The machinability of materials refers to the difficulty of cutting the excess part of materials to obtain qualified parts. Therefore, the cutting performance of materials is affected by many factors, mainly the physical and mechanical properties of workpiece materials, tool materials and geometric angles, cutting parameters and so on.
The traditional concept generally believes that gray cast iron is a metal material with excellent machinability, so the research on the machinability of gray cast iron is rare. However, with the process of cutting automation, especially at present, enterprises generally adopt CNC machine tools and automatic machining lines, high-speed cutting technology and high-quality tools. The cutting performance of gray cast iron is very sensitive to the wear and service life of tools, which seriously affects the production efficiency. In order to reasonably evaluate the cutting performance of gray cast iron, it is usually evaluated by tool durability, machining surface quality, metal removal rate, power consumption, machining accuracy, cutting force, cutting temperature, chip control and chip breaking difficulty.
1. Cutting heat method of gray cast iron
Cutting heat and cutting temperature of gray cast iron are important physical phenomena in the process of metal cutting. Most of the work done during metal cutting is converted into heat energy. In addition to the heat energy generated during cutting, there are also surface energy of new surface, residual strain energy of machined surface and chip, but this part of energy only accounts for 1% ~ 3% of the total energy. In the total heat energy converted, except for a very small amount of heat radiation, the rest are used to heat chips, workpieces and cutting tools. In the process of metal cutting, the cutting temperature generated by cutting heat, as an important physical index, has an important impact on tool life and part processing quality. Therefore, the cutting heat of gray cast iron is an important aspect of metal cutting. In the cutting process, the cutting heat mainly comes from the friction work consumed by the friction between the cutting and the rake face, the workpiece and the rake face, and the deformation work consumed by the elastic deformation and plastic deformation of the cutting layer metal under the action of the tool.
Generally speaking, when cutting gray cast iron, the highest point of tool temperature is a point near the tool tip. The farther away from the machined surface, the lower the temperature. At present, the research on cutting temperature at home and abroad mainly focuses on the measurement of cutting temperature and the simulation of cutting temperature field. Usually, the cutting temperature is characterized by the average temperature of the cutting area, that is, the contact area between the tool rake face and the chip. The commonly used measurement methods of cutting temperature of gray cast iron are generally divided into measurement method and calculation method. The actual measurement methods generally include thermocouple method, metallographic observation method and photothermal radiation method. These methods generally measure a certain position of the turning tool, according to the distance between the measuring point and the tool tip, and then use the reverse method of heat conduction to obtain the experimental temperature value of the turning tool. The calculation method is mainly based on a certain assumption of cutting heat source, using the theory of solid heat conduction and energy conversion to deduce and analyze the cutting temperature and the distribution of temperature field.
In recent years, through the actual measurement of the temperature of a measuring point of the tool during turning at home and abroad, combined with the finite element heat transfer simulation, the accurate measurement of the tool tip temperature is realized. It provides a new method to study the measurement of cutting temperature in turning process, so as to better study the cutting thermal characteristics in cutting process.
2. Tool wear method
As the direct executor of gray cast iron cutting process, the tool inevitably has the phenomena of wear and damage in the cutting process of workpiece. In the traditional gray cast iron cutting process, the wear state of the tool is mainly judged by the operator according to the processing time and the noise in the processing process, or the damage degree and wear amount of the tool are measured and evaluated by the shape, appearance and surface quality of the blade during or after the processing.
Using different sensors and different working principles, the main methods to detect tool wear are optical measurement, resistance measurement, ray measurement and so on. The optical measurement method is to observe and analyze the wear area and geometry of the tool wear area. The larger the damage area of the blade, the greater the tool wear in the cutting process of gray cast iron. The resistance measurement method mainly uses the contact conductivity between the blade and the sensor to calculate the wear width of the blade as the conductive area decreases with the tool wear. X-ray measurement method is to measure the radioactive material particles through the X-ray measuring device in the process of cutting gray cast iron, and use the size of the radiation dose to reflect the size of tool wear. However, these methods have obvious weaknesses, or can not detect the real-time measurement of the tool in the machining process, or are greatly affected by the tool displacement and chip deposition on the tool, so they can not accurately measure the state of the cutting edge of the tool.
With the development of modern science and technology, the measurement of tool wear has changed from shutdown measurement to on-line detection. Scholars at home and abroad have conducted extensive and in-depth research on tool real-time monitoring. Through the intelligent combination of modern computer technology and information acquisition and processing system, different forms of wear states on the blade can be accurately detected.
3. Vibration detection method
Vibration signal is considered to be a signal with high sensitivity to tool wear and damage. It is closely related to the cutting force and the dynamics of gray cast iron cutting system. Detecting vibration acceleration is a commonly used monitoring method at present. It is more commonly used in vibration engineering. It has the characteristics of convenient sensor installation, easy measurement signal extraction, simple test instrument and so on.
4. Workpiece surface roughness
Surface roughness is the evaluation of all kinds of small convex and concave undulations on the machined surface, and it is the micro geometric error of the workpiece surface. In the process of gray cast iron cutting, with the passage of gray cast iron cutting time and the change of tool wear, the machined surface roughness of the workpiece also changes. Therefore, the wear condition of the tool can be evaluated indirectly. In recent years, there are more and more measuring instruments and methods for surface roughness. At present, people mainly measure and evaluate the surface roughness of workpieces by contour method and area method. According to the measurement principle, the instruments measured by contour method can be divided into stylus method, light cutting method and microscope interference method. The corresponding instruments used include stylus surface roughness meter, light cutting microscope and interference microscope. This method is more suitable for laboratory environment, and can directly obtain the evaluation parameter “surface finish” of surface roughness. Area method mainly includes roughness comparison template, instrument based on capacitor principle, pneumatic instrument and instrument based on reflection measurement principle. This method is mainly based on the principle of optical reflection, and the relative value of workpiece surface roughness is obtained. Its test efficiency is high. Sample calibration is required in advance, which is greatly affected by cutting fluid, chip, workpiece material, vibration, etc.
5. Cutting force
In the process of cutting, the cutting force of gray cast iron is one of the most important output parameters. In the cutting process, the cutting force directly affects the generation of cutting heat, and further affects the tool wear, durability and machined surface quality. At the same time, cutting force is the necessary basis for calculating cutting power, designing and using machine tools, cutting tools and fixtures. In the process of cutting, the change of workpiece material and cutting conditions will cause the change of cutting force. There is a direct relationship between the cutting force of gray cast iron and tool wear. When the tool is cutting the workpiece, the growth rate of cutting force is linear with the tool wear rate. In the process of cutting gray cast iron, with the aggravation of tool wear, the cutting force of gray cast iron will also change accordingly. The component of cutting force and the change of torque can be measured through force measuring sensors, so that the wear state of tools can be detected indirectly.
The measuring device and process of cutting force of gray cast iron are relatively simple and direct, which can dynamically study the changes of cutting performance in the cutting process. People have done a lot of research on evaluating the cutting performance of gray cast iron by cutting force combined with other parameters. Seker et al. Compared and studied the cutting force, tool wear, workpiece surface roughness, chip deformation and chip breaking performance of several newly developed low-carbon and high sulfur free cutting steel materials in the turning process. The cutting force and tool wear were used to evaluate the effect of sulfur, rare earth and Bismuth on the machinability of austenitic stainless steel. The addition of sulfur, rare earth and bismuth alloy elements reduces the cutting force, reduces the wear of tools and improves the shape of chips.