The angle made by the helix of the thread at the pitch diameter is known as what?

The angle made by the helix of the thread at the pitch diameter is referred to as the lead angle. This angle is significant in understanding the mechanics of how a screw or threaded component engages with another part. The lead angle influences the amount of force needed to turn the screw and the efficiency of its movement. A steeper lead angle often means less radial force is required to move the screw axially but might increase the risk of stripping the threads.

The lead angle is defined geometrically as the angle between the helix and a line parallel to the axis of the screw at the pitch diameter, highlighting the relationship between the thread's geometry and its mechanical behavior. This relationship is crucial for engineers when designing threaded fasteners to ensure they function effectively in their intended applications.

The other terms listed, such as truncation, lead, and angle of thread, refer to different aspects of thread design and geometry, but they do not specifically define the angle at the pitch diameter, making lead angle the most accurate term in this context.