If your screw is not as perfect as you desire you can use screw compensation.
http://www.linuxcnc.org/docview/html//config_ini_config.html#sub:%5BAXIS%5D-Section
There are two types of compensation
COMP_FILE_TYPE = 0
COMP_FILE_TYPE = 1
The file format is:
You can name the file anything you like
Place this in each AXIS section you have a comp table for.