Official Answer
Answer
The surface finish, particularly on the inner and outer edges of the spiral ring, acts as a series of micro-notches. A rough finish (Ra > 1.6 \mu m) increases the stress concentration factor $K_t$, accelerating crack initiation under cyclic axial loads. Similarly, if the groove has tool marks from a dull lathe bit, these become initiation sites for 'groove cracking'. In high-cycle applications (e.g., $10^7$ cycles), rings are often vibratory tumbled to achieve a smooth, deburred finish (Ra 0.4-0.8 \mu m), which significantly shifts the S-N curve upwards, following the relationship $\sigma_a = \sigma_f'(2N_f)^b$.
