Terminology for Fitting Tolerances (Excerpt from JIS B 0401-1:2016)
2026/05/07
[Scope of Application]
This standard specifies ISO codes for dimensional tolerances applicable to dimensions related to the length of geometric forms (cylinders and two parallel planes lying opposite each other).
| Terminology | Definition | Corresponding English Terms (for reference) |
|---|---|---|
| 1) Basic Terms | ||
| Size and Shape | A geometric shape defined by dimensions related to length or angle. | feature of size |
| Represented Envelope | A theoretically accurate outer shape defined by a drawing or other related documentation. | nominal integral feature |
| Hole | The internal dimensional form of a workpiece, including non-cylindrical internal dimensional forms (e.g., the inner diameter surface of a hole). | hole |
| Reference hole | A hole selected as a reference in the hole-based fit method. | basic hole |
| Shaft | The external geometric form of a workpiece, including non-cylindrical external dimensions (e.g., the outer diameter surface of a shaft). | shaft |
| Reference shaft | Shaft reference A shaft selected as a reference in the shaft reference fit method. | basic shaft |
| 2) Terms related to dimensional tolerances and dimensional differences | ||
| Dimensional size | The size of a feature in its perfect form as defined by the drawing. | nominal size |
| Fitting size | The size of the fitting shell. | actual size |
| Tolerance limit | The maximum size permitted for the shape. | limits of size |
| Upper limit of size, ULS | The maximum size that is acceptable for a given size profile. | Upper Limit of Size |
| Lower Limit of Size (LLS) | The minimum size permitted for a size profile. | lower limit of size |
| Size difference | The value obtained by subtracting the reference value from a given value. | deviation |
| tolerance | Tolerance above or below the nominal size. | limit deviation |
| Upper tolerance ES (used for internal dimensions) es (used for external dimensions) | The difference between the upper tolerance size and the nominal size. | upper limit deviation |
| Lower tolerance EI (used for internal dimensions) ei (used for external dimensions) | The difference between the lower tolerance size and the indicated size. | lower limit deviation |
| Basic tolerance | A tolerance that defines the position of the size tolerance zone in relation to the nominal size. | fundamental deviation |
| Δ value | A variation value added to a fixed value to obtain the fundamental tolerance for an internal dimensional form. | Δ value |
| Size tolerance | The difference between the upper and lower tolerance limits. | tolerance |
| Size tolerance limit | A specified characteristic value that defines the upper and/or lower limits of the tolerance. | tolerance limits |
| Basic size tolerance, IT | All tolerances belonging to the ISO code system for dimensional tolerances. The letters IT stand for “International Tolerance.” | standard tolerance |
| Basic size tolerance class | A set of size tolerances characterized by a common identification symbol. | standard tolerance grade |
| Size tolerance range | The range of variation in size within the tolerance limits. | tolerance interval |
| tolerance class | A combination of the underlying tolerance and the basic size tolerance grade. | tolerance class |
| 3) Terms related to fits | ||
| Clearance | The difference between the size of the hole and the size of the shaft when the shaft diameter is smaller than the hole diameter. | clearance |
| Minimum clearance | (In a clearance fit) The difference between the lower tolerance of the hole and the upper tolerance of the shaft. | minimum clearance) |
| Maximum clearance | (In a tight or intermediate fit) The difference between the upper tolerance of the hole and the lower tolerance of the shaft. | maximum clearance |
| Clearance | The value obtained by subtracting the shaft diameter from the hole diameter when the shaft is larger than the hole, prior to fitting. | interference |
| Minimum interference | The difference between the upper tolerance of the hole and the lower tolerance of the shaft (in a tight fit). | minimum interference |
| Maximum interference | (In a tight or intermediate fit) The difference between the lower tolerance of the bore and the upper tolerance of the shaft. | maximum interference |
| Fit | The relationship in which an outer-dimension profile and an inner-dimension profile (a hole and a shaft of the same shape) fit together. | fit |
| clearance fit | A fit in which a gap always exists between the hole and the shaft when they are assembled. That is, when the lower tolerance of the hole is equal to or greater than the upper tolerance of the shaft. | clearance fit |
| Interference fit | A fit in which there is always a gap between the hole and the shaft when assembled. That is, when the upper tolerance of the hole is equal to or less than the lower tolerance of the shaft. | interference fit |
| Intermediate fit | A fit in which either a gap or a clearance exists between the hole and the shaft when assembled. | transition fit |
| Fit width | The arithmetic sum of the dimensional tolerances of the two dimensions that constitute the fit. | span of a fit |
| 4) Terms related to the ISO fit system | ||
| ISO fit system | A fit system consisting of a hole and a shaft, dimensioned according to the ISO code system for dimensional tolerances. | ISO fit system |
| Hole-based fit system | A fit in which the basic tolerance of the hole is zero. That is, the lower tolerance is zero. | Hole-basis fit system |
| Shaft-basis fit system | A fit in which the basic tolerance of the shaft is zero. That is, the upper tolerance is zero. | shaft-basis fit system |
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