How Is the Machining Accuracy of CNC Machining Classified?
Machining accuracy refers to the precision of product manufacturing and is a mechanical term used to evaluate the quality of machined surfaces. Machine accuracy is generally denoted by IT, which stands for the grade determining the precision of dimensions, commonly referred to as tolerance. The smaller the tolerance grade, the higher the precision of the workpiece, but the greater the difficulty of machining.
Generally, according to the international tolerance grade classification, IT6 is widely applied in critical machining tolerances. IT6 indicates that the mating surfaces have high uniformity requirements, ensuring excellent mating properties, stable and reliable performance, and enabling machining centers to perform various processes such as drilling, milling, boring, reaming, tapping, and rigid tapping. How are the precision levels of these machining processes classified and defined?
Milling Machine
Milling is the most critical machining process in a machining center, and it can be said that all machining processes in a machining center are carried out through milling. It is classified into climb milling and conventional milling based on the direction of spindle movement and workpiece feed. Using rotating multi-edge tools, CNC machining centers achieve efficient and common material removal. It is suitable for machining flat surfaces, grooves, various complex non-planar surfaces, and curved mold surfaces, among other special profiles.
The machining accuracy tolerance grade for precision milling in milling can generally reach IT16–IT8. Under this tolerance standard, the surface roughness of the workpiece is 0.63–5 μm.
Drilling Machine
Drilling is another important machining process in machining centers. It is a basic method for hole machining. Drilling is often performed on drilling machines and lathes, and some high-precision drilling can also be completed using machining centers.
Drilling has lower processing accuracy, with tolerance grades generally only reaching IT10. The surface roughness of the workpiece is typically 12.5–6.3 μm. However, after drilling, reaming and boring are often used for subsequent semi-finishing and finishing operations.
Boring
Boring is generally used for processing inner diameters. It is a machining process that uses tools to enlarge holes or cut circular contours of inner diameters, typically used from semi-rough machining to finishing. For steel-type workpieces, the tolerance grade for boring accuracy can reach IT9–IT7, with a surface roughness of 2.5–0.16 μm.
In current mechanical processing operations, grinding has the highest precision requirements and the lowest tolerance grade. In contrast, stamping and die casting have higher tolerance grades. Understanding the precision tolerances of each process allows us to fully leverage the high precision capabilities of CNC machining centers.