Analysis of Tool Selection in CNC Lathe Processing
The reasonable selection of turning tools directly affects the service life and processing efficiency of the tools, as well as the processing cost. The selection of turning tools needs to be determined by combining multiple factors and then using reasonable cutting parameters to achieve the goals of optimal quality, fastest speed, and lowest cost in processing.
CNC lathe machining, as a common method in mechanical processing, has different cutting characteristics due to the different materials of parts. Therefore, the most suitable tool should be selected based on the processing situation. Choosing the correct and appropriate turning tool is beneficial for ensuring stable machining quality, reducing tool wear, and reducing machining costs. Choosing a suitable tool can be considered from two aspects: the usage requirements of the turning tool and the characteristics of the turning tool itself.
1. Requirements for the use of turning tools
During the process of tool cutting, the workpiece is subjected to friction and deformation, and the tool also bears the corresponding cutting force. The selection of the tool will directly affect the quality of the workpiece. To ensure the efficient, energy-saving, stable, economical, and quick tool replacement of CNC lathe processing, the following requirements are required for the tools used in CNC lathes.
1.1 Strength and rigidity of cutting tools
The cutting tool should have high strength, good stiffness, and vibration resistance so that the machining can be carried out in a stable state. Otherwise, it will directly affect the machining quality. For example, when turning inner holes, the aperture and length of the parts have significant limitations on the selection of tools. Deep hole cutting often causes vibration of the tool holder, which not only causes wear on the cutting edge but also may produce vibration patterns on the machined surface, affecting the quality of the machined surface. Therefore, small-diameter hole cutting usually uses hard alloy or tungsten steel tool holders, and medium and above diameter cutting uses vibration reducing tool holders.
Cutting tools should have high economic efficiency, which is conducive to controlling processing costs. When selecting tools, try to choose tools with higher cost-effectiveness while meeting processing requirements. For example, cutting tools made of high-speed tool steel material are not only cheap but also easy to grind and can be used repeatedly. Under the premise of meeting processing requirements, there is no need to use alloys or other expensive cutting tools, reducing processing costs.
1.3 High precision and strong adaptability
When selecting cutting tools, appropriate tools should be selected based on the characteristics of the processing materials. The selected tools should have strong adaptability to the processing, ensuring the accuracy and quality of the processing. For example, when processing materials with relatively good toughness, such as aluminum, in order to achieve better surface quality and dimensional accuracy, the impact of the extrusion force generated between the workpiece and the tool during processing on surface quality should be considered. When processing nylon or other polymer materials, the low melting point characteristics of the material should also be considered. When selecting cutting tools, the heat generated during processing should be minimized as much as possible to avoid the impact of processing heat on the surface of the workpiece.
1.4 Meet the requirements of process parameters
The selected tool can meet the process requirements of cutting speed, feed rate, back feed, and other aspects. Processing efficiency is an important reference factor during machining, especially during rough machining. In order to quickly cut off the cutting part in a short period, larger cutting speeds, feed rates, and backdraft are often used to improve processing efficiency and shorten processing time effectively. During this process, the tool needs to withstand larger cutting forces, which have certain requirements for the material and tip radius of the tool.
1.5 Service life
The good wear resistance and long service life of cutting tools can not only directly reduce the frequency of tool replacement and processing costs but also ensure the dimensional stability and surface quality of the workpiece. To reduce tool wear, the hardness of the tool should be as much as possible greater than the hardness of the workpiece material in order to reduce tool wear. For example, when machining low-hardness workpieces such as nylon, the difference in wear between sharp steel and alloy knives may not be significant. However, when machining 45 steel or other hard workpieces, the wear of alloy knives will be significantly smaller than that of sharp steel knives. To ensure the service life of the cutting tool, the tool material and cutting parameters should also be reasonably selected based on the material of the workpiece to be machined. For example, when processing stainless steel workpieces, due to the good toughness of stainless steel, the iron chips cut during the machining process make it easy to adhere to the cutting edge, reducing the cutting performance of the tool and leading to the phenomenon of tool breakage. Therefore, when processing stainless steel materials, tools with materials that are not easily prone to tool breakage are often selected, and small back drafts and low feed speeds are used for processing.
During batch processing, replacing tools or blades is inevitable. To reduce the impact of replacing tools or blades on machining size and efficiency and to avoid tool alignment, it is an effective method to require easy replacement or disassembly of tools. The blades and tool holders should be universal, standardized, serialized, and standardized. After replacing tools or blades, the position of the tooltip will not change significantly, and the relative position accuracy of the tool holder should be high.
2. Characteristics of cutting tools
2.1 Tool materials
The materials used for machining parts are different, and the materials used for cutting tools are also different. The selection of cutting tool materials mainly considers three properties of the materials: cold hardness, red hardness, and toughness. In simple terms, cold hardness refers to the hardness of a material in the cold state, i.e., at room temperature, also known as wear resistance; Red hardness refers to the hardness of a cutting tool at high temperatures; Toughness refers to the ability of a tool to withstand vibration and impact loads. During the cutting process, the cutting head must withstand high pressure and temperature. Therefore, in addition to sufficient strength, toughness, and wear resistance, the blade material must also have high red hardness. Common cutting tool materials include high-speed steel, hard alloy, and coated cutting tools.
2.1.1 High-speed steel
High-speed steel is an alloy steel with a high content of tungsten and chromium, also known as white steel or front steel, with a higher hardness of 62 HRC-65 HRC, which is about 2.7 times the hardness of 45 # steel. It has a certain degree of red heat hardness, temperature resistance up to 560 ℃ -600 ℃, good toughness and processing performance. High-speed steel cutting tools are easy to manufacture, easy to grind, and can be ground into various angles and shapes according to specific processing requirements. Especially when processing some formed surfaces, its advantages of easy grinding and forming are particularly prominent. After grinding and grinding, high-speed steel can obtain sharper blades, which generate less processing heat during processing and are suitable for processing plastic workpieces with lower melting points. High-speed steel is cheap, can be repeatedly ground and used, has high economic efficiency, and can effectively control the cost of use. However, due to its lower hardness than hard alloys and its tendency to wear during high-speed cutting, it is not suitable for high-speed cutting of steel parts, resulting in lower surface processing quality than alloy cutting tools. So, when processing materials with low hardness and low melting points, such as nylon and some polymer materials, high-speed steel knives have obvious comprehensive advantages. However, when processing workpieces with harder materials, high-speed steel knives should be avoided as much as possible.
2.1.2 Hard alloy
Hard alloys are formed from powders of refractory materials such as tungsten carbide, titanium carbide, and cobalt under high pressure and sintered at temperatures ranging from 1350 ℃ to 1560 ℃. It has extremely high hardness and can reach 92 HRA at room temperature, second only to diamond. At the same time, the red hardness is very good, and it can still maintain good cutting performance at around 1000 ℃, with high usage strength and bending strength of up to 100 kg/mm3-170 kg/mm3. So when processing some materials with high hardness, hard alloy cutting tools are widely used. Hard alloy cutting tools can be divided into welded and rack types: welded cutting tools only have a hard alloy tip, which has the advantage of being able to obtain the desired shape or angle through grinding like high-speed steel cutting tools, and can also be repeatedly polished and used; Its disadvantage is that after the tool is reassembled, the position of the tooltip relative to the tool table changes, and the tool must be readjusted before continuing processing, so it is often not suitable for batch processing. The rack-type tool is a standard alloy blade made from the cutting part of the tool head. When the blade is worn or damaged, it can be directly replaced. After replacing the blade, the position of the tooltip relative to the tool table does not change much. Adjust the tool compensation to continue processing and use. Its disadvantage is that standard alloy blades are often not easy to grind and polish, and usually the tooltip can only be scrapped after wear, which is not as economical as welded tools.
2.1.3 Coated cutting tools
There is a contradictory relationship between the two key indicators of tool performance, namely hardness and toughness. Materials with high hardness often have low toughness, and improving toughness often comes at the expense of hardness. A coated tool composed of one or more layers of metal or non-metallic compound films with high hardness and good wear resistance coated on a softer tool substrate effectively solves the contradiction between the strength and toughness of the tool.
The commonly used coated cutting tools refer to coating the surface of cutting tools made of high-speed steel, hard alloy, or other materials with good toughness and coating a thin layer of materials with high hardness, high wear resistance, and high temperature resistance. During cutting, the coating area forms a protective zone of chemical and thermal barriers, reducing direct contact between the tool material itself and the workpiece, thereby reducing the wear of the cutting edge. Compared with uncoated tools, coated tools have advantages such as high surface hardness, good wear resistance, stable chemical properties, heat resistance and oxidation resistance, low friction coefficient, and low thermal conductivity. Compared to uncoated tools, coated tools have a longer tool life of more than 3-5 times during cutting. Coated cutting tools can be divided into TiC, TiN, Al2O3 and other coatings based on different coating materials. The hardness, toughness, wear resistance, and other properties of coatings made of different materials also vary. Therefore, suitable coated cutting tools should be selected according to different processing requirements. For example, TiCN-based coatings are suitable for processing materials such as ordinary steel, alloy steel, stainless steel, and wear-resistant cast iron. When using them to process workpieces, the material removal rate can be increased by 2-3 times; CrN is a titanium free coating that is suitable for cutting titanium and titanium alloys, copper, aluminum, and other soft materials. It has good chemical stability and does not produce sticking chips.
Coated cutting tools are now widely used in metal cutting. However, coated tools also have certain drawbacks. The coating material covering the surface of the tool not only improves the machining characteristics of the tool but also reduces the sharpness of the tool edge to a certain extent. The machining heat generated in the cutting area is significantly increased, and the tool itself can usually withstand this heat. However, when processing some low melting point materials, this heat may cause hot melting on the machining surface of the workpiece; it will directly affect the dimensional accuracy and surface quality of the workpiece.
2.2 Selection of tooltip arc radius
When manufacturing or sharpening turning tools, in order to protect the blade tip, a blade tip arc is retained in the blade tip part, and its radius is a value that is difficult to measure accurately. The common radius value is about 0.2 mm-0.8 mm. If the hypothetical tooltip position is used as the cutting point, programming in CNC turning is very simple. But because any tool has a tip arc, the size of the tip arc does not matter when turning the outer cylindrical surface or end face. But when turning chamfers, cones, arcs, or surfaces, it will affect the machining accuracy of the parts, and there will be over-cutting and under-cutting phenomena when programming with hypothetical tooltip positions.
The machining performance of tooltips with different radii is also different. The smaller the arc radius of the tooltip, the sharper the tool, the smaller the machining friction, and the smaller the machining heat generated. At the same time, it can reduce machining vibration and improve the surface quality of the machining while also making the machining size more accurate and stable. However, due to the smaller arc radius of the tooltip, the sharper the tool tip, which is prone to wear and even tool breakage, resulting in a shorter lifespan. It is not suitable for cutting with large back drafts, so it is often used for precision machining tools. If the arc radius is too small, it will also have an impact on the quality of the processed surface, causing obvious grooves on the processed surface and reducing the quality of the processed surface.
The larger the arc radius of the cutting tip, the stronger the strength of the cutting tip. The cutting tip is also sturdier, wear-resistant, and less prone to edge breakage. It can be used for cutting with a large back draft and high feed speed, which is beneficial for improving machining efficiency. However, the larger the radius of the tooltip, the larger the contact surface between the tool and the workpiece, and the greater the friction surface and extrusion force in the processing area, which can cause vibration and is not conducive to ensuring the stability of the processing size. Therefore, it is often used for rough machining.
2.3 Blade angle and shape
The tool angle is an important parameter used to determine the geometric shape of the cutting part of the tool. The selection of blade shape requires comprehensive consideration of processing form, cutting edge strength, clamping strength, economy, etc. For example, cylindrical turning tools have various shapes, including 120 ° for regular hexagons, 90 ° for squares, 60 ° for regular triangles, 35 ° for diamonds, 55 °, 85 °, and so on. Among them, the diamond-shaped blades of 35 °, 85 °, and equilateral triangle blades of 60 ° are particularly common. Although they are all cylindrical blades and can be used to process the outer circle, their processing characteristics also have certain differences.
The diamond-shaped 35 ° blade has a relatively large offset angle, which can reduce the interference between the secondary cutting edge and the machined surface. It has a wide applicability when processing cylindrical surfaces and can process nonmonotonic cylindrical contours. However, the diamond-shaped 35 ° blade has a thinner blade tip and poor cutting-edge strength, making it unsuitable for enduring large cutting forces for long periods and machining with an offset end face.
The diamond-shaped 85 ° blade has good cutting-edge strength, can withstand large cutting forces, and can simultaneously meet the machining requirements of both the outer and end faces. It is widely used in rough and fine machining, but due to its small secondary deflection angle, it is not suitable for machining nonmonotonic contour outer circles.
The performance of a 60 ° blade with a regular triangle is between that of a 35 ° and 85 ° diamond blade. It has both a strong cutting edge and a certain angle of secondary deviation and can be used for machining some nonmonotonic outer circles. Compared to diamond-shaped blades, the number of blade tips used for a regular triangle 60 ° blade is also different: usually, diamond-shaped blades only have two blade tips on one side to use, while regular triangle 60 ° blades have three blade tips on one side to use. Therefore, regular triangle 60 ° blades have a higher utilization rate and lower usage cost.
In summary, many factors affect the machining of CNC lathes, including various factors such as lathes, cutting tools, and programs, which need to be comprehensively considered. Among them, the selection factors of turning tools alone go far beyond the above analysis. The reasonable selection of turning tools directly affects the service life and processing efficiency of the tools and also affects the processing cost. The selection of turning tools also needs to be determined by combining multiple factors, and reasonable cutting parameters can be used to achieve the goal of optimal quality, fastest speed, and lowest cost in processing.
Author: Jiang Yi