And now a change of pace from rover design: a cat toy! The intent of this project was to build an enrichment toy for a cat. It provides a bit of physical activity and also provide rewards in the form of an occasional cat treat.
The design goals were:
- Printable on inexpensive 3D printers.
- Spherical exterior so a cat can bat at it and have it roll around.
- Hollow interior to hold some bits of cat treats.
- Holes in the exterior so treats would occasionally drop out as the cat plays with it.
- Can be pulled part for treat refill.
- Can be printed without print support material.
The simultaneous demands of spherical exterior, accessible hollow interior, and no support is a challenge for low-end hobbyist level 3D printers. Mid- and high-range printers have features like water-soluable support material which makes arbitrary shapes easy to build. Low end printer projects need to be designed to print on a flat print bed. In this case, the ball has to be printed as two hemispheres.
If each hemisphere had equal thickness walls throughout, the polar parts of the hemisphere would have been a challenge to print without supports. The top of the sphere will almost certainly sag due to overhang and ruining the spherical shape. Fortunately there was no requirement for the interior to be spherical and we could work around this issue by making the interior into a cone that can print without supports.
We then move on to the next challenge of joining two hemispheres. This was solved by printing a third piece – a springy clip that sits inside of our sphere’s “equator” and shaped to hold the two halves together. Here’s a cross-section view of the clip and the two hemispheres.
If this springy clip was too weak, the sphere will fall apart as the cat plays with it. If it is too strong, the human will have a hard time pulling it apart for refill. So it requires some tuning to find a sweet spot as every printer is a little different. Thanks to Onshape’s ability to adjust parameters, the lucky cat’s owner could fine-tune how much surface area to put on this clip and how much bending spring to put into its shape.
This cat toy is freely accessible in Onshape’s Public Documents library to be copied and modified, which is exactly what happened for at least one cat.