Dart hand spinner

I am a high school teacher, and I had made a hand spinner to teach my students how to made one by themselves.
http://www.thingiverse.com/thing:2197998
https://livingtechlearn.wordpress.com/2017/03/18/使用onshape繪製手陀螺/

Then I had heard some school think it is too danger.
So I draw a dart version. Be careful. :)

4mm steel ball *9
608ZZ *1
M3*5mm *3 ( Screws and nuts )