Thread geometry — Is the “major diameter” defined as the largest diameter of a screw thread (e.g., crest-to-crest on an external thread or root-to-root cylinder for an internal thread)?

Introduction / Context:
Understanding thread diameters—major, pitch, and minor—is essential for specifying fits, calculating strength, and selecting taps/dies. Mislabeling these diameters leads to tolerance stack-up problems and assembly failures. This item focuses on the definition of the major diameter.

Given Data / Assumptions:

• Claim: the major diameter is the largest diameter of a screw thread.
• Thread systems considered: metric ISO and unified (UNC/UNF), among others.
• We distinguish external vs internal threads but keep the definition consistent.

Concept / Approach:
The major diameter is the largest diameter of a thread form. For an external (male) thread such as on a bolt, the major diameter is measured at the crests. For an internal (female) thread such as in a nut or tapped hole, the major diameter corresponds to the imaginary cylinder that touches the roots of the internal thread profile. The pitch (effective) diameter lies between major and minor and is where the thread thickness equals the space between threads. The minor diameter is the smallest diameter of the thread form.

Step-by-Step Solution:
1) Recall thread diameter hierarchy: major > pitch > minor.2) Identify what “largest” maps to: major diameter.3) Confirm across thread types: definition remains largest for both internal and external threads.4) Conclude the statement is true.

Verification / Alternative check:
Check a thread data chart: for M10 × 1.5, the listed major diameter is approximately 10 mm for the external thread; pitch diameter and minor diameter are smaller values. Similar relationships appear in UNC/UNF tables (e.g., 3/8-16 UNC).

Why Other Options Are Wrong:

• Incorrect: Conflicts with formal definitions.
• Only applies to internal / external: The concept applies to both; only how it is realized (crest vs root cylinder) differs.
• Refers to pitch diameter: Pitch diameter is not the largest; it is the effective diameter for fit calculations.

Common Pitfalls:
Confusing pitch diameter with major diameter; assuming crest truncation changes which diameter is “major”; ignoring tolerance classes that slightly shift numeric limits but not definitions.

Final Answer:
Correct