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Difficulty: MediumScrews

In a mechanical shop, a technician selects a machine screw marked with the specification 1/4-20×1-1/2 in.1/4\text{-}20 \times 1\text{-}1/2\text{ in.} What does the number 2020 represent in this fastener designation?

  1. A
    The recommended screwdriver tip size required to install the screw
  2. B
    The tensile strength rating grade of the screw material
  3. The number of threads per inch along the fastener shankAnswer
  4. D
    The length of the threaded portion measured in sixteenths of an inch

Answer

The number 20 represents the thread pitch expressed as the number of threads per inch (TPI) along the fastener shank.
In standard imperial fastener notation (Unified National series), specifications follow the pattern [Nominal Diameter]-[Threads Per Inch] ×\times [Length]. In the designation 1/4-20×1-1/2 in.1/4\text{-}20 \times 1\text{-}1/2\text{ in.}, the number 20 indicates that the screw has 20 threads per linear inch (TPI), classifying it as Unified National Coarse (UNC).

Step-by-Step Solution

1
Analyze the standard Unified National imperial thread designation format.
The standard format is structured as: [Nominal Diameter] - [Threads Per Inch] ×\times [Fastener Length].
Imperial machine screws and bolts use this standardized notation to define thread geometry and physical sizing.
2
Map the specific values from 1/4-20×1-1/2 in.1/4\text{-}20 \times 1\text{-}1/2\text{ in.} to the standard notation components.
1/4 in.1/4\text{ in.} is the nominal major diameter, 2020 is the threads per inch (TPI, indicating Unified National Coarse / UNC), and 1-1/2 in.1\text{-}1/2\text{ in.} is the overall length.
The second parameter specifically identifies thread density per linear inch.

Key Concept

Unified National machine screw thread specification notation
Estimated Time:1m 0s
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Topics

Screws

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