Machining Errors And Tolerances of Parts

1. Machining error

Machining accuracy refers to the degree to which the actual values of the geometric parameters of the forging (size, shape and mutual position of geometric elements, microscopic unevenness of the contour, etc.) conform to the ideal design value after machining.

Machining error refers to the deviation of actual geometric parameters from their ideal design values. The smaller the machining error, the higher the machining accuracy.

Machining parts errors mainly have the following categories.

①Dimensional error: the degree of deviation of the actual size of the forging after processing from the ideal size. The ideal size refers to the average value of the largest and smallest limit sizes marked on the drawing, that is, the center value of the dimensional tolerance zone.

②Shape error: refers to the difference (or degree of deviation) of the actual surface shape of the processed forging from its ideal shape, such as roundness, straightness, etc.

③Position error: refers to the difference (or deviation) between the mutual position of the surface, axis or symmetric plane of the forging after processing for its ideal position, such as coaxiality, position, etc.

④ Surface micro-roughness: The micro-geometric error composed of small spacing and peaks and valleys on the surface of the processed forging. The microscopic unevenness of the forging surface is expressed by the evaluation parameter value of the surface roughness.

Machining errors are caused by many error factors in the process system.  Such as the principle error of the processing method, the positioning error of the forging, the manufacturing error and wear of the fixture and the tool, the manufacturing and installation error and wear of the machine tool, the error of the machine tool and the tool, the force, thermal deformation and friction vibration during the cutting process, There are also geometric errors of blanks and measurement errors in processing.

2. Geometric tolerance

In order to control the machining error and meet the functional requirements of the forging, the designer proposes the corresponding machining accuracy requirements through the forging drawing, and these requirements are given in the form of geometric tolerance.

The geometrical tolerance is the allowable variation range of the actual geometrical parameter value.

Relative to various machining errors, the geometrical tolerances are dimensional tolerances, shape tolerances, position tolerances, allowable values of surface roughness indexes, and tolerances of special geometrical parameters of typical parts.

A standard is a regulation that regulates technical requirements, and is a technical basis for common compliance within a certain range. The standards are issued at different levels. All over the world, the international standards (ISO) that enterprises abide by are common. my country's standards are divided into national standards (GB), industry standards (such as mechanical standards (JB)), local standards (DB) and enterprise standards. Local standards and enterprise standards are technical specifications formulated when there are no national standards and industry standards to be based on, and in a certain range, unified technical requirements are required.

In modern production, the manufacturing process of a mechanical product often involves many industries and enterprises, and some require international cooperation. In order to meet the technical coordination requirements between them, there must be a unified technical requirement of a common specification.

The range of standards is very broad and involves all aspects of people's lives. According to the target, it can be divided into basic standards, product standards, method standards and safety and environmental protection standards.