CHAPTER 3 : Ping-Pong Ball Minigun

PROJECT SIZE: Small
SKILL LEVEL: ★✩✩✩

Figure 3-1

THERE ARE TIMES WHEN THE COIL GUN will not bepowerful enough for the task that the Evil  Genius has in mind. Sometimes you need to totally overwhelm your enemy with superior firepower. This minigun design will fire ten ping-pong balls per second at considerable speed. An entire magazine of 50 ping-pong balls can be emptied in just five seconds.

It is a very easy project to make and requires little in the way of tools or special equipment, and all the parts can be obtained from the Internet or your local hardware store (see Figure 3-1). This project is seriously good fun and definitely more entertaining than blowing leaves about. However, it is strongly recommended that the Evil Genius have his minions on hand to pick up all the ping-pong balls, as this can become a bit tedious.

This design is described in such a way that you can adapt it for your particular leaf blower. Also, with the exception of a bit of glue, the project does not irreversibly alter the leaf blower, so it can resume its original use should there suddenly be any leaves to blow.

WARNING :

This is not a real gun, and ping-pong balls will not generally damage humans unless shot at very close range, at the eyes, or swallowed. However, a number of aspects of this project are dangerous.
■ It uses electricity, and we do not modify the electrics of the leaf blower. However, you should observe the safety instructions that came with your leaf blower.
■ Do not put your hands or fingers into the leaf blower when it is on or even just plugged in.
■ Do not place your eye or anyone else’s into the gun’s line of fire.

What You Will Need

The components for this gun are all readily available. Leaf blowers are all slightly different, so you will probably have to modify this design to work with your particular leaf blower. Before buying the parts, however, read through the whole project and try out a few tests on your leaf blower to make sure it is suitable, and also to determine roughly what length of pipe you will need. Obviously, the most important part of this project is the leaf blower. Various types of leaf blowers are available: electric ones, gas ones, ones that just blow, and ones that both blow and suck up leaves into an attached bag. The leaf blower used by the author is an electric one, which also has a bag for collecting the leaves. It is the aperture that the leaf collecting bag attaches to that is used to introduce the ping-pong balls.

If you have some other type of leaf blower, then as long as the air outlet is wide enough to accommodate a ping-pong ball and there is a spot in the air path where you can insert the balls, you should be able to make something. So, read through the rest of this chapter to understand the principals of what we are trying to do and then adapt them to work with your particular leaf blower. 

The balls need to be a good fit with the pipe. They should fit into the pipe easily and run slowly through it if you tip it. If the balls are a lot smaller than the pipe, they will not be fired out as fast as they could be. On the other hand, if they cannot move freely enough through the pipe, they will become stuck.

That is why a 40mm internal pipe diameter and 38mm-diameter balls are a perfect combination. This ensures there is just enough space for our round projectiles.

Be wary when buying your ping-pong balls, as the international standard size of balls since the year 2000 Olympic Games is now 40mm. Prior to that, 38mm balls were the most popular, especially in China. Plus, 38mm balls travel faster and can take more spin. This caused controversy at the time, as some saw the introduction of the bigger balls as a way of giving the non-Chinese competitors an advantage. This is clear evidence of the Evil Genius at work in the world of table tennis.

Plenty of people still sell 38mm balls (often from China), but check before you buy. You will also find “plastic” ping-pong balls on eBay. These are made of soft plastic, and tend to be made to far looser tolerances than proper celluloid balls. So even if these types of balls are advertised as 38mm, they may not be exactly 38mm, so try and avoid these, unless you already have some samples to try in a pipe.

It is also a good idea to take a ball along with you when you go to buy the pipe. In addition to the parts just listed, you will also need the following tools:

TOOLBOX :

■ Hacksaw
■ Scissors/craft knife
■ Epoxy resin glue or hot glue gun

Assembly

The Evil Genius still has a yard in which leaves must be blown, so as we mentioned earlier, this project does allow his leaf blower to revert to its original use in the fall. Better yet, it is an easy project to construct, since it is a simple design. The project adds three barrels to the leaf blower that are made of plastic wastewater pipe. These help channel the ping-pong balls, increasing the accuracy of the gun. There is no complex magazine or trigger mechanism; instead, the magazine is hinged onto the side of the leaf blower and the balls are simply tipped into the leaf blower.

When used as a gun, the leaf blower is held upside down to allow balls to be poured into the aperture where leaves are normally collected. The leaf blower used by the author is shown in Figures 3-2, 3-3, and 3-4. Unless you have the same model, you will probably find that your leaf blower is a different shape, but with a very similar basic design.

Familiarize yourself with the design of your leaf blower and compare it with the author’s. Turn it on, and if it has different settings, find the setting for maximum blowing power out of the nozzle. Also, look through it to see the path the air takes. You can also try dropping a ping-pong ball into it at the air-intake while it is running. Do not put your fingers inside the blower. This should be fired out, albeit not very fast or accurately. Work out where the balls will need to be introduced, as this will determine where you attach the magazine. 

Your leaf blower may have a round rather than rectangular nozzle. If this is the case, you can either fit the three waste pipe barrels in a row, as described in these instructions (and perhaps adapt things by fitting them in a triangular arrangement), or use just two barrels.

Figure 3-5 shows the design of the project. This is a simple design; the leaf blower does not need a lot of modification. Essentially, it just needs barrels to direct the balls accurately and a container for the balls. The container is hinged so that to fire the gun, balls are simply tipped into the leaf blower. As we mentioned earlier, the leaf blower is actually held upside down to allow the balls to be tipped into the inlet, where they fall into the path of the air jet, which whisks them into the barrels and fires them out the other end. The barrels are wedged into place with blocks of expanded polystyrene packing material that has been cut to the right size. This also has the effect of blocking most of the air that would otherwise flow around the pipes, thus maximizing the flow through the pipes.

The following sections take you through the construction step by step.

Step 1. Cut the Barrels

The PVC pipe can be easily cut with a hacksaw. You will get a cleaner, more perpendicular edge if you use a large sturdy hacksaw, but if none is available, then a small hobby hacksaw will work fine. They should be cut to a length where one end of the barrel protrudes from the nozzle of the leaf blower by about an inch (25mm) and the other end stops about three inches from the air outlet of the blower (see Figure 3-5).

This should allow the maximum flow of air into the pipes. Having the pipes protrude from the end of the leaf blower serves to make the device look more gun-like. Without it, the enemy of the Evil Genius might get the misleading impression that they are simply about to be blown to death and fail to take the Evil Genius seriously. That is something no Evil Genius will tolerate. The PVC pipe is surprisingly easy to cut. When cutting, first draw a line around the pipe where you want to make the cut. Then, unless you have a large sturdy hacksaw and vice, cut around the line, shifting position as you start to cut into the middle. 

Once cut, use a file or knife to scrape around the newly cut rim and remove any burrs. You should then be able to put a ball in one end and have it freely roll to the other and fall out. Having cut the barrels, tape them together with duct tape a few inches back from each end, as shown in Figure 3-6. This combines them into a
single solid structure. As mentioned earlier, if the pipes will not fit into the leaf blower side by side, you can experiment with either taping them together in a triangular pattern, or just using two barrels.

Before you fix anything in place, hold the pipes where they leave the leaf blower, turn the leaf blower on, and find the position for the pipes where you get the strongest blast of air through them.

Step 2. Fit the Barrels

The pipes are held in place by blocks of expanded polystyrene packing material (Figure 3-7). These are slightly larger than the gap they are filling and wedge the pipes firmly in place. They do not provide a complete seal for the pipes, but they do block most of the air flow around the pipes, so nearly all the air is directed into the pipe. Figure 3-8 shows the block for the nozzle end, and Figure 3-9, the one for the outlet end. Notice in Figure 3-9 how the pipes line up with the air outlet from the outlet vent at the bottom of the figure.

Before we go any further, let’s do a quick test of our gun. First of all, select a disposable minion and stand them about 20 ft (9m) from the gun. Now set the leaf blower to blow mode, turn it on, and pop a ball in front of one of the pipes near the air outlet. This should fire the ball at a considerable speed at the minion. Repeat the experiment, throwing a handful of balls into the “firing chamber” and watch the minion dance for your amusement. Picking a minion that wears glasses will reduce the chances of damaging their eyes.

Step 3. Create the Magazine

A large plastic water bottle was used to hold the balls. The top of this, where it narrows to the cap, is cut in such a way as to leave a flap that can be bent into a hinge (Figure 3-10), producing a shape that is basically cuboid. The hinged flap is then glued to the side of the leaf blower. To fire the gun, simply tip some balls from the magazine into the leaf blower inlet. After much experimentation with various trigger mechanisms, this was found to be the most effective way of controlling the firing of the gun.

Testing

It is now time to try out the gun. The gun should be used indoors, or you are likely to lose the ping-pong balls. Once again, a minion is required as a target. Stand at least five yards (meters) away from the minion. After giving them strict orders not to step on any of the pingpong balls, fire up the leaf blower and tip in a few
balls to get your aim right. Once your aim is accurate, pour in the rest of the balls as quickly as possible.

Finding the Muzzle Velocity

We can measure our muzzle velocity using the same approach we did with the coil gun (see Chapter 1). We will only need a single ping-pong ball and the impact target must be placed much further away. The arrangement we used is shown in Figure 3-11. 

As a target for measuring the speed of the balls,  a wall, window, or other solid object is much better than a minion. Minions are soft, and while they do make a noise when hit by a ball, a considerable delay occurs as the message slowly propagates through their nervous system and eventually reaches their brain. This delay tends to spoil the accuracy of the figures.

Your laptop should be positioned close to the target rather than the leaf blower to reduce the extraneous noise created by the leaf blower. The leaf blower is very noisy, so it is much harder to tell from our sound trace exactly where the ball left the nozzle of the gun. A trace is shown in Figure 3-12. The range between when the ball leaves the gun and when it hits the test target is marked on the diagram. The only way to determine the moment the ball leaves the gun is to repeatedly play part of the sound wave. Your built-in signal processor (brain) will be able to separate the “pop” as it leaves the nozzle from the general roar of the
leaf blower.

From this trace, we can see that our ball took 0.6 – 0.08 or 0.52 seconds to travel 5.5 meters, indicating an average velocity of 10.6m/s, or 24 miles per hour. Being very light and having a large surface area, the ping-pong ball will lose speed quickly, so the muzzle velocity is likely more than this figure.

An alternative to measuring the velocity with sound would be to use video. Use a similar arrangement to the sound, but set up a video camera at right angles to the flight of the balls. If you know the number of frames per second used by the video camera, then by estimating the distance the ball has moved between frames, you can work out the velocity.

The only problem with this approach is that you need a lot of room so you can put the video camera far enough away from, and to the side of, the flight of the balls. The author did not have a lot of success with this approach; the balls just moved too fast. But you may have more room for such experiments and a faster camera.

Theory

A leaf blower is a low-pressure air pump. It is designed to blow out as much air as possible, as fast as possible. By reducing the effective area of the end of the nozzle by fitting the barrels over just half the area, we can increase the velocity of the air traveling through the barrels. Since the leaf blower is open at both ends, if we increase the resistance to flow too much—say, by just having one barrel and blocking up the rest of the tube—then the air may simply bounce off the obstruction and come out the back of the blower, or create a vortex that merely circulates the air and hence the balls.

Trial and error was used to determine the best number of barrels to maximize the speed of the balls and the fire rate. For the author’s leaf blower, this turned out to be three barrels, which conveniently fit across the width of the blower.

Summary

This is one of the most enjoyable and easily completed projects in the book. As you would expect, applying a few kW of power to a bunch of ping-pong balls is always going to produce some exciting results. In the next chapter, we will embark on another weapon-type project, but this time it will be a model weapon, based on a laser and some servo motors.