Tolerance
Quick Definition
Tolerance is the permissible range of variation in a dimension or characteristic, defining the acceptable upper and lower limits within which a manufactured part is considered correct.
Tolerance explained
No manufacturing process is perfect — every machined, cast, stamped, or moulded feature has some variability. Tolerance defines the acceptable band within which that variability is allowed without affecting function or interchangeability. A shaft specified as 50h6 has a nominal diameter of 50 mm, with tolerance grade h6 specifying upper deviation = 0 and lower deviation = −16 µm, giving a tolerance band of 16 µm (0.016 mm).
ISO 286 establishes the international system for limits and fits. Tolerance grades (IT1 through IT18) define the tolerance magnitude — IT1 is the tightest (precision gauges), IT18 is the loosest (rough casting). Fundamental deviation (specified by a letter: A–ZC for holes, a–zc for shafts) positions the tolerance band relative to the nominal size. Together they define fits: clearance (shaft always smaller), interference (shaft always larger), or transition (may be either).
Tighter tolerances cost more — the machining process must be more controlled, measuring more frequent, and scrap rates tend to increase. Engineers must specify the tolerance that is just tight enough for the function, not the tightest achievable. A bearing seat requires IT6 or better; a painted external panel may accept IT12–IT14. Over-tolerancing adds cost without benefit.
Synonyms & Related Terms
Formula
Tolerance = Upper Limit − Lower Limit
Real-World Example
A 6205 deep groove ball bearing has a 25 mm bore, which fits on the shaft, and a 52 mm outside diameter, which seats in the housing. If the housing bore is specified as 52 H7 (upper deviation +30 µm, lower 0), its allowed range is 52.000–52.030 mm, a tolerance of 0.030 mm. The engineer inspects the machined housing with a bore gauge: a reading of 52.015 mm is accepted; 52.035 mm is rejected. The controlled fit locates the bearing outer ring without excessive stress.
Why It Matters
Tolerance is the language of interchangeable manufacture. Without standardised tolerances, every part would require individual fitting — the basis of artisan production before the industrial revolution. Modern mass production depends entirely on parts being made to tolerance so they assemble correctly with zero individual fitting. Incorrect tolerance specification is one of the most common engineering drawing errors, causing either assembly problems or unnecessary manufacturing cost.
Industry Applications
Related Standards
Frequently Asked Questions
What is the difference between tolerance and allowance?
Tolerance is the permitted variation within a single part dimension. Allowance is the intentional clearance or interference between two mating parts in a fit — it is the difference between the maximum shaft and the minimum hole (or vice versa). Allowance defines the tightest fit possible; tolerance defines the variability within each part.
What is a clearance fit, interference fit, and transition fit?
Clearance fit: the shaft is always smaller than the hole (always a gap). Used for rotating parts, sliding assemblies. Interference fit: the shaft is always larger than the hole (always compressed together). Used for permanent assembly, hub-to-shaft retention. Transition fit: shaft and hole overlap partially — assembly may be either clearance or interference depending on which end of tolerance each part is.
What does H7/h6 mean on an engineering drawing?
H7 specifies the hole: uppercase H means the lower deviation is zero (hole nominal = lower limit); 7 is IT grade 7. h6 specifies the shaft: lowercase h means upper deviation is zero (shaft nominal = upper limit); 6 is IT grade 6. H7/h6 is a "locating clearance" fit — the shaft fits the hole with minimal clearance for accurate positioning.
What is geometric dimensioning and tolerancing (GD&T)?
GD&T (ASME Y14.5) and its ISO equivalent (ISO 1101) extend dimensional tolerances to geometric controls: straightness, flatness, circularity, cylindricity, perpendicularity, angularity, position, runout, and profile. GD&T communicates functional requirements precisely, allowing maximum allowable variation consistent with correct function — reducing over-tolerancing and manufacturing cost.
Why is it more expensive to achieve tighter tolerances?
Tighter tolerances require: slower machining speeds (less material removed per pass), more tool changes (worn tools introduce error), more frequent measurement, temperature-controlled environments (thermal expansion matters at micrometre level), and more skilled operators. Grinding can achieve IT5–IT4; lapping can reach IT1–IT2 — but at significantly higher cost per part.
How do I choose the right tolerance for a specific application?
Match the tolerance to the function. Bearing seats: IT6–IT7. General machined fits: IT8–IT9. Keyway slots: IT9. Rough machined features: IT11–IT12. When in doubt, use the coarsest tolerance that still ensures correct function — this minimises cost without compromising reliability.
See Also
- Surface Roughness
- Bearing
- Shaft
- Keyway