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.
Draftspersons and design engineers use the PDF to accurately model components in 3D CAD software (like SolidWorks, Autodesk Inventor, or Creo). Inputting the correct undercut parameters ensures that interference checks in digital assemblies mimic real-world behavior perfectly. 2. CNC Programming (G-Code generation) Machinists programming CNC lathes require the exact width ( ) and radius (
(often designated as DIN 76-1 Form A) is a German technical standard for thread undercuts on external metric threads . Form A is the "normal" or standard length version used on shafts to allow a mating part to be screwed flush against a shoulder. Key Specifications for DIN 76 Form A Application : Used for external threads (bolts/shafts).
When a thread is machined using traditional turning or rolling methods, the tool cannot stop instantly. As the tool lifts away from the workpiece, it leaves behind a section of incomplete or shallow threads. This zone is called the thread run-out. If a mating nut attempts to flush up against the shoulder, it will jam on these incomplete threads, leaving an unwanted gap. 2. Thread Undercut (Freistich) din 76-a pdf
(Note: Always verify with an official, current DIN 76 PDF copy for manufacturing tolerances, as fine pitches will alter these values). How to Call Out DIN 76-A on Technical Drawings
Below is a general reference table derived from the standard for standard metric coarse threads: Thread Pitch ( Undercut Diameter ( Undercut Width ( Fillet Radius ( (e.g., M3) 0.7 mm (e.g., M4) 0.8 mm (e.g., M5) 1.0 mm (e.g., M6) 1.25 mm (e.g., M8) 1.5 mm (e.g., M10) 1.75 mm (e.g., M12) 2.0 mm (e.g., M16)
The English translation is prepared by the DIN language service; however, the standard notes that “in case of doubt, the German‑language original shall be considered authoritative.” Without an undercut, a nut or a part
Remember: The depth of the undercut must be slightly deeper than the thread root diameter.
A version used when space is limited for technical reasons. Form C Internal Threads The normal case for holes and nuts. Form D Internal Threads A short version for internal threads with limited depth. Comparison of DIN 76 Sub-Standards
The key takeaway is that . ISO 3508:1976 covered only thread run‑outs, and ISO 4755:1983 covered only undercuts for external threads. DIN 76‑1 covers both run‑outs and undercuts for both external and internal threads , making it a one‑stop reference for metric thread‑relief design. Key Specifications for DIN 76 Form A Application
Standard thread undercut for external threads (male fasteners).
The standard is a cornerstone of precise mechanical design, ensuring that external metric threads can be manufactured efficiently and assembled without mechanical interference. By understanding how the thread pitch dictates the undercut width, diameter, and radius, engineers can create robust designs that mitigate stress concentrations and facilitate smooth manufacturing workflows.