How Is The Machining Accuracy Of Cnc Machining Classified?
Machining accuracy in CNC machining plays a vital role in determining the quality and precision of finished components. It is evaluated using international tolerance grades (IT grades), which indicate how much a machined part may deviate from its nominal dimensions. This article explores how machining accuracy is classified based on different CNC processes and their respective tolerance levels.
Understanding IT Grades
The IT grade defines the allowable deviation from a specified dimension. For example, IT6 is a common tolerance grade used in precision applications. It ensures excellent mating surfaces, high repeatability, and reliable performance. Moreover, IT6 applies to various CNC operations such as drilling, milling, boring, reaming, tapping, and rigid tapping.
Accuracy by CNC Process
Milling Tolerances and Surface Finish
Milling is the primary process in CNC machining centers. It uses rotating multi-edge tools to cut the workpiece, enabling efficient shaping of flat surfaces, grooves, complex curves, and mold cavities.
- Types: Up milling and down milling
- Tolerance Grade: IT16 to IT8 (for finishing)
- Surface Roughness: 0.63 to 5 μm
Drilling Precision and Finish Level
Drilling provides a fundamental method for hole processing. Technicians perform it on drilling machines, lathes, or CNC machining centers.
- Precision Level: Lower compared to milling
- Tolerance Grade: Up to IT10
- Surface Roughness: 12.5 to 6.3 μm
Following drilling, operators often use reaming or boring to improve hole accuracy.
Boring Capabilities for Internal Diameters
Boring enlarges or refines existing holes. It is commonly used for internal diameter machining and ranges from semi-finish to precision finish stages.
- Tolerance Grade: IT9 to IT7 (for steel parts)
- Surface Roughness: 2.5 to 0.16 μm
Grinding achieves the highest machining accuracy among metal processing methods. It delivers the lowest tolerance grades and finest surface finishes. In contrast, processes like stamping and die casting offer higher tolerance grades and less precision. Therefore, understanding tolerance standards helps manufacturers select the right process and maximize precision output.