(Undercut Diameter): The diameter of the recessed portion. It is always smaller than the minor diameter of the thread. (Undercut Width): The longitudinal width of the recess.
When a designer wants an external thread undercut according to DIN 76‑1, Form A, the drawing will typically include a note such as:
Using these specific dimensions (often roughly 3x the thread pitch in width) ensures compatibility across different manufacturers. BAER Tools Thread Relief Explained: Function, DIN 76 & Practical Use
Standard thread undercut for internal threads (female fasteners). din 76-a pdf
Without an undercut, a nut or a part with a flat bearing face cannot be screwed flush against a bolt head because of the incomplete "run-out" threads.
is the nominal outer diameter of the thread (e.g., for an M10 bolt with a 1.5mm pitch,
) of the thread. The standard specifies four critical parameters: (Undercut Diameter): The diameter of the recessed portion
Understanding DIN 76-1 Form A: The Definitive Guide to Thread Undercuts
Standard undercut for internal threads (female/tapped holes). Form D: Short undercut for internal threads.
, subtract the value in the table from the nominal major diameter of your thread ( ). For example, for an M10 coarse thread ( Technical Drawing Callouts When a designer wants an external thread undercut
is the German standard (Deutsches Institut für Normung) that defines the dimensions and shapes of thread runouts and undercuts. Specifically, DIN 76-A refers to the design for metric ISO threads (standard fastening threads).
You can often find these technical documents through professional engineering document suppliers or the official DIN Media website. 4. Practical Application in Engineering
A typical designation for a thread undercut according to this standard looks like this: