How to classify the processing accuracy of CNC machining?

The classification of CNC machining accuracy needs to comprehensively consider the tolerance grade (IT system), machine tool performance indicators (positioning/repetitive positioning accuracy) and specific process capabilities. When actually selecting, it is necessary to match the processing method and machine tool grade according to the functional requirements of the parts. For example, aerospace parts require high-precision machine tools (IT5 grade), while agricultural machinery parts can be satisfied with ordinary grade (IT8 grade).

1. Classification based on IT tolerance grade

The internationally accepted IT tolerance grade system (ISO standard) divides machining accuracy into 20 grades (IT01 to IT18), and the smaller the value, the higher the accuracy. The specific classification is as follows:

High precision grade: IT01 to IT5, with a tolerance range of microns to submicrons, suitable for precision instruments, aerospace and other fields. For example, mirror grinding can reach IT5 or above, with a surface roughness Ra≤0.01μm.

Medium precision grade: IT6 to IT8, with a tolerance range of microns to tens of microns, used for automotive parts, mold processing, etc. For example, fine turning can reach IT7, with a surface roughness Ra=0.16μm.

Ordinary grade: IT9 to IT18, with a tolerance range of more than 100 microns, suitable for agricultural machinery and rough processing scenarios. For example, rough milling is IT11~IT13, with a surface roughness Ra=20μm.

2. Classification based on machine tool performance

According to the national standard (GB/T25372-2010), machining centers are divided into three levels according to accuracy:

General level (P level)

Positioning accuracy: ≤0.02mm, repeated positioning accuracy ≤0.015mm.

Applicable to general mechanical processing, such as electronic devices and general parts manufacturing.

Precision level (M level)

Positioning accuracy: ≤0.012mm, repeated positioning accuracy ≤0.008mm.

Used in high-demand fields such as precision molds and medical devices.

High-precision level (G level)

Positioning accuracy: ≤0.004mm (some ultra-precision machine tools can reach nanometer level), repeated positioning accuracy ≤0.003mm.

Applied to ultra-precision processing scenarios such as optical components and microelectronics.

3. Subdivision by processing capabilities and processes

High-precision processing

Tolerance: micron level (IT5 or above), surface roughness Ra≤0.1μm, such as mirror turning and ultra-precision grinding.

Medium-precision machining

Tolerance: 0.01~0.1mm (IT6~IT8), surface roughness Ra=0.8~1.6μm, such as fine milling and semi-finishing boring.

Low-precision machining

Tolerance: ≥0.1mm (below IT9), surface roughness Ra≥1.6μm, such as rough turning and drilling.

4. Extended classification of special machine tools

Some standards further refine the grades, for example:

Ultra-precision machining center: positioning accuracy 0.002~0.004mm, used for optical devices and minimally invasive surgical instruments.

Multi-axis linkage machining center: complex surface machining is achieved through linkage of more than five axes, and the accuracy can reach IT7 level.