This Instructable will teach you how to cast an aluminum slingshot capable of great power and suprising accuracy. The actual casting method used is "lost foam casting", in which foam is carved into the final design and buried in sand. Metal is poured into the foam, vaporising it and filling the mold cavity to make the final object. The basic slingshot design is Joerg Sprave's Rambone slingshot, although I did not use any templates in the creation of this slingshot.
I decided to do this one day when I had seen a video on making a lost foam aluminum slingshot. I had tried greensand casting (https://www.instructables.com/id/Cast-an-Aluminum-B...) and lost wax casting (https://www.instructables.com/id/Cast-a-Metal-Ring/), so I decided that I should try this last common method of casting in order to more fully master the subject. I also got hooked on slingshots after seeing Joerg Sprave's videos on YouTube.
Enough talking, let's get slingshotting (yes, that is an actual word)!
Step 1: Acquire Foam
First off, get some foam. I believe polystyrene foam (aka Styrofoam) works best for this, so try to get some. It should be very common - I heard that furniture stores might even give you some for free. I started with some oddly-shaped box inserts made of Styrofoam and began to butcher them with a large kitchen knife (pic 2). Once these were cut into nicer flat sheets (pic 3), I glued them (woodglue) together to make a thicker block (with an extra-thick part for the handle) and compressed (using iron railroad spikes) them to get rid of any space in between the seams (pics 4 and 5). Basically, you need a block of Styrofoam; you may already have yours in the right shape.
Step 2: Make the Template
You will also most likely want a template to carve your slingshot from. I just freehanded mine on some paper using a ruler for guidance. You could also download one, but I wanted my slinghshot to be personal. Next, copy the template so that you have 2 or more slingshot templates. Use tape to secure these to the front and back of your Styrofoam block. I had to cut one template to accomodate the extra-thick section of block where the handle was going to be.
Step 3: Carve It Up!
Using a coping saw blade removed from its saw body, cut the general outline of the slingshot from the foam block. I learned that when doing this, try to cut bigger than the outline so there will be more room for error and filing. After you have the rough shape cut out, take a woodworking file and file it exactly how you want the finished slingshot to look (pics 2 & 3). Make sure that this is precisely the shape you want in the end, because it is much easier to file Styrofoam than it is to file aluminum.
Also, if you see that your slingshot handle is not centered (like I did), you can cut it off with a sharp (non-serrated) knife and then re-glue it on with SuperGlue. This worked fairly well and definitely saved my slingshot from looking loopsided.
To finish the foam mold, take some sandpaper and go over the slingshot a few times. I cannot stress how important it is for the slingshot now to look just like you want it to look later, because it is quite hard to file imperfections out of aluminum. I had a lot of filing to do, and my slingshot wasn't as smooth as I wanted. Make it pretty now and save yourself the effort later. The slingshot form should look nice and rounded (pics 4 & 5).
Step 4: Bury the Slingshot With Sand!
Bwa ha ha ha ha!
Find a container suitable to hold your slingshot in, with some extra room on all sides. I used a cut-off vinegar container. Place some dry sand into the bottom of this container. If possible, sift the sand with a sieve. I thought my sand would be fine enough, but looking back, if I had sifted it, I probably wouldn't have had so much filing to do in the end. Make the sand as fine as possible.
With the bed of sand in the bottom of the container, place the slingshot in upside-down and fill the container all the way with the sand. Fill to about 1/2" above the slingshot. Tap the container on the side a few times to help the sand settle in to the fine detail of the slingshot. Your slingshot should be just below the surface of the sand; take your finger and uncover the tip, as in picture 4. This is where the metal will be poured into.
Before you begin the actual casting process, make a sprue by cutting the bottom off of a soup can. Place this over the uncovered tip of the slingshot (pic 6). You will pour the metal into this can and from there it will flow into the slingshot mold, vaporizing the foam and taking its place while it solidifies. The sprue helps by providing a place to have extra liquid aluminum, thus providing more pressure and forcing the aluminum to completely fill the mold.
Step 5: Pour the Slingshot
You probably already know this, but one can't be too careful. Molten aluminum is DANGEROUS! 1220°F will seriously put a damper on your fun if it gets on you. Wear gloves, safety glasses, jeans, and heavy boots. Don't do anything stupid. There we go. Done with the "safety talk"!
With your mold ready to go, light your furnace. Your furnace could be a paint can with a hole near the bottom of the side for a metal pipe to provide extra air from a ShopVac blower. Place charcoal in the paint can, light it, turn on the ShopVac, and you have a furnace! You could also build something more complicated like what I have in the video - a helium tank lined with refractory and capped with a refractory lid. David Nash on YouTube has some good tutorials on doing this kind of furnace if you are interested.
With your furnace lit, place aluminum in your crucible and put the crucible in the furnace. If you use a paint can furnace, you could also use a soup can crucible. No modifications needed. However, I bought a clay-graphite crucible from http://www.budgetcastingsupply.com/. If you used a soup can crucible, you can just pick it up from the furnace with pliers from the furnace. However, since I have a more fragile crucible, I got a friend to weld some crucible tongs from mild steel stock.
Phewft! By the time you're done reading all that, your aluminum should be melted! Make sure that you put enough aluminum into the crucible to fill up the entire slingshot mold. Once it is all molten, remove the crucible from the furnace. Add about 1/2 tbsp. each of washing soda (sodium carbonate) and table salt (sodium chloride) to the melt to remove gas bubbles and slag, respectively. Skim off the created slag with a spoon or, like I have, a flat steel rod.
This is the best part! It is time to pour the molten aluminum! As you pour, make sure to admire the one-of-a-kind shiny liquid streaming from your crucible. To actually pour the aluminum, pick up your crucible with your tongs/pliers and bring it over to your mold. Evenly and steadily pour the aluminum into the sprue (the cut-off soup can), filling the sprue at least 1/2 way full. The vaporizing Styrofoam may make a fun little burst of flames or smell awful, but you are doing this outside... right? Pour excess aluminum out of the crucible onto some dry dirt or sand, then let everything cool for at least half an hour.
While you let the aluminum and the furnace cool, you may want to clean up. My forearms resembled those of a coal miner when I was done casting. The video of me pouring my slingshot is below:
Step 6: Rough Finishing
This is where the hard work begins. Once you have let your slingshot cool for at least half an hour, remove it from the sand. My freshly-removed slingshot is depicted in picture 1. Next, cut off the sprue soup can (pic 2). I didn't want to cut across the sprue because that would make the end of the handle too flat, so I cut the sprue out of the excess aluminum (cuts visible in picture 3). Your slingshot is now free!
With a file, begin to shape the slingshot. I started by getting rid of the corners at the end of the handle (pic 5). Then, I went over the entire slingshot, striving to remove all the pits and bumps (pic 6). I mostly succeeded.
Next, use a circular file/rasp tool to make grooves in the tips of the fork (pics 7 & 8). These are for attaching the bands later. Your slingshot should now be generally as blemish-free as you want it to be, because the sanding in the next step will not remove large pits. Keep up the good work - it will pay off!
Step 7: Final Finishing
Grab a fairly rough grit of sandpaper and give your slingshot a good once-over. I found that using the paper like a shoeshiner's rag worked well for making the curves smooth (pic 1). I also just sanded back and forth on other parts (pic 2). Work your way up the grits, going as high as you dare. I only had about 400-grit, so that's where I stopped. Then, if you have a sanding sponge of very fine grit, by all means use it! Mine worked pretty well for shining the slingshot.
If you have buffing equipment, I am sure your slingshot would look even more dazzling with buffing. However, I didn't have this, so I stopped with the sanding sponge.
Step 8: Bands and Pouch
Once the slingshot is very shiny and smooth, add the bands to the slingshot. I used Theraband black (or some knockoff) configured in two sides with three bands each, 1"x8", for a total of six strips on the slingshot. Most people use Theraband gold, but I found some Theraband black at a local chiropractor, so that is what I went with. If you end up using TB gold, Joerg Sprave's band dimension calculator is very helpful for determining how much TB gold to use. For the pouch, I used a scrap piece of orange (weird, huh?) leather. I drilled two holes in it and rounded the pouch off with normal scissors. I will not discuss how to attach your bands and pouch (I am no expert) and will instead refer you to this great video, shown below:
Step 9: End Product
As you can see from the above images, the slingshot turned out quite beautifully! Although there are some minor pits and imperfections, I am exceedingly pleased with the results. The slingshot is shiny and smooth while also having a durable construction that will make it a lasting weapon to enjoy. I hope yours turns out just as well - a job well done is quite satisfying.
I shot a short video of me shooting some clay balls at my chimney. You should not shoot your house anywhere else! Also, please remember to wear safety glasses. The power behind this slingshot it incredible, and you really don't want to ruin your vision with a stupid mistake.
I really like how the slingshot is powerful enough to make the clay balls explode on impact. Also, as you can see from the video, it is currently winter. Sad. The Theraband black elastic bands don't work very well in the winter, so I can't do a whole lot besides shoot once and then go back inside to let the rubber warm up again. When spring rolls around, I plan on perfecting my shooting technique and then going squirrel hunting for meat and hide. I will be using steel ball bearings as ammunition; lead balls are also an excellent choice, although you should wash your hands after using them.
To wrap up this Instructable, I wanted to encourage any of you who are looking at all these steps and feeling intimidated. This was my first time doing a lost foam casting and I really didn't know much about it, and the slingshot turned out quite nicely! As for the amount of work involved, I am absolutely certain that it is worth it for the fun of slingshot shooting. Now, go and do likewise!