Straightness, flatness, roundness, cylindricity… Do you know all these Tolerance of Form and Position well?


Do you know what Tolerance of Form and Position is?

    Geometric tolerance refers to the allowable variation of the actual shape and actual position of the part from the ideal shape and ideal position.

 

    Geometric tolerance includes shape tolerance and position tolerance. Any part is composed of points, lines, and surfaces, and these points, lines, and surfaces are called elements. The actual elements of the machined parts always have errors relative to the ideal elements, including shape errors and position errors. This type of error affects the function of mechanical products, and the corresponding tolerance should be specified during design and marked on the drawing according to the specified standard symbols. Around the 1950s, industrialized countries had form and position tolerance standards. The International Organization for Standardization (ISO) published the geometric tolerance standard in 1969, and recommended the geometric tolerance detection principle and method in 1978. China promulgated shape and position tolerance standards in 1980, including testing regulations. Shape tolerance and position tolerance are referred to as shape tolerance for short.

 

    The processed parts not only have dimensional tolerances, but also inevitably have differences between the actual shape or mutual position of the points, lines and surfaces that constitute the geometric features of the part and the shape and mutual position specified by the ideal geometry. This difference in shape is the shape tolerance , and the difference in mutual position is the position tolerance, collectively referred to as the tolerance of form and position.

 

    When we talk about “Tolerance of Form and Position”, it is both theoretical and practical expertise, how much do you know about it? In production, if we misunderstand the geometric tolerance marked on the drawing, it will cause the processing analysis and processing results to deviate from the requirements, and even bring serious consequences.

    Today, let us systematically understand the 14 shape and position tolerances.

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14 Internationally Unified Geometric Tolerance Symbols.

01 Straightness

Straightness, which is commonly referred to as straightness, indicates the condition that the actual shape of the straight line elements on the part maintains the ideal straight line. The straightness tolerance is the maximum variation allowed by the actual line to the ideal line.

Example 1: In a given plane, the tolerance zone must be the area between two parallel straight lines with a distance of 0.1mm.

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02 Flatness

    Flatness, commonly known as flatness, indicates the actual shape of the plane elements of the part, maintaining the ideal plane condition. The flatness tolerance is the maximum variation allowed by the actual surface from the ideal plane.

Example: A tolerance zone is the area between two parallel planes at a distance of 0.08mm.

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03 Roundness

    Roundness, commonly referred to as the degree of roundness, indicates the condition that the actual shape of a circular feature on a part remains equidistant from its center. The roundness tolerance is the maximum variation allowed by the actual circle to the ideal circle on the same section.

Example: The tolerance zone must be on the same normal section, the area between two concentric circles with a radius difference of 0.03mm.

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04 Cylindricity

    Cylindricity means that each point on the contour of the cylindrical surface on the part is kept equidistant from its axis. The cylindricity tolerance is the maximum variation allowed by the actual cylindrical surface to the ideal cylindrical surface.

Example: A tolerance zone is the area between two coaxial cylindrical surfaces with a radius difference of 0.1 mm.

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05 line profile

    Line profile is the condition that a curve of any shape maintains its ideal shape on a given plane of a part. Line profile tolerance refers to the allowable variation of the actual contour line of a non-circular curve.

 

06 surface profile

 

    Surface profile is the condition that any surface on a part maintains its ideal shape. Surface profile tolerance refers to the allowable variation of the actual contour line of a non-circular surface to an ideal profile surface.

Example: The tolerance zone is between two envelopes 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, which is commonly referred to as the degree of parallelism, indicates the condition that the measured actual elements on the part are kept 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 a cylindrical surface with a reference parallel diameter of Φ0.03mm.

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08 Verticality

    Perpendicularity, which is commonly referred to as the degree of orthogonality between two elements, means that the measured element on the part maintains a correct 90° angle with respect to the reference element. Perpendicularity tolerance is the maximum variation allowed between the actual direction of the measured element and the ideal direction perpendicular to the datum.

 

09 slope

    Slope is the correct condition of any given angle between the relative orientations of two features on a part. The slope tolerance is the maximum variation allowed between the actual orientation of the measured feature and the ideal orientation at any given angle to the datum.

Example : 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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10 position degrees

    Position degree refers to the accurate condition of points, lines, surfaces and other elements on the custom cnc milling part relative to their ideal positions. The position tolerance is the maximum allowable variation of the actual position of the measured element relative to the ideal position.

Example: When the mark SΦ is added before the tolerance zone, the tolerance zone is the inner area of the sphere with a diameter of 0.3mm. The position of the center point of the spherical tolerance zone is the theoretically correct dimension relative to datums A, B and C.

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11 coaxial (concentric) degrees

    Coaxiality, commonly known as the degree of coaxiality, means that the measured axis on the part is kept on the same straight line relative to the reference axis. The concentricity tolerance is the allowable variation of the measured actual axis relative to the reference axis.

 

12 Symmetry

    The degree of symmetry means that the two symmetrical central elements on the part are kept in the same central plane. The symmetry tolerance is the amount of variation allowed by the symmetry center plane (or center line, axis) of the actual element to the ideal symmetry plane.

Example: Tolerance zone is the area between two parallel planes or straight lines with a distance of 0.08mm and arranged symmetrically with respect to the datum center plane or center line.

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13 round beating

    Circular runout is the condition in which a surface of revolution on a aluminum cnc parts maintains a fixed position relative to a datum axis within a defined measurement plane. Circular runout tolerance is the maximum variation allowed within a limited measurement range when the measured actual element rotates a full circle around the reference axis without axial movement.

Example : The tolerance zone is the area between two concentric circles perpendicular to any measurement plane, with a radius difference of 0.1mm and whose centers are on the same datum axis.

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14 full beats

    Full runout refers to the amount of runout along the entire measured surface when the machined metal parts is continuously rotated around the reference axis. The full runout tolerance is the maximum runout allowed when the measured actual element rotates continuously around the datum axis while the indicator moves relative to its ideal contour.

 

Example : The tolerance zone is the area between two cylindrical surfaces with a radius difference of 0.1 mm and coaxial with the datum.

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