Question 55 1AE01 - First Assistant Engineer

The Correct Answer is B **Why Option B ("national coarse thread") is correct:** Illustration GS-0038 typically depicts a standard V-shaped thread profile, which is characteristic of the Unified Thread Standard (UTS) or, in older terminology, the National Thread Series. The "National Coarse" (NC) thread (now designated UNC – Unified National Coarse) is the most common, general-purpose thread used for fasteners like bolts and screws. Key features of this thread type include: 1. **V-Profile:** The flanks (sides) of the thread are straight and meet at a 60-degree included angle. 2. **Rounded/Flattened Crest and Root:** Standard external threads often have a slightly rounded or truncated (flattened) crest and root, differentiating them from theoretical sharp V-threads. 3. **Application:** Since this illustration is typically used to represent a generic, standard fastener thread, the National Coarse (or Unified Coarse) designation is the appropriate identification for the most common thread form used in mechanical engineering and manufacturing. **Why the other options are incorrect:** * **A) acme thread:** Acme threads have a trapezoidal profile with a 29-degree included angle between the flanks, significantly shallower and broader than the standard 60-degree V-profile illustrated in standard fastener diagrams. They are primarily used for power transmission (lead screws) rather than fastening. * **C) square thread:** Square threads have flanks that are parallel to the thread axis, forming a true rectangular or square profile. Like acme threads, they are used for high-load power transmission, not general fastening. This shape is fundamentally different from the 60-degree V-profile. * **D) cannot be determined from information provided:** While a precise measurement (pitch, diameter) is needed to distinguish between National Coarse (NC/UNC) and National Fine (NF/UNF), the *type* of thread (the geometrical form) shown in a standard mechanical drawing illustration of a fastener thread is universally recognized as the 60-degree V-thread standard. Among the choices provided, National Coarse is the designation for this standard geometry when used as a fastener thread.