GD&T is the abbreviation of Geometric Dimensioning and Tolerancing, a symbolic engineering language widely used in the world, which clearly and precisely defines the size, shape, direction and position of the geometric product function, so that the product can be On the premise of meeting its functional needs, obtain looser tolerance requirements as much as possible to effectively reduce manufacturing costs; the continuous improvement of GD&T popularization and application can effectively reduce or eliminate the drawings of the same product in different countries, or the same country but different departments Differences in understanding (R&D, technology, manufacturing, quality inspection); GD&T originated in the 1940s and has been widely used in many industries such as national defense, aerospace, automobiles, engineering machinery, electronic appliances, and medical equipment , has a decisive influence on a global scale.

## GD&T generally refers to tolerance of form and position

Geometric tolerances are generally also called geometric tolerances, including shape tolerances and position tolerances. 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.

The processed parts will have dimensional tolerances, so the actual shape or mutual position of the points, lines, and surfaces that constitute the geometric features of the part are different from 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, and these differences are collectively called Geometric tolerances.

Form Tolerance (GD&T):1. Straightness The 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 straight lines that are not straight.

2. Flatness The symbol is a parallelogram, which is an index to limit the amount of variation between the actual plane and the ideal plane. It is a requirement for unevenness of the plane.

3. Roundness The symbol is a circle (○),