Connect motor shaft to anything else with a set-screw coupler. These couplers are made of machined aluminum and have a spiral cut that makes them slightly flexible so they can be fit to two shafts even if they are not perfectly co-linear and will help reduce bending effects. 

The way they are cut, create a spring effect in the X Y & Z linear axis but not in the rotational axis. That means that you will not get increased backlash - so they're suitable for precision CNC work.

This coupler will connect a 8mm diameter shaft to an 12mm diameter shaft.  Comes with two sets of double set-screws (two per side) for secure attachment. 

• d1=8mm / d2= 12mm 
• Outer diameter (D): 32mm
• Length (L): 40mm