Errors in Threads & Measuring the Pitch and Diameter of Screw Threads

Errors in Threads

     Errors in screw threads are related to the five elements of the screw threads. They are major diameter, minor diameter, pitch diameter, pitch and thread angle. If any errors are taking place in these five elements the produced screw is rejected. So, these elements are also be checked with proper gauging system carefully. The threads are produced by a point cutting tools.

     The errors in major and minor diameter cause interference of the mating threads, less root section, less wall thickness and poor contact of the flanks, which ultimately cause the weak in strength of the component. The errors in effective diameter also cause the interference of the flanks.

     The errors in pitch and thread angle also cause the progressive tightening of the mating parts due to the interference of the flank surfaces.

     Let us discuss some important errors in thread forms. They are

• Drunken error
• Pitch errors 

Drunken Error:  It is error due to the irregular form of helical groove on a cylindrical surface. In this case pitch measured parallel to the axis is always same, but problem is with the thread is not cut to its true helix.

Due to this flank surface will not be as a straight edge, it will be as curved form.

Pitch errors: 

    The pitch errors are due to improper ratios of cutting tool velocity to rotating velocity of the workpiece. these pitch errors are again classified as

• Progressive pitch erros
• Periodic pitch errors

Irregular errors

     Progressive errors: In this the pitch error results increasing of major or minor diameter or decreasing of major or minor diameter. It means the error may either in increasing order or decreasing order.

     Periodic errors: In this the pitch error causes the errors to repeat at certain time of interval.

     Irregular errors: These are the errors randomly take place on threads without any specific reason. These are the combination of all the errors take place on threads.

Types of Screw Threads

Screw threads are classified into two main types: external threads and internal threads.

External threads: These threads are formed on the exterior of a rod or shaft. They are also known as male threads.

Internal threads: Threads that are formed on the inside of a hole. They are also known as female threads.

External threads and internal threads mate with each other to create a threaded connection. The threads on the male and female parts of the connection must be of the same type and size in order for the connection to be secure.

There are many different types of screw threads, each with its own unique properties. Some of the most common types of screw threads include:

Metric threads: Metric threads are used in most countries outside of North America. They are identified by a letter M followed by a number, which indicates the major diameter of the thread in millimeters. For example, an M6 thread has a major diameter of 6 millimeters.

Unified threads: Unified threads are used in North America and some other countries. They are identified by a letter U followed by a number, which indicates the major diameter of the thread in inches. For example, an UNC #8 thread has a major diameter of 0.25 inches.

Pipe threads: Pipe threads are used for threaded pipe connections. They are identified by a letter NPT followed by a number, which indicates the major diameter of the thread in inches. For example, an NPT 1/2 thread has a major diameter of 0.5 inches.

Uses of Screw Threads

Screw threads have a versatile range of applications, thanks to their ability to provide secure connections and control motion. Some common uses include:

Fastening: The primary purpose of screw threads is to fasten two or more objects together. They create a secure bond that can be easily tightened or loosened when needed.

Locking: Screw threads can be used to lock two or more objects together so that they cannot be easily disassembled. This feature is vital in various mechanical and engineering applications.

Transferring motion: Screw threads can be used to transfer motion from one object to another. This principle is widely utilized in the mechanics of gears and lead screws in machinery.

Sealing: Screw threads can be used to create a seal between two objects. Pipe threads, for instance, are crucial for ensuring leak-free connections in plumbing systems.

Measuring the Pitch and Diameter of Screw Threads

The pitch of a screw thread is the distance between the crests of two adjacent threads. The diameter of a screw thread is the distance across the threads.

To measure the pitch and diameter of a screw thread, you can use the following tools:

Vernier caliper: A measuring tool that can be used to measure the pitch and diameter of a screw thread.

Pitch gauge: A tool that can be used to measure the pitch of a screw thread.

Procedure for Measuring Screw Threads

1. Measure the major diameter of the screw thread with a vernier caliper.
2. Place the vernier caliper around the screw thread, making sure that the jaws are parallel to the axis of the screw thread.
3. Read the major diameter of the screw thread on the vernier scale.
4. Measure the pitch of the screw thread with a pitch gauge.
5. Insert the blades of the pitch gauge into the screw thread.
6. Find the blade that fits snugly into the screw thread.
7. The pitch of the screw thread is the number of threads that are contained in one inch of the screw thread.

Once you have measured the pitch and diameter of the screw thread, you can identify the type of screw thread using a thread chart.

Conclusion

Screw threads are an essential component of many different types of machines and devices. They are used to fasten objects together, lock objects in place, transfer motion, and create seals. Understanding the different types of screw threads and how to measure them is an essential skill for anyone who works with threaded fasteners. By measuring the pitch and diameter of a screw thread, you can identify the type of screw thread and ensure that you are using the correct fastener for the job.