Building Joanne's Firepit
To make something in cast iron, you first have to have a pattern. The pattern looks just like the thing you're going to make, except its a little bigger to account for shrinkage as the iron cools in the mold. Traditional patterns are made of wood, so I had to basically make a big salad bowl. But it has to be pretty strong to withstand the beating it gets when the foundry "rams up" the mold, so its made of many layers of pine glued together. Each layer is made of pie-shaped pieces cut so that the grain of the wood is edge-on around the bowl, so that the wood will cut well in the lathe.
The first step was to make a layout drawing, from which to identify the pieces to be made and determine their dimensions.
You can see the layers on the layout. On the right you can see how the layers are stacked in steps, so the inside cuts could be done one step at a time. On the left in the shadow are tangent lines drawn at angles in 5 degree steps from the edge of the bowl to the bottom, to locate the starting point of each outside cut.
Using the layout to get the dimensions, I cut a cardboard template for all the pie-shaped pieces on each of the four steps. For each step, the outside diameter is the same on all layers but the inside diameter varies. So each template has several inside diameters drawn on it.
Next I got a bunch of "Perfect Plank", bits of clear pine joined together thats made for patternmaking. I used the templates to figure out how much was needed for all the different layers, then resawed and planed the planks (they come 1" thick, but the layers were 1/4" and 3/8".) I marked out all the pieces using the templates, being careful to line them up along the grain, and cut out the pieces on a bandsaw.
Next came the task of gluing them all together.
Above is a shot where I was working on it in my collection. It shows a couple of steps finished, one being glued up, and several rings for the top step that have been glued around but not put together yet.
Below are the finished steps in the back of my van, along with the layout and the pile of templates. The big step isn't dry yet so its still clamped up.
I had been working on gluing everything in between classes; in class I made more pieces to glue up. Now I took it all back to Gary's shop.
The next step was the critical one: cutting the inside and outside shape on Gary's big patternmaker's wood lathe. If you're not careful or unlucky, the pattern can shatter while you're cutting it.
First I cut the inside, one step at a time, because the bowl is most fragile when its only supported by the base. Thats our fearless teacher Gary watching to see that I do it right.
When the inside was finished, I turned it all around and mounted it on Gary's biggest faceplate, with half-round braces inside to make it more sturdy. The faceplate ended up being the follow block for the first casting.
You can just see the circles on the base that indicate the starting point for each angled cut. Thats fellow student Bud in the background. He took the picture of Gary and me.
Using the big lathe is really fun. Here I'm about half done with the outside.
Once the bowl was cut I was a long way from being finished. I made a mold for the legs and cast them in grey plastic epoxy resin, then cut them down and fastened them to the bottom with patternmaker's Bondo. This took an extra long time because I misjudged their proper position and cut them down too much, so I had to use a lot of Bondo. Its not my favorite substance.
I also had to make the decorative pieces on the top several times until they looked right. The the whole thing must be carefully sanded to eliminate pockets that might catch the molding sand, and lacquered so it will pull out of the mold. I also had to finish the follow block.
Finally its all done and ready for the foundry. Here is the finished pattern on the counter at the aluminum foundry. Next to it is the follow block made out of the faceplate.
At the foundry they put it inside a two-part "flask" that holds the sand used for the mold. First they put it in one half of the flask upside-down as in the picture, and pound sand around it to fill the flask and make the bottom half of the mold. Then they turned it over, took out the follow block (that kept sand from getting inside the bowl) and attached the other half of the flask. Sand was pounded into that half to finish the mold. They took the two halves of the mold apart, removed the pattern, cut some holes and channels for the metal to flow in, and put the halves of the mold back together. When a bunch of different customers' molds are ready, they melt a batch of metal and pour all the molds. The metal cools pretty quick, and they knock the sand away to reveal the finished cast piece.
Because the wood pattern is relatively fragile (but mine stayed together!) we first cast it in aluminum. I spent quite a bit of time fixing up the aluminum casting so that it was clean and smooth with no holes or rough sides, and now its the pattern. Several cast iron firepits were cast using the aluminum pattern, with more to come.