In 2008, I spent a year in China and collected many tiny accessories and hand-made decorative items.  One of my favorite things to buy for relatives and friends were hallowed cut jade balls.

I remember being struck by the intricacy of the design, but also it was a curiosity how jade balls were made, since the outer shells were a closed loop, it didn’t make sense how the inside shells were allowed to move freely.

One shell at a time, start at the inner shell

So, when Prof. Gershenfeld asked us to design something that could not be done with a subtractive process, I immediately thought of hallowed cut jade balls.

This was also a good opportunity for me to learn how to use Solidworks.  Although it looks simple in form, the design took me a while to perfect, using a different approach each time.  The final method came to me after thinking about how an additive process would work, so instead of trying to do it all at once, I began doing each shell one-by-one, starting with the inner shell and making my way to the outer shell.

The design for each shell is to make two concentric circles, then cut both circles in half to form a thick semi-circle shape.  Then draw a middle line along the cut, and do a revolve operation 360° around to form the shell.

One revolution at a time, make sure you’ve closed the loop

After the revolve, you can perform 3 extrusion cuts on the sphere in orthogonal directions, perpendicular to the front, right and top planes.

The best thing is then to continue the process for the middle, and outer shells.  Just make sure that you size the outside shells appropriately as the surface area increases, so does your thickness and the diameter of the extrusion cut should also increase.

Finally, when you’ve finished all of your layers, you can chamfer the edges of the holes to give the jade ball a softer appearance and so it will be easier on the fingers when you try to spin the inside shells around.

The extruded cut is done across the entire shell

All and all, a very fun exercise.  It has been sent to the 3D printer, but I am awaiting the results.  Until then, here is something interesting for you to know: a hallowed cut jade ball is actually done by a subtractive process! HAH! I bet you didn’t see that coming, did you?  But still, the principle applies, it’s not something a machine can do via subtractive process, but you need special tools and nimble fingers to carve it out of a big chunk of jade.

And that’s what makes the hallowed cut jade ball so special.

The final design

And… here’s the final output.

Parts move freely after soaking in acidic solution.