In limit, fit, and tolerance terminology for manufacturing metrology, what is the quantity called that equals the algebraic difference between the minimum limit (the lower permissible size) and the basic (nominal) size of a feature?

Introduction / Context:

In engineering metrology and the ISO system of limits and fits, the terms basic size, limits of size, and deviations precisely describe how far an actual or permissible size can vary from its intended nominal (basic) value. Correctly identifying which deviation is measured from the minimum or maximum limit is essential for interpreting drawings and calculating fits between mating parts like holes and shafts.

Given Data / Assumptions:

• Basic (nominal) size is the target dimension used to derive limits and tolerances.
• Lower limit is the smallest permissible size; upper limit is the largest permissible size.
• Deviations are algebraic differences taken with signs relative to the basic size.

Concept / Approach:

The lower deviation is defined as the algebraic difference between the lower limit of size and the basic size. Similarly, the upper deviation is the algebraic difference between the upper limit and the basic size. The term fundamental deviation denotes the deviation (upper or lower, depending on the letter position such as H or h) that defines the position of a tolerance zone relative to zero line in the ISO system.

Step-by-Step Solution:

Verification / Alternative check:

On a drawing, if a 25 H7 hole is specified, the basic size is 25 mm. The lower deviation for ‘‘H’’ holes is 0, so lower limit − basic size = 0 → lower deviation = 0, matching the definition. For shafts (e.g., h), the upper deviation is 0, and the lower deviation is negative.

Why Other Options Are Wrong:

• Upper deviation: uses the upper limit, not the lower limit.
• Fundamental deviation: a positional deviation defining the tolerance zone location; not specifically the lower-limit difference here.
• Actual deviation: difference of an actual measured size from the basic size, not a limit-based quantity.
• Limit of size: the permissible extreme sizes themselves, not their difference from basic size.

Common Pitfalls:

• Forgetting the sign convention (algebraic differences may be negative).
• Confusing actual measurement deviations with limit-based deviations used in specifications.

Final Answer:

lower deviation