I used an UP! 3D printer and it worked very well. It took me a while to optimise it, but I'll post that in an other Instructable.
Step 3: 3D Print
I used White ABS with normal definition. I suppose that PLA would also work fine.
Step 4: Clean
If you used a RepRap 3D printer, you will need to remove the support plastic with tweezers or needle-noze pliers.You might also want to use sandpaper to remove the rough texture.
Step 5: What you need
Once you have the core, get 14 eraser tips. They come in many colors, so you have the choice!
Step 6: Shine
If you want to be fancy and have a shiny surface, you can use a little bit of acetone. Be careful because it actually melts the plastic so use a brush (real hair because synthetic ones will also melt). If you put to much acetone on the little harms, they will shrink and the tolerances will change.
Step 7: Insert
Insert the erasers!
Step 8: Touch up
If you DID abuse on acetone, you will need to put a little bit of glue for the assembly.
Step 9: Erase
SURPRISE! The eraser-ball erases!
Step 10: Play
It also easily becomes a distracting office game.