Monthly Archives: May 2012

Zheng3 Penny Ballista

This nefarious device can easily launch a U.S. penny across a room when printed at 1:1 scale. It’s one of the primary engines used in a game of Seej.

This is another design inspired by The Art of the Catapult.

The nock on this model is the very tip of the throwing arm from the Zheng3 Penny Catapult, turned on its side and modified a bit so that it fits smoothly into the firing groove.

I repurposed the winding keys from the Zheng3 Penny Catapult to hold the ballista’s rubber bands in place.

To assemble:

Construction is really simple! Needlenose pliers are recommended, but not strictly necessary.

You’ll need two elastic bands to complete this ballista. Tie a single band through the bore on one of the two D-shaped locking keys. Repeat the process with the other locking key.

Feed the end of one band through the rectangular bore on the ballista’s left post. Insert the tenons of the locking key into the square mortises on the post to lock the band in place. You’ll be tempted to repeat the process with the other side, but don’t give in. It will save you precious minutes of frustration.

Unlike the Zheng3 Penny Catapult, these locking keys should be a snug fit. If some settling of the plastic during printing has made the mortises too small, trim the tenons a bit with your blade of choice.

Center the nock on the knot that connects the two rubber bands. Make sure that the penny’s cradle is facing forward.

Feed the end of the band that’s been locked down through the rectangular bore on the nock. Tie the free end of the band to the second band. Now feed the free end of the second band through the outside of the rectangular bore on the right post and loop it through the second locking key.

The nock has a wee nub on its underside. Slide this nub into the ballista’s firing groove; it will improve accuracy.

Load a penny into the nock, pull it back, and fire away. Be safe! Don’t hit the cat.

Flagrant stagecraft alert: There’s a piece of hookup wire holding the nock in firing position for dramatic flair.

You can download this model for free here. You might also be interested in the Seej Starter Set.

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She comes in colors

Kristen Wiig’s recent exit from SNL accompanied by The Rolling Stones’ She’s a Rainbow got me thinking about printing in different colors. Black and natural ABS are nice, sure, but red’s looking really sharp.

It seems to print more reliably than black or natural. Maybe I’m getting better at printing, maybe it’s the plastic, maybe it’s Maybelline.

Bonus: As any Warhammer 40K Ork will tell you, red things go faster.

Grab the STL here from Thingiverse.


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Cross-disciplinary confusion

So this is my first month or so doing 3D printing. I’m not an engineer and I’ve never touched AutoCAD.

My background is in video games; I’ve been doing 3D art in Maya day in and day out since the turn of the century.
Modeling for video games has always been about optimization. Every polygon has to contribute to the shape of the mesh. No duplicate vertices, no stray edges. And above all, keep your triangle count as low as possible.

That’s changed somewhat since my days of making 100-poly models for DreamCast hardware. And get off my lawn, you kids with your normal mapping and 30K triangle characters and your ZBrush.

One thing I’ve discovered is that when it comes to this particular kind of 3D printing, it’s OK to be a little loosey-goosey. I don’t have to join every vertex to a neighbor, I can have a few stray edges here and there. A model isn’t going to explode during a render because I didn’t weld the arm to the body properly.

Take a look at the original 4-hole gear model for Etchasketchulator. I spent way too much time painstakingly welding verts, especially in the holes for the grub screws. The gear teeth are connected to the top and the bottom faces of the gear. Everything is watertight– you could throw this model (tessellated) into an N64 and it would render, albeit at 3FPS.

By the time I got around to making a second, thicker version of the gear, I’d learned a lot. The gear teeth are just kind of close to the gear faces– no welding.  I used a Boolean operation to cut the holes in the sides of the gear shaft.

A Boolean is a modeling tool that uses brute force calculation to add, subtract, or intersect two pieces of geometry.

Old-school video game modelers may suppress a shudder at the use of a Boolean. They generally create way more geometry in the wrong places than they’re worth, at least in realtime rendering.

In 3D printing, I can be sloppy. It’s very liberating.

The Penny Catapult

Download the STL here!


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Here’s a basic torsion catapult. It uses one or more elastic bands to launch a U.S. penny if you print it at 1:1 scale. You probably won’t get enough torque to be impressive with less than 3 bands.

Construction is snap-together, but it wouldn’t hurt to put a drop of super glue on the joints for durability.

Feed an elastic band through each of the bores on the side braces. Tie it through the bore on each winding key. Slip the end of the throwing arm through the rubber bands. Repeat with as many rubber bands as will fit through the holes.

Twist each key 10 times and then lock it in place by inserting the locking tenons into the square holes on the side braces. The more twists, the more launching power you’ll have. Careful, pennies can sting, especially if you take an Abe to the forehead.

You can change the payload’s arc by raising the front of the catapult.

Inspiration for this catapult came from The Art of the Catapult. You won’t find a better introduction to medieval siegecraft than this book. It’s got plans to build all kinds of stuff from catapults to trebuchets. Very kid-friendly.

If you read The Art of the Catapult you’ll learn that “catapult” is more of a catch-all term for this breed of siege engine. This model would more properly be called a mangonel.

If you’re looking to build stuff with a PG rating, Whoosh Boom Splat: The Garage Warrior’s Guide to Building Projectile Shooters is a better bet.

Flagrant stagecraft alert: the clever-eyed among you will note the use of black electrical tape to keep the throwing arm cocked for the photo.

ReplicatorG Settings:

HBP: 120° C
1.8mm Black ABS
Using Raft
Layer Height: .25
Number of shells: 1
Feedrate: 25 mm/sec
Travel Feedrate: 55