Thread specification systems: The standard set of threads per inch assigned to various nominal diameters (for example, UNC or UNF selections) is referred to as the ________.

Introduction / Context:
When specifying Unified inch threads, designers choose a nominal size and a series that sets the threads per inch (TPI). Selecting the correct series affects strength, engagement length, and assembly.

Given Data / Assumptions:

• The question is about the standardized TPI associated with diameters.
• Examples include UNC (coarse), UNF (fine), and UNEF (extra fine).
• We are not describing a geometric distance such as pitch in metric terms.

Concept / Approach:

In the Unified system, a thread series defines the TPI for each nominal diameter. By contrast, “thread pitch” is the distance between threads used in metric designation (for example, M10 x 1.5 has pitch 1.5 mm). Lead and major diameter are distinct parameters.

Step-by-Step Solution:

Verification / Alternative check:

Handbooks list tables by series, showing TPI per diameter. Metric tables list pitch directly rather than series, reinforcing the distinction.

Why Other Options Are Wrong:

Lead: axial advance per turn; unrelated to TPI assignment.

Major diameter: largest thread diameter; does not define TPI.

Thread pitch: in inch threads, TPI is used, not pitch in distance units; the question asks for the standard listing across diameters, which is the series.

Common Pitfalls:

Mixing inch-series terminology with metric pitch conventions; use series (UNC/UNF) for TPI-based inch threads.

Final Answer:

Series of thread