What are the characteristics of CNC turning?
2021-10-26

What are the characteristics of CNC turning?

CNC turning is a method of cutting the workpiece on the lathe by rotating the workpiece relative to the tool. Turning is the most basic and common cutting processing method. Most workpieces with revolving surfaces can be processed by turning methods, such as inner and outer cylindrical surfaces, inner and outer conical surfaces, end surfaces, grooves, threads, and rotary forming surfaces. Common lathes can be divided into horizontal lathes, floor lathes, vertical lathes, turret lathes, and profiling lathes, and most of them are horizontal lathes.


Due to the development of modern technology, various high-strength and high-hardness engineering materials are increasingly used. Traditional turning technology is difficult or impossible to process certain high-strength and high-hardness materials. The modern CNC turning technology makes it possible and has achieved obvious benefits in production.


1. Introduction to the characteristics of turning

(1) High turning efficiency

Turning has a higher efficiency than grinding. Turning uses large cutting depths and high workpiece speeds. The metal removal rate is usually several times that of grinding. Once clamping can complete multiple surface processing while grinding requires multiple installations, the extra time is short, and the position accuracy between the processed surfaces is high.


(2) The cost of equipment investment is low. 

When the productivity is the same, the investment of the lathe is better than that of the grinder, and the cost of the auxiliary system is also low. For small batch production, turning does not require special equipment. In contrast, large batches of high-precision parts require CNC machine tools with good rigidity, high positioning accuracy, and high repeat positioning accuracy.


(3) Suitable for small batch flexible production requirements. 

The lathe itself is a flexible processing method with a wide range of processing. The lathe is easy to operate and fast in turning and clamping. Compared with grinding, CNC turning can better meet the requirements of flexible production.


(4) Hard turning can make the parts obtain good overall machining accuracy.

Most of the heat produced in CNC turning is taken away by the cutting oil, and it will not produce surface burns and cracks like grinding. It has excellent processing surface quality and precise processing roundness, ensuring high position accuracy between processing surfaces.


2. Cutting tool materials and their selection

(1) Coated carbide tools

Coated cemented carbide tools are coated with one or more coatings with good wear resistance on the toughened cemented carbide tools. The coating usually plays the role of the following two aspects: On the one hand, it has a much lower thermal conductivity coefficient than the tool matrix and the workpiece material, which weakens the thermal effect of the tool matrix. On the other hand, it can effectively improve the friction and adhesion of the cutting process and reduce the generation of cutting heat. Coated carbide cutting tools have greatly improved strength, hardness, and wear resistance compared with cemented carbide cutting tools.


(2) Ceramic tool

Ceramic tools have the characteristics of high hardness, high strength, good wear resistance, good chemical stability, good anti-adhesive performance, low friction coefficient, and low price. In normal use, the durability is extremely high, and the vehicle speed can be several times higher than that of cemented carbide. It is especially suitable for high-hardness material processing, finishing, and high-speed processing.


(3) Cubic boron nitride tool

The hardness and wear resistance of cubic boron nitride is second only to diamond, and it has excellent high-temperature hardness. Compared with ceramic tools, its heat resistance and chemical stability are slightly worse, but its impact strength and crush resistance are better. It is widely used in cutting hardened steel, pearlitic gray cast iron, chilled cast iron, and high-temperature alloys. Compared with cemented carbide tools, its cutting speed can even be increased by order of magnitude.


3. The selection of cutting oil

(1) Tool steel tools have poor heat resistance and lose hardness at high temperatures. Therefore, cutting oil with good cooling performance, low viscosity, and good fluidity is required.


(2) When high-speed steel cutting tools perform high-speed rough cutting, the cutting amount is large and a large amount of cutting heat is generated, so cutting oil with good cooling properties should be used. If high-speed steel tools are used for medium and low-speed finishing, low-viscosity cutting oil is generally used to reduce the friction and bonding between the tool and the workpiece, inhibit the formation of cutting edges, and improve the machining accuracy.


(3) Cemented carbide tools have higher melting point and hardness, better chemical and thermal stability, and much better cutting and wear resistance than high-speed steel tools. Active sulfur cutting oil can be used in general processing. If it is heavy cutting, the cutting temperature is very high, and it is easy to wear the tool very quickly. At this time, inactive vulcanized cutting oil should be selected, and the flow of cutting oil should be increased to ensure sufficient cooling and lubrication.


(4) Ceramic, diamond, and cubic boron nitride cutting tools have high hardness and wear resistance. Generally, low-viscosity inactive vulcanized cutting oil is used during cutting to ensure the surface finish of the processed workpiece.


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