https://en.wikipedia.org/wiki/Trapezoidal_thread_forms modified stub acme shape https://en.wikipedia.org/wiki/Trapezoidal_thread_forms ACME threaded rod http://en.wikipedia.org/wiki/Trapezoidal_thread_forms Acme screws https://en.wikipedia.org/wiki/Trapezoidal_thread_forms ACME threaded rod ACME