Manufacturing is never perfect. We can never manufacture a perfect form (e.g., a block with 5 × 3 × 4 inch dimensions). Variability is an inherent characteristic of manufacturing. Sources of variability are abundant and include the skills of the machine operator (machinist), the accuracy and age of the machine, the ambient conditions, and the condition and age of the cutting tool. Producing parts with perfect dimensions would be impossible and prohibitively expensive. Thus, we attempt to control manufacturing variability rather than entirely eliminate it.
In addition to controlling physical attributes of manufacturing such as training operators well and maintaining the machines and cutting tools, we ...