Confusion about the lead screws sold on the store -> 8mm Metric Acme Lead Screw The page claims that they are: - ACME - Metric - Trapezoidal ACME is the imperial standard and not generally referred to as trapezoidal. And has a 29 degree thread angle. Metric *are* trapezoidal and have a 30 degree thread angle... So, which is it really? ACME or Metric Trapezoidal?