Thread profile features: What is the correct term for the bottom surface that joins the sides of two adjacent threads within a screw thread form?

Introduction / Context:
Thread elements have specific names that matter for inspection, strength, and manufacturing. This question focuses on identifying the lower surface of the thread valley.

Given Data / Assumptions:

• We are identifying a geometric feature of the thread profile.
• Options include parameters (lead, pitch) and surfaces (crest, root).
• We assume Unified/metric standard profiles.

Concept / Approach:

The root is the bottom of the thread groove between flanks. The crest is the top of the thread. Lead and pitch are distances, not surfaces. Proper root radius/flatness affects stress concentration and fatigue life.

Step-by-Step Solution:

Verification / Alternative check:

Profile diagrams in standards clearly label the root at the base of the thread, consistent across thread systems.

Why Other Options Are Wrong:

Lead and pitch: movement and spacing terms, not surfaces.

Crest: the opposite (top) feature.

Common Pitfalls:

Interchanging crest and root in notes; this can lead to incorrect callouts for radii or flats, impacting strength.

Final Answer:

Root