I've finished building the X-carriage assembly and now working mating it with the Z-axis. The wheels for the Z axis travel seem to be awfully tight despite adjusting the eccentric spacers. On Mark Carew's video at 44:31 he states that the wheels should be good and tight. How tight should they be? Some wheels move freely when spun with a fingertip, others move with some mild friction, others barely move at all and seem very tight. What is the preferred amount of friction?