Official Answer
Answer
The edge margin $Y$ is the distance from the groove to the end of the shaft or bore. It must be sufficient to prevent the material from shearing off under axial load. The formula is $Y = \frac{3P}{\pi \cdot D \cdot \sigma_y}$. If $Y$ is too small, the material behind the groove will experience shear failure or 'blowout'. In aerospace applications where weight is critical, $Y$ is optimized but generally kept to at least $3 \times$ the groove depth $d$ to maintain a safety factor of 2.0 or higher against the yield strength of the host material.
