: Maximum width of the undercut including the transition angle.
If your drawing is specifically detailing metric fine threads or requires specific tolerances, you may include the thread size alongside it:
When a drawing specifies a "DIN 76-A" feature, it instructs the machinist to create a specific groove (undercut) at the end of an external thread where it meets a solid shoulder. This ensures that the mating internal thread can be screwed completely up to the shoulder face without binding on the incomplete threads. Thread Run-out vs. Thread Undercut din 76-a pdf
This document applies to both external threads (e.g., on bolts and threaded studs) and internal threads (e.g., in blind tapped holes) that use ISO metric coarse or fine pitch threads [9†L3-L6] [10†L7-L8].
The standard is split into parts, and the variable "$a$" is a critical dimension within them. : Maximum width of the undercut including the
For official documentation, you can purchase the full standard from DIN Media or Accuris Standards Store . Din 76-1 | PDF | Screw | Metalworking - Scribd
| Thread pitch P [mm] | Groove width, g1 [mm] | Groove width, g2 max | Groove diameter, d3 max [mm] | | :--- | :--- | :--- | :--- | | 0.2 | 0.6 | Form A: 3.5 × P = 0.7 | 0.05 | | 0.25 | 0.6 | Form A: 0.9 | 0.1 | | 0.3 | 0.8 | Form A: 1.1 | 0.1 | | 0.35 | 0.8 | Form A: 1.2 | 0.2 | | 0.4 | 1.0 | Form A: 1.4 | 0.2 | | 0.45 | 1.0 | Form A: 1.6 | 0.2 | | 0.5 | 1.0 | Form A: 1.8, Form B: 2.5 × P = 1.3 | 0.2 | | 0.6 | 1.2 | Form A: 2.1, Form B: 1.5 | 0.3 | | 0.7 | 1.2 | Form A: 2.5, Form B: 1.8 | 0.4 | | 0.75 | 1.2 | Form A: 2.6, Form B: 1.9 | 0.4 | | 0.8 | 1.6 | Form A: 2.8, Form B: 2.0 | 0.4 | | 1.0 | 2.0 | Form A: 3.5, Form B: 2.5 | 0.6 | Thread Run-out vs
: Using DIN 76-A ensures that manufacturers and engineers globally use the same dimensions for consistent assembly fits. Accessing the Full Standard
The search for a is ultimately a search for clarity in thread design. DIN 76‑A (Form A of DIN 76‑1) provides a standardized, proven set of dimensions for external thread undercuts that balance tool clearance, stress distribution, and assembly reliability.
For most professionals, the small purchase price is a worthwhile investment in and having the latest, correct version .
To ensure you are using compliant data for aerospace, automotive, or industrial manufacturing, consider these avenues: