My First 3d Printed Project: The Crawling Terminator Hand

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I was always interested in 3d printing for the purpose making functional mechanisms. I still had all my mechanical drawings from that previously mentioned Terminator project and decided to 3d print an articulated robot hand. Not just any robot hand, but something capable of dragging itself along under its own power. I wanted to see what 3d printed mechanisms could do.

The Terminator hand design was based upon the T-800 Endoskeleton Arm that used to be available from Sideshow Collectibles. That model was apparently molded directly from a working terminator arm made at Stan Winston Studios for use in the movies. It had the screw heads and accommodations for finger linkages already laid out and the dimensions were exactly what they needed to be to recreate the functional mechanism.

T-800EndoskeletonArm.JPG

The mechanisms for the T-800 hands were originally fabricated using traditional machining techniques ( of course). This meant there were plenty of places to grip the parts in a vise or a 4 jaw chuck for the machining process. These features translated well to the technology of FDM 3d printing, where it is desirable to have flat surfaces to attach to the print bed.

 Finger Parts being Grown on 3d Printer

Finger Parts being Grown on 3d Printer

 Finished Parts

Finished Parts

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If I Had It All To Do Over Again: The Thrashing Torso v.2.0


We live in interesting times.

 

3d printing is becoming a mature technology and can be very useful and making mechanisms. I once heard 3d printing described as a lifestyle enhancement. Speaking as someone who is tired of expending my valuable hours planted in front of a lathe or milling machine, I heartily agree.

 

Man hours are expensive. Robot hours not so much. The more work that can be delegated to some form of CNC (computer numerical control) machine, the better.

 

3d printing has its strengths and it has its weaknesses. The trick is to play up its strengths and avoid the weaknesses. Over the past several years I've been experimenting with 3d printing parts for animatronics mechanisms with mixed results. Sometimes plastic just won't do and metal fabrication comes back into the picture. However, there is a lot plastic can do.



 

The New and Improved Thrashing Torso:

 

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Since I built the original thrashing torso, all those years ago, a few things have changed.

 

3d printing is one of these changes. Another change is the introduction of new off-the-shelf products meant for use in Halloween haunted attractions. I've recently been introduced to the Spider Joint from Spider Hill Prop Works. It is a versatile plastic joint used in conjunction with 1 inch pvc pipe to create body armatures. The new Thrashing Torso also uses a 12-volt motor from Frightprops, another haunted attraction oriented business. Additionally, a plastic halloween skeleton is used. Some modifications are required and this proved to be the most time-intensive part of the build.  

 

A few other refinements have been incorporated into the design. Bungee cord is used instead of springs and clothesline from the hardware store is used instead of steel cable.

 

All of these changes make the finished mechanism much easier to make, the parts are easier to acquire, and everything is much lighter.

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The Breakdown:

 

Man Hours Required: ~30 hrs.

 

I spent about a half a day designing the mechanism in Fusion 360 and I estimate another half a day setting up for the 3d printing and cleaning up the parts as they came out of the printer. Only another day was spent assembling the mechanism. Modifying and mounting the plastic skeleton took the better part of two more days, which includes futzing around with clearances and tensioning the bungee cords.  Considering how much time the first Thrashing Torso took to build, this is great.


 

Cost: ~$158 (total)

 

    Plastic Skeleton $40

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Plastic Skeleton Label

 

    Fright Props motor $25

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    Spider Joints (x8)  $28

Spider Joint.JPG

 

    Misc. Components and Fasteners ~$40

 

    ABS Printer Filament Roll $25



 

Some Thoughts on 3d Printing:

 

There has been quite a bit of hype about 3d printing over the past few years. 3d printing is incredibly useful but let me come right out and say that it can be a pain in the ass. The technology is getting better and better all the time, but like any fabrication technique, it takes time to master. This is especially true when 3d printing functional mechanisms. In the next post (or three) I am going to discuss 3d printing and its applications to making animatronics, as well as some of the pitfalls you may be able to avoid.

 

The Trashing Torso Summarized

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Mechanical Thrashing Torso Project Summary:

Time Required: ~80 hrs.

Cost: ~$200

These are estimates based upon my recollection of this project from 25 years ago, so take them with a grain of salt.


 

Topics Covered:

 

The Design Process:

The design process was been broken down to the basic steps and then applied to this specific project.

Translating the geometry of organic movement into mechanical movements is a fundamental aspect of the design process so I tried to be as clear as possible about how this is done.

 

Materials and Found Objects:

Always try to use off-the-shelf components whenever possible.

Aluminum angle and channel stock available from any hardware store was used throughout the Thrashing Torso project.

Repurposed electrical cable and PVC pipe was used for the rib cage.

That electroformed skull and jaw were definitely NOT off the shelf items, and a plastic Halloween prop skull would probably work just as well.

 

Fasteners:

Screws versus rivets. Basic rule: rivets are good for things that will never need to come apart. Screws for everything else.

 

Cable Actuation:

A very rudimentary application of cables to drive the mechanism was demonstrated. This was suitable for this application but not a typical example. More refined applications of cable control will be explored in future projects.

 

Spring Return:

Gravity was used to move the Thrashing Torso into its slumped position with the assistance of springs. It is best not to rely too much on springs. They can be finicky and prone to fail. A trained gunsmith or an engineer can make use of springs to their greatest advantage but the rest of us are generally just guessing when we slap springs into a project.

 


Electric Motors:

The way I chose a motor for this project 25 years ago is pretty much the same way I do it now. I get as big a motor as I can to do the job, within reason. Motors that don’t have to work hard last a long time.

 

Cranks:

The use of cranks should be well understood by anyone who has peddled a bicycle.  

 

Control of Movement and Quality of Movement:

Make the best use of leverage. Initially, I learned to use leverage by doing it badly. Best to understand leverage and how to use it to its best advantage as soon as possible. A lack of leverage and mechanical advantage will result in jerky, uncontrolled movements, if anything moves at all. The leverage in the mechanical Thrashing Torso is achieved by keeping the drive cable up off the segmented spine mechanism with metal angle brackets with holes drilled in them. It was crude and involves a lot of friction but it worked.

Controlling a complex puppet is often dependent upon limiting movements to what is absolutely necessary. Sometimes this can be determined only through trial and error. Start with your best guess as to what is necessary and adapt your approach as needed. Let the mechanism tell you what it needs. All the various joints of the Thrashing Torso move in the same plane, forward/back, and each joint is limited as to how far it can pivot forward and backward.




What We Didn’t Get Into: Fabrication Techniques

Fabrication is a huge topic and I really didn’t want to get into it here. There are a couple of books I will recommend for those who are interested in learning the basics of fabrication.

 

Book Recommendations:

“Making Things Move” by Dustyn Roberts

“Robot Builder’s Bonanza” by Gordon McComb

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For those with only limited experience these books are full of really valuable information.




 

CAD Model of the Mechanical Thrashing Torso:

 

I’ve created a CAD model of the Thrashing Torso and uploaded it to GrabCad. It includes some minor refinements in the design but it is fundamentally the same as what is presented here. Anyone interested in making their own Thrashing Torso can download the files and get all details and dimensions.

 

grabcad.com/library/animatronic-thrashing-torso-1

Trashing Torso Mechanism

My First Animatronics Project: the Mechanical Thrashing Torso

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Introducing the Mechanical Thrashing Torso! It was created back in my art student days as part of a haunted house attraction. It is a good example of what can be accomplished with limited experience, tools, and materials. The Thrashing Torso consists primarily of a segmented spinal column driven by a cable pull from a single electric motor. After 25 years it is still functional, though it has seen some maintenance and upgrades in that time.

 

Tools and Equipment Used:

  • Hacksaw

  • Electric hand drill

  • Drill press

  • Hand file

  • Bench vice

  • Pliers

  • Screwdrivers

  • Crescent wrench

  • Rivet gun

  • Tap handle

  • Taps

  • Drill bits

Materials:

  • Extruded aluminum square tubing

  • Aluminum flat stock

  • Misc. fastener hardware (nuts, washers, screws, pop rivets)

  • Misc. springs

  • Misc. wire and connectors

  • .125” steel cable

  • PVC pipe

  • Salvaged chunks of electrical cord (rib cage)

  • DC motor

  • Power supply

  • Misc. brackets and “things”

 

This materials list is left pretty vague because a lot of scavenged materials went into this project and it was a long time ago.

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