The cutting accuracy requirements during large CNC processing

1. Dimensional accuracy requirements

Linear dimensional accuracy: In large-scale CNC processing, the accuracy requirements for linear dimensions are usually determined according to the specific use of the workpiece and industry standards. Generally speaking, the dimensional accuracy of ordinary mechanical parts may be controlled between ±0.05mm - ±0.1mm. For example, in the processing of automotive parts, the dimensional accuracy requirements of some non-critical parts such as engine brackets may be relatively low, and the assembly and use requirements can be met within this range. However, for large structural parts in the aerospace field, such as aircraft wing beams, the dimensional accuracy requirements are extremely high, which may reach ±0.01mm - ±0.02mm, because the accuracy of these parts is directly related to the flight performance and safety of the aircraft.

Roundness and cylindricity accuracy: For rotating parts, roundness and cylindricity are important accuracy indicators. In general industrial applications, the roundness accuracy may be required to be between ±0.01mm - ±0.03mm. For example, in the processing of large motor shafts, in order to ensure the smooth operation of the motor, the roundness accuracy of the shaft needs to be controlled within this range. The cylindrical accuracy requirement is similar, which takes into account the shape accuracy of the cylindrical surface over the entire length. For high-precision equipment such as hydraulic cylinders, the cylindrical accuracy may be as high as ±0.005mm - ±0.01mm to ensure the sealing performance and movement accuracy of the piston in the cylinder.

Flatness accuracy: The plane machining accuracy of large workpieces is also crucial. In the machining industry, the flatness accuracy of ordinary planes is generally required to be between ±0.02mm - ±0.05mm. For example, in the machining of machine tool beds, the flatness accuracy will affect the installation accuracy and machining accuracy of the machine tool. For high-precision optical platforms and other equipment, the flatness accuracy requirements are extremely high, which may reach ±0.002mm - ±0.005mm to meet the high-precision requirements of optical instrument installation and use.

2. Shape accuracy requirements

Contour accuracy: The contour accuracy of large CNC-processed workpieces refers to the degree of conformity between the actual contour after processing and the designed contour. In the mold processing industry, the cavity contour accuracy requirements for injection molds, die-casting molds, etc. are very high. Generally speaking, the contour accuracy error may be controlled between ±0.03mm - ±0.08mm, depending on the size and accuracy level of the mold. For example, the contour accuracy requirements of small precision injection molds may be around ±0.03mm to ensure the dimensional accuracy and appearance quality of plastic products.

Surface accuracy: For large workpieces with complex surfaces, such as ship propellers, automobile body cover molds, etc., surface accuracy is the key. Surface accuracy includes the shape accuracy and surface quality of the surface. In the automotive industry, the surface accuracy of the body panel mold is required to be high, and its shape error is usually controlled between ±0.05mm - ±0.1mm to ensure that the stamped body panel has a good appearance and assembly accuracy. In high-end shipbuilding, the surface accuracy of the propeller is required to be higher because it directly affects the hydrodynamic performance of the propeller. The shape error may need to be controlled between ±0.03mm - ±0.05mm.

3. Position accuracy requirements

Hole system position accuracy: In large-scale mechanical structures and equipment, multiple holes are often involved in the processing, such as the workbench of machine tools, large-scale building steel structures, etc. The position accuracy of the hole system is crucial for subsequent assembly and function realization. Generally speaking, the position accuracy of the hole center distance is required to be between ±0.03mm - ±0.1mm. For example, in the wall panel processing of large printing presses, the position accuracy of the hole system used to install components such as roller shafts needs to be controlled between ±0.03mm - ±0.05mm to ensure the parallelism and transmission accuracy between rollers.

Position accuracy between features: In addition to the hole system, there are also strict requirements for the position accuracy between different features on the workpiece (such as bosses, grooves, keyways, etc.). In the processing of mechanical transmission parts, such as the housing of a large gearbox, the position accuracy between the keyway and the shaft hole may be required to be between ±0.03mm - ±0.08mm to ensure the correct installation and power transmission accuracy between the gear and the shaft.

4. Surface roughness requirements

General processing surface: For most surfaces after large CNC processing, there are certain requirements for surface roughness (Ra value). In ordinary machining, the roughness Ra value of non-matching surfaces may be between 3.2μm - 12.5μm. For example, for non-critical surfaces such as the housing of large equipment, the roughness can meet the appearance and basic protection requirements when it reaches this range. For matching surfaces with relative motion, such as guide rails, sliders, etc., the roughness Ra value is generally required to be between 0.8μm - 3.2μm to reduce friction and wear and ensure the smoothness of movement.

High-precision surface: In some precision machinery and optical equipment, the requirements for surface roughness are higher. For example, in the processing of high-precision optical lens molds, the roughness Ra value of the mold cavity surface may need to reach between 0.1μm and 0.4μm to ensure the surface quality and optical performance of the optical lens. In the processing of high-precision bearing rings, the roughness Ra value of the inner and outer surfaces is also required to be between 0.2μm and 0.4μm to improve the rotation accuracy and service life of the bearing.

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