Threads terminology: In a screw thread, which term refers to the smallest diameter of the thread profile (measured at the root for external threads and at the crest for internal threads)?

Introduction / Context:
Accurate thread measurements are essential for strength, fit, and manufacturing. Understanding major, minor, and pitch diameters prevents errors in taps, dies, gaging, and callouts.

Given Data / Assumptions:

• We are asked for the smallest diameter associated with the thread form.
• We assume standard Unified or ISO metric thread definitions.
• We distinguish between major, minor, and pitch diameters.

Concept / Approach:

The minor diameter is the smallest diameter of the thread profile. For external threads (bolts), it occurs at the roots; for internal threads (nuts), it occurs at the crests. Major diameter is the largest; pitch diameter is where the thread thickness equals the groove thickness.

Step-by-Step Solution:

Verification / Alternative check:

Thread tables list sizes including major, pitch, and minor diameters, all consistent with this definition.

Why Other Options Are Wrong:

Internal/external thread diameter: imprecise, not standard names for these dimensions.

Major diameter: the largest diameter, not the smallest.

Common Pitfalls:

Confusing pitch diameter with minor diameter; pitch diameter governs fit, but it is not the smallest value.

Final Answer:

Minor diameter