I made this adapter for a project where I need to attach a grinder disk to an 8mm shaft to drive it using a 775 DC motor. 

The idea is that this will press fit into an 8mm OD linear motor shaft. 

I have also designed the shaft to go all the way through the part for reinforcement. 

Keep in mind this part is plastic after all, it is on you to make sure you use good printing techniques and materials to ensure your safety.