The shape tolerance
The shape tolerance
Shape tolerance is also called general geometric tolerance, including the shape tolerance and location tolerance. Any parts is composed of dot, line, face, these dot, line, face are called elements. Machining parts after the actual elements relative to the ideal always have error, including the shape error and position error. This kind of error affects the function of mechanical products, design the corresponding tolerance shall be established and according to the standards set by the symbolic mark on the drawing. Before and after the 1950 s, the industrialised countries physical tolerance standard. In 1969, the international organization for standardization (ISO) released form tolerance standard, 1978 recommended shape tolerance test principle and method. In 1980, China promulgated the shape and position tolerance standard, including testing requirements. Shape tolerance and location tolerance merge to form a tolerance for short.
Processed parts will have size tolerance, and therefore constitute actual part geometry feature point, line and plane shape or mutual location and ideal geometry shape and position there is a difference, these differences in shape is the shape of tolerance, and the difference of each position is the position tolerance, these differences are collectively referred to as the shape tolerance (Geometric tolerances).
Bullet
Geometric tolerance includes shape tolerance and position tolerance, and position tolerance includes orientation tolerance and positioning tolerance. The specific contents and tolerance representation symbols are shown in the figure below:
Shape tolerance
1. The straightness symbol is a short horizontal line (-), which is an index to limit the variation of the actual straight line to the ideal straight line. It is a requirement for the non straightness of a straight line.
2. The flatness symbol is a parallelogram, which is an index to limit the variation of the actual plane from the ideal plane. It is a requirement for uneven plane.
3. The circularity symbol is one circle (○), which is an index to limit the variation of the actual circle to the ideal circle. It is the requirement for the circular contour of parts with cylindrical surface (including conical surface and spherical surface) in a normal section (the surface perpendicular to the axis).
4. The cylindricity symbol is a circle (/ ○ /) sandwiched between two oblique lines, which is an index to limit the variation of the actual cylinder to the ideal cylinder. It controls the shape errors in the cylinder cross section and shaft cross section, such as roundness, prime line straightness, axis straightness and so on. Cylindricity is a comprehensive index of various shape errors of a cylinder.
5. The symbol of line profile is an upward convex curve (-), which is an index to limit the variation of actual curve to ideal curve. It is a requirement for the shape accuracy of non-circular curves.
6. The surface contour symbol is that the upper part is a semicircle and the lower part is a horizontal one. It is an index to limit the variation of the ideal surface of the actual surface. It is the requirement for the shape accuracy of the surface.
orientation tolerance
1. Parallelism (‖) is used to control the deviation of the measured feature (plane or straight line) on the part from the direction of the datum feature (plane or straight line), that is, the measured feature is equidistant from the datum.
2. Perpendicularity (⊥) is used to control the deviation of the measured element (plane or straight line) on the part from the direction of the datum element (plane or straight line), that is, the measured element is required to be 90 ° to the datum.
3. The inclination (∠) is used to control the deviation of the direction of the measured element (plane or straight line) on the part from the datum element (plane or straight line) by a given angle (0 ° ~ 90 °), that is, the measured element is required to form a certain angle (except 90 °) to the datum.
Positioning tolerance
1. Coaxiality (◎) is used to control the different axial degrees of the measured axis and the reference axis that should be coaxial in theory.
2. The symmetry symbol is three horizontal lines with a horizontal length in the middle. It is generally used to control the non coincidence degree between the measured elements (central plane, central line or axis) that are required to be coplanar in theory and the datum elements (central plane, central line or axis).
3. The position degree symbol is a circle with two straight lines perpendicular to each other, which is used to control the change of the measured actual element relative to its ideal position, which is determined by the datum and the correct theoretical size.
Runout tolerance
1. The circular runout symbol is a slash with an arrow. The circular runout is the difference between the maximum and minimum readings measured in a given direction by a fixed position indicator during one cycle of non axial movement and rotation of the measured actual element around the reference axis.
2. The total runout symbol is two oblique lines with arrows. The total runout is the continuous rotation of the measured actual element around the reference axis without axial movement. At the same time, the indicator moves continuously along the ideal prime line. The difference between the maximum and minimum readings measured by the indicator in a given direction.
Shape error
It refers to the geometric errors that may occur during machining of geometric elements such as points, lines and surfaces on parts.
For example, when machining a cylinder, the diameter of each section of the shaft may be different, or the section of the shaft may not be round, or the axis may not be straight, or the plane may be warped and unequal.
position error
It refers to the possible relative position error of structural elements on parts during machining.
For example, the rotation axes of the step shaft may be offset.
There is an efficient measurement method for measuring various form and position errors, that is, the data acquisition instrument can be directly used to connect various indicators, such as dial indicator, etc. the data acquisition instrument will automatically read the measurement data and analyze the data without manual measurement and data analysis, which can greatly improve the mechanical measurement efficiency.
Measuring instruments: deflectometer, dial indicator (or other indicator), data acquisition instrument
Measurement principle: the data acquisition instrument can read the data in real time from the dial indicator and calculate and analyze the form and position error. Various form and position error calculation formulas have been embedded in our data acquisition instrument software. There is no need to calculate the cumbersome data manually, which can greatly improve the accuracy of measurement.
Schematic diagram of measurement:
Advantages:
1) Improve the measurement efficiency at a lower cost: compared with similar products, the cost is very low and the measurement efficiency is greatly improved;
2) Improve the accuracy of measurement: the traditional method of visual observation by surveyors is easy to lead to wrong measurement results;
3) Data traceability: save data records and trace and analyze them. The traditional mode has poor traceability due to the lack of real-time records;
Collapse and edit the shape tolerance of this section
Shape tolerance refers to the total amount of variation allowed by the shape of a single actual element.
Shape tolerance is expressed by shape tolerance zone. The shape tolerance zone includes four elements: the shape, direction, position and size of the tolerance zone. The shape tolerance items include: straightness, flatness, roundness, cylindricity, line profile and surface profile. (Note: the first item shall be added before the straightness tolerance value Φ， To indicate that the shape of the tolerance zone is a cylindrical surface (refer to GB / t17852-1999)
The popular point is the elements related to shape.
Fold edit the positional tolerance of this section
Positional tolerance refers to the total allowable variation of the position of the associated actual feature to the datum.
Fold edit the orientation tolerance of this segment
Orientation tolerance refers to the total allowable variation of the associated actual feature in the direction of the datum. Such tolerances include parallelism, perpendicularity and inclination.
Fold edit the runout tolerance of this section
Runout tolerance is a tolerance item given based on a specific detection method. Runout tolerance can be divided into circular runout and full runout.
Fold edit the positioning tolerance of this section
Positioning tolerance is the total amount of allowable change in the position of the associated actual feature to the datum. Such tolerances include coaxiality, symmetry and position.
Collapse and edit the tolerance icon of this segment
There are 14 geometric tolerances of parts, including 6 shape tolerances and 8 position tolerances, which are listed in the table below.
Straightness
Straightness refers to the condition that the actual shape of the linear elements on the part maintains an ideal straight line. That is commonly referred to as straightness.
Straightness tolerance is the maximum allowable variation of actual line to ideal line. That is, it is given on the drawing to limit the allowable variation range of actual line machining error.
Flatness
Flatness represents the actual shape of the plane elements of the part and maintains the ideal plane. That is commonly referred to as flatness.
Flatness tolerance is the maximum allowable variation of the actual surface from the plane. That is, it is given on the drawing to limit the allowable variation range of the actual surface machining error.
Roundness
Roundness refers to the condition that the actual shape of the round element on the part is kept equidistant from its center. That is, the degree of roundness.
Roundness tolerance is the maximum allowable variation of the actual circle to the ideal circle on the same section. That is given in the drawing to limit the allowable variation range of the machining error of the actual circle.
Cylindricity
Cylindricity means that the points on the outline of the cylindrical surface on the part are equidistant from its axis.
The cylindricity tolerance is the maximum allowable variation of the actual cylinder to the ideal cylinder. That is given in the drawing to limit the allowable variation range of the actual cylindrical surface machining error.
Line profile
Line profile is the condition that the curve of any shape maintains its ideal shape on the given plane of the part.
The tolerance of the non-circular contour line is the allowable deviation of the contour line. That is, it is given in the drawing to limit the allowable variation range of actual curve processing error.
Surface profile
Surface profile refers to the condition that the surface of any shape on the part maintains its ideal shape.
Surface profile tolerance refers to the allowable variation of the actual contour line of non-circular surface to the ideal contour surface. That is, it is given in the drawing to limit the variation range of actual surface machining error.
Parallelism
Parallelism refers to the condition that the measured actual feature on the part maintains an equal distance from the datum. That is, the degree of parallelism.
Parallelism tolerance is the maximum allowable variation between the actual direction of the measured element and the ideal direction parallel to the datum. That is, it is given in the drawing to limit the allowable variation range of the measured actual elements from the parallel direction.
verticality
Perpendicularity refers to the condition that the measured feature on the part maintains a correct angle of 90 ° relative to the datum feature. That is, the degree of orthogonality between the two elements.
Perpendicularity tolerance is the maximum allowable variation between the actual direction of the measured element and the ideal direction perpendicular to the datum. That is, the maximum allowable variation range given on the drawing to limit the deviation of the measured actual elements from the vertical direction.
Inclination
Inclination is the correct condition that the relative direction of two elements on the part maintains any given angle.
Inclination tolerance is the maximum allowable variation between the actual direction of the measured element and the ideal direction at any given angle to the datum.
Degree of symmetry
Symmetry refers to the state that two symmetrical central elements on the part remain in the same central plane.
Symmetry tolerance refers to the allowable variation of the symmetry center plane (or center line and axis) of the actual element to the ideal symmetry plane. The ideal symmetry plane refers to the ideal plane common with the datum symmetry plane (or centerline and axis).
Coaxiality
Coaxiality refers to the condition that the measured axis on the part is kept in the same straight line relative to the reference axis. That is commonly referred to as the degree of coaxial.
Coaxiality tolerance is the allowable variation of the measured actual axis relative to the datum axis. That is, it is given in the drawing to limit the allowable variation range of the measured actual axis from the ideal position determined by the reference axis.
Position degree
Positional degree refers to the accuracy of the point, line, surface and other elements on the part relative to its ideal position.
Positional tolerance is the maximum allowable variation of the actual position of the measured element relative to the ideal position.
Circular runout
Circular runout refers to the condition that the rotating surface on the part maintains a fixed position relative to the reference axis in the limited measuring surface.
Circular runout tolerance refers to the maximum allowable variation within the limited measurement range when the measured actual element rotates a whole circle around the reference axis without axial movement.
Total beat
Total runout refers to the runout along the whole measured surface when the part rotates continuously around the reference axis.
Total runout tolerance is the maximum allowable runout when the measured actual element rotates continuously around the reference axis and the indicator moves relatively along its ideal contour.
Problems needing attention in folding and editing this paragraph
(1) The geometric tolerance content is represented by frame. The frame content is from left to right. The first frame is always the geometric tolerance bullet, the second frame is the tolerance value, and the third frame is the benchmark, even if the leader is led out from the right end of the frame (2) When the measured feature is the center feature, the arrow must be aligned with the relevant dimension line When the common axis of the feature is not included in the common axis, only the common axis of the feature must be included in the common axis segment (the marking method of arrow pointing to axis or center line in the new standard has been abolished)
(3) When the measured feature is a contour feature, the arrow direction is generally perpendicular to the feature However, for roundness tolerance, the direction of the arrow must be perpendicular to the axis
(4) When the tolerance zone is a circle or cylinder, a symbol shall be added before the tolerance value“ Φ ", the tolerance value is the diameter of the circle or cylinder. This situation only exists when the measured element is the axis. The tolerance zone of coaxiality is always a cylinder, so the tolerance value is always preceded by a symbol" Φ "; the perpendicularity of the axis to the plane and the positional degree of the axis generally adopt the cylinder tolerance zone, and the symbol shall also be added before the tolerance value" Φ "(5) for some additional requirements, the corresponding symbols are often added after the tolerance value. For example, the (+) symbol indicates that the measured element is only allowed to be waist drum shaped convex, (-) indicates that the measured element is only allowed to be saddle shaped concave, (>) It shows that the error can only be gradually reduced in the direction of the small end of the symbol If the geometric tolerance is required to comply with the maximum material requirements, the symbol ○ m shall be added. Words can be used as additional instructions on the top and bottom of the frame For example, the description of the number of measured elements shall be written above the tolerance frame; Explanatory notes (including requirements for measurement methods) shall be written below the tolerance frame For example: 300mm away from the shaft end; Within a, B, etc
Geometric tolerance is the geometric accuracy requirement for the shape and position of workpiece elements in order to meet the functional requirements of products. The shape and position of the measured actual elements are limited by geometric tolerance.
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