Introduction: Cast an Aluminum Slingshot!

Picture of Cast an Aluminum Slingshot!

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 ( and lost wax casting (, 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

Picture of 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

Picture of 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!

Picture of 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!

Picture of 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

Picture of 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 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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!


KiaraW1 (author)2016-02-23

It seems like aluminum can be cast into just about anything. I've seen many different projects where they have used aluminum to create just about anything. In this case you made a sling shot. Did you mold the aluminum into the shape yourself?

GoldCivetta (author)2015-09-14

cool project dude

lazyboy395 (author)2015-02-26

This looks a lot like JoergSprave's Ram-bone slingshot, only it's aluminium.

You could probably double the bands(w/ Theraband Gold, one of the strongest type) w/out breaking the slingshot, and use steel bearings. With those additions, it could probably even destroy solid concrete.

PopsicleGhoul (author)lazyboy3952015-03-31

Yeah, I modeled it after his. Thanks for the tip about the bands! I think I must have banded the slingshot wrong, because they broke after a few days of shooting. I will definitely try the TBG! I like the prospect of demolishing concrete!

ericcartman1717 made it! (author)2015-02-04

i did not have a foundry at the time so I couldnt melt aluminum but I was able to melt zinc so I used it. its 1 lb and 14 oz so its heavy but I like it

Wow, that slingshot is absolutely gorgeous! I bet it feels awesome to heft it in your hand! Well done!

thank you

tgranig (author)2014-12-16

do you watch youtube videos of "joergspreve" and "the art of weapons" ? :D
very cool instructable :)

PopsicleGhoul (author)tgranig2014-12-16

Thank you! And yes, I frequently watch both channels. They do some pretty crazy stuff. :)

sakko303 (author)PopsicleGhoul2014-12-28

I am inspired! I have seen a few crucible/casting type projects online and really want to try it out! Have you ever watched King of Random? he intends to release a video very soon about an aluminum can ...forge? is that the correct term? I dont know... anyway. please consider instructables for your squirrel meat and hides!

PopsicleGhoul (author)sakko3032014-12-28

I am pleased to have inspired the love of metal casting in you! I do follow the King of Random quite eagerly - he does some really cool stuff! I look forward to seeing his new video. Also, I will certainly make an Instrctable about doing something with squirrels bagged with a slingshot (once the weather warms up!). I imagine there are some tasty squirrel recipes... :)

tytower (author)2014-12-26

Styrofoam cups disolved in a little acetone goes to a rubbery ball ,pieces of which can be torn off and pushed into any hole you want plugged like roofing iron. It sets hard as and no more leaks

PopsicleGhoul (author)tytower2014-12-27

Wow, thanks for the great information! I will be sure to remember this helpful tip for my next lost foam casting.

spark master (author)2014-12-18

Cool instructable !

This should people get over their fear of flaming metal.

Thanks! And flaming/molten metal is quite awesome to watch, indeed.

it is bacon (American smoked bacon), or peeing off a tall structure.... UHOH Santa will be all over me for that one!

Attention, bacon and flaming metal... Sanity has left the building!

pick a holiday and enjoy!

wmrhere (author)2014-12-20


First, I and 8 generations before me have all resided here in the USA, into colonial america, not the UK.

Second, the propulsion of a projectile from your catapult defines it as such. A catapult, uses a arbalète mechanism like a spear-gun. Yes, a slingshot means exactly that and yes that is what is. It relies on centrifugal force to launch the projectile.

It may seem a small issue to refer to an object or process by an incorrect term, but it does diminish credibility.

As with any major or minor technical issue, communicating or accurately describing methods, materials and systems are the key to replication. Whether or not Gran-Paw made you, what he called a slingshot it is in fact a CATAPULT. USA or UK it is the same.

PopsicleGhoul (author)wmrhere2014-12-21

So... I don't mean to start any ongoing exchange here, but I believe that my use of the term "slingshot" is correct. If you Google image search "slingshot," you will find pictures of the same type device I made. While the term "catapult" is also correct, it is most commonly used in the UK (I verified this on a few forums). A Google image search for "catapult" primarily returns pictures of the medieval siege weapon. Thus, I could have used either term and I would be correct. Additionally, King David used a "sling," which relies on spinning, rather than a "slingshot/catapult," which relies on elastic stretching force. I hope this helps with any confusion or misunderstanding.

wmrhere (author)2014-12-21

Btw your casting will be most reliable if you minimize the handle to yoke voids. They could result in a dangerous failure.

PopsicleGhoul (author)wmrhere2014-12-21

Yeah, this was my first lost foam casting. Hopefully my next one will be improved from experience.

wmrhere (author)2014-12-21

My mis-statement
This catapult uses a arbalète mechanism like a spear-gun. A
catapult can use a number of mechanisms/ methods to store energy. Look into punkin chunkin.

macrumpton (author)2014-12-19

Great ible!

I used a similar process back when I was in art school in the last century.

We used a sand that had some clay mixed in, so if you squeezed some it would hold its shape when you released it. We also used masking tape to cover the foam, and this made for a smooth surface (with a slight wrinkle from the tape surface). You could also tape a piece of paper onto the foam for larger flat areas. The surfaces that resulted were very smooth with no voids. One last tip, we made the sprue out of foam as well. Two more thoughts: You can get large flat panels of white foam at Home Depot and similar stores in the insulation section and I think you could also use a hot wire cutter on the foam to make it easier to carve.

BTW Be careful about the fumes from lost foam casting. My teachers told us it contained cyanide, so make sure you are in a well ventilated area.

PopsicleGhoul (author)macrumpton2014-12-19

Thank you! Also, thank you very much for your deeply insightful comments and suggestions. I am excited to try them out on my next lost foam casting project! They should be helpful!

macrumpton (author)PopsicleGhoul2014-12-20

Feel free to ask any questions that you may have about the process.

We also used another unconventional process borrowed from the automotive industry. It is a variation on traditional lost wax casting. First you then make your piece and sprue out of brown micro-crystalline wax, find a cardboard box that is at least 5 inches bigger than your piece (on each side) and mix up regular construction sand with some liquidy epoxy until it is thoroughly coated and put about 4 inches of the coated sand in the bottom of the box.
Coat your wax piece with a paint like substance we called a zircon wash (which did such a great job of holding detail you could see fingerprints in the wax on the final piece) and carefully pack the coated wax in the box with the remaining epoxy sand. After a couple of hours the epoxy is cured and you pop the box in a preheated 300° oven big enough to hold your box and over the next couple of hours the wax melts through the zircon wash into the sand. While the wax is melting out you heat up your metal (I used this process with both aluminum and bronze). When you are ready put one of those ceramic funnels over your sprue and pour the metal into the cavity. The beauty of this technique is that after pouring the metal the epoxy in the sand starts to break down, and by the time the metal is cool the epoxy is so degraded the sand breaks off in big chunks with a few whacks with a hammer. There is very little cleanup on the finished piece, and the quality was as good as plaster investment casting.

Errol1951 (author)2014-12-19

When using molten aluminium the slightest bit of moisture can cause extreme explosions so be very careful. when Molten aluminium covers water the water explodes followed by another explosion when the aluminium is in the air and can cause massive damage. I worked in the Aluminium smelting industry for 17 years and we had to be very careful with moisture around it

PopsicleGhoul (author)Errol19512014-12-19

Ahhh yes... very good point! I believe I mentioned this somewhere in a comment, but I have had a few "exciting" experiences with those explosions. You see, molten aluminum on concrete causes extremely violent and forceful explosions of both liquid and solid metal at ~1200F. While fun to watch (at a distance), this is definitely something to be avoided! Metal casters should always make sure their tools aren't wet and should also put all scooped-off dross on a metal surface to avoid explosions. Thanks for the input!

HoldOnTight (author)2014-12-19


wmrhere (author)2014-12-19

Good info on simple Al casting. BUT
The etymology of this mis-used term "slingshot", can be debated but king David of biblical fame did not have access to any old inner tubes for his catapult. Citing wikipedia as a legitimate source for info is a joke, Google - wikipedia unreliable info. Anyone can pontificate on any wiki subject but verification is not reliable. Are american Indians from India. Wrong is wrong irrespective of tradition or vintage.

PopsicleGhoul (author)wmrhere2014-12-19

Ahh yes - good point. It seems there is a fair bit of confusion on naming Internet-wide. King David in the Bible used a sling, which is spun around the head. The term slingshot is commonly used to refer to what I made in the US. Catapult, I believe, is more common in the UK. However, the word catapult can also refer to the Roman siege weapon thar uses a flinging arm. I hope that helps. :)

wmrhere (author)2014-12-18

This frame is not a "slingshot" which is an entirely mechanism this is a catapult

Spokehedz (author)wmrhere2014-12-18


spark master (author)wmrhere2014-12-18

in the US it IS a sling shot, in UK and Europe it is catapult, kinda like foot ball and soccer. But I do agree with you, but most people in the US would not picture this as a catapult, that is reserveved for a Roman catapult or Bucking Ass.

amulder1 (author)2014-12-18

Nice Instructable. Personally I'd do some strength testing, if it fails due to a casting defect it won't be pretty. For the surface you could try and use solder to fill the gaps.

I'm also planning on making a wooden Slingshot (one day...), I like the design "doberman knives" makes on his YT channel. In one of his older vid's he shows how he makes them.

PopsicleGhoul (author)amulder12014-12-18

Thank you! I'll have to check out the video you referenced... I bet stained wood and Theraband gold would look pretty nice. Also, regarding the strength testing, it is a very good idea - even just trying to bend the arms by hand is helpful in making sure that the slingshot is structurally sound.

amulder1 (author)PopsicleGhoul2014-12-18

Look for the older how to off the high end slingshot. It's a laminated piece with hardwood, micarta and felt. The video is in 5parts and from last year(2013)

gabrieltaft (author)2014-12-16

Thanks for the instructable! The slingshot looks amazing. Wish you shot something that explodes in your video :p

Yeah, totally! I hope to buy some of those little caps for toy guns and take out all the powder to make explosive ammunition. I'll have to post a video to this Instructable when that happens! :)

I also wonder if the shock of hitting a hard surface would detonate dynamite...

Wolfbane221 (author)2014-12-16

cool instructable! Would you say water displacement in a graduated cylinder would be the easiest way to measure the volume of aluminum you need?

Thank you! The graduated cylinder would work well, although for the sprue to fill with metal and supply the shrinkage, you should have extra aluminum melted. I just filled my crucible full and then poured what fit. The extra molten aluminum in the crucible can be made into ingots for later. Good luck!

About This Instructable




More by PopsicleGhoul:Build a Milk Jug Raft!Fading RGB Keyboard LightCast an Aluminum Slingshot!
Add instructable to: