Understanding Dimensional and Geometric Tolerances in Manufacturing
Dimensional Tolerances
Dimensional tolerances define the acceptable variation in the dimensions of a manufactured part. They are crucial for ensuring that parts fit and function correctly together.
Key Definitions:
- Shaft: Any cylindrical or prismatic part designed to fit inside another element.
- Hole: Any prismatic or cylindrical housing into which a shaft is inserted.
- Dimension or Measure: A numerical value expressing length or angle.
- Nominal Dimension: The theoretical value of a dimension, used as a reference for defining size limits.
- Tolerance: The allowable margin of error in manufacturing for the nominal dimension and geometric shape of a piece, indicated numerically or symbolically. Calculated as T = DM – Dm (where DM is the maximum dimension and Dm is the minimum dimension).
- Tolerance Zone: The area within which the actual dimension of a part must fall. The amplitude of this zone is the value of the tolerance.
- Effective Dimension: The actual dimension of the part after manufacturing.
- Limit Dimensions: The extreme values that determine the maximum and minimum dimensions of a piece.
- Maximum Dimension: The largest acceptable dimension in manufacturing.
- Minimum Dimension: The smallest acceptable dimension in manufacturing.
- Good Measure: A dimension obtained during machining that falls within the tolerance zone.
- Deviation or Difference: The difference between a specific dimension and the nominal dimension.
- Upper Deviation: The algebraic difference between the maximum dimension and the nominal dimension.
- Lower Deviation: The algebraic difference between the minimum dimension and the nominal dimension.
- Reference Line or Zero Line: The line that corresponds to the nominal dimension, serving as a reference for all other dimensions.
- Fit: The relationship between two mating parts, a shaft and a hole.
- Maximum Clearance: The difference between the maximum dimension of the hole and the minimum dimension of the shaft. (JM = DM – dm)
- Minimum Clearance: The difference between the minimum dimension of the hole and the maximum dimension of the shaft. (Jm = Dm – Dm)
- Maximum Interference: The difference between the maximum dimension of the shaft and the minimum dimension of the hole. (AM = dM – Dm)
- Minimum Interference: The difference between the minimum dimension of the shaft and the maximum dimension of the hole.
- Tolerance for Adjustment: The sum of the individual tolerances of the shaft (t) and the hole (T).
- Types of Fits:
- Mobile: A fit where there is always clearance.
- Fixed: A fit where there is always interference.
- Interference: A fit that can be either mobile or fixed.
Geometric Tolerances
Geometric tolerances specify the allowable variations in the form and position of a part’s features. They are essential for ensuring the proper function and assembly of complex parts.
Form Tolerances
Form tolerances define the permissible deviations of a single feature from its ideal geometric shape.
- Straightness: The property of a line being straight.
- Flatness: The property of a surface being flat.
- Roundness: The quality of a circle, a perfect circular approach.
- Cylindricity: The approximation of the lateral surface of a cylindrical piece to an ideal straight cylinder.
- Profile Accuracy: The degree of approximation of a profile to a desired theoretical shape.
- Surface Accuracy: Refers to the quality and form of any surface.
Position Tolerances
Position tolerances define the permissible deviations in the relative position of two or more features.
- Parallelism: The quality of being parallel between a line or a plane and a reference.
- Squareness: The quality of being perpendicular between a line or a plane and a reference, which can be a plane or a line.
- Tilt: The approximation of a plane or a line to a desired angle with respect to another plane or line.
- Position: The degree of accuracy in the position of a point in a plane or space.
- Runout: The degree of approximation of the position of the center of a circle or a cylinder with respect to the desired position.
- Coaxiality: The degree of coincidence of the axes of two right cylinders, one of which is taken as a reference.
- Symmetry: Sets the offset about an axis or plane.