This is designed to fit a writing utensil, such as a crayon or writing pen. The ball is 50mm. The hole in the middle is 9mm. Utensil is held in place by a two each of 1/4-20 set screws. The set screw holes will need to have the threads cut buy a 1/4-20 tap available at any hardware store. Instead of a 1/4-20 setscrew, a 6mm-1 could be substituted.