Here is a helpful, practical article summarizing the key points of (the current standard: DIN 76-1:2016-11 – "Thread run-outs and thread undercuts for metric ISO threads according to DIN 13").
: It gives the threading tool a clear exit path, protecting the machinery and ensuring clean thread profiles. Technical Specifications and Dimensions
DIN 76 defines four distinct forms (A, B, C, D) categorized by their geometry and application: Thread Relief Explained: Function, DIN 76 & Practical Use din 76-a pdf
: Undercut diameter. This is always smaller than the minor diameter of the thread. : Minimum width of the undercut.
Because DIN standards are copyrighted technical documents, from public domains. You must purchase them. Here is a helpful, practical article summarizing the
: By providing a smooth radius in the undercut, it helps distribute mechanical stress, reducing the risk of fatigue failure at the thread-to-shank transition. Standardization
This article serves as a comprehensive resource. We will explain what DIN 76-A is, why it is critical for reducing stress concentrations, how to interpret its tables, and—most importantly—how to access a legitimate DIN 76-A PDF for your workshop or design office. This is always smaller than the minor diameter of the thread
When a threaded section transitions abruptly to a plain shaft (a sharp corner), it creates a tremendous stress concentration. Under cyclic loads or vibration, this sharp corner becomes a failure point, leading to cracks and eventual fracture. The undercut defined in DIN 76-A rounds this corner, distributing stress evenly.
The complete technical tables, which list exact millimeter dimensions for every thread size (M3, M10, etc.), are found in the official DIN 76-1 PDF available through DIN's official website or authorized resellers like specific dimensions