Spheres or spherical forms in digital fabrication are usually approximated in a Cartesian coordinate system. An apparent example of this approximation can be found in a 3D-printed sphere, where its layers become distinct and visible as they get closer to the poles. Cartesian coordinate machines require a minimum of four degrees of freedom and ideally five to process the geometry that is perpendicular to the spherical surface. However, the machines that are able to perform the task are hard to afford at the individual or small fab lab level, especially when the size of objects are bigger than 300mm in diameter.

Inspired by the MIT Center for Bits and Atoms’ MTM Snap project and Eggbot from the Evil Mad Scientist Lab, Spheremill tackles this problem by introducing a spherical coordinate system. The prototype uses THK R-guides and a belt drive mechanism to move the spindle along the polar angle θ. Two rotary axis modules were used for the azimuthal angle φ to secure enough torque when processing the stock. The machine is able to process spherical stocks of up to 550mm in diameter with the maximum depth of 150mm.

The chassis is mostly made of a 12-mm high-density polyethylene (HDPE) sheet with a snap-fit joint design borrowed from the MTM Snap. This minimizes tedious bolting work while providing enough accuracy in assembly. The total material cost is approximately $1500. This can be lowered significantly if the R-guides are replaced with a DIY solution.

The project was commissioned by Gwacheon National Science Museum and was nominated as a Fab Replication Grant Finalist.

Machine view from the top 

Preparing the sphere stock

Test drive result

Shuttle Radar Topography Mission (SRTM) Depth map of North and South America and its full 3D milling result

SRTM(Shuttle Radar Topography Mission) Depth map of North and South America and its 3D milling result