PComp : The Tail Saga – episode 3

The Bone System

Beware of obsession! Play the game of obsession for the road, not for the goal…

Obviously i needed something solid inside the fluffy tail to able to swing it with a servo. Could be a piece of wire, or some laser cut acrylic to be fancy.

But i wanted it to move like a tail! So the spine inside the tail must have joints inside it. Could be to wires looped inside each other. Could be two acrylics screwed to each other to be fancy.

But something more challenging? By this point i thought the whole thing is practically done and i can make the design more… interesting! So i drew a challenge in my head: A complete bone system that can be 3D printed in ONE print with the joints already interlocked.

it’s doable. it’s one of those doable things. I’ve printed interlocking designs before. I did actually try and look for other 3D printable joints designs online. But couldn’t find anything worthwhile. I had printed interlocking designs before. It’s one of the unique things about 3D printing: You can build things inside each other! And the secret? All the parts must be in some way connected to the bed. Look at these designs for example:

NASA ISS 3D Printable Wrench

A few years ago NASA sent a normal FDM 3D printers (Much like the ones we use at iTP) to the International Space Station. A while later, they asked for a new wrench for a common screw inside ISS. So NASA sent them a file for them to 3D print and after a few hours, they had a new ratchet wrench complete with moving parts inside, all in one print. The secret? Everything inside is connected the print bed.

So that was the basis of the design. Two interlocked cylinders.

First Sketch

And to make sure they stay inside eachother, i would make one concave and one convex. Basically the outside of a donut and inside of a slightly larger donut.

So i got the designing in 3D, and the actual bump in the road: I never used parametric design programs! i did do some low-poly modeling, but this was a different thing altogether. Programs like Rhino, Catia, SolidWorks, Inventor, etc. are somewhat similar in functionalities. I chose to use Autodesk’s Fusion 360. I used it a bit before and really liked the aesthetics of it. Plus its free for students and need broadband connection… both of which i have at iTP and didn’t back home. Simple things. So this was a good reason for me to learn Fusion 360 on the way.

Basic Shape
Donut Design

A test print of this should i could print the parts safely with a margin of 1mm between them.

All of this took a week. Mostly redesigning the same concept over and over just because i didn’t anticipate some steps in the process.

In my next iteration, i sought advice from design people on the floor and outside iTP, They couldn’t much to the design and i proceeded to make the design more sophisticated:

Sketches – Second Stage

This is the final design for the joint. I printed a test, increased the margin to 1.5mm, and after 30min of print, i had a functional, interconnected joint!

The key for this design is the connection between the core of donut to one of the axles. The process of connected two taller object in air is called “Bridging”. Bridges are done a lot in 3D printing, and the key is to make sure you close the gap as close as possible before making a bridge. Here you can see the moment of bridging in my print:

Final Joint

Now i just needed to add some bones to these joints. for that i tried to first have an outline of the necessary angles the tail going to be at and design according to that. You’ll see that in the end i followed the joint outlines used in 3D rigging systems, but a more streamlined shape:

Bone Angle Study
General Shape
Convex design – not that good!


Concave design – This works better!
Sketch to Polygon
Initial Design
Final Design – Ready to Print

Now to reiterate the point of connecting to bed for printing interlocked objects, look at the footprint of the first layer of my design and how it grows and intertwines with the other parts without touching it:


Print Progression

And the final result:

A fully functional bone system with interlocked joints that prints in one 3D print already assembled

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