The CAD model is a perfect theoretical representation of a product. When parts are manufactured to the same specification, each one is slightly different from all the others. Tolerance design ensures that the product meets the engineering requirements despite this part variation.

To determine how much tolerance can be permitted, you must consider the accumulation or stackup of variation in individual dimensions. Dimensional variation combines to produce variation in a critical distance in the final product, usually between two different parts in the assembly. For each critical distance, you must determine an acceptable range of values within which the assembly functions as per the requirement.

Tolerance stackup analysis helps you to understand the relationship between dimensional variation and functional requirements.