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)?
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AInternal thread diameter
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BMinor diameter
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CMajor diameter
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DExternal thread diameter
Answer
Correct Answer: Minor diameter
Explanation
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:
Recall definitions: major = largest, minor = smallest, pitch = halfway in profile thickness.Identify the smallest diameter as “minor diameter.”Confirm this applies to both external and internal cases (location on profile differs, definition does not).Select “Minor diameter.”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