My Z axis is not homing correctly. 

The most likely causes of the Z stepper motor not homing are:

1. There is electrical noise in the Z home switch caused by the Z stepper motor wires. You can separate the home switch wire and wrap every 5 or 6 loops like in the picture.

                    

2. The threaded Z rod and nut are binding. The stepper motors only have so much torque and cannot drive through the binding. This can be solved by reducing friction between the rod and nut. The screw should turn easily. Possible solutions: • try spraying dry lube (graphite, Teflon, or silicon) on the threaded rod. The optimal nut flat selection is as loose as possible without axial movement of the threaded rod. • make sure the aluminum helical coupler has space left between the helical cut to help with misalignment. Tighten the coupler to the Z stepper motor shaft, then pull the coupler down, stretching it about 1 mm. While stretched, tighten the set screws to the threaded rod.

3. Stepper motor current too low – The stepper motor driver is not supplying enough current for the stepper motor to drive the screw. Turn up the stepper motor current on the A4988 driver.

4. Stepper motor current too high – The stepper motor driver is heating up and going into protection mode. Once it cools it turns back on. This can look like erratic stepper motor movement. Turn down the stepper motor current on the A4988 driver.