Parts will deviate from perfect form and be rejected. Tolerances are added to reduce cost. As feature tolerances depart from MMC towards LMC, LMVC limits the range beyond which failure may occur. When the difference between MMC and LMC is the maximum tolerance allowed by functions of fit and performance, cost is optimized. A drawing that optimizes fit performance and cost is the legitimate license for quality control personnel to do their job.
Cost decreases exponentially as tolerances increase.
The goal for optimizing cost is to specify design tolerance that is greater than Six Sigma process capability. Cost savings become insignificant beyond the Six Sigma critical point. Therefore, cost is optimized based on comparing the process tolerance against the design tolerance. Optimizing means that the greatest difference exists between assuring interchangeable fit at MMC and meeting performance at Least Acceptable Material Condition. Thomas Berilla has more than 40 years of experience as an engineer for private industry and government agencies.