Used for internal threads (blind tapped holes) to allow the tap to clear the bottom of the hole.
: Form A features a rounded transition to minimize the notch effect , which prevents stress fractures in high-load components. Dimensions :
However, as noted above, DIN 76‑1 extends these international documents by:
Here is the summarizing the key content of DIN 76-1 as found in the standard (not the PDF file itself, but the data you would extract from it): din 76a pdf
: For external threads, the undercut diameter is smaller than the thread's minor diameter to ensure clear tool exit and flush assembly.
What (e.g., M8 x 1.25) are you currently designing?
Reducing stress concentrations at the sharp corner of the shoulder. The "Form A" Designation Used for internal threads (blind tapped holes) to
Because the required measurements change depending on the diameter and pitch (e.g., M10 1.5 vs M20
: These define the transition zone where the thread profile fades into the shank. DIN 76 provides "normal" and "short" lengths to accommodate different manufacturing tolerances and assembly requirements .
: Used in pumps and pressure vessels to ensure safety under high-pressure conditions. How to Find a DIN 76-A PDF What (e
Proper sealing or high-fatigue applications require specific surface roughness values inside the undercut groove, which are detailed in the official text. How to Call Out DIN 76-A on Engineering Drawings
The "DIN 76a PDF" you are searching for is the official document for DIN 76-1. However, it is important to be aware of copyright laws and sourcing practices.
Without a proper undercut according to DIN 76, a nut might not be able to sit flush against the bearing surface of a bolt. The undercut provides a "clearance" zone where the cutting tool exits the workpiece, ensuring:
| Dimension | Symbol | Value/Calculation | Description | | :--- | :--- | :--- | :--- | | | P | - | The fundamental parameter; all other dimensions are derived from P . | | Undercut Width | g2 | 3.5 x P (standard) 2.5 x P (short) 5 x P (long) | The width of the groove cut next to the shoulder. The standard value allows the use of standard tools. | | Run-Out Length | x | 3.5 x P (normal) 6.5 x P (long) | The length over which the thread gradually fades out. | | Undercut Radius | r | 0.5 x P (approx) | The radius at the bottom of the undercut, which helps reduce stress concentration. |
If you are looking for sheet metal bending radii, the standard is often confused with "76".