3 Secrets About Metal Cutting That Nobody Will Tell You
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3 Secrets About Metal Cutting That Nobody Will Tell You

Views: 157     Author: Site Editor     Publish Time: 2023-08-15      Origin: Site

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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. To achieve this process, there must be a relative movement between the workpiece and the tool, that is, the 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

●Introduction to Metal Cutting


Metal cutting is a process in which cutting tools are used to remove excess material from a workpiece, 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 performed 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 there are various forms, they have common phenomena and laws in many aspects. These phenomena and laws are the common basis for learning various cutting methods.


●Subject Content


Overview

The main contents include chip formation and deformation in metal cutting, cutting force and cutting work, cutting heat and cutting temperature, tool wear mechanism and tool life, cutting vibration, and machining surface quality.


Chip Formation Mechanism

From a mechanical point of view, according to the simplified model, the formation process of metal chips is similar to that of pushing a stack of cards to other positions with a tool. The mutual slip between the cards means that the shear deformation of the metal cutting area passes through this. After this kind of deformation, when the chips flow from the front of the tool, further friction deformation occurs at the interface between the tool and the chip. 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.

Metal cutting

Built-up Tumor

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 a wedge-shaped chip material that has undergone severe deformation tends to accumulate 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 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 the built-up edge during finishing. The generation, growth, and shedding of the built-up edge is a cyclical dynamic process, which causes the actual rake angle and cutting depth of the tool to change accordingly, causing fluctuations in cutting force and affecting processing stability. In general, when the cutting speed is very low or very high because there is no necessary condition to produce a built-up edge, no built-up edge is generated.


●Technical Points


Cutting Force

When cutting, the front and back of the tool are both subjected to normal force and frictional force. These forces form a resultant force F. In external turning, the resultant cutting force F is generally decomposed into three mutually perpendicular component forces: tangential force F── It is perpendicular to the tool base surface in the direction of cutting speed, often called the main cutting force; radial force F──in a plane parallel to the base surface, perpendicular to the feed direction, also called thrust; axial force F──in In a plane parallel to the base plane and parallel to the feed direction, it is also called 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, resistance wire and piezoelectric crystal dynamometers are more commonly used. After the dynamometer is calibrated, the size of each component of the cutting process can be measured.


Cutting Heat

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 using cutting fluid, the cutting heat on the tool, workpiece and chips is mainly carried away by the cutting fluid; when the cutting fluid is not used, the cutting heat is mainly carried away or transmitted by the chips, workpiece and tool, of which the heat carried away by the chips is the largest, and the heat is transferred away. 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.

Metal cutting

Cutting Temperature

During the cutting process, the temperatures in the cutting zone are 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 machined surface, the machining accuracy, 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 the 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

The wear of the tool during cutting is a comprehensive result of the physical and chemical effects of cutting heat and mechanical friction. Tool wear is manifested as wear bands, nicks, and chips 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.

Metal cutting

Tool Life

The cutting time elapsed before the tool starts to cut and reaches the tool life criterion is called tool life. The tool life criterion generally uses a predetermined value of tool wear. The occurrence of a certain phenomenon can also be used as a criterion, such as vibration intensification, The roughness of the machined surface is deteriorated, chip breaking and chipping are not good. 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 working-hour quota are often determined according to the processing conditions according to the principle of the lowest production cost or the highest productivity.


Machinability

This 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. 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 of the tool and the wear and cutting conditions are the 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.


Cutting Vibration

During the cutting process, mechanical vibrations such as free vibration, forced vibration, or self-excited vibration are often generated between the tool and the workpiece. Free vibration is caused by some sudden shocks to the parts of the machine tool, and it will gradually attenuate. Forced vibration is caused by the continuous alternating interference force inside or outside the machine tool, and its influence on cutting depends on the size and frequency of the interference force. Self-excited vibration is the initial vibration caused by the sudden interference force between the tool and the workpiece, which changes the rake angle, clearance angle, and cutting speed of the tool, as well as vibration coupling, etc., and obtains the period from the steady-state energy. The energy of sexual action promotes and maintains 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.


Chip Control

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 use two or three-digit coding to represent the shape of various chips, and it is generally believed that short-curved chips are reasonable chip-breaking shapes.


Cutting Fluid

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.

Metal cutting

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