Quick Definition
Quick Answer: Thread Addendum is the radial distance from the pitch line or pitch diameter to the crest of a thread.
Thread Addendum is the radial distance from the pitch line or pitch diameter to the crest of a thread. It describes the portion of the thread profile extending outward from the pitch line.
What Is Thread Addendum?
In thread geometry, Addendum describes the portion of a thread profile located between the pitch line and the crest.
For an external thread, the addendum extends outward from the pitch diameter toward the Major Diameter.
For an internal thread, the corresponding geometry is interpreted within the internal thread profile according to the applicable thread form and standard.
Addendum is primarily a thread geometry term and is particularly useful when analyzing theoretical thread profiles and standard thread forms.
Why Is Thread Addendum Important?
Addendum helps define the relationship between the pitch diameter and the crest of a thread.
It is important for understanding:
- Thread profile geometry
- Major Diameter
- Pitch Diameter
- Thread Height
- Standard thread forms
Understanding Addendum is particularly useful when interpreting detailed thread diagrams, engineering drawings, and thread specification data.
Addendum vs Dedendum
Addendum and Dedendum describe two different portions of a thread profile.
| Term | Definition |
| Addendum | Radial distance from the pitch line to the crest |
| Dedendum | Radial distance from the pitch line to the root |
Together, they describe the radial proportions of the thread profile.
The relationship between these dimensions depends on the applicable thread form and standard.
Addendum vs Major Diameter
Addendum and Major Diameter are related, but they are not the same dimension.
For an external thread, the Major Diameter represents the diameter measured across the thread crests.
Addendum represents the radial distance from the pitch line to those crests.
Therefore:
Major Diameter is a diameter dimension, while Addendum is a radial dimension.
For a simplified symmetrical external thread:
Addendum ≈ (Major Diameter − Pitch Diameter) / 2
The actual relationship may vary depending on the thread form, truncation, and applicable standard.
Addendum vs Thread Height
These terms should not be treated as interchangeable.
Thread Height generally describes the overall radial height of a thread profile between the crest and root.
Addendum describes only the portion extending from the pitch line to the crest.
The remaining portion between the pitch line and root is generally associated with the Dedendum.
This distinction is important when interpreting detailed thread geometry.
Frequently Asked Questions
- What is Addendum in a screw thread?
Addendum is the radial distance from the pitch line to the thread crest.
- Is Addendum the same as Thread Height?
No. Addendum represents only the portion above the pitch line, while Thread Height generally describes the overall radial height between crest and root.
- Is Addendum related to Major Diameter?
Yes. For an external thread, Addendum describes the radial difference between the Pitch Diameter and the Major Diameter.
- What is the opposite of Addendum?
The corresponding portion below the pitch line is called Dedendum.
- Does every thread have the same Addendum?
No. Addendum depends on the thread form, pitch, dimensions, truncation, and applicable standard.
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