. A two piece mold would be needed to make an exact copy of the handle, but this was just a test so I didn't even try. The pipe was then inverted, the steel plate removed and the original handle dug out of the sand. I just blew out any loose sand that fell back into the mold cavity.
I have a small propane torch, but no O2. So I knew melting any amount of aluminum would be slow. I used a large old spoon that I used for lead casting, and held the spoon and aluminum to the flame. I didn't get enough aluminum melted to fill the mold, and didn't feel like wasting a bottle of propane so I just poured what I did have melted into the mold. After a few minutes I removed the aluminum from the sand, then melted a larger chunk of lead and poured it into the mold.
Here's what came out of the mold. To the left is the aluminum, and to the right is the lead. You can see the middle part of the lead casting isn't very smooth. Looks like the mold is only good for one good casting. The aluminum part really is very smooth, I was a little surprised it worked that well. You can see dimples in both pieces though, that's the larger clay particles. I think once the clay is screened it should work very well.
Now on to the foundry to do some real melting. The main thing with small home foundry is a refractory cement to insulate the firing chamber. Hmm, still no fire clay to use. But the bentonite might just work. The worst that can happen is that it cracks and breaks apart.
Here is the start of the furnace. The mud pie at the bottom of the picture will be the bottom of the furnace. If it looks like an inverted air cleaner cover, you're right, that's what it is. I used some wire mesh to help reinforce the cement. The galvanized cylinder above is the housing from an old propane heater. I used the burner for a different project and never had used the housing. This will form the main outside housing of the furnace. The rusty old steel pipe to the right will be the crucible. I welded a 1/4" thick steel plate to form the bottom.
For a refractory mix I used:
4 Pounds sand based concrete mix
2 Pounds silica sand
1 Pound Clay
I have no idea if it will hold up to the heat. If it doesn't I'm not out much. About $3 worth of materials is all. While the base was setting up, I made a liner out of sheet metal to cast the refractory material into. I also added some more wire mesh inside to reinforce the refractory material. I cut and inserted a 10" length of EMT as an air supply tube.
I bucket mixed another 40 pounds of the refractory material to fill the furnace.
I must admit, I'm very impatient. In fact, so much so that I couldn't wait for the refractory to cure. No, I didn't fire it up wet, but I did make another uninsulated furnace to do a quick melt. I just used an old metal 5 gallon bucket, cut a hole in the side for an EMT conduit air hole and fired it up with charcoal. I used about a 3" long piece of 4" diameter steel pipe as a support for the crucible. I used a vacuum cleaner with the hose in the blow hole, rather than the suck hole. (That sounds weird doesn't it?)
Once the coals were burning I moved the air supply a little closer to the air inlet, that really got things going in a hurry. I didn't take long to melt the aluminum, maybe 12 or 14 minutes. Then I dropped in a few more pieces of aluminum, when they melted the crucible was not quite 1/2 full.
I made a mold in my green sand using a piece of 2" OD pipe, about 5 inches long. I used a 6" diameter PVC pipe section to hold the sand. The sand was added about 1" at time and rammed in with a wood 1x2. Once filled to the top, I tapped on the 2" form pipe to loosen it, and pulled it from the sand. I brought out an old muffin tin to cast any extra aluminum as ingots.
Here is a picture with the cylinder I cast getting trued up in the lathe. On the ways of the lathe are two ingots I cast from the extra aluminum.
As you can see from the picture, it worked. Just goes to show that a quickie casting foundry can be made very cheaply and very quickly. I'll have to do a little research on how strong cast aluminum is. The plan is to turn the solid aluminum bar into bulkheads for my new 1.75" outboard rocket motors.
Here you can see the crucible with a couple of 1/2" diameter tabs welded on, and the tool to pick up and move the crucible. The tool is primitive, yet effective. I split some 1 1/4" EMT down the length, then heated it with a propane torch and bent it to form the arms. The arms were drilled out to form a hinge so the crucible stays level.
Here I'm setting up for the first firing of the new foundry. It's been over two weeks since the refractory was cast, so it should be dry enough for a firing. At the bottom left is the quickie furnace I made without refractory cement. You can see the vacuum I use for an air supply. The amount of air going into the furnace is regulated by moving the nozzle closer or farther away.
Here it is with a fire started, you can see all the aluminum pieces in the crucible.
Now there is molten aluminum in the crucible, I added more aluminum pieces as more room became available. I'm sure a lid would help keep in heat and speed things up. But it really doesn't take much time as it is. Unless I want to melt some brass, I don't think I'll bother with a lid.
Here the crucible is as full as I want it. You can see the slag I skimmed off in a pile to the right. From what I understand you're really not supposed to work with molten metal over concrete. It is recommended to work over dry sand for safety.
Here is the result of the casting. Six aluminum ingots. I could have poured more, but leaving a little metal in the crucible helps get the next melt started. There's a little over 3 pounds of aluminum in the six ingots.
The furnace seems to have held up well, the refractory cement (so far) didn't crack. The outside of the furnace didn't get hot enough to burn the paint or masking tape on it. I've been working on making more green sand and a box to hold the sand, cope and drag I guess it's called. I finished grinding the last of the 10 pound bag of cat litter. It looks like I'll need more clay, I'm using about 30% clay to 70% silica sand by volume.