Basic Thread Terms

April 19, 2024 11:00 am

Threads are an essential part of manufacturing, and their use can be found in almost every industry. They play a vital role in joining two parts together. However, understanding the terminology used in threads can be confusing for those unfamiliar with the field. In this article, we will discuss some of the essential terms related to screw threads.

Pitch Diameter

The pitch diameter (also referred to as the effective diameter) is the diameter of an imaginary cylinder, which intersects it so that the distance between two intersection points at opposite flanks of a thread flanks equals half the nominal pitch of the thread (This definition is valid only for n parallel threads).

The allowance (see below) between the pitch diameters of the female and male parts of a threading assembly determines whether they can be successfully mated together.

Major Diameter

The major diameter of a thread is the diameter of an imaginary cylinder that touches the largest diameter of a thread. The nominal value of the major diameter equals the nominal thread size. For example, in a UNC 1/4-20 thread, the nominal major diameter is 1/4″ (6.35 mm)

External Thread

  • The major diameter is measured on the crest of the thread.
  • The major diameter is a critical dimension to control. Together with the pitch diameter, it forms the addendum of the thread and controls if the screw would fit on a bolt. The permissible range for the major diameter is derived from its class.

Internal Thread:

  • The major diameter is measured on the root of the thread.
  • The major diameter is not a critical dimension. It only has a requirement to be equal to or larger than the nominal thread diameter.

Minor Diameter:

The minor diameter is the diameter of an imaginary cylinder that touches the smallest diameter of a thread. It depends on the pitch and the thread’s angle.

External Thread:

  • The minor diameter is measured on the root of the thread.
  • The minor diameter is not a critical dimension. It only has a maximum requirement to ensure that the root is relieved.

Internal Thread:

  • The minor diameter is measured on the crest of the thread.
  • The minor diameter is a critical dimension. Together with the pitch diameter, it controls if the bolt would fit on a screw.

TIP: You can get the value of all these terms for any thread size with this Online Thread Calculator

Pitch & Lead

Pitch is the axial distance between adjacent repeating features on a mechanical element. (e.g., screw thread, knur, saw blade)

The lead is the linear distance the thread travels due to a full 360° rotation.

In a single start thread, the lead equals the pitch. In multi-start threads, the lead equals the pitch multiplied by the number of starts. This means that in a multi-start thread, the lead will always be larger than the pitch.

 

Lead Angle

The lead angle measures the inclination of a screw helix from a plane that is perpendicular to the screw’s axis. In more simple words, it is the proportion between the screw’s lead and its diameter.

As seen in this chart, a screw with a fine pitch and a large diameter will have a small lead angle, whereas a screw with a coarse pitch and a small diameter will have a large lead angle.

Length of engagement

Length of Engagement is the axial distance of full threads in contact between the male and female parts of a threading assembly. It is usually measured in multiples of the pitch. For example, 5P means a length of engagement of five times the pitch.

Allowance

Thread allowance is the distance in maximum material condition between the nominal thread diameter and the crest of an external thread.

The thread class defines it, and it indicates how tight or loose the thread is when assembled.

A screw assembly with a small allowance provides better accuracy but is harder to assemble. A larger one is easier to assemble but provides lower accuracy.

The allowance size does not influence a thread’s production difficulty or price.

Tolerance

The width of the tolerance field of a diameter on the actual thread profile. (Pitch, Major & Minor diameters).

A wide tolerance is easier and cheaper to produce. However, it results in a larger spread between threads. On the other hand, a narrow tolerance is more challenging and expensive to produce, but it yields a smaller spread between threads.

Class

To manufacture or measure a thread, one has to know the maximum and minimum permissible values of the basic dimensions. These values are calculated according to the thread class. The class is a number-letter code added after the thread’s description. For example 1A or 2C in inch threads and H7 or g6 in metric threads. The code indicates which allowance and tolerance (see above) should be used to calculate the maximum and minimum allowed diameters.

 

(*) All images provided courtesy of MachiningDoctor.com

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