A product is made of many parts that have been made from different materials. Many electronic products use parts made from plastics, sheet metal, machined parts, rubber, castings, etc. Parts made from these processes have unique properties, and manufacturing dictates the tolerances that can be specified with these parts. Sheet metal parts require a much wider tolerance band compared to machined parts, Plastic molded parts, made from mature molds and processes, have low variation in dimensions from batch to batch. Machined parts will vary from batch to batch, and many operators tend to make parts I on the high side of the tolerance, so they can be able to reduce some dimensions in the future if the parts fail to pass inspection tests.
Knowledge of manufacturing processes, and proper use of six sigma design for mechanical parts will reduce the need for conservative design thereby decreasing the costs of the product as well as providing for high-quality parts.