Strong aluminum coupling for connecting your stepper motor to a M8 rod.
You can fix the coupling using the hexagonal nuts to the shaft of the
stepper motor. Put a M8 nut onto the M8 threaded rod and insert the M8
rod fully into the coupling. Then tighten the M8 nut to fix the rod. You can use grub screws to fasten the coupling to the motor shaft.
Design and author are available here: http://reprap.org/wiki/Mendel:_Prusa_durable_z-axis_motor_coupling