What is the formula to calculate allowance?

• A. MMC external feature + LMC internal feature
• B. MMC internal feature - MMC external feature
• C. MMC external feature - LMC internal feature
• D. LMC external feature - MMC internal feature

Answer: (B)

The formula for calculating allowance is based on the principle of maximum material condition (MMC) and the relationship between internal and external features. In this context, allowance refers to the intentional difference between the maximum material limits of mating parts, which is significant for ensuring proper fit and function.

The correct approach is to subtract the maximum material condition of the external feature from that of the internal feature. This is because the internal feature at MMC represents the state where it is at its largest permissible size, while the external feature at MMC is the smallest permissible size. By performing this subtraction, one can determine the allowance, which indicates how much clearance or interference is provided when the parts are assembled.

This calculation is essential in design and manufacturing to ensure that parts fit together correctly, avoiding issues such as excessive play or tightness in the assembly process. Understanding how to apply the MMC concept to both internal and external features allows engineers and designers to create more reliable and functional products.