A precision part is not simply a part with the smallest number on every dimension. Good tolerance design concentrates control where fit, motion, sealing or product performance actually depends on it.
Concentrate precision at the interface
Identify the surfaces and dimensions that control fit or function. Over-tolerancing unrelated features can increase cost and lead time without improving the assembled product.
Consider material and process
Material grade, heat treatment, part geometry and surface finish can change the practical process route. Thin sections and interrupted geometry may also move during heat treatment or stress relief.
Agree on measurement before machining
Datum references, inspection method and environmental conditions should be clear before production begins. A measurement result is only meaningful when supplier and customer evaluate the same requirement in the same way.