In production, if we misunderstand the geometric tolerances marked on the drawings, the processing analysis and processing results will deviate from the requirements, and even lead to serious consequences. Today, let us systematically understand the 14 geometric tolerances.
Let me show you the key points first. The following table shows the internationally unified 14-item geometric tolerance symbols. This is very important.
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01 Straightness
Straightness, commonly known as straightness, refers to the fact that the actual shape of the straight line elements on the part maintains an ideal straight line. Straightness tolerance is the maximum allowable variation of an actual line from an ideal straight line.
Example 1: In a given plane, the tolerance zone must be in the area between two parallel straight lines with a distance of 0.1mm.
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Example 2: Add the mark Φ before the tolerance value, then the tolerance zone must be within the area of the cylindrical surface with a diameter of 0.08mm.
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02 Flatness
Flatness, commonly known as flatness, indicates the actual shape of the plane elements of the part and the condition of maintaining an ideal plane. Flatness tolerance is the maximum allowable variation of an actual surface from an ideal plane.
Example: The tolerance zone is the area between two parallel planes 0.08mm apart.
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03 roundness
Roundness, commonly known as roundness, refers to the fact that the actual shape of a circle element on a part is equidistant from its center. The roundness tolerance is the maximum allowable variation of the actual circle from the ideal circle on the same cross-section.
Example: The tolerance zone must be on the same normal section, and the radius difference is the area between two concentric circles with a tolerance value of 0.03mm.
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04 Cylindricity
Cylindricity means that all points on the cylindrical surface contour of the part are equidistant from its axis. Cylindricity tolerance is the maximum allowable variation from an actual cylindrical surface to an ideal cylindrical surface.
Example: The tolerance zone is the area between two coaxial cylindrical surfaces with a radius difference of a tolerance value of 0.1mm.
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05 line profile
Line profile refers to the condition that any curve of any shape maintains its ideal shape on a given plane of the part. Line profile tolerance refers to the allowable variation of the actual contour of a non-circular curve.
Example: The tolerance zone is the area between two envelopes enclosing a series of circles with a diameter of 0.04mm. The centers of the circles lie on lines with theoretically correct geometric shapes.
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06 Surface contour
Surface contour refers to the condition in which an arbitrary-shaped surface on a part maintains its ideal shape. Surface contour tolerance refers to the actual contour line of a non-circular surface and the allowable variation from the ideal contour surface.
Example: The tolerance zone is between two envelope lines enveloping a series of balls with a diameter of 0.02mm. The centers of the balls should theoretically be located on the surface of the theoretically correct geometric shape.
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07 Parallelism
Parallelism, commonly known as the degree of parallelism, indicates that the actual elements being measured on the part remain equidistant from the datum. Parallelism tolerance is the maximum allowable variation between the actual direction of the measured element and the ideal direction parallel to the datum.
Example: If the mark Φ is added before the tolerance value, the tolerance zone is within the cylindrical surface with a reference parallel diameter of Φ0.03mm.
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08 Verticality
Perpendicularity, commonly known as the degree of orthogonality between two elements, indicates that the measured element on the part maintains a correct 90° angle relative to the datum element. The verticality tolerance is the maximum amount of variation allowed between the actual direction of the feature being measured and the ideal direction that is perpendicular to the datum.
Example 1: If the mark Φ is added before the tolerance zone, the tolerance zone is perpendicular to the cylindrical surface with a datum diameter of 0.1mm.
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Example 2: The tolerance zone must be located between two parallel planes that are 0.08mm apart and perpendicular to the datum line.
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09 inclination
Inclination refers to the correct condition of maintaining any given angle between the relative directions of two elements on a part. Slope tolerance is the maximum amount of variation allowed between the actual orientation of the feature being measured and its ideal orientation at any given angle to the datum.
Example 1: The tolerance zone of the measured axis is the area between two parallel planes with a tolerance value of 0.08mm and a theoretical angle of 60° with the datum plane A.
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Example 2: Add the mark Φ before the tolerance value, then the tolerance zone must be located within a cylindrical surface with a diameter of 0.1mm. The tolerance zone should be parallel to plane B perpendicular to datum A and at a theoretically correct angle of 60° to datum A.
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10 position degrees
Position refers to the accuracy of points, lines, surfaces and other elements on the part relative to their ideal positions. Positional tolerance is the maximum allowable variation in the actual position of the measured element relative to its ideal position.
Example: When the mark SΦ is added before the tolerance zone, the tolerance zone is the area inside the ball with a diameter of 0.3mm. The position of the center point of the ball tolerance zone is the theoretically correct size relative to datums A, B and C.
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11 degrees of coaxiality (concentricity)
Coaxiality, commonly known as coaxiality, indicates that the measured axis on the part remains on the same straight line relative to the reference axis. Coaxiality tolerance is the allowable variation of the actual axis being measured relative to the reference axis.
Example: When the tolerance value is marked, the tolerance zone is the area between cylinders with a diameter of 0.08mm. The axis of the circular tolerance zone coincides with the datum.
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12 degrees of symmetry
Symmetry refers to the state that the two symmetrical center elements on the part remain in the same central plane. The symmetry tolerance is the allowable variation of the symmetry center plane (or center line, axis) of the actual feature from the ideal symmetry plane.
Example: The tolerance zone is the area between two parallel planes or straight lines that are 0.08mm apart and are symmetrically arranged with respect to the datum center plane or center line.
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13 circle jump
Circular runout means that the surface of revolution on the part maintains a fixed position relative to the datum axis within a limited measurement plane. Circular runout tolerance is the maximum variation allowed within a limited measurement range when the actual element being measured rotates around the reference axis for a complete revolution without axial movement.
Example 1: The tolerance zone is the area between two concentric circles that are perpendicular to any measurement plane, have a radius difference of 0.1mm, and have their centers on the same datum axis.
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Example 2: The tolerance zone is the area between two circles with a distance of 0.1mm on the measuring cylinder at any radius position coaxial with the datum.
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14 full beats
Total runout refers to the runout along the entire measured surface when the part continuously rotates around the reference axis. The total runout tolerance is the maximum amount of runout allowed when the actual element being measured rotates continuously about the datum axis while the indicator moves relative to its ideal contour.
Example 1: The tolerance zone is the area between two cylindrical surfaces with a radius difference of 0.1mm and coaxial with the datum.
