: Fasteners with a thread undercut do not meet the minimum breaking load requirements specified in DIN EN ISO 898-1 Accessing the PDF and Papers
This standard applies to metric ISO threads (DIN 13) and defines the undercut relief at the end of a threaded section to allow the nut to be screwed on tightly against a shoulder.
The actual dimensions vary based on the pitch of the thread. According to technical summaries often found in a DIN 76-1 PDF , standard values include: Metric Pitch ( Undercut Width (Normal/Form A) Undercut Diameter Where to Find the Official PDF din 76a pdf
DIN 76-1:2004-06 THREAD RUN-OUTS AND THREAD UNDERCUTS - PART 1: FOR I. DIN 76-1 - 2016-08
The standard is typically divided into sections based on the type of thread: : Fasteners with a thread undercut do not
Applies to bolts, screws, and similar components with external or internal ISO metric threads (coarse or fine pitch) as per DIN 13-1. Undercut Types: Type A (Normal Design): Used when no other specifications are provided on drawings. Type B (Short Design):
DIN standards are protected by copyright. You generally cannot legally download a free, complete, unwatermarked PDF of the current DIN 76a from public websites. However, here are your legitimate options: DIN 76-1 - 2016-08 The standard is typically
DIN 76‑1 has a long history, reflecting decades of industrial experience. The standard has been regularly updated to incorporate changes in manufacturing practice, align with new international norms, and improve clarity.
The standard categorizes undercuts into several forms based on the thread type and the available space on the component: BAER Tools : The standard "long" design for external threads
: Using these specifications prevents interference issues during tightening. If a bolt lacks a proper undercut, it may bottom out against its own thread run-out before the head makes full contact with the surface.
: A shorter alternative used when space or structural limitations demand a compact thread relief area. 2. Structure of the DIN 76 Standard