How to Read Screw Thread Designation?

Introduction

Concentrate on the format when reading screw thread designations. In the case of metric threads, “M” stands for metric threads and is followed by the nominal diameter and pitch in millimeters (for example, M12 x 1.75). The nominal diameter in inches is listed first for Unified National threads (such as 1/4-20 UNC), then the threads per inch and the thread series.

Screw threads are critical components used in a wide range of mechanical applications, from fastening parts together to creating precision movements. Understanding the thread designation is essential for selecting the right screw for specific purposes. This article aims to explain the key elements in screw thread designations and how to interpret them correctly.

Metric Thread Designation:

Metric threads are identified by the letter “M” followed by a number that represents the nominal diameter in millimeters. After the “x,” another number indicates the pitch, which is the distance between two adjacent thread crests. A metric screw, for instance, with a pitch of 1.75mm and a diameter of 12mm is designated as “M12 x 1.75”.

Unified National Thread Designation (UN):

Unified National threads, commonly used in the United States, are specified by two numbers separated by a hyphen. The first number represents the nominal diameter in inches, and the second number indicates the number of threads per inch. For instance, “1/4-20” refers to a Unified National thread that has 20 threads per inch and a 1/4-inch diameter.

Thread Series:

In Unified National threads, a thread series is denoted by additional letters after the thread designation. Common thread series include UNC (Unified National Coarse) and UNF (Unified National Fine). UNC threads are more widely used for general-purpose applications, while UNF threads are employed in applications requiring higher strength and precision.

Thread Depth and Angle:

Thread depth and thread angle are crucial parameters in screw thread designations. The thread depth refers to the distance from the crest to the root of the thread, while the thread angle is the angle between the flanks of the thread. These specifications are crucial for ensuring proper engagement and load-carrying capacity of the screw.

Special Thread Designations

In some cases, screw threads may have additional designations to indicate specific features. For example, “RH” denotes a right-hand thread, while “LH” indicates a left-hand thread. Threads with a helix angle greater than 45 degrees are considered “fast threads” and are designed for rapid assembly and disassembly.

Understanding screw thread designations is vital for selecting the right screws for different applications. Whether dealing with metric or Unified National threads, knowing how to interpret nominal diameter, pitch, thread series, depth, and angle empowers engineers and manufacturers to make informed decisions for their projects.

Video 01: How to Read a Metric Screw Thread Callout