A general tolerance of 0.1 mm is suitable for most turning jobs and helps keep costs under control. Tighter tolerances are worthwhile when parts need to fit together, such as bearing shoulders, seal faces, mating diameters, and threads that must engage with predefined components. Surface finish is distinct from dimensional size. A shaft can meet exact drawing dimensions across the board and still leak if the surface is too rough for a seal. It is therefore best to specify which surfaces are sealing or sliding faces, and which are purely cosmetic. Material behavior plays a role as well. Stainless steel work-hardens, brass machines smoothly, and thin-walled aluminum can deflect once released from the chuck. For long or slender parts, we plan workholding and cutting sequences to accommodate that movement rather than trying to correct it later. We also consider how the part will be measured. Tolerances that cannot be checked with simple calipers, micrometers, or gauges typically mean increased inspection time. It is better to agree on this before production begins than to debate it after delivery. Only call out truly critical dimensions and leave the rest to general tolerances. You will get parts that fit properly without paying for precision that the assembly does not require.