As a veteran in machining, you have read countless drawings and processed countless parts. When we talk about "form and position tolerances", it is both theoretical and practical professional knowledge. How well do you know about it? In production, if we misunderstand the form and position tolerances marked on the drawings, the processing analysis and processing results will deviate from the requirements, and even bring serious consequences. Today, let us systematically understand the 14 form and position tolerances.
Let me first show you the key points. The following table is the internationally unified 14 form and position tolerance symbols, which is very important.
01 Straightness
Straightness, which is usually called the degree of straightness, indicates the condition that the actual shape of the straight line element on the part remains an ideal straight line. The straightness tolerance is the maximum allowable variation of the actual line to the ideal straight line.
Example 1: In a given plane, the tolerance zone must be in the area between two parallel lines with a distance of 0.1mm.
Example 2: Add the symbol Φ before the tolerance value, then the tolerance zone must be in the area within the cylindrical surface with a diameter of 0.08mm.
02 Flatness
Flatness, or the degree of flatness, indicates the actual shape of the planar element of the part, maintaining the ideal plane. Flatness tolerance is the maximum variation allowed by the actual surface from the ideal plane.
Example: The tolerance zone is the area between two parallel planes with a distance of 0.08mm.
03 Roundness
Roundness, or the degree of roundness, indicates the condition that the actual shape of the circular element on the part is equidistant from its center. Roundness tolerance is the maximum variation allowed by the actual circle from the ideal circle on the same section.
Example: The tolerance zone must be on the same positive section, and the area between two concentric circles with a radius difference of 0.03mm tolerance value.
04 Cylindricity
Cylindricity indicates that the points on the cylindrical surface contour on the part are equidistant from its axis. Cylindricity tolerance is the maximum variation allowed by the actual cylindrical surface from the ideal cylindrical surface.
Example: The tolerance zone is the area between two coaxial cylindrical surfaces with a radius difference of 0.1mm tolerance value.
05 Line profile
Line profile refers to the condition that a curve of any shape on a given plane of a part maintains its ideal shape. Line profile tolerance refers to the allowable variation of the actual contour line of a non-circular curve.
Example: The tolerance zone is the area between two envelopes that envelop a series of circles with a tolerance of 0.04mm in diameter. The centers of the circles are located on the line with the theoretically correct geometric shape.
06 Surface profile
Surface profile refers to the condition that a surface of any shape 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 from the ideal contour surface.
Example: The tolerance zone is between two envelopes that envelop a series of balls with a diameter of 0.02mm. The centers of the balls should theoretically be located on the surface with the theoretically correct geometric shape.
07 Parallelism
Parallelism, which is usually referred to as the degree of maintaining parallelism, indicates the condition that the actual elements measured on the part are kept at an equal distance relative to the reference. 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 symbol Φ is added before the tolerance value, the tolerance zone is within the cylindrical surface with a diameter of Φ0.03mm parallel to the datum.
08 Perpendicularity
Perpendicularity, which is usually referred to as the degree of orthogonality between two elements, indicates that the measured element on the part maintains the correct 90° angle relative to the datum element. Perpendicularity tolerance is the maximum allowable variation between the actual direction of the measured element and the ideal direction perpendicular to the datum.
Example 1: If the symbol Φ is added before the tolerance zone, the tolerance zone is perpendicular to the cylindrical surface with a diameter of 0.1mm on the datum surface.
Example 2: The tolerance zone must be located between two parallel planes with a distance of 0.08mm and perpendicular to the datum line.
09 Inclination
Inclination is the correct condition that the relative directions of two elements on a part maintain any given angle. The inclination tolerance is the maximum variation allowed between the actual direction of the measured element and the ideal direction 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 distance of 0.08mm and a theoretical angle of 60° with the datum A.
Example 2: If the symbol Φ is added before the tolerance value, the tolerance zone must be located in a cylindrical surface with a diameter of 0.1mm. The tolerance zone should be parallel to the plane B perpendicular to the datum A and have a theoretical correct angle of 60° with the datum A.
10 Position
Position is the accurate condition of the points, lines, surfaces and other elements on the part relative to their ideal positions. The position tolerance is the maximum variation allowed between the actual position of the measured element and the ideal position.
Example: When the symbol SΦ is added before the tolerance zone, the tolerance zone is the area inside a sphere with a diameter of 0.3mm. The position of the center point of the spherical tolerance zone is the theoretically correct size relative to the datums A, B and C.
11 Coaxiality (concentricity) degree
Coaxiality, commonly known as the degree of coaxiality, indicates the condition that the measured axis on the part remains on the same straight line relative to the reference axis. The coaxiality tolerance is the amount of variation allowed for the measured actual axis 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 is consistent with the reference.
12 Symmetry
Symmetry indicates the condition that two symmetrical center elements on a part remain in the same center plane. The symmetry tolerance is the amount of variation allowed for the symmetrical center plane (or center line, axis) of the actual element to the ideal symmetry plane.
Example: The tolerance zone is the area between two parallel planes or lines with a distance of 0.08mm and symmetrically arranged relative to the reference center plane or center line.
13 Circular runout
Circular runout indicates the condition that the rotating surface on the part remains in a fixed position relative to the reference axis within the limited measuring surface. The circular runout tolerance is the maximum variation allowed within the limited measuring range when the actual measured element rotates one full circle around the datum axis without axial movement.
Example 1: The tolerance zone is the area between two concentric circles perpendicular to any measuring plane, with a radius difference of 0.1mm and the centers on the same datum axis.
Example 2: The tolerance zone is the area between two circles with a distance of 0.1mm on the measuring cylindrical surface at any radial position coaxial with the datum.
14 Total runout
Total runout refers to the runout along the entire measured surface when the part rotates continuously around the datum axis. The total runout tolerance is the maximum runout allowed when the actual measured element rotates continuously around the datum axis and the indicator moves relative to its ideal profile.
Example 1: The tolerance zone is the area between two cylindrical surfaces with a radius difference of 0.1mm and coaxial with the datum.
Example 2: The tolerance zone is the area between two parallel planes with a radius difference of 0.1mm and perpendicular to the datum.
