Metal cutting is a material removal and forming method in the metal forming process, and it still occupies a large proportion in today’s mechanical manufacturing. The metal cutting process is a process in which the workpiece and the tool interact. The tool cuts the excess metal from the workpiece to be processed, and under the premise of controlling the productivity and cost, the workpiece can obtain the geometric accuracy, dimensional accuracy and surface quality that meet the design and process requirements. In order to realize this process, there must be relative movement between the workpiece and the tool, namely cutting movement, which is provided by the metal cutting machine tool. Machine tools, fixtures, tools and workpieces constitute a machining process system. Various phenomena and laws of the metal cutting process will be studied in the motion state of this system
Metal cutting is a process in which cutting tools are used to remove excess material from a workpiece, so as to obtain parts that meet requirements such as shape, dimensional accuracy, and surface quality. Three conditions must be met to realize this cutting process: there must be relative movement between the workpiece and the tool, that is, cutting motion; the tool material must have a certain cutting performance; the tool must have appropriate geometric parameters, that is, the cutting angle. The metal cutting process is carried out by machine tools or hand-held tools. The main methods include turning, milling, planing, grinding, drilling, boring, gear processing, scribing, sawing, filing, scraping, grinding, reaming, Tap thread, sleeve thread, etc. Although its forms are diverse, they have common phenomena and laws in many aspects. These phenomena and laws are the common basis for learning various cutting methods.
A brief history
The research on the principle of metal cutting began in the middle of the 19th century. In 1851, the Frenchman M. Kokkila first measured the torque of a drill bit when cutting cast iron and other materials, and listed the table of the work required to remove a unit volume of material. In 1864, the Frenchman Jossell first studied the geometrical parameters of the cutting tool. The influence of force In 1870, the Russian… Ji Mei first explained the formation process of chips, and put forward the view that the metal material is not only squeezed but also sheared in front of the tool. In 1896, the Russian Brix began to introduce the concept of plastic deformation into metal cutting. At this point, the chip formation has a more complete explanation. In 1904, the British J.F.
Nicholson manufactured the first three-way dynamometer, which made the research level of cutting force a big step forward. In 1907, American F.W. Taylor studied the influence of cutting speed on tool life and published the famous Taylor formula. In 1915, the Russian…Usachev inserted a thermocouple into a small hole near the cutting edge to measure the temperature of the tool surface (often called the artificial thermocouple method), and used experimental methods to find out the difference between this temperature and the cutting conditions From 1924 to 1926, British EG Herbert, American H. Shore, and German K. Kotwin independently measured the average temperature (often called Natural thermocouple method).
From 1938 to 1940, Americans H. Ernst and M.E. Merchant used high-speed cameras to photograph the chip formation process through a microscope, and used friction to analyze and explain the formation mechanism of intermittent and continuous chips. Since the 1940s, scholars from various countries have systematically summarized and developed the research results of predecessors, made full use of modern technology and advanced testing methods, made many new achievements, and published a large number of papers and monographs. For example, the Americans S. Lamaringam and J.T. Black used a scanning electron microscope to dynamically observe the chip formation using a micro-cutting device in 1972, and obtained the experimental basis for explaining the chip formation with dislocation mechanics.
The main content includes the formation and deformation of chips in metal cutting, cutting force and cutting work, cutting heat and cutting temperature, tool wear mechanism and tool life, cutting vibration and surface quality.
Chip formation mechanism
From a mechanical point of view, according to the simplified model, the forming process of metal chips is related to pushing a stack of cards 1, 2, 3, 4, etc. to positions 1, 2, 3, 4, etc. with a tool. (Figure 1 [Schematic diagram of chip formation process]) The situation is similar. The mutual slippage between the cards means that the shear deformation of the metal cutting area after this deformation, when the chips flow from the front of the tool, it produces further steps at the interface between the tool and the chip. The friction deformation. Generally, the thickness of the chip is greater than the cutting thickness, and the length of the chip is shorter than the cutting length. This phenomenon is called chip deformation. The shear deformation caused by the metal being squeezed by the front of the tool is a characteristic of the metal cutting process. Due to different workpiece materials, tools and cutting conditions, the degree of chip deformation is also different, so various types of chips can be obtained.
When cutting general steel or other plastic materials at low and medium speeds, there is friction between the chip and the front of the tool. If the thin layer on the chip immediately in front of the tool is separated from the chip matrix under the action of higher pressure and temperature But it is bonded on the front of the tool, and then is bonded layer by layer, and it tends to accumulate a wedge-shaped chip material that has undergone severe deformation near the tip of the tool, which is called a built-up edge. The hardness of the built-up edge is more than twice that of the base material, and it can actually replace the cutting edge. The bottom of the built-up edge is relatively stable. There is no obvious dividing line between the top and the workpiece and the chips. It is easy to break and fall off. Some of them are taken away with the chips and some remain on the processing surface, which makes the workpiece rough. Therefore, we must try to avoid or inhibit the formation of built-up edge during finishing. The generation, growth and shedding of built-up edge is a periodic dynamic process (according to the measurement, its shedding frequency is 30-170 times/sec), which causes the actual rake angle and cutting depth of the tool to change accordingly, causing The cutting force fluctuates, which affects machining stability. In general, when the cutting speed is very low or very high, because there is no necessary conditions for generating built-up edge (the friction between the larger chip and the front of the tool and a certain temperature), no built-up edge is generated.
The Technical Points Of Metal Cutting
During cutting, the front and back of the tool are subjected to normal force and friction. These forces form a resultant force F. In external turning, the resultant cutting force F is generally decomposed into three mutually perpendicular component forces (Figure 3 [Resultant cutting force and Force component]): Tangential force F──It is perpendicular to the tool base surface in the cutting speed direction, often called the main cutting force; radial force F──In a plane parallel to the base surface, it is perpendicular to the feed direction, Also known as thrust; axial force F──in a plane parallel to the base plane and parallel to the feed direction, also known as feed force. In general, F is the largest, and F and F are small. Due to the different grinding quality and wear conditions of the tool’s geometric parameters and the change of cutting conditions, the ratio of F and F to F varies in a wide range.
The actual cutting force in the cutting process can be measured with a force dynamometer. There are many types of dynamometers, the more commonly used are resistance wire and piezoelectric crystal dynamometers. After the dynamometer is calibrated, the size of each component of the cutting process can be measured.
When cutting metal, the work done by the shear deformation of the chips and the work done by the friction between the front and back of the tool are all converted into heat. This heat is called cutting heat. When cutting fluid is used, the cutting heat on the tool, workpiece and chips is mainly taken away by the cutting fluid; when cutting fluid is not used, the cutting heat is mainly taken away or transmitted by the chips, workpiece and tool, and the heat carried away by the chips is the largest. Although the heat to the tool is small, the temperature on the front and back affects the cutting process and the wear of the tool, so it is very necessary to understand the law of cutting temperature changes.
During the cutting process, the temperature of the cutting zone is different, forming a temperature field for the temperature distribution of the chips and the workpiece. This temperature field affects the deformation of the chips, the size of the built-up edge, the quality of the processed surface, the accuracy of the machining, and the wear of the tool. Affect the increase in cutting speed. Generally speaking, the metal in the cutting zone becomes chips after being sheared and deformed, and then further violently rubs against the front of the tool. Therefore, the highest point of temperature distribution in the temperature field is not at the edge with the greatest positive pressure, but at the front The upper part is some distance from the cutting edge. The temperature distribution in the cutting area must be measured by manual thermocouple method or infrared temperature measurement method. The temperature measured by the natural thermocouple method is only the average temperature of the cutting zone.
Tool wear during cutting is a comprehensive result of physical and chemical effects caused by cutting heat and mechanical friction. Tool wear is manifested by wear bands, nicks and chipping on the back of the tool, crescent-shaped wear that often appears on the front, and sometimes oxidation pits and groove-like wear on the auxiliary back. When these wears extend to a certain extent, the tool will fail and cannot be used. The factors for the gradual wear of tools usually include abrasive wear, adhesive wear, diffusion wear, oxidative wear, thermal cracking wear, and plastic deformation. Under different cutting conditions, especially at different cutting speeds, the tool is affected by one or more of the above-mentioned wear mechanisms. For example, at lower cutting speeds, tools are generally damaged due to abrasive wear or adhesive wear; at higher speeds, diffusion wear, oxidative wear and plastic deformation are prone to occur.
The cutting time elapsed before the tool starts cutting and reaches the tool life criterion is called tool life (previously called tool durability). The tool life criterion generally adopts a certain predetermined value of tool wear. The appearance of a certain phenomenon can also be regarded as Criteria, such as vibration intensification, deterioration of machined surface roughness, poor chip breaking and chipping, etc. After reaching the tool life, the tool should be reground, indexed or discarded. The sum of the tool life of the tool before it is discarded is called the total tool life.
In production, the tool life and the proposed man-hour quota are often determined according to the processing conditions according to the principle of the lowest production cost or the highest productivity.
Refers to how easy it is for a part to be cut into qualified products. According to the specific processing objects and requirements, it can be used as criteria such as the length of the tool life, the quality of the processed surface, the level of metal removal rate, the size of the cutting power and the difficulty of chip breaking. In production and experimental research, it is often used as an indicator of the machinability of a certain material, and its meaning is: when the tool life is minutes, the cutting speed allowed for cutting the material. The higher it is, the better the processability is, and it usually takes 60, 30, 20 or 10 minutes.
Processing surface quality
Usually include surface roughness work hardening residual stress, surface cracks and metallographic microstructure changes. There are many factors that affect the quality of the machined surface in cutting. For example, the cutting edge radius of the tool and the built-up edge are the main factors that affect the surface roughness; the cutting edge blunt radius and wear and cutting conditions of the tool are the main factors that affect the surface roughness. The main factors of work hardening and residual stress. Therefore, in production, the quality of the machined surface is often improved by changing the geometry of the tool and selecting reasonable cutting conditions.
During the cutting process, free vibration, forced vibration, or self-excited vibration (chatter) and other types of mechanical vibration are often generated between the tool and the workpiece. Free vibration is caused by some sudden impacts on machine tool parts, and it will gradually attenuate. Forced vibration is caused by continuous alternating interference forces inside or outside the machine tool (such as unbalanced machine tool moving parts, intermittent cutting, etc.), and its impact on cutting depends on the size and frequency of the interference force. Self-excited vibration is caused by the sudden interference force between the tool and the workpiece (such as encountering hard points during cutting), which causes the initial vibration to change the rake angle, the clearance angle and the cutting speed of the tool, and the coupling of vibration modes, etc. Obtain periodic energy from steady-state energy to promote and maintain vibration. Generally, various primitive self-excited vibrations may be generated according to cutting conditions, and the chatter marks left on the machined surface will produce more common regenerative self-excited vibrations. The above-mentioned various vibrations usually affect the surface quality of the added tool, reduce the life of the machine tool and the tool, reduce the productivity, and cause noise, which is extremely harmful and must be eliminated or reduced.
Refers to controlling the shape and length of chips. By controlling the curling radius and discharge direction of the chips, the chips collide with the workpiece or the tool, and the curling radius of the chips is forced to increase, and the stress in the chips is gradually increased until the curling radius of the broken chips can be changed by changing the chip’s curling radius. Thickness, chip flutes or chip breakers are ground on the front of the tool to control, and its discharge direction is mainly controlled by selecting a reasonable entering angle and blade inclination. Modern people have been able to express the shape of various chips with two or three digit codes, and it is generally considered that the short arc chip is a reasonable chip breaking shape.
Also called cooling lubricating fluid, it is used to reduce friction during the cutting process and lower the cutting temperature to improve tool life, processing quality and production efficiency. Commonly used cutting fluids include cutting oil, emulsion and chemical cutting fluid.
Production application editing Voice
When designing and using machine tools and tools, it is necessary to apply the data on cutting force, cutting temperature and tool cutting performance in cutting principles. For example, when determining the basic parameters such as the maximum torque and rigidity of the machine tool spindle, the data of the cutting force should be used; when developing new materials with high cutting performance, the law of tool wear and breakage should be mastered; the effect of thermal deformation should be analyzed in the cutting process. When the machining accuracy is affected, it is necessary to study the cutting temperature and its distribution; in the automatic production line and digital control machine tool, in order to make the machine tool work normally and stably, and even realize unmanned operation, it is necessary to apply the related chip formation and its control.
Research results, and realize automatic compensation of tool wear and automatic alarm of tool damage during processing. For this reason, various countries have developed a wide variety of sensors for online detection of tool wear and damage, most of which use cutting force or torque, cutting temperature, and tool wear as sensing signals. In addition, in order to make full use of machine tools, improve processing economy and develop computer-aided manufacturing (CAM), it is often necessary to apply optimized data such as cutting conditions, tool geometry, and tool life. Therefore, the principle of metal cutting is increasingly widely used in production. Various countries have accumulated a large amount of cutting data through cutting tests or on-site collection, and used mathematical models to express the tool life, cutting force, power and surface roughness equivalent to cutting. The relationship between the conditions is then stored in the computer, and a metal cutting database is established or compiled into a cutting data manual for users to check and use.
General operating procedures editing Voice
The operator must pass the examination and hold the “Equipment Operation Certificate” of this machine tool before operating this machine tool.
Earnestly do it before work:
- 1. Read the shift record carefully to understand the operation status and existing problems of the machine tool in the previous shift.
- 2. Check the machine tool, workbench, guide rail and all main sliding surfaces. If there are obstacles, tools, iron filings, impurities, etc., they must be cleaned, wiped clean, and oiled.
- 3. Check the workbench, guide rail and main sliding surface for new pulling, grinding, or bruising. If any, notify the team leader or equipment personnel to check together, and make a record.
- 4. Check the safety protection, braking (stop), limit and reversing devices should be complete and intact.
- 5. Check that the operating handles, gates, switches, etc. of machinery, hydraulics, and pneumatics should be in non-working positions.
- 6. Check that each tool rest should be in a non-working position.
- 7. Check that the electrical distribution box should be closed firmly and the electrical grounding is good.
- 8. Check that the amount of oil in the oil storage part of the lubrication system should meet the requirements and be well sealed. The oil indicator, oil window, oil cup, oil nozzle, oil line, oil felt, oil pipe and oil separator should be complete and installed correctly. According to the lubrication instruction chart, do manual oiling or motorized (hand position) pump oiling, and check whether oil is coming from the oil window.
- 9. For machine tools that are parked for more than one shift, they should be idle for 3 to 5 minutes in accordance with the operating procedures and requirements of the manual and the use of hydrostatic devices (see Appendix I for details).
- ① Whether the control handles, gates, switches, etc. are flexible, accurate and reliable.
- ② Whether the safety protection, brake (stop), interlock, clamping mechanism and other devices are working.
- ③ Check whether the movement of the mechanism has enough travel, adjust and fix the limit, fixed-range stopper and reversing bumper, etc.
- ④ Check whether there is oil in the lubricated parts of a motorized pump or a hand pump, and whether the lubrication is good.
- ⑤ Whether the movement, working cycle, temperature rise, sound, etc. of mechanical, hydraulic, static pressure, pneumatic, profiling, profiling and other devices are normal. Whether the pressure (hydraulic pressure, pneumatic pressure) meets the requirements. After confirming that everything is normal, you can start working.
For the equipment that is continuously transferred, the transferee should check the above (Article 9) together. After the transfer is clear, the transferee can leave. For equipment that is succeeded from another shift, if it is found that there is a serious violation of operating procedures in the previous shift, the team leader or equipment personnel must be notified to check it together and make a record, otherwise it will be handled in accordance with the operating procedures of this shift.
After the equipment is overhauled or adjusted, the equipment must also be inspected in detail according to the above (Article 9), and work can only be started after it is deemed that everything is correct.
Earnestly accomplish in work:
- 1. Stick to your post, operate carefully, and don’t do anything that has nothing to do with your work. Stop the machine when leaving the machine due to accidents, and turn off the power and air supply.
- 2. Process according to the process regulations. It is not allowed to arbitrarily increase the amount of feed, the amount of grinding and the cutting (grinding) speed. It is not allowed to use the machine tool beyond specification, overload, or weight. Rough use of precision machines and small use of large machines are not allowed.
- 3. The tool and workpiece should be clamped correctly and fastened firmly. Do not damage the machine during loading and unloading. Align the tool, and the workpiece is not allowed to be hit by a heavy hammer. It is forbidden to tighten the tool or workpiece by lengthening the handle to increase the torque.
- 4. It is not allowed to install thimble, cutters, knife sleeves, etc. which are not in accordance with the taper or hole diameter of the machine tool spindle taper hole, tailstock sleeve taper hole and other tool installation holes, surface nicks, and dirty.
- 5. The mechanical speed change of the transmission and feed mechanism, the clamping and adjustment of the tool and the workpiece, and the manual measurement of the workpiece between the working procedures should be carried out after the cutting and grinding are terminated, and the tools and abrasive tools should be stopped after the workpiece is moved away from the workpiece.
- 6. Knives and abrasive tools should be kept sharp, if they become dull or cracked, they should be sharpened or replaced in time.
- 7. During cutting and grinding, the tool and abrasive tool are not allowed to stop without leaving the workpiece.
- 8. It is not allowed to disassemble the safety protection device on the machine tool without authorization, and the machine tool without safety protection device is not allowed to work.
- 9. The hydraulic system is not allowed to adjust without authorization except for the throttle cutting.
- 10. Tools, workpieces and other sundries are not allowed to be placed directly on the machine tool, especially on the guide rail surface and the work table surface.
- 11. Frequently remove iron filings and oil stains on the machine tool, and keep the guideway surface, sliding surface, rotating surface, positioning reference surface and working table surface clean.
- 12. Pay close attention to the operation and lubrication of the machine tool. If you find abnormal phenomena such as malfunction, vibration, heat, crawling, noise, peculiar smell, bruises, etc., you should stop for inspection immediately and continue working after troubleshooting.
- 13. In the event of a machine tool accident, press the master stop button immediately, keep the accident scene, and report to the relevant department for analysis and processing.
- 14. Welding and repair welding of workpieces on the machine are not allowed.
Earnestly after work:
- 1. Put the mechanical, hydraulic, pneumatic and other operating handles, gates, switches, etc., to the non-working position.
- 2. Stop the machine tool and cut off the power and air supply.
- 3. Remove iron filings, clean the work site, and carefully clean the machine tool. Refuel and maintain the guide rail surface, rotating and sliding surface, positioning reference surface, and working table surface.
- 4. Carefully fill in the machine tool problems found in the shift in the shift record book, and do a good job of shifting.