Question 12 3AE02 - Third Assistant Engineer (Alt)

The Correct Answer is A ### Explanation of Why Option A ("40 threads per inch") is Correct The **pitch** of a screw thread is the distance a screw advances in one complete revolution. In many standard Imperial (or English system) micrometer screw gauges, the common specification for the spindle thread is **40 threads per inch (TPI)**. 1. **Pitch Calculation:** If there are 40 threads per inch, the pitch (P) is $1 \text{ inch} / 40 = 0.025$ inches. 2. **Micrometer Thimble Scale:** Since the pitch is $0.025$ inches, one full rotation of the thimble moves the spindle $0.025$ inches. The main barrel scale (sleeve) of the micrometer is usually marked in divisions of $0.025$ inches. 3. **Standard Design:** This pitch (40 TPI) is a standard design feature that allows the commonly used 25-division thimble scale to easily provide a least count of $0.001$ inches ($0.025 \text{ inches} / 25 = 0.001 \text{ inches}$). Therefore, 40 threads per inch is the standard pitch for an Imperial micrometer screw. ### Explanation of Why the Other Options Are Incorrect **B) 50 threads per inch:** While 50 TPI is a possible thread specification, it is not the standard or most common pitch used for typical industrial Imperial micrometers. Using 50 TPI would result in a pitch of $1/50 = 0.020$ inches, which is not compatible with the standard $0.025$-inch main scale divisions. **C) 75 threads per inch:** 75 TPI is too fine and is not the standard pitch used in commonly manufactured Imperial micrometers. This pitch would result in a very small advance per revolution ($1/75 \approx 0.0133$ inches), making it incompatible with the standard micrometer scale layout designed around 40 TPI. **D) 100 threads per inch:** 100 TPI is extremely fine (pitch of $0.010$ inches) and would rarely, if ever, be used for the main spindle screw of a standard micrometer gauge due to manufacturing difficulty, wear concerns, and incompatibility with the standard $0.025$-inch scale markings. (Note: Metric micrometers typically have a pitch of $0.5$ mm or $1.0$ mm, which translates to a much coarser thread count than 100 TPI).