Thread Inspection: An Overview of the Most Popular Methods

May 6, 2019 11:27 am

 

The goal of thread inspection is to ensure that a machined thread meets design requirements.

There’s a multitude of methods and tools that can help with the process, some more accurate than others. In this article, we’ll give you an overview of the advantages and disadvantages of the most popular methods used to inspect external threads together with the most recommended tools.

 

Anatomy of a thread

 

A thread is a uniform helical groove cut inside of a cylindrical workpiece, or on the outside of a tube or shaft.

Cutting threads by using a lathe requires a thorough knowledge of the different principles of threads and procedures of cutting. Hand coordination, lathe mechanisms, and cutting tool angles are all interrelated during the thread cutting process. Before attempting to cut threads on a lathe a machine operator must fully understand all the principles, terminology, and uses of threads.

 

Thread section

Image credit: smithy.com

 

The goal of inspecting (or measuring) a thread is making sure that the thread is cut at the right depth and that the pitch diameter matches the design specifications.

The pitch diameter is the diameter of an imaginary cylinder formed where the width of the groove is equal to one-half of the pitch.

Basic thread dimensions can be found in thread charts, which are generally present in most machining manuals and guide books. If you don’t have one, we recommend getting this.

Reading these charts is fairly simple, all you need to know is the major diameter of the part, something that can be easily measured with a caliper, and the number of pitches per inch, something that you can easily determine by using a thread pitch gauge like this. Once you have all this information you are ready to inspect your thread. Here are the most popular methods.

 

Go/No Go Gauges

Thread ring gage

Go Thread Ring Gage – 1/4″ –28- NF – Class 2A

Go/No Go ring gauges are the most commonly used tool for thread inspection. This method, however, provides limited results as thread gauges “only” say if the part is acceptable or not acceptable, but doesn’t provide a specific measurement. As the name suggests, ring gauges come in couples (the Go gage and the No Go gage) and use of them is very simple: if the thread is within the specifications, the Go gage will fit the part and the No Go gage won’t fit.

 

Advantages

  • The easiest way to inspect an external thread.
  • Doesn’t require any additional training or practice.

 

Disadvantages

  • This method only reveals if the thread is ”good” or ”bad”, but doesn’t provide any specific measurement.
  • Ring gauges need to be recalibrated from time to time, and the process is difficult and expensive.
  • Ring gauges are not universal. They can only be used for the specific thread size stated on the gauge.

 

Thread Wires

Thread wire set

Fowler Thread Measuring Wire Set

 

Thread wires are the most used method by those that want to obtain a more accurate result. These tools measure the pitch diameter on external threads.

Thread wires are definitely cheaper than ring gauges, but they come with a catch: they require the inspector to have a good level of dexterity as they are not easy to position and hold in place.

Wire sets come with charts that look like this:

 

thread chart

Image credit: machineshoptalk

 

These charts are used to determine the diameter of the wires you’ll need to use to measure the thread and the constant that you’ll need to subtract from the initial measurement to calculate the pitch diameter. To measure the thread using this method you’ll need 3 wires and a micrometer. Here are the steps to follow:

1. Place and hold two wires in two adjacent pitches on the top of the part

2. Place and hold the third wire in the pitch located across the part from where the two top wires are positioned

3. Use the micrometer to measure the distance between the top wires and the bottom wire

4. Subtract the constant that refers to the wires from the measurement to obtain the pitch diameter size.

 

This video will make it easier to understand the process.

 

Advantages

  • This is a very accurate method (assuming that the flank angle is correct).
  • It can be applied to all external threads.

 

Disadvantages

  • This method requires a calculation to find the correct measurement result.
  • It requires practice.
  • It only measures thread pitch diameter.

 

 

Thread Micrometers

Thread micrometer

Fowler Digit Micrometer with 60-Degree Point, 0-1″ Measuring Range

Thread micrometers are used to measure the pitch diameter of a thread. They are easy to use but they are not cheap. Unlike regular micrometers, thread micrometers have V-shaped anvils designed to fit into a 60° thread angle.

 

Advantages

  • Takes an accurate measurement (assuming correct flank angle).
  • It can be used on all threads with the same flank angle.

 

Disadvantages

  • More expensive than other methods.
  • Only measures thread pitch diameter.
  • All deviations from the correct flank angle will affect measurement accuracy.

 

Thread Triangles

thread triangle

Fowler Thread Triangle Gages

Thread triangles are little, hardened prisms designed to fit into a 60° thread angle. They are used in a similar way to the three-wire method, but the triangular shape and the rubber harnesses make them easier to place and hold while measuring the distance between them with a micrometer. Just like three wires sets, thread triangles come with a chart indicating the value that needs to be subtracted from the reading on the micrometer to obtain the measurement of the pitch diameter.

 

Advantages

  • It’s a very accurate method (assuming that the flank angle is correct)
  • It can be applied to all external threads

 

Disadvantages

  • It requires a calculation to find the correct measurement result
  • It requires practice
  • Only measures thread pitch diameter

 

What’s the best method?

Although there’s no simple and straightforward answer to this question, the three-wire method has been generally considered to be the most satisfactory and universal method when properly executed.

 

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