The precision index of CNC parts is an important standard to evaluate the quality of CNC machining, which covers many aspects to ensure that the parts meet the design requirements in terms of size, shape, position and so on. The following are the precision indicators of CNC parts sorted out according to authoritative sources:
First, dimensional accuracy
Definition: The degree of conformity between the actual size of the part after machining and the tolerance zone center of the part size is the dimensional accuracy. Dimensional accuracy is controlled by dimensional tolerances. Dimensional tolerance is the allowable variation in the size of a part during cutting. In the case of the same basic size, the smaller the dimensional tolerance and the higher the dimensional accuracy.
Influencing factors: mainly by the geometric accuracy of the machine tool, motion accuracy, tool wear, cutting parameters and other factors.
Second, shape accuracy
Definition: The degree of conformity between the actual geometry of the surface of the part machined by CNC lathe and the ideal geometry is the shape accuracy. There are 6 items to evaluate shape accuracy, such as straightness, flatness, roundness, cylindricity and line profile. Shape accuracy is controlled by shape tolerance, and each shape tolerance is divided into 12 accuracy levels except for 13 accuracy levels of roundness and cylindricity. Level 1 is the highest and level 12 is the lowest.
Influencing factors: including the geometric accuracy of the machine tool, the shape and wear of the tool, cutting force, vibration and so on.
Third, position accuracy
Definition: The actual position accuracy difference between the relevant surfaces of the parts after machining by CNC lathe is the position accuracy. There are eight items to evaluate position accuracy, such as parallelism, perpendicularity, inclination, coaxiality, symmetry, position, round runout and full runout. Position accuracy is controlled by position tolerances, which are also divided into 12 accuracy levels.
Influencing factors: mainly affected by the positioning accuracy of the machine tool, repeated positioning accuracy, fixture accuracy and wear, cutting force and so on.
Fourth, surface roughness
Definition: The microscopic geometry error of the surface of a part.
Influencing factors: including cutting parameters, tool materials, coolant use, etc.
Fifth, other precision indicators
Positioning accuracy: The degree of accuracy of the exponential control machine tool when it stops at a specified coordinate point reflects the consistency of the machine tool controller instructions with the actual position.
Repeated positioning accuracy: refers to the position deviation when the same position is stopped several times, which measures the stability of the machine tool.
Geometric accuracy: It involves the shape error of each moving axis of the machine tool and its associated parts, including straightness, perpendicularity, parallelism and planarity. For example, straightness measures the deviation between the actual moving trajectory of the axis of motion and the ideal straight line, which has a direct impact on the accuracy of machining the linear surface.
Spindle rotation accuracy: As the rotation center in machining, its rotation accuracy directly determines the hole processing and surface finish. It mainly includes radial runout, end runout and axial movement, which affect the dimensional accuracy and shape accuracy of the workpiece.
Reverse clearance: the non-working stroke caused by the elastic deformation of the transmission chain when the servo system of the exponential control machine tool changes the direction of movement. Reducing the reverse clearance is particularly important for improving machining accuracy and surface quality, especially in precision contour machining.
Indexing accuracy: It not only affects the angular position of the machining parts in space, but also affects the coaxial degree of hole processing.
Resolution and pulse equivalent: the size of the pulse equivalent determines the machining accuracy and surface quality of the CNC machine tool, the smaller the pulse equivalent, the higher the machining accuracy and surface quality of the CNC machine tool.
In summary, the precision index of CNC parts is a multi-dimensional evaluation system, covering many aspects such as size, shape, position and surface roughness. In the process of CNC machining, it is necessary to comprehensively consider these precision indicators, and ensure that the machined parts meet the design requirements through reasonable process planning, tool selection, cutting parameters setting and other measures.
