Definition
A plus/minus tolerance is the allowed deviation above and below a nominal dimension, or on only one side (unilateral).
What it means for the calculation
The tolerance directly decides which process is achievable and how it gets inspected. A tight tolerance of a few microns often needs grinding, honing or a CMM measurement instead of milling or turning alone, which raises cost and cycle time.
Where it appears on the drawing
Directly after a dimension, for example 20 ±0.05, or as two separate values for a unilateral tolerance (20 +0.10/-0).
Common mistakes
Assuming a symmetric tolerance when the drawing gives unilateral values. Also overlooking that a tighter tolerance on one dimension tightens the whole tolerance stack-up of related dimensions.
The tolerance directly decides which process is achievable and how it gets inspected. A tight tolerance of a few microns often needs grinding, honing or a CMM measurement instead of milling or turning alone, which raises cost and cycle time.
See how Blake reviews a partFrequently asked questions
What is the difference between bilateral and unilateral?
Bilateral allows deviation on both sides of the nominal dimension; unilateral only on one side.
Does ± always mean the same process?
No, the achievable tolerance depends on the process: rough milling, grinding or lapping each set a different limit.
What applies without an explicit ± value?
A general tolerance table in the title block or a referenced standard, such as ISO 2768, applies by default.