Aliens M41A Pulse Rifle - Measuring the Thompson.

Before proceeding with the rest of the pulse Rifle, I'll need to spend time looking closely at the Airsoft Tommy gun I'm using as the base. 

To start I'll need to remove the Thompson's stock, foregrip and rear sight.

The Stock and Foregrip are each held on by 2 screws.  Unscrew them and the parts pull right off. The gun's battery is normally housed in the stock, so i'll have to reroute those wires and create a new battery holder somewhere in the rifle.

Removing the rear sight was a bit more tricky, involving removing the upper receiver and unscrewing four small interior screws. With the grenade launcher and the reference model, we can start to see the basic shape of the pulse rifle.

I need an accurate digital representation of the Thompson to work around as I model the body of the pulse rifle. With calipers and a bit of patience, I reconstructed the Tommy Gun inside sketchup.

I didn't bother measuring and modeling the pistol grip, because there is no part of the Pulse Rifle body that touches that area. No need to waste time accurately creating something I won't need. 

The most important parts of my digital reconstruction are the places where the Pulse Rifle body will touch the Thompson. To confirm the accuracy of those areas, I created test geometries to 3D print and fit onto my real world Thompson. If the parts fit, my measurements are accurate, and I can get started on the hard part.

Aliens M41A Pulse Rifle - The Grenade Launcher

In the original Pulse Rifle the underbarrel grenade launcher is a shortened, repurposed pump action shotgun. To simplify this project, and because airsoft pump action shotguns are surprisingly expensive, we going to use the Launcher from the Thingiverse 3d Model as a substitute.

I adjusted the Pulse Rifle 3d Model to the correct scale using measurements from the Airsoft Tommy Gun and cut the launcher away from the rest of the rifle. A little clean up work later and we have the launcher as a single solid object.

To fit the 350mm long Launcher into my 150mm tall MakerBot Replicator it has to be cut into 3 parts using SketchUp. I also added attachment pegs to each section to simplify reassembly.

Front and Rear sections of the launcher are easy to print, but the center section will need support material. MakerBot Desktop’s supports are a pain in the ass, so instead I'll use Meshmixer. Meshmixer lets you create minimal supports only where you needed them. They work great, are easy to remove, and are much more efficient.

Estimated Print Time with Makerbot supports, 11 hours.  

Estimated Print Time with Meshmixer supports, 6 hours. 

The finished launcher looks great.  A little modeling putty and some sanding will have it ready for paint in no time. Unfortunately, something is off. Look closely at the small reference model I printed on my Form 1+.

The Thomson's barrel is about 30mm too short. Damn.  

After rechecking my measurements, I have to conclude the original Pulse Rifle didn't use the stock Thomson barrel.  I'll have to look into ways to fix this.   

Aliens M41A Pulse Rifle - Getting Started

There are dozens of Pulse Rifle kits available and sections of the Replica Prop Forum dedicated to building them properly. I however, being a bit of a design masochist, am going to build it my own way. I'm not breaking any new ground, every conceivable permutation of Pulse Rifle has already been done, but this project is just for fun.

From my research, I know the original Pulse Rifles were made from World War 2 era Thompson submachine guns. If you look closely at the gun Ripley is holding in the screenshot, you can see the charging handle, ejection port, upper receiver, and barrel.  

Or you can just trust me... Its in there.

I’m going to start my build from an Airsoft replica of a WWII M1A1 Thompson. Starting here will simplify my build by giving me something with known dimensions to work around. As a bonus, the final creation will fire Airsoft BB's. I'm also going to use this 3d model of a Pulse Rifle from Thingiverse as a guide. It’s the same 3d model Freeside used to print their rifle, and it will save me a lot of time working out the weapon’s proportions.

Aliens M41A Pulse Rifle

I recently had the pleasure of hanging out with the awesome people at Freeside, Atlanta’s Hackerspace. We were lucky enough to be in town for one of Freeside’s ongoing cosplay group build nights, where they are building equipment of a squad of Colonial Marines from the Aliens. I got to help sand their pulse rifle master…

That dust mask, is hiding a huge smile.  Image by Melinda Cross.

Which reminded me…

Long ago, during high school in Alabama, I wanted to build a Pulse Rifle. At the time I was interested in so many things and didn't have a clue how to do them. I also didn't have any money, and was a little worried how my parents, dyed into wool anti-gun new englanders, would respond to their son building a sci-fi machine gun in the basement.

But now I'm an Adult, with an income, and a bunch of 3D Printers…

So I'm going to build my own very own Pulse Rifle.

I'll post updates and files as I work, so you can come along for the ride.

Building the HMS Thunderchild

HMS Thunderchild

In HG Wells ‘War of the Worlds’ the HMS Thunderchild is only human creation to defeat a trio of Martian Tripods. The ship, described as an ‘iron clad torpedo ram’, charges three Tripods attacking a of fleet London evacuees at the mouth of the River Blackwater. Thunderchild rams one Tripod, destroys another with her guns, and consumes the third when her boilers explode.

The story of the Thunderchild is romantically British. A brave Royal Navy ship sacrificing itself for King and Country. What makes the story even more intreseting is that the ship is a Torpedo Ram, which is something of a historical fluke.

Torpedo Rams where an extremely short lived class of small warship in the 1880’s. They were small heavily armoured ships intended to attack enemy battleships at high speed with torpedoes and, if that failed, ramming. Like most novel weapons, they were great at capturing the public's attention and not much else. Only one was ever build, the HMS Polyphemus, and War of the Worlds is the only depictions of a torpedo ram in action.

Thunderchild by Henrique Alvim Corrêa,  an Illustration from the 1906 edition of War of The Worlds

Designing Thunderchild

For my version of Thunderchild I drew inspiration from the SMS Beowulf, a German ship and really life contemporary of Thunderchild. Launched in 1890, she would have been in the prime of her career during the 1899 War of the Worlds. The ship had a sloped hull and a bow with a reversed rake, broadly matching a torpedo ram’s shape. Beowulf’s artillery was mounted in twin side by side turrets, an anachronistic features that places the design firmly in another time.

An awesome working model of the SMS Beowulf built by Häfner Reinhold

Modeling the Thunderchild

Most of the ship is simple polygonal shapes, easy to model in Sketchup, but the hull presented a some special challenges.

The center section is a simple extrusion. I used Fredo 6’s ‘Round Corners’ plug-in to start the curvature of the bow and stern, then stretched the resulting geometry into shape with Fredo 6’s ‘Scale Tools’.

To create the reverse rake of the bow, I drew the basic curve and used Eneroth’s ‘Upright Extrude’ plugin to extrude it along a path while keeping the direction of the curvature constant. I intersected the resulting shape with the hull, and cut away the excess.

The ships stern proved to be more challenging. I made a section of the hull into a separate group and squashed it with Sketchup’s scale tool to create the bottom of the aft deck. (blue in the illustration) The transition between the hull and propeller is an extrusion of the hull profile along a path using ‘Upright Extrude’. (Teal) The final part is the proper shaft tunnel, which brings the two sides of the hull together and hides some of the less optimal parts of other shapes. (Magenta)  Once I had all the geometry, I intersected the shapes and cut away the excess.

The rest of the modeling was straightforward. I used the basic sketchup tools to create the upper decks, superstructure, armaments, and lifeboats.  The only exceptions are the three large turrets. They started as cylinders that I used Fredo 6’s ‘Round Corners’ and ‘Scale Tools’ to bend into shape. Everything was build as components, then exploded and attached together to make a single unified mesh.

Printing the Thunderchild

For this project I used a Formlabs Form 1+ desktop SLA 3D Printer.  Desktop SLA printing is great for small highly detailed models, but is limited to smaller build sizes. The Thunderchild model is 200mm long, which puts it at the upper limit of what is possible with a Form 1+. I had to position it runing corner to corner at about 45 degrees to fit it into the build volume. 

I used Meshmixer to hollow out the interior of the ship add drain holes in the stern. Hollowing the ship makes it more efficient to print on the Form 1+, and the drain holes let excess air and resin escape. The support structure was created by Formlabs Preform software. Print time at 100 microns is about 6 hours.

Removing the support structure was tricky, some of the supports where larger than the parts of the model they were supporting.

I gave the ship a quick coat of gray primer to even out the color and bring out the details. What amazes me about SLA printing is the level of detail you can get. The small cannon barrels are less than 1 mm in diameter. The little detail like stair treads, ventilators, and lifeboat dericks really bring the ship to life. Its not hard to picture a tiny crew running to get the Thunderchild ready of battle.

Download the Thunderchild from Thingiverse and print your own!

Thingiverse Link: www.thingiverse.com/thing:775961

Formlabs Tiny Television

Long ago in a place called suburbia, science fiction offered a future where anyone with fishbowl and a raygun could be a spaceman. The heros were good, the villains were evil, and the rockets had style. Propelled across the galaxy by plutonium and a can do-attitude, adventure was limited only by your imagination.

The Tiny TV is inspired by the Philco Predicta, an iconic if flawed hallmark of the 1950’s, and concept art by Dave Schultze. Built around a 2” LCD display from Adafruit, the Tiny TV was printed on the Form 1+ in Formlabs clear resin. The front lens was polished clear, and the body was sanded and painted. It works best when paired with hottest coming attractions of the 1950’s.

The Tiny Television was created in collaboration with Formlabs as part of a study into their outgoing marketing channels.

Small Motor Creations 6 - Orbital Gearboxes

Our little motors are great, but they spin so fast that its hard to use them directly for things that require lower RPM's, like wheels. To make the little FA-130's more useful we need to build a gearbox to slow the motor down and transform some of that speed into torque.

Orbital Gearboxes are one of my favorite solutions. They offer a range of input and output options in a nice compact package.  You'll find them in power drills, cars, cranes, pretty much everywhere there is an electric motor. These two gearboxes are bases on the same set of components, with different sections of the mechanism being used as the output.

In the Orange Gearbox Input through the Sun Gear at the center and output is through the Ring Gear that runs around the outside. The three Planet Gears act as idler gear, carrying movement from the Sun to the Ring.

The Sun Gear has 12 teeth and the Ring Gear has 60 teeth. In this case the Planet Gears don't affect the gear ratio because as idlers they are just carrying movement from one place to another. To get the gear ratio for this gearbox we divide the number of teeth on the output gear by the number of teeth in the input gear. That gives us a Gear Ratio of 5:1, so for every 5 turns of the input gear we get 1 turn of the output gear.

The Gray Gearbox uses the Sun Gear as its input and the Planetary Gear Carrier as its output while the Ring Gear is held stationary. The Carrier is the structure that holds the Three Planet Gears. As the Sun gears turns, the planets are spun between the Ring and Sun. The Carrier makes a revolution each times the Planets go around the Sun, Thus the name Orbital Gearbox.

The Gear Ratio for this gearbox is a bit more complex.  In this configuration the ratio is the number of teeth in the Sun Gear divided by the Sum of the Number of Teeth in the Ring and Sun Gears.  So in this case 12/(12+60) = .0138. For every input turn we get 0.0138 output turns. To clear things up we'll divide 1 by 0.0138, which is approximately 72. This gearbox has a ratio of 72:1, so for every 72 turns of the input we get 1 turn of the output.  And that is why I love orbital gearboxes, big ratios in tiny little packages.

For more information about orbital gearboxes I would highly recommend this article at Woodgears.ca  https://woodgears.ca/gear/planetary.html 

Download the Orbital Gearboxes Files From Thingiverse: http://www.thingiverse.com/thing:605701

FA-130 Size Motor At Pololu: www.pololu.com/product/77

Small Motor Creations 5 - Flywheel

The Flywheel is an unseen hero with so many amazing abilities. They are used in everything from Uninterruptible Power Supplies to the Attitude Control systems of spacecraft. A Flywheel's angular momentum can be used to to resist or impart a rotational force, or to store large amounts kinetic energy for reuse.

The mechanism behind this flywheel doesn't get much simpler. Its a spinning disk of mass contained within and enclosure. One of our little Fa-130 Motors spins the flywheel, and the enclosure lets us experiment without interrupting its rotation.

So what can we do with our flywheel?  Well first and foremost, It makes a great gyroscope.

The spinning flywheel is resistance to changes in in it angular momentum. So you can easily balance it on its edge. Interestingly, because there is so much energy in the momentum of the spinning wheel, it will keep balancing on its edge for about a minute after power is removed.

Electrical energy is being converted to kinetic energy by our little motor. That kinetic energy is stored in the momentum of the flywheel, and can be converted back to electrical energy.  Like most DC Motors, the our little Fa-130 will also function as a small generator. To test this we'll need a voltmeter. Hammerspace has a nice one from Fluke. 

With the Battery connected, the circuit shows 1.033 Volts.  Where using a 1.5 Volt D-Sized Battery, but the voltage is being drawn down by the current demands of spinning the flywheel.

When the battery is disconnected, the meter shows 0.338 Volts. That is voltage being generated by our motor as its being spun by the flywheel. This little motor makes a poor generator, but we're still recovering almost a third of what was initially in the circuit. 

As the Flywheels slows, the voltage produced drops. After 30 seconds, the flywheel is only producing 0.042 Volts.

This has been an interesting experiment, and if I get the time to revisit it I'd like to couple the Flywheel with an electronic controller to make a rudimentary Reaction Wheel.

Download the Flywheel Files From Thingiverse: www.thingiverse.com/thing:557984

FA-130 Size Motor At Pololu: www.pololu.com/product/77

Small Motor Creations 4- Impeller Pump

The Impeller Pump is a simple example of a mechanism that is essential to industrialized society, but is almost never seen.  Impeller pumps are found a garden fountains and basement sump pumps, water treatment plants, and anyplace else liquids need to be moved quickly and efficiently.

The principle behind the Impeller Pump is identical to the earlier Vacuum project.  The FA-130 motor spins Impeller blades, imparting centripetal acceleration on the water and throwing it outwards at higher pressure. This high pressure water exits through the outlet.  For and impeller pumps blades need to be submerged for it to function. So our little motor is going to getting wet in this project.

My knowledge of the complexities of and impeller pump is limited, so this project is a bit of an experiment. We're going to test what effect the diameter of the rotor has on the output of the pump. 

The smallest pump has a rotor diameter of 20mm, the medium size pump had a 30mm rotor, and the large pump has a 40mm rotor.

The pumps outlets are all sized to accept a piece of 4mm tubing. To test the pumps, I timed how long it takes each one to move 1 liter of water from one container to another. 

As a control, I used the 4mm tube to siphon 1 liter of water from one container to the other, which took 27 Minutes 8 Seconds, or 36 mL/Minute

The 20mm Pump moved 1 liter of water in 4 Minutes and 44 Seconds, or 211 mL/Minute

The 30mm Pump moved 1 liter of water in 5 Minutes 48 Seconds, or 172 mL/Minute

The 40mm Pump moved 1 liter of water in 9 Minutes 28 Seconds. or 105 mL/Minute

Alright, So all three of these pumps suck...

Its interesting that the pump with the smallest rotor diameter moved water the fastest. I speculate that the smaller rotor is accelerating less water at a time, letting it spin faster and move more water overall.

Download the Pump Files From Thingiverse: www.thingiverse.com/thing:545388

FA-130 Size Motor At Pololu: www.pololu.com/product/77

Small Motor Creations 3 - Vacuum

The Vacuum is a humble cousin of the mechanism you'll find in everything from your Dustbuster to a Roomba. A spinning rotor creates an air pressure difference, drawing air in through the nozzle and exhausting it out through the slots. Dust and dirt are drawn in with the airflow, and captured by a filter.

The rotor is attached to one of out little FA-130 motors.  As the rotor spins it imparts centripetal acceleration on the air between the vanes. The air is flung out, creating an area of low pressure at the rotors center. The suction nozzle attaches directly over this area of low pressure, and air exits through the slots around the outside of the Vacuum's body.

A small piece of toilet paper make a simple filter to capture the dust we pick up, otherwise it will just be blown out the exhaust. The Toilet paper is captured between the body and nozzle, with the excess torn away.

So how well does our tinny motor vacuum work? We can use a handful of sawdust from the Hammerspace dust collector to test it out. 

Well its no Dyson, but that's not half bad for our tinny little motor and a few hours of work.

Download the Vacuum Files From Thingiverse: www.thingiverse.com/thing:539986

FA-130 Size Motor At Pololu: www.pololu.com/product/77

Small Motor Creation 2 - Dart Thrower

The Dart Thrower is a simple version of the flywheel mechanism used in pitching machines and battery powered Nerf guns like the Barricade.  The principle of operation is simple.  Two flywheels are spun in opposite directions by the motor. The gap between the flywheels is slightly smaller the the diameter of the dart, and they will grip it as it passes between. Energy is transferred from the flywheels to the dart, and it goes forward, quickly.

The motor is directly attached to one Flywheel and the other is spun by the interconnecting gears. The gears spin the two Flywheels spin in opposite directions.

Now we have to see how well the Dart Thrower works by testing how far it can throw. I created a stand that grabs the bottom of the motor and holds the it pointed up 30 degree. Its taped to the floor, so it wont move during the test.

I used the Hammerspace workshop floor as a handy premade reference grid.  The floor tiles are 12 inches square, and for a bit of added precison, I laid down a tape measure staring at the base of the thrower.  

We're going to fire 10 darts from the Dart Thrower, which will give us a good idea for how far it can shoot using out little FA-130 size motor and 4.5 volts.

From above we can see that the darts cluster between 6 and 12 feet downrange. The furthest dart is 147 inches from the launcher and the closest at 70 inches.  The average of all the darts is 122 inches, or a little more then 9 feet.  Not bad for out tinny little motor.

The Dart Thrower is Sized to use Nerf N-Strike Style darts

Download the Dart Thrower Files From Thingiverse: www.thingiverse.com/thing:534969

FA-130 Size Motor At Pololu: www.pololu.com/product/77

Small Motor Creation 1 - Ducted Fan

A Ducted Fan is a little like an electric jet engine. They are often used in model aircraft to in place of jet engines, and hold a place of honor in every sci-fi and futuristic aircraft I've ever scene.  Ducted fans offer the promises of high power densities in small protected packages without all those unshrouded propellers tips whipping about. 

A Ducted Fan is made up of three primary parts: the impeller, the shroud, and the motor.  The motor is of out little Pololu FA-130 Motors.  The impeller blades are fixed a 45 degrees and the hub is large enough to cover the face of the motor. The shroud is a bit trickier.  For the ducted fan to work properly it has to come as close to the spinning ends of the impeller as possible without touching. It also has to have enough strength to hold the motor in place as it spins. I designed the shroud to have a 0.5mm gap between its inner surface and the impeller blades.

I wasn't expecting much from this project, Ducted Fans are notoriously hard to design. So I was shocked when it shot across the table after I applied power.  This little puppy produces some thrust!

We can quantify the thrust produced using a small digital scale and a test stand.  The test stand holds the Ducted Fan above the surface of the scale, so it can pull in air.  The fan is installed face down, with the exhaust pointing up.  Any thrust produced will push down against the surface of the scale through the test stand.  Thrust will be quantified as an increase in the weight registered by the scale.

To keep things simple, I set up the Ducted Fan, test stand and batteries on the scale and used the tare function to zero them out, so at the start of the test, the scale reads 0.000kg.

With the fan running the scale reads 0.012kg.  This little Ducted Fan, made from a $1.99 motor and some 3D Printed Parts is producing 12 grams of thrust! That's shocking!  In this test i'm running the motor at 3.6 volts, slightly above its design spec.  The Fan and motor weigh 24 grams, so it producing half it own weigh in thrust.

I wonder what would happen if I swapped this basic 130 size motor for one of the models design for high speeds?

You can download the Ducted Fan and the Test Stand from Thingiverse.

Ducted Fan: http://www.thingiverse.com/thing:530911

Test Stand: http://www.thingiverse.com/thing:530914

The FA-130 size motor I used is available on Pololu: www.pololu.com/product/77

10 Simple Creations With Motors

Digging through my project box, I found a stash of  Mabuchi FA-130's that I bough from Pololu long ago. 

Mabuchi FA-130's are great little project motors. The 130 size is commonly used in toys and inexpensive electronics, and they are available in a variety of voltages and ratings. These motors are rated at 1.5v to 3v, have a free running speed of 12,300 RPM, and cost $1.99 from Pololu.

I'm going to use these motors as the basis for 10 simple creations over the next few days. Each creation will make the this simple motor do something useful.

-Michael