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MindMap Gallery Machinery Manufacturing Basics
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Machinery Manufacturing Basics
Tool angle, cutting and chip parts, these contents enable students to comprehensively understand the basic principles, technological processes and technical applications of mechanical manufacturing.
Edited at 2024-03-07 11:15:24
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Machinery Manufacturing Basics
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- Outline
Machinery Manufacturing Basics
introduction
Advanced Manufacturing Technology
Computer Numerical Control Machine Tool CNC
Machining center MC
Flexible Manufacturing System FMS
Computer Integrated Manufacturing System CIMS
CAD and CAM integrated technology
Rapid prototyping technology RP
Concurrent Engineering CE
Virtual Manufacturing VM
machining methods
Isomaterial manufacturing: plastic forming
Additive Manufacturing: 3D Printing
Subtractive manufacturing
Cutting and Grinding
1.1 Metal cutting and tool basics
1.1.1 Geometric elements of workpiece and tool
Workpiece surface
Surface to be processed
machined surface
transition surface
Tool elements
Blade body
hilt
cutting part
Mounting surface
Knife wedge
Tool surface (one point, two lines and three sides)
tip
main cutting edge
Secondary cutting edge
rake face
main flank
secondary flank
1.1.2 Movement of tools and workpieces
main movement
The main movement provided by machine tools or manpower, which promotes relative movement between the tool and the workpiece, thereby bringing the front of the tool close to the workpiece
main direction of movement
The instantaneous main movement direction of the selected point of the cutting edge relative to the workpiece
Cutting speed Vc
The instantaneous velocity of the selected point of the cutting edge relative to the main motion of the workpiece
Feed motion
Movement provided by machine tools or manpower, which generates additional relative movement between the tool and the workpiece. Together with the main movement, the chips can be removed continuously or continuously to obtain the machined surface with the required geometric characteristics.
Feed speed Vf
The instantaneous speed of the selected point of the cutting edge relative to the feed motion of the workpiece
Synthetic cutting speed Ve
The instantaneous speed of the selected point of the cutting edge relative to the resultant cutting motion of the workpiece (the motion synthesized by the main motion and the feed motion)
1.1.3 Tool angle
stationary frame of reference
base pr
cutting plane
Orthogonal plane po
Assume working plane pf
Tool angle
Leading angle (kr): Leading angle is the angle between the main cutting edge and the feed direction measured from the base plane. It affects the thickness of the cutting layer, the direction of cutting forces, and the surface roughness of the workpiece.
Secondary slip angle (ks): The secondary slip angle is the angle between the secondary cutting edge and the feed direction measured in the feed plane, which affects the cutting conditions at the secondary cutting edge.
Rake angle (γo): The rake angle is the angle between the rake face and the base face measured in the main section. The rake face is the surface through which chips flow. The size of the rake angle directly affects the sharpness and cutting resistance of the tool. Increasing the rake angle can reduce cutting deformation, reduce cutting force and cutting temperature, and improve cutting efficiency. However, an excessively large rake angle will reduce the strength of the tool edge.
Relief angle (αo): The relief angle is the angle between the flank surface and the cutting plane measured in the cutting plane. The flank surface is opposite to the machined surface of the workpiece. The function of the relief angle is to reduce the friction between the tool and the workpiece and protect the tool from wear. Appropriate relief angle can improve the durability of the tool, but too large relief angle may lead to insufficient strength of the tool.
Edge inclination angle (λs): The edge inclination angle is the angle between the projection line of the main cutting edge on the base surface and the feed direction. Edge inclination affects how chips are formed and ejected.
Secondary relief angle (βo): The secondary relief angle is the angle between the secondary flank surface and the vertical plane in the feed direction. It is mainly used to reduce the friction between the secondary cutting edge and the workpiece.
The relationship between tool angle and tool working angle
Effect of cross feed movement on working angle
Changes in working cutting plane and working base surface
During transverse feed turning (such as cutting off, facing), since the tool moves in both transverse and radial directions at the same time, the trajectory formed by the cutting edge of the tool on the surface of the workpiece is no longer a straight line but part of the Archimedean spiral. In this case, the original cutting plane and base surface of the tool will be tilted accordingly, forming a new working cutting plane (Pse) and working base surface (Pre)
Changes in working front angle and working back angle
As the cutting edge of the tool cuts along a spiral path on the workpiece surface, the original rake angle (γo) and relief angle (αo) will be converted into working rake angle (roe) and working relief angle (a oe). As the transverse feed increases, the helix angle contacted by the cutting edge will also increase accordingly, resulting in an increase in the working rake angle and a decrease in the working relief angle.
Cutting performance and tool wear
An increase in the working rake angle means that the tool is sharper, which can reduce cutting resistance; while a decrease in the working rake angle may increase the friction between the tool and the workpiece in the cutting area, thereby accelerating tool wear.
Cutting force and processing stability
Different from radial feed, the cutting force distribution and direction generated during transverse feed change, which will affect the stability of the cutting process. If the feed amount is too large or uneven, it may cause cutting vibration or a decrease in machining accuracy. .
Workpiece surface quality
Changes in working angle will also affect the surface quality of the workpiece, such as smoothness, burrs, etc. Reasonable selection of feed amount and working angle will help to obtain better surface quality.
Effect of axial feed motion on working angle
During the turning process, the feed movement of the tool along the axis of the workpiece is mainly used in turning outer circles, inner holes and flat surfaces, etc.
The influence of tool installation height on working angle
When turning the outer circle: If the turning tool is installed too high, that is, the tool tip is higher than the rotation center of the workpiece, the rake angle will increase and the relief angle will decrease. An increase in the rake angle means a sharper cutting edge, but a decrease in the clearance angle will lead to an increase in the contact area between the tool flank surface and the workpiece, increasing friction, thereby increasing cutting resistance and heat generation, weakening the strength of the tool tip, and shortening the tool. life, and may affect the surface quality of the workpiece.
When turning inner holes: If the installation position of the turning tool is also too high, its impact on the working angle is opposite to that of turning the outer circle, that is, the rake angle decreases and the rear angle increases. Such an installation position may result in less smooth cutting, lower cutting efficiency, and may cause tool interference or instability.
The turning tool is installed too low: For turning the outer circle, when the turning tool tip is lower than the workpiece rotation center, the relief angle will increase and the rake angle will decrease. An increase in the clearance angle can reduce the friction between the flank surface and the machined surface of the workpiece, but a reduction in the clearance angle will make the cutting edge of the tool relatively blunt, increase cutting resistance, and is detrimental to cutting efficiency and tool life.
When turning internal holes: If the turning tool is installed too low, its impact on the working angle is similar to the above-mentioned case of turning the outer circle. The rake angle increases and the relief angle decreases. This may be beneficial in this case because it can make Cutting is smoother.
Effect of tool holder centerline deflection on working angle
1.1.4 Geometric and motion parameters and cutting methods in cutting
Feed (f): The displacement of the tool or workpiece along the feed direction per unit time, which can be expressed as feed per revolution or feed per minute.
Back cutting amount (a_p): also called cutting depth, it is the maximum size of the cutting layer perpendicular to the main movement direction
3 elements of cutting
Back knife amount ap(mm) ap= (dw - dm) / 2 (mm)
Feed amount f(mm/r)
Cutting speed vc(m/min) Vc=πdn/1000(m/min) n=1000vc/πd(r/min)
Cutting layer: The layer of material removed from the workpiece by the tool in one cutting action or stroke
Cutting Width (aw or ah): This is the size of the cutting layer measured along the transition surface of the workpiece (i.e. the newly machined surface), i.e. the width of the cutting edge action. In turning, the cutting width is equal to the effective length of the cutting edge of the tool
Cutting thickness (ac or ap): Also known as depth of cut or layer thickness, this is the dimension of the cutting layer measured perpendicular to the transition surface, i.e., the maximum distance a tool cutting edge can penetrate into the workpiece. In turning, the depth of cut is equal to the back engagement (a_p), while in other cutting methods such as milling, the depth of cut may refer to the maximum depth of the material cut by the cutting edge of the tool in a single pass.
Cutting layer cross-sectional area (AD or Ac): Also known as the cutting layer nominal cross-sectional area, it is the cross-sectional area of the cutting layer on the cutting layer size plane. The calculation method is usually the cutting width multiplied by the cutting thickness.
1.1.5 Tool materials
Tool material properties
High hardness, high wear resistance, sufficient strength and toughness, high heat resistance (thermal stability), good thermophysical properties and thermal shock resistance, good process performance
High-speed steel (HSS, High-Speed Steel): High-speed steel is a tool steel containing tungsten, chromium, molybdenum, vanadium, cobalt and other alloying elements. It has good hardness, wear resistance and red hardness (maintains hardness at high temperatures). capability), suitable for general cutting processing, such as turning, milling, drilling, etc. Common high-speed steel models include W18Cr4V, M2, M42, etc.
Cemented Carbide: Cemented carbide is a composite material sintered by tungsten carbide (WC) particles and binder metal (such as cobalt Co). Its hardness and heat resistance are much higher than high-speed steel, and it is suitable for high-speed steel. , cutting of high-precision and difficult-to-machine materials, such as titanium alloys, high-temperature alloys, hardened steel, etc. According to composition and use, cemented carbide is divided into P type (suitable for processing steel), M type (suitable for processing cast iron and non-ferrous metals) and K type (suitable for processing non-metallic materials), etc.
Ceramic tool materials: including Alumina-Based Ceramics, Silicon Nitride-Based Ceramics, etc., which have extremely high hardness, excellent wear resistance, and strong high temperature resistance, but have relatively low impact toughness. Poor, often used for high-speed finishing and dry cutting.
**Cubic Boron Nitride (CBN, Cubic Boron Nitride)** and diamond (Diamond): These two super-hard tool materials have a hardness close to or even exceed that of natural diamond. They have extremely high wear resistance and thermal stability, and are particularly suitable for Processing of extremely hard ferrous metals (such as quenched steel, chilled cast iron) and non-ferrous metals (such as aluminum alloys, copper alloys).
1.2 Deformation during metal cutting process
1.2.1 Overview
Primary Shear Zone or Primary Deformation Zone This is the most severely deformed part during cutting, located inside the cutting layer in front of the cutting edge. Features: Near the cutting edge, the metal material is subjected to great shear stress, shear slip occurs, and new grain boundaries are formed. Along with severe plastic deformation, obvious work hardening occurs here. Function: The cutting layer metal is forcibly separated by the cutting edge of the tool in this area, forming the basic shape of the chip.
Secondary Deformation Zone or Frictional Deformation Zone Located where the bottom of the chip contacts the rake face of the tool. Features: After leaving the cutting edge, the chips continue to contact the rake face and are further subjected to extrusion and friction, resulting in additional plastic deformation and heat. Function: This area determines the degree of plastic deformation of the chip, has an important influence on the curl and fracture form of the chip, and also affects the distribution of cutting force and cutting heat.
Tertiary Deformation Zone or Built-up Edge Region Near the point where the tool flank contacts the machined surface. Features: The machined surface is squeezed and rubbed by the tool flank, causing subtle plastic deformation and local work hardening. Function: The third deformation zone has a great impact on the surface quality of the workpiece, including surface roughness, residual stress distribution and potential microstructural changes.
1.2.2 Formation and control of built-up edge
Phenomenon: An accumulation of hard metal flakes formed on the rake face of the tool during metal cutting. Its formation is related to a variety of factors and brings a series of adverse effects to the processing process, such as increasing cutting force, reducing processing accuracy and surface quality, etc.
Causes
Cutting speed: Within a certain range, moderate cutting speed will promote the formation of built-up edge. When the speed is too low, built-up edge will generally not be generated due to the low temperature; when the speed is high, due to the increase in temperature, built-up edge may disappear due to the work hardening effect and will no longer occur.
Cutting temperature: Local high temperature causes the underlying metal of the chip to soften, making it easier to bond with the rake face of the tool.
main impact
positive influence
Protect the tool: The hardness of built-up edge is usually higher than that of the workpiece material, and it can replace the cutting edge in the cutting process to a certain extent.
Increase the actual rake angle: The formation of built-up edge is equivalent to forming an additional rake angle on the rake face of the tool, which helps to reduce cutting forces and improve cutting conditions, especially in the roughing stage, which is helpful to increase production efficiency
negative impacts
Affects the dimensional accuracy of the workpiece: Because the built-up edge is not fixed, but is constantly being generated and broken, this results in unstable cutting depth, which in turn affects the dimensional accuracy of the workpiece, especially the high dimensional tolerances required during finishing. more obvious under circumstances
Increased surface roughness: Small particles or fragments after broken built-up edge may remain on the surface of the workpiece, forming hard points and burrs, increasing the roughness of the machined surface and reducing surface quality.
Cause vibration and noise: The existence of built-up edge may cause vibration during the cutting process and increase the noise of the machine tool. In severe cases, it may also lead to a decrease in cutting stability and a shortened tool life.
Affects cutting force and power consumption: Although built-up edge can temporarily reduce cutting force, its instability and sudden rupture may cause cutting force fluctuations, impact the machine tool power system, and increase energy consumption.
Measures to control built-up edge
Cutting speed adjustment: Increase or reduce the cutting speed appropriately to avoid the medium-speed range where built-up edge occurs.
Material pretreatment: Change the hardness and plasticity of the workpiece material through heat treatment and other methods to reduce the probability of built-up edge.
Tool selection: Choose tool materials with better wear resistance and higher hardness, appropriately increase the rake angle, reduce the friction of the cutting edge, and finely grind the rake surface of the tool to reduce roughness.
Cutting fluid application: The use of cutting fluid can effectively reduce the temperature of the cutting zone, reduce the friction between the tool and the chips, thereby inhibiting the formation of built-up edge.
Optimize cutting parameters: reasonably set cutting depth and feed, reduce cutting force and cutting temperature, and prevent excessive adhesion between chips and tools.
Improved tool coating: Using tool coating technology with excellent wear resistance and anti-adhesion properties can reduce the possibility of built-up edge.
1.2.3 Main factors affecting cutting deformation
Workpiece material properties: Hardness of the material: Harder materials deform less during cutting because they are more difficult to plastically deform. Plasticity and toughness of materials: Materials with greater plasticity are more likely to undergo plastic deformation during the cutting process; materials with better toughness can better resist cutting forces and are less prone to large deformations.
Cutting speed: High-speed cutting can reduce cutting deformation, because the heat generated by fast cutting softens the material and reduces plastic deformation, but too high cutting speed will aggravate thermal deformation. Increased cutting speed will also cause the cutting temperature to rise, causing the material to deform due to thermal expansion.
Tool geometry parameters: The rake angle of the tool: increasing the rake angle reduces cutting deformation and vice versa. Leading angle: Increasing the leading angle will increase the cutting thickness, which may lead to greater plastic deformation.
Cutting amount: Feed rate: Increasing the feed rate will increase the cutting thickness and increase the degree of plastic deformation. Depth of cut: Increasing the depth of cut also increases the possibility of cutting deformation.
1.2.4 Types and control of chips
Ribbon Chip (Continuous Chip/Ribbon Chip): The inner surface is smooth, the outer surface is rough, and it is continuous in a long strip. It usually occurs when the cutting speed is high, the cutting thickness is small, and the rake angle of the tool is large. The advantage of strip-shaped chips is that the cutting force is stable and the cutting process is relatively smooth. However, too many continuous chips can easily wrap around the tool or workpiece, affecting chip removal and machining safety.
Serrated Chip / Segmented Chip: The surface of the chip is jagged and the inner surface may have cracks. This type of chip is common when the cutting speed is low, the cutting thickness is large, or the rake angle of the tool is small. The formation of cracked chips is accompanied by large impact and vibration, which has a certain impact on the life of cutting tools and machine tools.
Unit chip (Chip Fragmentation / Discontinuous Chip): Based on the extrusion chip, when the shear stress is large enough, the chip will be divided into independent units. This kind of chip is relatively broken and often occurs under poor cutting conditions. Or when the material hardness is high.
Powder Chip/Dust Chip: Commonly seen when processing brittle materials, the chips are small and irregular in shape, and scatter in all directions, posing a threat to the safety of the operator. At the same time, the machine tool cleanliness and environmental requirements are high.
Adjust chip measures
Adjust the cutting speed (v_c): Increasing the cutting speed can make the chips thinner, which is beneficial to the formation of continuous strip-shaped chips, reducing cutting force and tool wear, but too high a cutting speed may cause the cutting temperature to rise, which is harmful to the tool and workpiece material. adversely affect performance. On the contrary, reducing the cutting speed is conducive to the formation of squeeze cracks or short chips.
Adjust the feed (f): Increasing the feed tends to produce thicker chips, which may form squeeze cracks or intermittent chips. This is beneficial for reducing cutting force and tool wear, but may lead to an increase in cutting temperature and Tool life is reduced. Reducing the feed rate helps form thin, continuous chips.
Adjust the depth of cut (a_p): The depth of cut also has a great impact on chip formation. Deeper cutting depth will increase the cutting volume, which may lead to increased cutting forces and chip thickness, while shallow cutting is beneficial to the formation of thin and continuous chips.
Increase the rake angle (γo): Increasing the rake angle can reduce cutting deformation and cutting force, which is beneficial to the formation of continuous chips, but it may also reduce the strength and wear resistance of the tool. Adjust the main declination angle (Kr): Increasing the main declination angle helps to deform the chips and facilitates the formation of shorter chips, which facilitates chip removal; reducing the main declination angle helps to form continuous strip-shaped chips. Use chip breakers: Adding chip breakers to the tool design can change the chip deformation path, force the chips to break, and achieve a stable chip breaking effect.
1.3 Cutting force
1.3.1 Source of cutting force
When metal cutting, the force required by the tool to cut into the workpiece to deform the material being processed and turn it into chips is called cutting force.
Deformation resistance: elastic deformation resistance, plastic deformation resistance
Friction: friction between the chip and the rake face, friction between the flank face and the machined surface of the workpiece, friction within the deformation zone
1.3.2 Factors affecting cutting force
Influence intensity: back cutting amount ap>feed f>cutting speed vc
Workpiece material: Material hardness: The higher the hardness of the workpiece material, the greater the deformation resistance that needs to be overcome during cutting, so the cutting force will increase. Material strength: The greater the tensile strength and shear strength of the material, the greater the cutting force generated during cutting. Material plasticity and toughness: Materials with good plasticity and toughness will experience more plastic deformation during the cutting process, thereby increasing cutting forces. Work hardening tendency: Materials with strong work hardening will have greater cutting force during cutting.
Cutting conditions: Cutting speed (v_c): Generally speaking, as the cutting speed increases, the cutting temperature will increase accordingly, and the deformation resistance of the material will decrease. Therefore, high-speed cutting can reduce the cutting force, but after exceeding a certain speed, the thermal deformation caused by cutting heat will increase. Large cutting force. Feed (f) and back engagement (a_p): increasing these two will increase the cutting area, so the cutting force will increase. Cutting depth (a_p): When the amount of back cutting increases, the cutting force increases approximately in proportion to it.
Tool geometry parameters: Rake angle (γo): Increasing the rake angle can reduce the squeezing effect of the cutting edge, thereby reducing the cutting force. Relief angle (αo): An appropriate relief angle can reduce the friction between the tool and the machined surface, thereby reducing the cutting force. Leading angle (Kr): Changes in the leading angle will affect the distribution of cutting force and cutting thickness. Reasonable selection of the leading angle can optimize the cutting force. Edge inclination angle (λs): The edge inclination angle affects the formation and discharge of chips, and a suitable edge inclination angle helps reduce cutting forces. Tool nose arc radius (rε): Although it has little effect on cutting force, a larger tool nose arc radius will change the directional distribution of cutting force.
Tool wear: After the tool is worn, the sharpness of the cutting edge decreases, causing the cutting resistance to increase, and the cutting force will increase accordingly. Cutting fluid: The use of cutting fluid can play a cooling, lubrication and cleaning role, reduce the cutting temperature, reduce the friction coefficient, thereby reducing cutting force. Machine tool rigidity and dynamic characteristics: The rigidity and stability of the machine tool and the load-bearing capacity of the spindle bearing all affect the cutting force transmission and balance during the cutting process.
1.4 Cutting heat and cutting temperature
1.4.1 Generation and conduction
produce
Cutting heat is an inevitable phenomenon in the metal cutting process Deformation work: During the cutting process, the workpiece material undergoes elastic and plastic deformation in the shear zone due to being cut by the tool. This part of the energy is converted into heat energy and becomes one of the main sources of cutting heat. Friction work: The friction caused by the relative motion between the chip and the rake face, the machined surface and the flank face will also generate a large amount of heat. Among them, plastic metal materials mainly rely on deformation work, while brittle metal materials may rely more on frictional heat. Adiabatic shearing at the interface between the tool and the workpiece: At the cutting moment, due to the small contact area and short contact time, the heat cannot be conducted quickly, causing the local temperature to rise sharply.
conduction pathway
Influence intensity: back cutting amount ap>cutting speed vc>feed f
Chips: A considerable part of the cutting heat is discharged by the heat carried by the chips themselves. Especially during high-speed cutting, the proportion of heat carried away by the chips is relatively high. Tool: The cutting heat is transferred outward through the tool body, and then dissipated from the tool body into the air or taken away by the cutting fluid. Workpiece: The workpiece serves as a heat transfer medium. Part of the heat gradually diffuses into the workpiece through its own heat conduction, and then is dissipated through the overall thermal radiation and convection of the workpiece. Surrounding media: including cutting fluid, air or other cooling media, which remove heat through direct contact and convection heat transfer.
1.4.2 Cutting temperature and its influence on the cutting process
Tool wear: Increased cutting temperature will cause the hardness of tool materials to decrease, especially high-speed steel and carbide tools. The hardness loss is obvious at high temperatures, which will intensify tool wear and shorten tool service life. Tool failure: If the cutting temperature exceeds the heat resistance limit of the tool material, the tool may soften, oxidize, phase change, etc. In severe cases, it may cause surface melting or tool fracture, affecting processing quality and safety. Workpiece quality: High-temperature cutting will cause changes in the metallographic structure of the workpiece material, such as reduced hardness and increased plasticity. The processed workpiece may undergo thermal deformation, affecting dimensional accuracy and shape accuracy. At the same time, high-temperature cutting will also cause an oxide layer to appear on the surface of the workpiece, reducing the surface quality. Chip formation: Cutting temperature affects the degree and mode of chip deformation. At high temperatures, chips are more likely to form continuous chips, and the chips may be softened by high temperatures and adhere to the tool, forming built-up edge, which affects the stability of the cutting process. Cutting force and power consumption: As the cutting temperature increases, the deformation resistance of the workpiece material may decrease, resulting in a decrease in cutting force. However, high-temperature cutting also increases the friction between the tool and the chip, and the tool and the workpiece, which in turn may increase cutting forces and power consumption. Cooling lubrication: The cutting temperature determines the effect of the cooling lubricant. At high temperatures, good cooling lubrication can absorb a large amount of heat, lower the cutting temperature, reduce tool wear, and improve tool life and workpiece surface quality.
1.4.3 Measurement method of cutting temperature
Natural thermocouple method: Utilize the naturally formed thermocouple effect between the workpiece and the tool material to estimate the temperature by measuring the thermal electromotive force. This method is suitable when the tool and workpiece materials are quite different. Artificial thermocouple method: A small thermocouple is directly welded to the tool, such as the tool tip or the rake surface of the tool, to directly sense the temperature of the cutting point.
1.4.4 Factors affecting cutting temperature
Influence intensity: cutting speed vc> feed f> back cutting amount ap Cutting speed (v_c): The increase in cutting speed will significantly increase the temperature of the cutting area, because the faster the speed, the more cutting heat is generated per unit time. Feed (f): Increasing the feed means that the amount of metal removed per unit time increases, thus generating more cutting heat. Back cutting amount (a_p): The back cutting amount has a relatively small impact on the cutting temperature, but increasing the back cutting amount will lead to an increase in the cutting area, which may increase the temperature. Workpiece material: Thermal conductivity of materials: The smaller the thermal conductivity of the workpiece material, the less easily heat can be transferred, causing the cutting temperature to increase. Material hardness and strength: Materials with higher hardness and strength require greater cutting force when cutting and generate more cutting heat. Tool material and geometric parameters: Thermal conductivity of tool materials: Tool materials with good thermal conductivity can transfer cutting heat out faster. Tool geometric parameters: such as rake angle, relief angle, main deflection angle, etc., which affect cutting deformation, friction and chip formation, thereby affecting the generation of cutting heat. Tool wear: Tool wear will change the actual working angle and cutting edge status, resulting in an increase in cutting heat. Use of cutting fluid: The cooling effect of cutting fluid directly affects the cutting temperature. Suitable cutting fluid can effectively take away cutting heat and reduce the temperature. It can also lubricate and reduce friction and heat. Cutting environment: Whether there are good ventilation and heat dissipation conditions, as well as the rigidity and stability of the machine tool will also affect the cutting temperature. Processing methods: Different cutting processing methods, such as turning, milling, drilling, etc., will have different cutting temperatures due to their different cutting methods and cutting mechanisms. Cutting mode and chip form: Different cutting modes (such as continuous cutting, intermittent cutting) and chip forms (such as strip chips, squeeze chips, etc.) have different effects on the generation and dissipation of cutting heat.
1.5 Tool wear and durability
1.5.1 Tool wear patterns
Abrasive Wear: During the cutting process of the tool, the hard particles (such as carbides, oxides, etc.) present in the workpiece material cause scratches and scratches on the tool surface, resulting in the gradual loss of tool surface material. Adhesive Wear: During the cutting process, the atomic distance between the contact surfaces of the tool and the workpiece material decreases, resulting in intermolecular attraction, resulting in transfer between the tool material and the workpiece material, forming adhesive wear, that is, the tool surface material is "welded" to the workpiece or on chips. Diffusion Wear: In high-temperature and high-pressure cutting environments, diffusion occurs between atoms of the tool and the workpiece material. The tool material enters the workpiece surface or the workpiece material enters the tool surface, thereby changing the original composition and structure of the tool, resulting in tool surface hardness and wear resistance. reduce. Phase Transformation Wear: During the cutting process, the tool material undergoes phase transformation under the influence of high temperature. For example, the WC-Co bond of cemented carbide tools may soften, oxidize or decarburize at high temperatures, resulting in a decrease in tool hardness and increased wear. Fatigue Wear: Under repeated impact loads, microcracks on the surface or subsurface of the tool expand until they break, causing particles to fall off, which is caused by cyclic stress during the cutting process. Oxidative Wear: In an aerobic environment, tool materials react with oxygen at high temperatures to form oxides. These oxides fall off during the cutting process, causing tool wear. Plastic Deformation Wear: The tool material undergoes plastic deformation due to large cutting forces during the cutting process, causing the tool surface to become rough and the cutting edge to become blunt. Thermal Wear: The high temperature generated during the cutting process softens the tool material, losing its proper hardness and wear resistance, aggravating wear.
1.5.2 Tool wear process
Initial wear stage (also called running-in wear stage): In the short period of time when a newly ground tool is first used, due to the microscopic unevenness of the tool surface, the existence of microcracks, oxide layers or decarburization and other defects, and the very sharp cutting edge, the contact area between the tool and the workpiece is small, and the local pressure The stress is large, causing some parts of the tool to wear out quickly. At this stage, the irregular parts of the flank surface will be ground quickly, and the tool wears faster. Normal wear and tear stage: After initial wear, the rough parts of the tool surface have been worn away, and the tool has entered a relatively stable wear state. At this time, the wear rate increases roughly in a linear relationship with the cutting time, and the growth rate is slow. During this stage, the tool performance is relatively stable and good cutting conditions can be maintained. Rapid wear stage (also called extreme wear stage): When the tool is worn to a certain extent, its geometry changes significantly, the cutting edge is no longer sharp, the cutting force increases, the friction force and cutting temperature increase significantly, which will cause the wear rate to accelerate sharply. At this stage, the cutting efficiency of the tool is greatly reduced, the processing quality deteriorates, and even chipping and chipping may occur. Once the tool enters this stage, continued use will not only seriously affect product quality, but may also damage the machine tool or other components, so the tool should be replaced in time.
1.5.3 Factors affecting tool durability
Influence intensity: cutting speed vc> feed f> back cutting amount ap Cutting speed (v_c): Excessive cutting speed will cause the temperature of the contact area between the tool and the workpiece to increase, accelerating the wear and softening of the tool material. Feed (f) and cutting depth (a_p): Increasing the feed and cutting depth will increase the volume of material cut per unit time, generating more cutting heat and mechanical wear. Use of cutting fluid: Suitable cutting fluid can effectively cool the tool and workpiece and reduce thermal deformation and wear. Workpiece material: The hardness, strength, toughness and chemical stability of the workpiece material all affect tool durability. Hard materials and difficult-to-machine materials often accelerate tool wear. Tool material: The hardness, wear resistance, heat resistance, impact toughness and other properties of the tool material directly determine the durability of the tool. Such as high-speed steel, carbide, ceramic tools, cubic boron nitride (CBN) and diamond tools, etc., their durability varies under different working conditions. Tool geometry parameters: The angle design of the tool (rake angle, relief angle, main deflection angle, etc.) will affect the distribution of cutting force and chip formation. Reasonable selection of tool geometric parameters can improve tool durability. As the main deflection angle increases, the cutting temperature increases. Due to the increase in the nominal thickness of the cutting layer, the unit cutting edge load increases, resulting in a reduction in tool durability. The rake angle increases, the cutting temperature decreases, and the durability increases. Tool surface treatment: Tool surface coatings (such as TiN, TiCN, AlTiN, etc.) can improve the surface hardness and wear resistance of the tool, reduce the friction coefficient, thereby improving durability. The grinding quality of the tool edge is also important. The sharpness and surface roughness of the edge affect friction and wear during the cutting process. Assembly accuracy of machine tools and cutting tools: Whether the tool installation is accurate and stable, as well as the rigidity of the machine tool and spindle runout will all affect the stress condition and wear rate of the tool. Cutting process: Cutting methods (such as continuous cutting, interrupted cutting, light cutting, heavy cutting, etc.) and cutting sequence also affect tool durability. Cooling and lubrication system: The design and use of the cooling and lubrication system directly affect the dissipation of cutting heat and the lubrication condition of the cutting area. Good cooling and lubrication can significantly improve the durability of the tool.
Selection of reasonable tool durability values
Maximum productivity and durability Tp
Lowest cost durability Tc
Maximum profit durability Tpr
1.5.4 Damage to tools
Brittle damage: main forms: fragmentation, chipping, crack damage, and peeling Brittle breakage refers to the phenomenon that the tool suddenly breaks due to impact or heavy load during use, and the tool material fails to undergo sufficient plastic deformation. This type of damage usually manifests as rapid crack expansion and fracture, and the damaged surface is usually rough and has irregular edges. The causes of brittle breakage usually include: the tool material itself is highly brittle, there are cracks or defects on the tool surface, severe impact during the cutting process, excessive cutting speed, insufficient cooling leading to overheating of the tool, and unreasonable tool design. Plastic damage: Plastic damage refers to the fact that during the long-term cutting process of the tool, due to excessive accumulated plastic deformation, the tool material gradually softens and the wear increases until it is no longer possible to continue cutting. At this time, severe wear, deformation and peeling will occur on the tool surface. Factors affecting plastic damage include: insufficient thermal hardness and wear resistance of the tool material, improper selection of cutting parameters (such as excessive cutting speed, excessive feed, or excessive cutting depth), poor cooling and lubrication, inappropriate tool geometric parameters, etc. Measures to prevent tool breakage: Select the appropriate tool material: According to the workpiece material and processing conditions, select a tool material with sufficient hardness, strength, toughness and wear resistance. Optimize cutting parameters: Set the cutting speed, feed amount and cutting depth reasonably to reduce the cutting load of the tool, while paying attention to maintaining the stability of the cutting process. Improve tool design: Reduce cutting force and tool wear by optimizing the geometric parameters of the tool (such as rake angle, relief angle, main deflection angle, etc.), and improve the impact resistance of the tool. Good cooling and lubrication: Use efficient cutting fluid to ensure that the tool is fully cooled and lubricated during the cutting process, reducing thermal deformation and wear of the tool. Regular maintenance and replacement: Replace the tool in time according to the actual wear condition of the tool to avoid damage caused by excessive wear of the tool. Monitor the cutting process: Use modern monitoring technology, such as sound monitoring, vibration monitoring, etc., to monitor the status of the tool in real time and provide early warning of the risk of tool damage.
1.6 Machinability of workpiece material and its improvement
Measure material processability
Processability is measured by the tool durability T or the cutting speed VT under a certain durability.
Measure machinability by cutting force or cutting temperature
Measuring processability based on machined surface quality
Processability is measured by the ease of chip control or chip breaking
Ways to improve
Adjust the chemical composition of materials
Use appropriate heat treatment processes
1.7 Selection of tool materials and geometric parameters
Tool cutting performance: back cutting amount ap>feed f>cutting speed vc Lathe performance: feed amount>back cutting amount>cutting speed Workpiece material properties: The hardness, strength, toughness and cutting performance (such as thermal conductivity, thermal conductivity, work hardening degree, etc.) of the workpiece material directly affect the selection of tool materials. For example, when machining carbide or quenched steel, carbide or cubic boron nitride (CBN) tools are usually chosen; while when machining aluminum alloys or plastics, high-speed steel or carbide tools are chosen. Cutting conditions: Cutting parameters such as cutting speed, feed rate and depth of cut determine the cutting forces, cutting temperature and mechanical stress experienced by the tool. These factors all need to match the properties of the tool material. For example, high-speed cutting usually requires the selection of tool materials with high temperature resistance and good thermal stability. Tool geometry parameters: Rake angle (γo): Increasing the rake angle can reduce cutting deformation, cutting force and cutting temperature, but too large a rake angle will reduce the strength of the tool. Therefore, for hard and brittle workpiece materials, a larger rake angle can be selected; while for materials with good plasticity, the rake angle should be appropriately reduced. Relief angle (αo): Increasing the relief angle can reduce the friction between the tool flank surface and the machined surface, reduce cutting temperature and tool wear, but the relief angle should not be too large to avoid affecting the rigidity of the tool. Leading angle (Kr): The size of the leading angle affects the cutting depth and cutting width, which in turn affects the distribution of cutting force. For difficult-to-cut materials, a larger leading angle can be selected to reduce cutting force; while for conventional materials, an appropriate small tilt angle can improve cutting efficiency. Edge inclination angle (λs): The selection of edge inclination angle has an important impact on the formation and discharge of chips. An appropriate edge inclination angle can improve chip shape, reduce cutting force and tool wear. Tool tip arc radius (rε): Increasing the tool tip arc radius can reduce the stress concentration near the tool tip and improve tool durability. However, for occasions that require precision cutting, the tool tip arc radius should be reduced to improve machining accuracy. . Processing requirements: According to the requirements of dimensional accuracy, surface roughness and production efficiency of the workpiece, select tool materials and geometric parameters that can not only meet the processing quality but also ensure the tool life. Economic benefits: Considering the cost-effectiveness between tool cost and tool life, select a tool material and geometric parameter combination that has high economic benefits while ensuring processing quality.
1.8 Reasonable selection of cutting amount
1. Determination of the amount of back knife ap As long as the stiffness of the process system and the power of the machine tool allow, choose a larger back cutting amount as much as possible to reduce the number of feeds. When the precision requirements of the parts are high, you should consider leaving a finishing allowance. The finishing allowance is generally smaller than that left in ordinary turning, and is often 0.1 to 0.5 mm. 2. Feed amount f (some CNC machine tools use feed speed Vf) The selection of feed amount f should be compatible with the amount of back cutting and the spindle speed. On the premise of ensuring the processing quality of the workpiece, a higher feed speed (below 2000㎜/min) can be selected. When cutting off, turning deep holes or finishing turning, a lower feed speed should be selected. When the tool is idle, especially when "returning to zero" at a long distance, the feed speed can be set as high as possible. During rough turning, f=0.3~0.8㎜/r is generally used, during fine turning, f=0.1~0.3㎜/r, and during cutting, f=0.05~0.2㎜/r. 3． Determination of spindle speed (1) Spindle speed when turning the outer circle When bare turning the outer circle, the spindle speed should be determined according to the diameter of the part to be processed and the cutting speed allowed by the parts and tool materials, processing properties and other conditions. In addition to calculation and table selection, the cutting speed can also be determined based on practical experience. It should be noted that the low-speed output torque of CNC lathes with AC variable frequency speed regulation is small, so the cutting speed cannot be too low. After the cutting speed is determined, use the formula n =1000 vc/πd to calculate the spindle speed n (r/min) (2) Spindle speed when turning threads When turning threads, the spindle speed of the lathe will be affected by many factors such as the pitch P (or lead) of the thread, the frequency increase and decrease characteristics of the drive motor, and the thread interpolation operation speed. Therefore, different CNC systems are recommended. spindle speed selection range. Most economical CNC lathes recommend that the spindle speed n (r/min) when turning threads is: n ≤ (1200/P)-k(1) In the formula, P——pitch of thread to be processed, mm; k——Insurance coefficient, generally taken as 80.
1.9 Reasonable selection of cutting fluid
Cooling effect: Cutting fluid can quickly absorb and take away a large amount of heat generated during the cutting process, reduce the temperature of the cutting area, avoid overheating of the tool, reduce thermal deformation, and protect the performance of the tool and workpiece material. Lubricating effect: Reduce the friction coefficient between the tool and the workpiece, the tool and the chips, reduce cutting force, reduce tool wear, improve tool durability and workpiece surface quality. Cleaning function: remove chips and grinding chips during the cutting process, prevent them from accumulating on the surface of the tool or workpiece, keep the cutting area clean, and improve processing accuracy. Anti-rust effect: After cutting, the cutting fluid can form a protective film on the surface of the workpiece to prevent the workpiece and tool from oxidizing and rusting in the air. Other functions: Some cutting fluids can also help inhibit bacterial growth, improve environmental performance, or enhance the luster of metal surfaces. The types of cutting fluids mainly include: Water-based cutting fluid: such as emulsion, semi-synthetic cutting fluid and fully synthetic cutting fluid. Emulsions are formulated from mineral oil, emulsifiers and other additives to form a water-in-oil or oil-in-water emulsion through an emulsification process. Semi-synthetic and fully synthetic cutting fluids use water as the main carrier, combined with additives such as extreme pressure agents and rust inhibitors, and have good cooling performance and low environmental pollution. Oil-based cutting fluids: such as mineral oil cutting oils, extreme pressure cutting oils and active cutting oils, etc., mainly rely on the lubricating properties of oil to reduce friction. Their cooling effect is not as good as water-based cutting fluids, but their protective properties for tools are better. When selecting cutting fluid, the following factors should be considered: roughing finishing Cutting of difficult-to-machine materials Grinding Methods of using cutting fluid mainly include: Direct injection: Injecting cutting fluid through the machine tool's cooling system into the cutting area to maximize cooling and lubrication. Soaking or applying: For manual or simple machining, you may soak the tool or workpiece surface, or apply cutting fluid regularly during the machining process.
Cutting: Cutting refers to the operation of using tools to remove material from the workpiece during metal cutting. This is a dynamic process that includes relative movement between the tool and the workpiece. The cutting edge of the tool exerts sufficient pressure on the workpiece material to cause plastic deformation and fracture of the material, thereby removing excess material from the workpiece. Chips: Chips are the waste portion of the workpiece material removed by the tool during the cutting process. It is the result of the cutting process, usually showing different shapes and types (such as ribbon chips, squeeze chips, unit chips, broken chips, etc.). Its shape, size, thickness, etc. are related to the cutting conditions (such as cutting speed, feed rate, etc.) Feed, cutting depth, etc.), tool geometric parameters (such as rake angle, relief angle, leading angle, etc.), workpiece material properties and other factors are closely related.