The Home Foundry, Metal Casting - Part 1, by Dan in Oklahoma

Monday, Feb 15, 2010

I set out to learn how to sand cast aluminum and set up a metal working shop at my home more than two years ago. Let me start by saying that sand casting can not be learned overnight and although books are helpful, especially the David Gingery Book 1 "The Charcoal Foundry", there is no substitute for hands on experience. You have to get out there and try, try, try. To get started in aluminum casting it is critical to first come up with the casting sand. This was really difficult where I live, the sand must be very fine to gain the detail for the part being cast. And equally critical is adding the right amount of bentonite clay to hold the sand together. I wanted to come up with materials locally so that I would have a continuing supply. My journey started by the river where erosion had broken down the limestone to a workable fine grain sand. I shoveled about a hundred pounds and brought it home. After many hours of sifting I believed I was ready to go on to the next step. Again I walked near the river and searched out local reddish clay. Again I brought it home and ever so slowly got it sifted and cleaned up. For the first test batch, keep in mind that I was nowhere near even lighting a fire at this point, I mixed 7 cups of sand to 3 cups of clay. Then I added around 1 cup of water. I let the sand “Temper” overnight and the next day started pressing some metal parts into it to see if it would hold its shape. It was a failure, the casting sand would not stay together my clay was no good. I started looking for actual bentonite clay, there are no deposits within 200 miles of where I live, and then I discovered that plain cat litter is 100% bentonite clay.

Having cats I already had litter on hand, but the pieces were far to large to use, and I had to come up with a way of breaking it down. This was another adventure, I tried water, vinegar and a mortar and pestle to no avail. I had nearly given up when I decided to throw a few cups into my “Brass Shaker” or cartridge case tumbler. After 10 hours the cat litter was broken down enough to add to my sand, I still had to sift out the larger pieces. I mixed 7 cups of sand to 3 cups of bentonite clay. The ratio remains the same whether you are using cups or any other unit of volume. I added 1 ½ cup of water, mixed, and let it “temper” overnight. Sure enough this worked the sand held its shape but was still porous enough to let trapped gasses and steam out. I even experimented with molten lead on some simple open face castings. The coolest thing about sand casting is that all required tools and even the forge and forge fuel can be made at home costing very little money.

The Casting Flask

The casting flask is a two part box that is open at the top and bottom for compacting sand around the item you wish to replicate. It consists of a “Cope” top part and a “Drag” the bottom portion. Making your own casting flask is not hard I started with a 2 X 4 and cut it into 12" segments to make a total of 8 parts. 2 X 4's placed on top of each other give your flask a total depth of 7". This is a good starting point for replicating smaller parts. I am a huge sci-fi fan and my goal with the first castings was to make a “Lightsaber” or more precisely a “Lightsaber” pommel. If you wish to make larger parts you will need a larger flask, but it is important to START SMALL, there is still much to learn. You will also need some smaller board such as a segment of 1/4" plywood to make the guides or line up points. It is imperative that the Cope and Drag line up together each time you put them together, and that the two parts are as flush with each other as possible. Take four of your 12" segments and form your first square on a flat surface such as a table or board. When you are satisfied that your square is flush clamp it together tightly.
I found out the hard way that when you drive in the wood screws it can throw the whole thing off, if not clamped. I used a Work Mate bench as my clamp, you can use nails as well but the banging can quickly throw the boards out of alignment. A total of 8, 3 ½" wood screws, two at each corner, and I had the first half of the flask. Now form up the second half of the flask but instead of using a table, place your four segments atop your newly completed drag. This ensures a good fit, clamp it, then run in your wood screws. You now have a cope. It is time to make your alignment guides, start by cutting two triangle shapes from a 1/4" piece of plywood or whatever you have laying around. The triangles need to be roughly 6 X 6 X 6, the triangles will be attached to your cope. Again clamp your cope and drag together making sure the two halves are as perfectly aligned as you can get them. It took me two wood clamps and the work mate to lock things down. Take your first triangle and put it halfway between the cope and the drag, making sure half the triangle is facing point down towards the bottom of the drag. Nail it in place, the nails will only go into the cope, not the drag, I used small 1" nails.

Next, cut two small 1 X 5" strips from your plywood and place them tightly against the triangle point on the drag only. Nail them in place, what you are doing is ensuring that the two halves of your flask fit together at the same point whenever they are separated. Repeat the process on the opposite side of the flask. Next you need to add handles to both parts, when the cope and drag are “rammed up”, or filled with compacted wet sand, they can be quite heavy. You will be moving them around often when you are making the mold or “drawing the mold”. I used some scrap 1 X 1" boards cut into 4" lengths and nailed them to the sides opposite the guides. Lastly, you need to give the entire inside of the flask a heavy coat of lacquer. Clear spray paint or plain paint will work also. The casting sand will be wet when used and you don’t want the moisture creeping into your boards. You now have a Casting Flask, these cost $70 to $300 if ordered off eBay or other vendors. There are other types of casting flasks but this type will be used for most home castings. Next up, the forge and crucible.


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