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Molded Coil Endcaps Coil Stage Design Completed Endcaps

After the end caps were completed, they were positioned along

the barrel and holes were drilled so the IR detector beams could

shine across the barrel. Then the coils were wound carefully in

between the end caps with the aid of my vice grip. Each inner coil

layer is secured with a single layer of packing tape.

Placement of Coil

Endcaps

Coil Winding Completed Coils

Construction

End Caps

The first components to be constructed were the magnetic coil end caps. Mixing and molding the iron-epoxy compound was challenging to

say the least. The mixture ratio has to be just right. If theres too much epoxy, the end cap wont be ferrous enough and might as well be a

chunk of plastic. If theres too much iron, the piece will conduct eddy currents and possibly crack when coming out of the mold. Also, the

right amount of compression has to be applied to the mold- too much compression will cause the epoxy to ooze out, leaving an overly

iron-rich piece inside. I had to make 16 end caps to get 9 that were good- meaning I had a 56% success rate with this process. Each

piece took 48 hours to dry, making the effort long and frustrating. Lastly, the end caps were fitted with brackets to mount the infrared

projectile sensors and coil magnet wire.

Coils

Projectiles

An advantage to using steel nails as projectiles is the ease of fabrication, which is a huge plus when you need to make twenty of them.

Each projectile was simply measured, cut, and filed into shape. Maintaining a precise length was important to insure that each projectile

extends far enough to trip the first stage IR trigger when pushed out o f the magazine.

Finished Projectiles

Initial Testing

struction - Delta-V Engineering http://www.deltaveng.com/gauss-machine-gun/constr

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components used to switch the coils on and off. Two thick copper

bus bars connected to the battery terminals run the length of the

housing down either side. Between the copper rails are the eight

IGBTs, one to switch each coil. Between each IGBT is a small

circuit board containing the voltage suppression electronics.

Initially, each voltage suppression circuit board contained four

series diodes, which I mistakenly thought was the fastest way to

quench the coil current. Later, I learned that the single

diode/resistor method was much faster, so each circuit board was

removed and upgraded. Lastly, each stage has a red LED that

lights when the IGBT is conducting. This provides a fault indicator

if a coil is erroneously switched on as well as a cool racing LED

effect during each shot.

IGBTs Installed Copper Busbar Fabricated Busbars Installed

Switch Indicator LED

Early Voltage Suppression

Circuit Boards

Circuit Boards Installed

Frame

The gun frame had to be built twice. The first attempt was based on a design that just

couldnt be built with hand tools, so I redesigned it to use pre-existing aluminum cross-

sections to minimize parts, cuts and joints. The second design was stronger and easier

to build, but several parts still had to be redone once, twice, and even three times until I

had something that was solid and respectable. By the time the aluminum frame was

complete and ready to accept the gun components, I had probably ingested more

aluminum sawdust than a healthy human being ever should, but it was worth it. The

final product is extremely rigid and sturdy, with no rattles or creaking even when tested

under heavy loads (such as when I accidentally ran into it head-first while vacuuming the

floor one day).

Lots of additional components were built into the frame. The pistol-grip and trigger are hugely improved from the CG-33. The ergonomic

grip is solidly bolted into the frame, and the trigger pulls smoothly and comes to a confident stop just as the shot is triggered. The select

fire switch, power switches, battery monitor LEDs, circuit board mounting brackets, safety fuses and a targeting laser were all designed,

built, redesigned and rebuilt and mounted into the frame during this phase as well.


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Foregrip Cut Vicegrip Used as Brake Completed Foregrip

Led Panel Battery LEDs Installed Modified Laser Pen

Laser, Switches & Fuses Grip & Trigger Completed Gun Frame

Circuit Boards

A circuit that makes perfect sense on paper wont work when you simulate it, and a

circuit that works perfectly in a simulator wont work when you test it on a breadboard,

and a circuit that works perfectly on a breadboard wont work once you solder it

together. Then you spend three weeks troubleshooting it, only to find that it starts

working again for no reason at all. Thus went the process for the CG-42s circuit boards.

In the end, everything worked out. The layouts of all the circuit boards were sketched on

paper before-hand to fit into their small spaces while remaining neat and organized.

Cramped, messy circuit boards make it easy for shorts and loose connections to occur.

When your circuits are connected to two giant flammable batteries that you hold right

next to your face when you shoot the gun, thats some pretty serious business.Power Supply Circuit Board

Projectile Feed System

This was a challenge to build, as moving parts usually are. For

the coilgun to operate reliably, the projectile feed system will have

to operate smoothly and consistently even when actuated 10

times per second. The difficulty began with the magazine. Two

magazines, three followers and four magazine springs were

designed, built, and failed before I came to a good design thatcould feed the projectiles without jamming. The first magazine was proven wrong with my hammer.

The final magazine is built from bended aluminum sheets joined

with epoxy, and the spring was manually bent from steel wire.

Instead of using a mechanical latch, the magazine is held in place

by a Neodymium magnet.

The injector solenoid required some modification to get it to

function properly. The stock return spring didnt provide enough

force to retract the solenoid to its starting position when subjected

to friction against the spring-loaded projectiles. Using two stock

Final Magazine Stenciled

on Aluminum

Magazine Plate Cut Out Magazine, Follower, and

Base Plate


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springs provided enough force, but the action was mushy and

inconsistent. Using a trick from freshman physics lab, I measured

the springs force constant and purchased a new single spring that

provided double the force. This fed the projectiles much more

reliably. After some time spent adjusting the positioning and

stroke-length of the solenoid, I finally had a mechanism capable of

sustaining full-auto projectile feed.

Spring Bending

Assembled Injector

Mechanism

Injector in Aluminum

Housing

Final Testing

After many months spent connecting all of the components together and troubleshooting all sorts of problems, the whole coilgun was

assembled and ready to go. The first full-power test shots were fired on July 1st, 2013. The first shot was a success. Immediately after

the second shot, the IGBT controlling the first stage failed dead-short and exploded. The battery fuse failed to trip, allowing the first stage

coil to melt and burn quite thoroughly. The entire incident can be observed below:

After making th is video I was so amped up on science I couldnt sleep until two in the morning

Fortunately, the circuit board fuses worked properly and protected the rest of the components. The cause of IGBT failure was later

determined to be excessive voltage allowed by a blown resistor in the voltage suppression circuit. The first stage coil had to be replaced,

and all eight voltage suppressors were refitted with higher wattage resistors to widen the safety margin. Thanks to requirement 4

(maintainability) the repairs were quick and the gun was back up and running in less than two weeks. The results of the full performance

characterization are shown below.


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Final Testing Results

With a total efficiency of 7.0% the results are extremely pleasing (compared to a typical 2% coilgun). As expected, performance had

improved since initial testing. This is due to the resistors that I added to the voltage suppression circuits to quench the coil current more

quickly. This drastically reduces suck-back, especially in later stages where the projectile is moving very fast. The efficiency of the first and

last coils dropped (relative to the other stages), likely due to induction of eddy currents in the aluminum gun frame which the first and last

coils are now mounted into. As to what causes the performance drop on the 6th stage, I have no theories. Oddly, looking back at the initial

test with 7 stages, the 3rd-to-last stage performed low as well.

Completion

After an eternity of sawing, filing, drilling and soldering I suddenly realized that there was nothing left to be done and the gun was finished.

Somehow everything turned out better than I expected, from the appearance of the gun to the performance numbers. All of the goals were

met, except rate of fire which came in at 7.7 rounds/sec. This could be increased by changing a single resistor, but I chose to keep it this

way since the coils wont hea t up as quickly.

The battery powered coilgun equation was remarkably accurate in determining the power required. This is because the actual acceleration

of the projectile was very close to being constant, which was the key assumption of that equation. The efficiency guess (6-10%) was also

fairly close. However, the 40m series resistance prediction was way off- actual series resistance was closer to 100m due to

unexpectedly high internal resistance within the battery cells. The coils only get about 190A instead of the 300A I planned for, but since

the 300A was way over-designed, the coils still get enough current to meet the projectile power demands. This means I could have gotten

away with using the smaller 2500mAh batteries, but the bigger batteries provide the advantage of being able to power far more shots.

While testing and making the video, I put about 100 rounds through the gun and the batteries only dropped from 50.20 to 49.03V. I could

probably fire several hundred more rounds before the batteries drain to the minimum 36V and need a recharge.

In the end, Im very happy with how it turned out. Holding the gun makes you feel like a mad scientist, and firing full-auto bursts gives the

shooter a terrifying sense of excitement and destructive power. Project complete!

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