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Questions and Answers
What is the purpose of using a sine bar or angle plate in measuring principle?
What is the relationship between form and orientation tolerances when applied to a single feature?
What is the purpose of a parallelism tolerance for a flat surface?
What is the role of a datum plane or axis in perpendicularity tolerance?
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What is the purpose of applying orientation tolerancing for features of size?
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What is the relationship between perpendicularity tolerance and parallelism tolerance?
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What is the purpose of controlling flatness on both sides of a feature?
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What is the role of the feature control frame in orientation tolerancing?
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What is the purpose of applying a parallelism tolerance to one surface of a feature?
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What is the relationship between orientation tolerance and form tolerance?
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Study Notes
Orientation Tolerances
- Orientation tolerances control angularity, parallelism, and perpendicularity.
- When applied to plane surfaces, orientation tolerances control flatness if a flatness tolerance is not specified.
- The general geometric characteristic for orientation is termed angularity, which describes angular relationships between lines, surfaces, or features.
Angularity Tolerance
- Angularity is the condition of a surface or axis at a specified angle (other than 90o) from a datum plane or axis.
- An angularity tolerance for a flat surface specifies a tolerance zone defined by two parallel planes at a specified basic angle from a datum plane or axis.
- The surface of the considered feature must lie within this tolerance zone.
Perpendicularity Tolerance
- Perpendicularity is the condition of a surface at 90o to a datum plane or axis.
- A perpendicularity tolerance for a flat surface specifies a tolerance zone defined by two parallel planes perpendicular to a datum plane or axis.
- The surface of the considered feature must lie within this tolerance zone.
Parallelism Tolerance
- Parallelism is the condition of a surface equidistant at all points from a datum plane.
- A parallelism tolerance for a flat surface specifies a tolerance zone defined by two planes or lines parallel to a datum plane or axis.
- The line elements of the surface must lie within this tolerance zone.
Measuring Principle
- In theory, it is necessary to determine that every point on the centreline of the hole falls within the tolerance zone.
- Measuring systems using a fitted gaging mandrel are usually considered acceptable.
- The gaging mandrel must be straight and true, made of tool steel, hardened and ground, and its diameter must fit snugly in the hole.
Applying Orientation Tolerances to Flat Surfaces
- Orientation tolerances, when applied to plane surfaces, control angularity, parallelism, and perpendicularity.
- The feature control frame is associated with the size dimension of the feature requiring control.
Reference to a Datum
- An orientation tolerance indicates a relationship between two or more features.
- Whenever possible, the feature to which the controlled feature is related should be designated as a datum.
Measuring Principle for Flat Surfaces
- The part to be measured is set up on a surface plate in such a way that the surface of the part, were it perfectly formed, would be parallel with the surface plate or with an auxiliary square or angle plate.
- When necessary, a sine bar or angle plate is used.
Applying Form and Orientation Tolerances to a Single Feature
- When both form and orientation tolerances are applied to a feature, the form tolerance must be less than the orientation tolerance.
- An example is shown in the given figure where the flatness of the surface must be controlled to a greater degree than its orientation.
Orientation Tolerance Controlling Form
- Under certain circumstances, such as for very thin parts, it is often desirable to control flatness on both sides.
- This is accomplished by applying a parallelism tolerance to one surface of the feature and making the opposite side the datum feature.
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Description
This quiz covers the concepts of orientation tolerances, including angularity and perpendicularity, in the context of geometric dimensioning and tolerancing (GD&T).
