This instructable will show you how I made my monstrous tennis ball launcher (cannon) from mainly scrap metal and just a few things from eBay. Also included are tips and potential improvements for certain areas in both design and construction. And of course, how could I nearly forget, how to have lots of fun with it.
Propulsion source: Compressed air from compressor or foot pump (depends how lazy I’m feeling)
Barrel dimensions:90mm OD x 70mm ID x 500mm
Main ammunition: Tennis ball
Max distance of main ammunition: 100m
Secondary ammunition: anything you can stuff down the barrel with wadding at bottom

I first came up with the idea of making a tennis ball launcher to help enthuse school children (year group 9) about mathematics by getting them to aim for a target on the ground behind a wall of paper so that they had to use complex equations and mechanics principles. I did think about making a ping pong ball launcher to the same general design but scaled down as this would have been cheaper and easier, but then I thought that it would be much more fun for them to use a cannon outside. Using this instructable you could still make the ping pong cannon, but what says I love my dog more than a tennis ball cannon strapped to the roof of your car in the dog park car park.

At the end of the instructable I will include the relevant equations of motion that i hope to be able to teach the year 9's. I will also include a link to a simulator that also accounts for drag .

Step 1: The Frame

This cannon frame was constructed from 6x30mm flat bar steel but would have been easier to construct it from wood or aluminium as the bending of the steel was quite labour intensive.
The frame design is very strong, being held together with M6 bolts. Holes in the bottom parts of the frame allow pegs to be used to hold the cannon to the ground. There are also markings on the slide to indicate the angle that it will produce which will come in very helpful when predicting the distance. (Even though you can't see it in the pictures, they are there)

  I started by cutting all of the bar to length and then filing them whilst clamped together to make them exactly the same length.

Step 2: Drilling

Next step is to drill all of the holes, all with exactly the same spacing so that they all line up when you bolt the peices together.
At first I tried drilling the holes with a power drill, but it turned out that the mild steel that I had bought wasn't quite as mild as I thought, it actually took me about 3minutes to drill one 6mm diameter hole in a 6mm thick bar. I thought that this was terrible progress so in the end I just took all of the parts to school to use the pillar drill. This also meant that I had access to specific clamps which are designed to make drilling easier. Although I used these clamps it is not neccesary for you to. Instead you could just get a friend to help.

The wood at the back of the machine vise in the picture was implemented so that both pieces were clamped evenly instead of only the bottom one being held(the vise jaws are not perpendicular).
I drilled multiple parts at the same time becausethat way I could ensure that the pieces would match up. Infact I actually used this method of clamping two parts together for the entire project, using the same two holes from the one part to template all of them.

Step 3: Bending

Sorry that I couldn't get any pictures of this one but it is a bit difficult to hold a blowtorch, hammer and a digital camera all at the same time. As you can guess by this statement, I am not a woman.
To bend all of the parts that needed bending first I marked with chalk the line that needs bending(chalk is not affected by the heat), heat the intended area to cherry-bright red(that way it will still be plyable after positioning in the vise). Place in the vise so that the line is approx.2mm above jams. Start beating with a hammer as close to the vise a possible(to reduce unneccesary warping) until the bend is a right angle.
In heinsight it would have been faster, and probably cheaper to buy some brackets, but this would have meant more bolts and less strength, oh, and of course less satisfaction.
After all of the parts had been bent, I realised that all of the parts with holes in had had there holes miss shapen so back out with the power drill to make those holes round again.

Step 4: Frame Assembly

This is where I bolted together all of my metal and find out that my "skill" as a designer had payed off, giving just enough space between the bolts(for most of them any way) to fit in the socket. and all of this without any forethought.
There were only 2 holes that couldn't fit a bolt through and that was because I made a bend too close to the hole.
The small metal pieces that look like brackets are made to slide up and down the rails(sorry should have mentioned before, the long peices on the floor are layered, so there is one long, one short going perpendicularly and then another long one on top).

Step 5: Plumbing

First job was to empty the fire extinguisher(I did mine by shooting sweet potatoes out of the hose over the wall.
Next step was to take the top off of the extinguisher. I did this by clamping the top in the vise and trtying to twist the bottle(this didn't work) so then I decided to attach a G-clamp to the bottle for some extra leverage and then it came off surprisingly easyily.
I chose to use a fire extinguisher bottle as the air resevoir because I know that it will not fail, it has been made to european standards.
This also means that I can take it in to school without getting into trouble.
Pressure vessels can be very dangerous and legally if you want to take it off of private property in the UK then it needs to be certified as being safe by a recognised professional. This can cost alot of money to have done, whereas a fire extinguisher is already certified.
Because I am using a fire extinguisher as the resevoir, I feel a lot safer going up to higher pressures than I would do with a home made resevoir that many counterparts use, which means that I have the potential for greater performance from the cannon.

Because a fire extingusher is designed to contain dry CO2, it is not a good idea to keep the bottle for more than 5 years running with air. This is due to the fact that when air is compressed, water vapour in the air forms clouds and can rust the bottle if it has not been given a protective coating.

Step 6: Find All the Plumbing You Need

Find all of the plumbing you need to go from the fire extinguisher bottle to the end of the valve. I couldn't find anything with the correct thread size to go directly in to the bottle so I had to cut up the top of the extinguisher which I did find a coupler for.
Other than this coupler I used a 22mm compression tee, and a 22mm ball valve.
The reason for the tee instead of going straight to the valve is that the cannon will need a source of compressed air. On the tee I araldited a bike inner tube valve in place as well as clamping it between a modified olive and the nut. The olive was modified just by filing the top off so it wasn't as sharp. Didn't want it cutting through the rubber. To use the araldite I first cleaned everything by sanding and then rinsing in methylated spirits to remove any grease and oils from the skin. After their rinse, to stop debris or grease form contacting the part i used a metal bar to lift the part out of the meths and then burnt it off.

In the photo with all parts lined up, ignore the last 2 parts as they come next

Step 7: Valve to Barrel

To link the barrel and the valve together, their needs to be a coupler in between. I made this one from chipboard itha nut from the valve in the middle so that I can screw the barrel onto the assembly.
As can be seen by the photos, first I started by drawing out a circle with the same internal diameter as the barrel(67mm), from there I started cuting out with a jigsaw, that was until it started sparking and I discovered that my skill at using this tool is rubbish. So I rough cut it out of the sheet. Because of this minor inconvenience I had to come up with an idea of how to make the piece of wood round and to the correct size.
My solution
Step1: Put the cut out block in the vice and cut along tangents to the circle until all of the excess i could remove was gone.
Step2: Drill a 6mm hole in the centre of the circle(there was a mark left by the compasses) and put a short threaded rod(or     bolt)(M6)through the hole with a nut on the end.
Step3: Put the end of the threaded bar into the drill chuck.
Step4: Clamp the handle of the drill in the vise and do the nut up to tighten it against the chuck.
Step5: Turn the drill on and start sanding(my drill has a button on so that i don't have to continuously have to hold the trigger, but if yours doesn't you could use tape or similar) making sure to wear safety glasses as the setup is not ideal and potentially dangerous. Another safety and comfort precaution would be to use a dust mask or piece of cloth over the mouth and nose as it can get very dusty.
Yet another consideration that I didn't think of at the time would be to put the drill in the vise so that when you turn it the top comes towards you so  that when you sand in the bottom half, your face doesn't turn a nice woody colour.
Keep sanding until the barrel is a tight fit around the wood.

After this external shaping, I drilled out the centre to 30mm (using a forstner bit) because it was the closest to the 27mm diameter of the nut.

Araldite is one of my best friends, I use it for quite a few parts in numerous projects. Here I used it to hold the nut in the wood, it was good because it also made a nice seal. I used the presision epoxy for this project because it bonds to metal, rubber and wood.

Step 8: The Barrel

Here I had to drill a hole for the barrel to pivot.
The pole for the pivot was 22mm but I only had a 20mm drill,  so fist I drilled it and then I melted the remainder away by heating up the pole and poking it through the plastic.

By the way, the barrel is made from nylon and is just the right size, so I will give a link to the ebay supplier I got it from.

Step 9: Cannon Assembly

The pole used to pivot on was meant to go through the 2 holes at the top of the frame but, I didn't have a 20mm drill bit for metal(from there I would have filed it to the correct size) at home so I just left them as 6mm.
Because the pipe was 22mm and the hole was 6mm, I cut the pole a bit short, araldited a length of 15mm PVC pipe in it then araldited some bolts into it with the head on the inside so that approx. 10mm was sticking out of either end. I then bolted this into the frame with some quite strenuous bending and finally dissmantling.

To aim the cannon we have to be able to set the angle and lock it. For this I needed something going down from the bottle to the rails that allowed for 2 pivot points.
To fix these metal bars to the bottle I drilled a 20mm hole in both of the bars shown in the first and last photos, and filed them out to 22mm. Then to the diameter of the bottle I fixed 2 sections of pipe in line with each other using jubilee clips. To do this I first drilled holes through the pipes near to the ends then tried to elongate them to accomodate the band with a milling drill bit that i had. BIG MISTAKE. The bit snapped when I was trying to do this and cut me(they really should have said on the packaging that it wasn't suitable for metal), I should have probably got some steri strips on it but a plaster did me fine. After that I ended up filing the holes.
It was essential that the ends of the pipe are straight and preferably have a large radius put into them but not very deep(not very circular and shallow groove) this all allows for a better fit around the bottle.

To lock the cannon in place I made the slides mentioned earlier slightly bent in the middle so that in one section of them they can slide freely but at a different section, the mechanism jams, locking the angle. To move between the sections I just kick the metal to the left or to the right.

Step 10: Finished/equations

Well there it is, I have finished my project.
In the picture that is me proudly standing by my machine.
To use the cannon to devastating affect: simply pump up the cylinder to upto 55psi (I dare not go further) put the tennis ball in the barrel and sharply, hit the lever to open it as fast as you can. For alternative ammunition you could put a potato down the barrel, or an apple, keeping in mind that the softer the thing that you put in the lower the pressure you can use or you end up making a mess in the barrel. If your chosen ammo doesn't fit snggly you can shove screwed up newspaper down the barrel as wadding and will make your ammo go further. The first time I put a potato down the barrel I didn't ever see it again. I aimed at the wall in my garden but apparently a little too high.Pumped it up to 30psi and let loose. It actually went over the wall, over the road, over the big sycamore tree, over the 1st block of flats and finally I heard a crack. It may have touched down, and smashed a roof tile. I did go looking for it but it must have rolled into their gutter. Oops! (in total it would have traveled at least 50m, and would have kept on going if that pesky roof didn't get in the way.)

As promised at the start of the instructable, here are the relevant equations
maximum height=(initial velocity*sin(angle fro horizontal))^2  / 19.6

maximum distance=2*initial velocity^2 *sin(angle from horizontal) *cos(angle from horizontal) / 9.8

These equations are true under the assumption that drag due to air resistance is negligable, so the link below will send you  to a website that has a simulator for a projectile with air resistance included in its calculations. I can not vouch for the integrity of this software as I have not yet tested it against a real projectile.

The drag coefficient of a sphere is 0.51 according to wikipedia so you may want to look elsewhere for it.

The only real improvements that I could recommend are making the aiming mechanism more rigid and the same about the connector between valve and barrel.

Step 11: Further Improvements

As this is an instructable, I tried to keep all of the parts in this project, within the capabilities of most DIY'ers, but as I have access to a metal workshop then I may aswell make my work the best that it can be.
The first improvement that I made was to replace the piece of wood that connects the barrel to the rest of the assembly. The replacement was made from a 70mm square bar (from the scrap metal merchant) and a picture of it is shown.
The first external diameter is 91mm to match the external diameter of the barrel. The other external diameter is 67mm to be ever so slightly larger than the internal diameter of the barrel (66.9mm), this ensures a tight fit which will give the barrel more support than if it were the same size.
Although the photos don't show it very clearly, there are actually 2 internal diameters. One is for the smaller diameter of the compression nut, this helps to give an air tight seal. the second is for the larger diameter of the nut. it is actually about 1mm smaller than the large diameter of the nut, this wasn't by design but turned out for the best as I didn't need to use an adhesive to keep it in place (just a hammer).
The holes in the corners are 6mm and allow the flange to be bolted onto the barrel. If you do make this project, please remember to put the nut onto the valve and then mark both the nut and the flange to get the orientation correct when you put the nut into it, when I put the nut in I knew that there was no way of getting it out without destroying the flange.
I made the entire flange from aluminium on the lathe, which is why it has such a good finish. I thought about buffing the aluminium after but realised that it would just look tacky if I did. To get the square round, I had to use a 4 jaw chuck. an easy method of getting the material in the centre is to mark the centre of the jaws and the centre of each side of the material and line them up. I only recently came up with this method and think that is much easier than the alternative.

In the barrel there are 2 sets of holes, one set was a mistake. I put the flange in place and started to drill with a 6mm through it's holes just to get the holes in the correct place. I took off the flange and forgot to change the drill bit. I needed a 5mm hole not a 6mm hole because the tapping size is 5mm. (for those of you who don't know much a bout thread making, to start off you make a hole in the material that is slightly smaller than the minimum on the external thread and then tap the hole. A tap is an internal thread making tool. the drill size for M6 is 5mm, it is not 1mm difference for all threads).
Anyway after that embarrasment, I repositioned the flange, started off the holes, and this time drilled the correct size.

When my friends started playing with my new toy I realised that the speed at which you open the valve makes a considerable difference to the distance that the chosen projectile would travel. The faster it opens the further the projectile goes. To make this work more effectively I would advise coming up with some mechanism to open it with pneumatics or solenoids. As the  systems that I have thought of would  be quite expensive it would be much appreciated if someone could come up with a cheap solution and post it in the comments section.

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