Geometric Tolerancing - University of...

Distorted Objects by Suzy Lelievre

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Geometric Tolerancing

Baek-Ki-Kim-Twisted Stool

Mesh

Geometric Tolerancing What is it ?

Geometric Tolerancing • Unlike Dimensional Tolerance that concerns itself with size control,

Geometric Tolerancing concerns itself with SHAPE CONTROL • Geometric Tolerancing influences the manufacturing and inspection

process chosen

• Geometric Tolerancing is required in features in industries such as • Aerospace component manufacture • Automobile component manufacture • Precision machinery manufacture • Biomedical component manufacture

• Geometric Tolerancing is used where high interchange of parts is required

• Geometric Tolerancing and Dimensional Tolerancing are linked

Basic Concepts of Geometry Tolerancing

• Geometry Tolerances are specified according to functional requirements: Use it if it is needed, if not DO NOT as it adds to the expense of the manufacture, inspection of a component and clarity of an engineering drawing.

• Depending on the feature to be toleranced and the way in which it is

dimensioned, the tolerance zone will be one of: • The space between two parallel lines • The space within a circle • The space between two concentric circles • The space within a cylinder • The space between two co-axial cylinders • The space between two equidistant surfaces • The space within a sphere

Geometry Tolerancing Symbolic Structure

Interpretation of Geometry Tolerancing Flatness

• A Flatness tolerance is used to control the flatness of a surface. The tolerance zone is a specified space separating two parallel planes between which the surface to be controlled must lie.

Straightness • A straightness tolerance may be used to control:

• 1. Straightness of a line • 2. Straightness of an axis in a single plane • 3. Straightness of the axes of solids of revolution

Perpendicularity • A perpendicularity tolerance is used to control:

• 1. a line with respect to a datum line • 2. an axis with respect to a datum plane

• 3. a surface with respect to a datum axis • 4. a surface with respect to a datum plane

Position • A position tolerance is used to control the location of a feature by

limiting its deviation from a specified true position. • 1. a hole axis • 2. a surface

Position (Concentricity) • A concentricity tolerance is used to control a condition in which two or

more features such as circles, spheres cylinders, cones or hexagons are required to share a common centre or axis

Case Study – Qantas A380 Oil Stub Pipe Late in 2010, a Qantas A380 suffered a catastrophic failure of one of its four main Rolls-Royce engines. Investigations point to a failure in an oil stub pipe caused by a manufacturing defect.

Position (Symmetry) • A symmetry tolerance is used to control a condition in which one or more

features are symmetrically disposed either side of a centre line (axis) or centre plane (median) of another feature which is specified as the datum.

Cylindricity • A cylindricity tolerance specifies a tolerance zone consisting of an annular space between

two co-axial cylinders having a difference in radii equal to the specified tolerance.

Angularity • An angularity tolerance is used to control angular relationships of any angle between

straight line (axes) or surfaces with straight line elements such as flat or cylindrical surfaces

Circularity • A circularity tolerance specifies the width of an annular tolerance zone, bounded by two

concentric circles in the same plane within which the circumference of the feature must lie.

Parallelism • A parallelism tolerance is used to control the orientation of features related to one another

by an angle of zero degrees.

Example (A Bad One)

Effects of Combining Linear and Geometric Tolerances

• In tolerancing features of size on mating components where ease of assembly is important, it should be realised that the least favourable condition for assembly occurs when mating sizes are the maximum material size allowable by the individual tolerance of each component.

• Greater variations in shape geometry can be accepted as the mating sizes approach their least material size.

Virtual Size • Virtual size refers to the dimension of the overall envelope of perfect form which touches the

highest points of a feature.

• For a shaft, it is the maximum measured size plus the actual effect of form variations such as flatness, straightness, roundness, cylindricity and profile.

• For a hole, it is the minimum measured size minus the actual effect of form variations such as flatness, straightness, roundness, cylindricity and profile.

Interpret this Example of Geometric Tolerancing