A number of angles and measurements defines the thread of the screws and bolts.
All are of importance.
The lead is the distance through which a screw advances with one turn in the direction of the shaft. There are single lead threads and multiple lead threads. Multiple lead threads are able to feed faster with fewer revolutions.
The pitch is the distance between the corresponding threads in the axial direction.
The effective diameter is the midway point between the crest and root. In other words, the point where the internal thread and external thread overlap when screws are meshed together.
The effective diameter is,
along with pitch, one of the
most important dimensions
in threads.
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the Basic terms subpages
for further information!