THIS WORK IS OUTDATED. CHECK OUT https://www.thingiverse.com/thing:7195072 for my new version.

Warning: This work is NOT the final phase. There will be a updated version within 1~2 months.

Warning: It contains parts (edge_1 and tiny_edge) which are difficult to print. A small line width is recommended for these parts.

Warning: Before the next version is released, be sure that you have a super glue.

Recommended for PETG or ABS.

1. Print all the files, except tiny_edge and corner.
2. Glue edge_1 to edge_2, which would make a complete edge.
3. Glue cap to core_2. adjust the position of the cap using center so that the center fits closely to the sphere in the center of core_2.
4. install four of the center onto the core_2 and slide them to the flat side of the core_2.
5. Put glue on the four center and stick the other fore center on them.
6. Install the edge to the assembly.
7. Push core_1 to the assembly.
8. Make some turns and see if it works well. Then pull the core_2 out of it.
   9_1. It works well: go to Step 10.
   9.2 There are still space around the core, from which you can see the inner sphere: if you don't like it, print tiny_edge and install them.
   9.3. the edge parts are too loose: Print tiny_edge and corner and install them.
9. Glue core_1 to the assembly.
10. Well done!

There's space for improvments.. and I'm looking forward to release the NEXT version, for which there's fewer parts, fewer steps to assembly and smoother turns!