This is a Spool-holder for the Tevo Flash 3D printer. You will need 4 pieces of 17 x 35 x 10mm steel ball bearings. The spool-holder is adjustable in its width by turning the knob. The bracket that holds the whole spool holder can be inserted from the backside of the printer by sliding out the housing of the electronics a bit. You can buy the ball bearings here on Amazon: 5 pcs ball bearings : http://www.c3d.at/g?i=bb5&p=6 10 pcs ball bearings : http://www.c3d.at/g?i=bb10&p=6