CFS Thread Design & Compatibility 2 — Questions and Answers
Question 1: What does the thread tolerance class 6H/6g denote in the ISO metric system?
- Coarse pitch with tight tolerance
- Standard internal/external thread tolerance for general use (Correct answer)
- Fine pitch with loose tolerance
- Left-hand thread designation
Correct answer: Standard internal/external thread tolerance for general use
6H denotes the internal (nut) thread tolerance and 6g denotes the external (bolt) thread tolerance; together they represent the standard general-purpose fit in ISO metric threading.
Question 2: Which thread form has a 60° included angle and is the most widely used in the United States?
- Buttress thread
- ACME thread
- Unified National thread (Correct answer)
- Whitworth thread
Correct answer: Unified National thread
The Unified National (UN) thread form uses a 60° included angle and is the dominant fastener thread standard in the United States.
Question 3: A bolt is specified as 3/8-16 UNC. What does '16' represent?
- Thread length in millimeters
- Number of threads per inch (Correct answer)
- Thread tolerance class
- Nominal diameter in 1/16 inch increments
Correct answer: Number of threads per inch
In the designation 3/8-16 UNC, '16' is the number of threads per inch (TPI), which defines the pitch of the thread.
Question 4: Which condition describes the maximum material condition (MMC) for an externally threaded fastener?
- Largest pitch diameter allowed (Correct answer)
- Smallest pitch diameter allowed
- Mean of min and max pitch diameter
- Pitch diameter equal to nominal
Correct answer: Largest pitch diameter allowed
For an external thread, MMC corresponds to the largest allowable pitch diameter, meaning the most material is present on the bolt.
Question 5: What is the primary reason UNF (fine thread) fasteners are preferred over UNC in high-vibration aerospace applications?
- Lower manufacturing cost
- Greater tensile strength per unit length and better self-locking tendency (Correct answer)
- Easier hand assembly
- Compatibility with metric tooling
Correct answer: Greater tensile strength per unit length and better self-locking tendency
UNF threads have a smaller pitch, increasing the number of threads per inch, which raises the stripping torque threshold and resists vibration loosening better than coarse threads.
Question 6: In thread gaging, what does a GO gage verify?
- The thread is not too large (checks maximum material limit) (Correct answer)
- The thread is not too small (checks minimum material limit)
- The thread pitch diameter exactly equals nominal
- The thread lead angle is correct
Correct answer: The thread is not too large (checks maximum material limit)
A GO gage checks that the thread does not exceed its maximum material condition by verifying that the gage can be assembled through the full thread engagement length.
Question 7: What thread series designation is used for threads on thin-walled tubing and fittings where a fine pitch is needed but UNF designation is not applicable?
- UNEF (Correct answer)
- UNS
- UN8
- UNC-3A
Correct answer: UNEF
UNEF (Unified National Extra Fine) threads are used for thin-walled sections, instrument applications, and fitting connections where standard fine threads are too coarse.
CFS Thread Design & Compatibility 2
What does the thread tolerance class 6H/6g denote in the ISO metric system?