Symbols of geometric tolerance and surface roughness and their meanings

2022-11-14 08:41:26 Jiaxing Focusimple Electronics Co., Ltd. Viewd 68

Any part is composed of three elements: point, line and surface. After machining, the actual elements of the part often have errors compared with the ideal elements, including shape error and position error. In industrial design, mechanical parts need to be tested according to the form and location tolerance standard. When designing the RF connector, the corresponding tolerance shall be marked on the drawing according to the specified standard symbol. When machining, due to different machining methods and workpiece materials, the machined surface will have small spacing and small peak valley unevenness which is surface roughness. Form and position errors and surface roughness will affect the performance of mechanical parts. In this chapter, we will introduce the symbols and significance of geometric tolerance and surface roughness. A thorough understanding of the theory is an important prerequisite for design and practical production applications.


Form tolerance and position tolerance


Form tolerance:It is the basic geometric tolerance that determines the shape of the workpiece. It’s an independent feature, can be determined independently without a datum.


Position tolerance:It refers to the tolerance that determines the position of corresponding features relative to a certain datum. Before specifying the positional tolerance, the datum must be determined, so the positional tolerance is the geometric tolerance of the feature associated with the datum.


Classification and symbol

# The feature control frame of form tolerance does not include a datum frame, while the feature control frame of position tolerance must have a datum corresponding to it.



Straightness is applicable to the description of line features. It is used to describe the straightness of line features. It is a form tolerance and does not require a datum.

The content in the shape control box above indicates that the axis of the cylinder must be located within a cylinder with a diameter of 0.1 mm.



Flatness refers to the state that all elements of a surface are on a plane.

Flatness control: it refers to the geometric tolerance that limits the allowable unevenness error of a surface.

Flatness tolerance zone: two parallel planes with all elements in the middle. The first plane of the tolerance zone is composed of three highest points on the controlled surface, and the second plane is a parallel plane that deviates from the first plane and controls the distance between tolerance values. As shown in the figure above, the tolerance zone of flatness is two parallel planes with a distance of 0.2mm.

Tolerance value: the distance between two faces. As shown in the figure above, the tolerance value is 0.2mm.



Verticality indicates the degree of change of line element, surface, center plane, tangent plane or axis in the direction of 90 ° (vertical) relative to the datum.

Type: face to face, face to line, line to line, line to line



Coaxiality specifies the coaxiality of the axis of two cylindrical surfaces. It is a positional tolerance and requires a reference datum.


Datum A is located on the diameter dimension line, which means that the datum feature is the axis of the revolving body. The shape control frame above shows that the axis of the cylinder pointed by the arrow must be located in the cylinder with datum axis A as the axis and the coaxiality of the diameter of 0.03 mm.


Positional degree

The positional degree indicates the position of the dimension feature in the datum system and the change degree of the relative real position.

Circular runout

Circular runout describes the runout of any circular part when the workpiece rotates.

The shape control frame in the above figure shows that if it rotates 1 circle around the datum axis in a straight line, the radial runout of the cylindrical surface pointed by the arrow of the marking line on any measuring plane perpendicular to the datum axis line shall not exceed 0.03 mm.


Total runout

Total runout describes the runout of the entire rotating surface during rotation.


The shape control frame in the above figure shows that when the cylindrical part is rotated in a straight line around the datum axis, at any point on the cylindrical surface, the total radial runout of the cylindrical surface pointed by the arrow of the marking line shall not exceed 0.03 mm.


Surface roughness


Surface roughness refers to the roughness of the machined surface with small spacing and small peaks and valleys. The distance (wave distance) between two wave peaks or two wave troughs is very small (below 1mm), which belongs to micro geometric shape error. The smaller the surface roughness, the smoother the surface.

The surface roughness code specified in the national standard consists of specified symbols and relevant parameters.


Evaluation parameters

(1)Arithmetic mean deviation of profile Ra

(2)Maximum height of contour Ry

Symbols and codes

a)  Basic symbol, obtained by any processing method;

b)  Obtained by removing materials;

c)  Obtained without removing materials。


Ra-- The arithmetic mean of the absolute value of the contour offset within the sampling length L.

Rz-- The sum of the average values of the five largest contour peak heights and the average values of the five largest contour valley depths within the sampling length.

Ry-- The distance between the contour peak line and the contour valley bottom line within the sampling length L.

Generally, if Ra/Rz/Ry is not marked in the symbol, it defaults to Ra.

Examples of RA

Examples of Rz、Ry

Dimensioning method

a. The tip is perpendicular to the machined surface, pointing from the outside of the material to the material surface, and the long stroke is always on the edge;

b. Digital dimension: the same as dimension;

c. All surfaces shall be marked only once; It can be marked on contour line (or extension line) and dimension limit;

d. exceptional case:

1)For the most frequently used code: if Ra is all the same, the code will be marked in the upper right corner; If most of Ra are the same, they can be uniformly marked on the upper right corner of the drawing with the word "Others" added. In both cases, the figure is magnified by 1.4 times;

2)Continuous surfaces are marked only once;

3)The discontinuous same surface shall be connected with a thin solid line and marked only once。、

Symbol and direction

Metric and English Comparison Table of Surface Roughness