We know that ceramic cutting tools are very suitable for machining hardened steel, heat-resistant alloys or high-temperature alloys, but operators often mistakenly follow the processing methods of cemented carbide. They need to untie this “carbide complex” and consider the special requirements of ceramic tool processing. Choose the appropriate insert shape and the rigidity of the machine tool, tool bar and fixture according to the processing material.
Carbide Tool Complex: Misuse of Ceramic Tools
High temperatures are the main enemy of carbide machining. Therefore, most operators will reduce the cutting speed and increase the feed rate, and when the situation is bad, they will further reduce the spindle speed. However, this machining method that is most suitable for carbide is precisely the biggest taboo in ceramic tool machining, because most of the problems encountered in ceramic tool machining are caused by insufficient cutting speed and excessive feed rate.
High temperature has a different effect on ceramics than on carbide milling tools. During the cutting process, the material being cut is pushed away from the shear area on the tool rake face, and the cutting heat also accumulates in this area. As the cutting speed increases, the heat generated in the shear area cannot be carried away by the waste chips in a short time, which will form a high temperature and produce a softening effect.
Cutting Features of Ceramic Tools
The melting point of cemented carbide is about 1199℃, and high temperature can easily cause deformation and damage to the matrix of cemented carbide inserts. Therefore, reducing the cutting speed can often ensure the reasonable life of cemented carbide inserts. The melting point of ceramic materials is as high as 1999℃, so the high temperature generated during high-speed machining is beneficial to ceramic inserts.
The most suitable cutting speed for ceramic inserts is much higher than that of cemented carbide inserts. The high temperature effect generated during high-speed cutting will soften the processed material, thereby greatly reducing the resistance during cutting. Therefore, under the same conditions, choosing ceramic inserts, which are more fragile than cemented carbide inserts, can easily achieve the same cutting effect as cemented carbide inserts. Sometimes using ceramic tools can increase the material removal rate from hundreds of feet per minute to thousands of feet per minute.
The right combination of cutting speed and feed rate will create an ideal environment for ceramic inserts in the shearing area. But reducing the spindle speed will cause the tool to spark – leading to the failure of the blade and tool.
The material, coated or uncoated, of ceramic inserts is based on a silicon nitride or aluminum oxide matrix. Silicon nitride-based ceramic tools generally have good toughness and are more suitable for rough turning and milling of forgeable cast iron, ductile iron and other difficult-to-process cast iron and high-hardness alloys. In addition to being very suitable for machining cast iron, ceramic tools based on silicon nitride are also suitable for machining steel materials with a hardness lower than HRC65. They can be used in turning rolls and high-temperature alloy machining where whisker-reinforced ceramics cannot be used because the speed is too low. When turning and milling cast iron, a surface linear speed of 1524m/min can achieve the most economical tool life.
Classification and Application of Ceramic Tool Materials
Alumina-based ceramics have good wear resistance and moderate hardness, and are the most economical ceramic tool materials. However, they should be avoided for intermittent, collision or high-hardness material processing. Alumina-based ceramics are mostly used for semi-finishing and finishing of gray cast iron. The high compressive strength of this material makes it very suitable for boring cast iron. However, alumina-based ceramics have poor thermal shock resistance and are therefore not suitable for using coolants during processing.
New alumina-based ceramics containing silicon carbide (SiC) single crystals or whiskers have high melting points, high strength, and good chemical stability, wear resistance, and thermal shock resistance. Whiskers increase the fracture strength of ceramic materials.
Whisker-reinforced ceramic inserts rarely break or break as catastrophically as traditional carbide inserts. Usually, whisker-reinforced ceramic inserts are only gradually worn out in a predictable damage mode.
Whisker-reinforced ceramics are stronger than other ceramic materials and are very suitable for machining high-temperature alloys and similar materials such as hardened steel, high-hardness cast iron, plasma spraying and welding surface processing. For example, when using whisker-reinforced ceramics to machine high-nickel alloys, the interface temperature can reach 982°C and the material removal rate can reach more than 10 times that of carbide tools. The high strength of whisker-reinforced ceramics makes them very suitable for intermittent turning, milling and die/mold processing.
Due to their good resistance to thermal shock, whisker-reinforced ceramic tools can be used for dry cutting, wet cutting or intermittent cooling without worrying about chipping or thermal cracking.
Coated whisker-reinforced ceramics are very suitable for continuous semi-finishing and finishing operations and similar light and medium-intensity operations that require long tool life. The life of coated ceramic tools is three times that of uncoated ceramic tools, but they are not suitable for processing under harsh conditions, such as milling and intermittent cutting.
Ceramic tools are not recommended for machining titanium metals. Titanium has a very low ignition point, and ceramic tool processing will inevitably generate high temperatures, which can easily cause fires.
Clamping and Rigidity Requirements for Ceramic Tools
The rigidity of the tool bar is as important as the rigidity of the machine tool. In a high-volume environment, ceramic inserts must be clamped on special tool bars that prevent the inserts from making small movements. Tool bar rigidity is particularly important in turning with long overhangs. Large overhangs make it easier for the tool bar to produce small deflections during high-speed cutting, and deflections will cause vibrations that damage the ceramic tool. Therefore, the overhang length of tool bars for ceramic inserts should be as short as possible, because the force generated by the tool bar deflection increases cubically with the length of the overhang. That is to say, if other conditions remain unchanged, the tool bar overhang increases by 1 times, and the tool bar deflection increases to 8 times the previous one.
Boring tool bars usually have a larger aspect ratio than external turning tool bars, so it is reasonable to use heavy metal and carbide boring bars. In general, nickel-based alloys can be processed using steel boring tools with an aspect ratio of 3 times, heavy metal boring tools with an aspect ratio of 5 times (such as heavy metal anti-vibration tool bars), and carbide boring tools with an aspect ratio of 7 times.
