I call these "3D Bot Bits", since most of the resulting creations are robot-looking.
There were a few things I knew I wanted to achieve in this design:
1 - Give the joints as much range of motion as possible
2 - Make the joints tight enough to have friction to hold specific positions (not collapse with gravity)
3 - Allow the joints to flex a bit without breaking
4 - Allow for customization - so new parts could easily be designed and added to kits
Basic Design Principle
While it's pretty easy to make these two parts, what added to the challenge was making them in a way that gave them a flat enough base to print reliably (I don't like using rafts and supports) and let them snap together and apart without too much effort.
With these design goals and principles in mind, here's a general description of how I went about the modeling:
1 - Created a sphere of around 10mm radius then duplicated it and scaled up the duplicate to a 12mm radius. The smaller sphere is now inside and centered with the larger sphere.
2 - Flattened both spheres by taking away approximately 4mm from the bottom and 4mm from the top (to make them more like bulging puck shapes).
3 - Subtract the smaller sphere from the larger, so that the larger sphere becomes a shell.
4 - Create some gap between the outer surface of the inner smaller sphere and the inner surface of the outer shell - I used about 0.3mm radius here by pushing the inner surface of the outer shell to be bigger, but you could also just scale the X/Y of the outer shell object too.
5 - Create a break in that outer shell object so that it can stretch to allow the inner sphere to be snapped in and snapped out in the finished object.
6 - Add a connector "axel" between the parts which is 1-2mm smaller than that break in the outer shell so that it fits into the break easily for connecting parts.
Strength and Stretch
The position of the part during printing is actually quite important - which is why I designed it this way. The part that grabs onto the ball - the "Gripper" - is where most of the stress will be. As the ball is snapped in and out when the parts are put together and taken apart, the Gripper will stretch. If that stretch was done along the Z-axis part of the built part (the up-and-down axis), the prt would break quite quickly given that is the weakest part of 3D Printed parts. But with the Gripper part printed laying down, it makes it much stronger and actually allows it to flex quite a bit without breaking.
|A Custom Bot Bit: Robot Hand|
Customizing Bot Bits
Once I had a simple ball and gripper pair working well, I started creating some alternative parts with different configurations and shapes.
One technique I use when building models like this is to never permanently merge the component parts of an object - or to keep copies of the component parts so they can be re-used.
This made it super simple to make new connecting Bot Bits which were shaped like a "T" or with two grippers on either end, or an "X" shape to allow more complex builds.
Since my initial creations looked like Robots (hence the name Bot Bits), I decided to make some super-custom parts too - like hands and sneakers and face parts. This is where I see the most interesting potential of this design - allowing others to create their own Bits to make specific types of Bots.
The Bot Bit Kit
With a good feeling about the most simple "complete" robot that can be built with these Bot Bits, I created a simple Bot Bit Kit - which contains all the parts needed to make the basic Bot. This can be printed in one shot on my Lulzbot TAZ4 in 2.5 hours very reliably.
The basic parts in this kit can also be put together in different ways to make many other creatures or designs - take a look at the pictures at the end of this post for some ideas.
Here is a link to the Bot Bit Kit model. If you print it, please comment back here with some pictures of the creation you made with this kit!