Why is there the concept of tolerance and fit? All manufactured products, no matter how sophisticated equipment is used, no matter how much effort is made, their size and shape cannot be exactly consistent with theoretical values. This is the gap between ideal and reality!
So how to meet the interchangeability requirements of parts? That is to say, among a batch of parts or components of the same specification, any one of them can meet the specified performance requirements without any selection or additional modification. This requires that the dimensions of the produced parts should be within the allowable tolerance range.
01
Terms related to tolerance
During the processing of parts, due to the influence of machine tool precision, tool wear, measurement errors, etc., it is impossible to process the size of parts absolutely accurately. In order to ensure interchangeability, the processing error of the part size must be limited within a certain range, and the amount of variation in size must be specified.
1) Basic size
According to the strength and structural requirements of the part, the size determined during design.
2) Actual size
Dimensions obtained by measuring.
3) Limit size
Two limit values for allowable size variation. It is determined based on the basic size. The larger of the two limit values is called the maximum limit size; the smaller one is called the minimum limit size.
4) Size deviation (referred to as deviation)
The algebraic difference of a dimension minus its base dimension. Dimensional deviations are:
Upper deviation = maximum limit size - basic size
Lower deviation = minimum limit size - basic size
The upper and lower deviations are collectively referred to as limit deviations, and the upper and lower deviations can be positive, negative or zero.
The national standard stipulates that the code name of the upper deviation of the hole is ES, the code name of the lower deviation of the hole is EI; the code name of the upper deviation of the shaft is es, and the code name of the lower deviation of the shaft is ei.
▲ Tolerance zone diagram
5) Dimensional tolerance (tolerance for short)
The amount of variation in size allowed.
Dimensional tolerance = maximum limit size - minimum limit size
= upper deviation - lower deviation
Because the maximum limit size is always greater than the minimum limit size, that is, the upper deviation is always greater than the lower deviation, so the dimensional tolerance must be a positive value.
6) Zero line, PR zone and tolerance zone diagram
The zero line is a reference line used to determine the deviation in the tolerance zone diagram, that is, the zero deviation line. Usually the zero line represents the basic size. Mark "0", "+", "-" on the left end of the zero line, the deviation above the zero line is positive; the deviation below the zero line is negative. The tolerance zone is an area defined by two straight lines representing the upper and lower deviations. The width and position of the tolerance zone are the two elements that constitute the tolerance zone.
7) Standard tolerance and standard tolerance grade
Standard tolerances are any tolerances listed in national standards to determine the size of the tolerance zone. A standard tolerance class is a class that determines the degree of dimensional accuracy. Standard tolerances are divided into 20 grades, namely IT01, IT0, IT1~IT18, which represent standard tolerances, and Arabic numerals represent standard tolerance grades, among which IT01 grade is the highest, grades are lowered in turn, and IT18 grade is the lowest. For a certain basic size, the higher the standard tolerance level, the smaller the standard tolerance value, and the higher the accuracy of the size.
8) Basic deviation
It is used to determine the upper or lower deviation of the tolerance zone relative to the zero line position. Generally, it refers to the deviation close to the zero line. When the tolerance zone is above the zero line, the basic deviation is the lower deviation. When the tolerance zone is below the zero line, the basic deviation is the upper deviation.
According to actual needs, the national standard stipulates 28 different basic deviations for the hole and the shaft respectively, as shown in the figure below. The basic deviation values of holes and shafts can be found from relevant tables.
▲ Basic deviation series
It can be seen from the above figure that:
1) The basic deviation code is represented by Latin letters, the capital letter represents the basic deviation code, and the lowercase letter represents the basic deviation code of the axis. Since the basic deviation is only used to indicate the size of the tolerance zone in the figure, one end of the tolerance zone is drawn as an opening.
2) The deviation from A~H is the lower deviation, J~ZC is the upper deviation, and the upper and lower deviations of JS are +IT/2 and -IT/2 respectively.
3) The basic deviation of the axis is the upper deviation from a~h, the lower deviation from j~zc, and the upper and lower deviations of js are +IT/2T and -IT/2 respectively. Another deviation of holes and shafts can be calculated from the basic deviation and standard tolerance.
02
Associated terms
In machine assembly, the relationship between the tolerance zone of holes and shafts with the same basic size and combined with each other is called fit. Due to the difference in the actual size of the hole and the shaft, a "play" or "interference" can occur after assembly. In the fit between the hole and the shaft, the algebraic difference obtained by subtracting the size of the shaft from the size of the hole is positive when it is positive, and when it is negative it is interference.
(1) Types of coordination
Fits are divided into three categories according to their gap or interference:
picture
1) Clearance fit
The tolerance zone of the hole is above the public relations zone of the shaft, and any pair of holes and the shaft match will become a fit with clearance (including the minimum clearance of zero), as shown in Figure a above.
2) Interference fit
The tolerance zone of the hole is below the tolerance zone of the shaft, and any pair of holes and the shaft are matched as a fit with interference (including a minimum clearance of zero), as shown in Figure b above.
3) Overfitting
The tolerance zone of the hole overlaps with the tolerance zone of the shaft, and any pair of holes and the shaft are matched, which may have a gap or an interference fit, as shown in Figure c above.
(2) Coordinating benchmark system
The national standard stipulates two benchmark systems, as shown in the figure below.
picture
▲ Two benchmark systems
1) base hole system
The basic deviation is a system in which the tolerance zone of a certain hole and the tolerance zone of the shaft of the basic deviation constitute a kind of cooperation, as shown in figure a. That is to say, the position of the tolerance zone of the hole is fixed in the fit of the same basic size, and different fits are obtained by changing the position of the tolerance zone of the shaft. The hole made by the base hole is called the reference hole. The national standard stipulates that the lower deviation of the reference hole is zero, and "H" is the basic deviation code of the reference hole.
2) Base shaft system
The basic deviation is a system in which the tolerance zone of a certain shaft and the tolerance zone of holes with different basic deviations constitute a system of various fits, as shown in Figure b. That is to say, the position of the tolerance zone of the shaft is fixed in the fit of the same basic size, and different fits are obtained by changing the position of the tolerance zone of the hole. The hole made in the center of the base shaft is called the reference shaft sleeve. The national standard stipulates that the upper deviation of the reference shaft is zero, and "h" is the basic deviation code of the reference shaft.
It can be seen from the basic deviation series diagram that:
In the base hole system, the reference hole H is matched with the shaft, a~h (11 types in total) are used for clearance fit; j~n (5 types in total) are mainly used for excessive fit; (n, p, r may be excessive fit or interference fit); p~zc (12 types in total) are mainly used for interference fit.
In the basic shaft system, the reference axis h is matched with the hole, A~H (11 types in total) are used for clearance fit; J~N (5 types in total) are mainly used for excessive fit; (N, P, R may be excessive fit or interference fit); P~ZC (12 types in total) are mainly used for interference fit.
03
shape tolerance
Shape tolerance refers to the total variation allowed by the shape of a single actual element. Form tolerances are expressed in form tolerance zones. The shape tolerance zone includes four elements such as the shape, direction, position and size of the tolerance zone. The shape tolerance items include: straightness, flatness, roundness, cylindricity, line profile, and surface profile.
1) Straightness
Straightness refers to the condition that the actual shape of the straight elements on the part maintains the ideal straight line. This is what is commonly referred to as flatness. The straightness tolerance is the maximum variation allowed by the actual line to the ideal line. That is, given on the drawing, it is used to limit the allowable variation range of the actual line processing error.
picture
▲Pattern example 1: In a given plane, the tolerance zone must be the area between two parallel straight lines with a distance of 0.1mm.
picture
▲Pattern example 2: If the mark φ is added before the tolerance value, the tolerance zone must be within the area of the cylindrical surface with a diameter of 0.08mm.
2) Flatness
Flatness refers to the actual shape of the plane element of the part and the condition of maintaining the ideal plane. This is what is commonly referred to as smoothness. The flatness tolerance is the maximum variation allowed by the actual surface to the plane. That is, it is given on the drawing to limit the allowable variation range of the actual surface processing error.
picture
▲Pattern example: The tolerance zone is the area between two parallel planes with a distance of 0.08mm.
3) Roundness
Roundness is the condition of the actual shape of a feature representing a circle on a part, equidistant from its center. That is commonly referred to as the degree of roundness. The roundness tolerance is the maximum variation allowed by the actual circle to the ideal circle on the same section. That is, given on the drawing, it is used to limit the allowable variation range of the machining error of the actual circle.
picture
▲Pattern 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.
4) 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. That is, given on the drawing, it is used to limit the allowable variation range of the actual cylindrical surface machining error.
picture
▲Pattern example: The tolerance zone is the area between two coaxial cylindrical surfaces with a radius difference of 0.1mm.
5) 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. That is, given on the drawing, it is used to limit the allowable variation range of the actual curve processing error.
picture
▲Pattern example: The tolerance zone is the area between two envelopes that enclose a series of circles with a diameter of 0.04mm. The centers of the circles lie on lines of theoretically correct geometry.
6) 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. That is, given on the drawing, it is used to limit the variation range of the actual surface processing error.
picture
▲Pattern 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.
04
position tolerance
The position tolerance refers to the total amount of variation allowed by the position of the associated actual element to the datum.
(1) Orientation tolerance
Orientation tolerance refers to the total amount of variation allowed by the associated actual feature to the datum in the direction. This type of tolerance includes three items: parallelism, perpendicularity, and inclination.
1) 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.
picture
▲Example of pattern: 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.
2) 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.
picture
▲Explanation of the legend: If the mark φ is added before the tolerance zone, then the tolerance zone is perpendicular to the reference plane and within a cylindrical surface with a diameter of 0.1mm.
picture
▲ Legend: The tolerance zone must be located between two parallel planes with a distance of 0.08mm and perpendicular to the reference line.
3) 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.
picture
▲Explanation of the legend: the tolerance zone of the measured axis is the area between two parallel planes whose distance is 0.08mm and which form a theoretical angle of 60° with the datum plane A.
picture
▲Explanation of the legend: If the mark 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 plane B perpendicular to datum A, and form a theoretically correct angle of 60° with datum A.
(2) Positioning tolerance
The positioning tolerance is the total amount of variation allowed for the position of the associated actual feature relative to the datum. This kind of tolerance includes three items: position degree, coaxiality degree and symmetry degree.
1) Position degree
Position degree refers to the accurate condition of points, lines, surfaces and other elements on the 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.
picture
▲ Legend: When the mark Sφ is added before the tolerance zone, the tolerance zone is the inner area of the ball 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.
2) Coaxiality
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.
picture
▲Concentricity tolerance legend: 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.
3) 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.
picture
▲Legend description: The tolerance zone is the area between two parallel planes or straight lines with a distance of 0.08mm and symmetrical arrangement with respect to the datum center plane or center line.
(3) Runout tolerance
Runout tolerance is a tolerance item given based on a specific detection method. Runout tolerance can be divided into circular runout and full runout.
1) Circle beating
Circular runout is the condition in which a surface of revolution on a part 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.
picture
▲ Legend 1: 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.
picture
▲ Legend 2: The tolerance zone is the area between two circles with a distance of 0.1mm on the measurement cylinder at any radial position coaxial with the datum.
2) full beating
Full runout refers to the amount of runout along the entire measured surface when the part 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.
picture
▲ Legend 1: The tolerance zone is the area between two cylindrical surfaces with a radius difference of 0.1mm and coaxial with the datum.
picture
▲ Legend 2: The tolerance zone is the area between two parallel planes with a radius difference of 0.1mm and perpendicular to the datum.
Here, it is the following table, hurry up and collect it~
