I bought the 8 mm lead screw ( http://openbuildspartstore.com/8mm-metric-lead-screw-acme/ ) and the matching (?) nut ( http://openbuildspartstore.com/8mm-acme-nut-block/ ) for my Z axis. Wow, the nut is super tight on the lead screw. It is so tight it is barely movable by twisting it with my hands, the motors will never be able to drive this. This can't be right. Am I doing something wrong or am I missing something here? Grease? Do they "wear in" over time or something?