The discussion focuses on calculating the axle diameter using bending moments and tensile strength of materials. The user is attempting to determine the plastic modulus (Sx) and is seeking guidance on accessing steel tables for selecting the appropriate axle diameter. Participants emphasize the importance of considering both bending and torsional stresses in the analysis, especially for rotating shafts, and suggest consulting machine design textbooks for comprehensive guidance. The conversation also touches on practical considerations, such as using standard stock sizes for cost-effectiveness and ensuring compatibility with bearings. Overall, the thread highlights the complexities of shaft design and the need for thorough calculations to ensure safety and functionality.