Acme threads are used in lathe lead screws. This form of thread enables the easy engagement of the half nut. The metric acme thread has an included angle of 30°.
The reasons for rough and broken threads are
Thread cutting tool is blunt
The size of hole or diameter of the blank for bolt is not correct
The flutes of cutting tool are filled with chips
The distance between one point on a screw thread and the corresponding point on the next thread, measured parallel to the axis is called as Pitch of Thread.
Lead is the distance of a threaded component moves along the matching component during one complete revolution. For a single start thread the lead is equal to the pitch.
The American Acme form has an included flank angle of 29-degrees. The metric Trapezoidal thread is at 30-degrees.