Cutting ToolsV.Umasankar SMBS1
Cutting Tools
One of most important components in machining process Performance will determine efficiency of operation Two basic types (excluding abrasives)
Single point and multiple point
Must have rake and clearance angles ground or formed on them2
CuttingCutting-Tool Materials
Toolbits generally made of seven materials
HighHigh-speed steel Cast alloys (such as stellite) Cemented carbides Ceramics Cermets Cubic Boron Nitride Polycrystalline Diamond3
Cutting Tool Properties
Hardness
Cutting tool material must be 1 1/2 times harder than the material it is being used to machine.
Capable of maintaining a red hardness during machining operation
Red hardness: ability of cutting tool to maintain sharp cutting edge Also referred to as hot hardness or hot strength4
Cutting Tool Properties
Wear Resistance
Able to maintain sharpened edge throughout the cutting operation Same as abrasive resistance
Shock Resistance
Able to take the cutting loads and forces
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Cutting Tool Properties
Shape and Configuration
Must be available for use in different sizes and shapes.
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HighHigh-Speed Steel
May contain combinations of tungsten, chromium, vanadium, molybdenum, cobalt Can take heavy cuts, withstand shock and maintain sharp cutting edge under red heat Generally two types (general purpose)
MolybdenumMolybdenum-base (Group M) TungstenTungsten-base (Group T)
Cobalt added if more red hardness desired7
Cast Alloy
Usually contain 25% to 35% chromium, 4% to 25% tungsten and 1% to 3% carbon
Remainder cobalt High hardness High resistance to wear Excellent red-hardness red-
Qualities
Operate 2 times speed of high-speed steel highWeaker and more brittle than high-speed steel high8
Carbide Cutting Tools
First used in Germany during WW II as substitute for diamonds Various types of cemented (sintered) carbides developed to suit different materials and machining operations
Good wear resistance Operate at speeds ranging 150 to 1200 sf/min
Can machine metals at speeds that cause cutting edge to become red hot without loosing harness9
Manufacture of Cemented Carbides
Products of powder metallurgy process
Tantalum, titanium, niobium Blending Compaction Presintering Sintering10
Operations
CementedCemented-Carbide Applications
Used extensively in manufacture of metalmetalcutting tools
Extreme hardness and good wear-resistance wear-
First used in machining operations as lathe cutting tools Majority are single-point cutting tools used singleon lathes and milling machines
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Types of Carbide Lathe Cutting Tools
BlazedBlazed-tip type
CementedCemented-carbide tips brazed to steel shanks Wide variety of styles and sizes Throwaway inserts Wide variety of shapes: triangular, square, diamond, and round
Indexable insert type
Triangular: has three cutting edges
Inserts held mechanically in special holder12
Reasons Indexable Inserts More Popular than Brazed-Tip Tools Brazed1. 2.
3.
4. 5. 6.
Less time required to change cutting edge Amount of machine downtime reduced considerable thus production increased Time normally spent in regrinding eliminated Faster speeds and feeds can be used Cost of diamond wheels eliminated Indexable inserts cheaper than brazed-tip brazed13
CementedCemented-Carbide Insert Identification
American Standards Association has developed system by which indexable inserts can be identified quickly and accurately Adopted by manufacturers Table 31.1 in text
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Qualities of Tungsten Carbide Tools
Determined by size of tungsten carbide particles and percentage of cobalt1. 2. 3. 4. 5.
Finer the grain particles, lower the tool toughness Finer the grain particles, higher tool hardness Higher the hardness, greater wear resistance Lower cobalt content, lower tool toughness Lower cobalt content, higher hardness15
Additive Characteristics
Titanium carbide
Addition provides resistance to tool cratering Content increased
Toughness of tool decreased Abrasive wear resistance at cutting edge lowered
Tantalum carbide
Addition provides resistance to tool cratering
Without affecting abrasive wear resistance
Addition increases tool's resistance to deformation16
General Rules for Selection of Proper Cemented-Carbide Grade Cemented1. 2.
3.
4.
5.
Use grade with lowest cobalt content and finest grain size Use straight tungsten carbide grades to combat abrasive wear To combat cratering, seizing, welding, and galling, use titanium carbide grades For crater and abrasive wear resistance, use tantalum carbide grades Use tantalum carbide grades for heavy cuts in steel, when heat and pressure might deform cutting edge17
Coated Carbide Inserts
Give longer tool life, greater productivity and freer-flowing chips freerCoating acts as permanent lubricant
Permits higher speed, reduced heat and stress
Two or three materials in coating give tool special qualities
Innermost layer of titanium carbide Thick layer of aluminum oxide Third, very thin layer titanium nitride18
Coatings
Titanium carbide
High wear and abrasion resistance (moderate speed) Used for roughing and finishing Extremely hard, good crater resistance Excellent lubricating properties Provides chemical stability Maintains hardness at high temperatures19
Titanium nitride
Aluminum oxide
Tool GeometryTerms adopted by ASME
SIDE RELIEF SIDE CLEARANCE20Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
CuttingCutting-Tool Terms
Front, End, Relief (Clearance)
Allows end of cutting tool to enter work Permits side of tool to advance into work
Side Relief (Side)
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CuttingCutting-Tool Terms
Side Cutting Edge Angle
Angle cutting edge meets work
Positive Negative - protects point at start and end of cut
Nose Radius
Strengthens finishing point of tool Improves surface finish on work Should be twice amount of feed per revolution
Too large chatter; too small weakens point22
Side Rake
Large as possible to allow chips to escape Amount determined
Type and grade of cutting tool Type of material being cut Feed per revolution Formed by side rake and side clearance23
Angle of keenness
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Back Rake
Angle formed between top face of tool and top of tool shank
Positive
Top face slopes downward away from point Top face slopes upward away from point
Negative
Neutral24
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
CementedCemented-Carbide Cutting-Tool CuttingAngles and Clearances
Vary greatly Depend on three factors
Hardness of cutting tool Workpiece material Type of cutting operation
May have to be altered slightly to suit various conditions encountered25
Cutting Speeds and Feeds
Important factors that influence speeds, feeds, and depth of cut
Type and hardness of work material Grade and shape of cutting tool Rigidity of cutting tool Rigidity of work and machine Power rating of machine
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To obtain maximum efficiency
Machining with Carbide ToolsPrecautions in machine setup
Rigid and free from vibrations Equipped with heat-treated gears heatSufficient power to maintain constant cutting speed Cutting tool held as rigidly as possible to avoid chatter
Cutting operation
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Suggestions for Using CementedCemented-Carbide Cutting Tools
Work Setup
Mount work in chuck or holding device to prevent slipping and chattering Revolving center used in tailstock for turning work between centers Tailstock spindle extended minimum distance and locked securely Tailstock should be clamped firmly to lathe bed28
Suggestions for Using CementedCemented-Carbide Cutting Tools
Tool Selection
Use cutting tool with proper rake and clearances Hone cutting edge Use side cutting edge angle large enough tool can be eased into work Use largest nose radius operating conditions permit29
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
CementedCemented-Carbide
Capable of cutting speeds 3 to 4 times highhighspeed steel toolbits Low toughness but high hardness and excellent red-hardness redConsist of tungsten carbide sintered in cobalt matrix Straight tungsten used to machine cast iron and nonferrous materials (crater easily) Different grades for different work30
MetalMetal-Cutting
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Turning
High proportion of work machined in shop turned on lathe
Workpiece held securely in chuck or between lathe centers Turning tool set to given depth of cut, fed parallel to axis of work (reduces diameter of work)
Chip forms and slides along cutting tool's upper surface created by side rake32
Turning
Assume cutting machine steel: If rake and relief clearance angles correct and proper speed and feed used, a continuous chip should be formed.33
Planing or Shaping
Workpiece moved back and forth under cutting tool
Fed sideways a set amount at end of each table reversal
Should have proper rake and clearance angles on cutting tool34
Plain Milling
MultiMulti-tooth tool having several equally spaced cutting edges around periphery Each tooth considered single-point cutting tool single(must have proper rake and clearance angles) Workpiece held in vise or fastened to table
Fed into horizontal revolving cutter Each tooth makes successive cuts Produces smooth, flat, or profiled surface depending on shape of cutter35
Plain Milling
36Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Inserted Blade Face Mill
Consists of body that holds several equally spaced inserts
Required rake angle Lower edge of each insert has relief or clearance angle ground on it
Cutting action occurs at lower corner of insert
Corners chamfered to give strength37
Face Milling
38Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
End Milling
MultiMulti-fluted cutters held vertically in vertical milling machine spindle or attachment Used primarily for cutting slots or grooves Workpiece held in vise and fed into revolving cutter End milling
Cutting done by periphery of teeth39
Nomenclature of an End Mill
40Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Nomenclature of an End Mill
41Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Drilling
MultiMulti-edge cutting tool that cuts on the point Drill's cutting edges (lips) provided with lip clearance to permit point to penetrate workpiece as drill revolves Rake angle provided by helical-shaped flutes helical
Slope away from cutting edge Angle between rake angle and clearance angle42
Angle of keeness
Characteristics of a Drill Point
Chip formation of a drill
Cutting-point angles for standard drillCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Cutting FluidsTypes Fluids and Applications
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Cutting Fluids
Essential in metal-cutting operations to metalreduce heat and friction Centuries ago, water used on grindstones 100 years ago, tallow used (did not cool) Lard oils came later but turned rancid Early 20th century saw soap added to water Soluble oils came in 1936 Chemical cutting fluids introduced in 194445
Economic Advantages to Using Cutting Fluids
Reduction of tool costs Reduce tool wear, tools last longer Increased speed of production Reduce heat and friction so higher cutting speeds Reduction of labor costs Tools last longer and require less regrinding, less downtime, reducing cost per part Reduction of power costs Friction reduced so less power required by machining46
Heat Generated During Machining
Heat finds its way into one of three places
Workpiece, tool and chips
Act as disposable heat sink
Too much, cutting edge will break down rapidly, reducing tool life
Too much, work will expand47
Heat Dissipation
Ideally most heat taken off in chips Indicated by change in chip color as heat causes chips to oxidize Cutting fluids assist taking away heat
Can dissipate at least 50% of heat created during machining
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Characteristics of a Good Cutting Fluid1. 2.
3. 4.
5.
Good cooling capacity Good lubricating qualities Resistance to rancidity Relatively low viscosity Stability (long life)
6. 7. 8. 9.
Rust resistance Nontoxic Transparent Nonflammable
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Types of Cutting Fluids
Most commonly used cutting fluids
Either aqueous based solutions or cutting oils Cutting oils Emulsifiable oils Chemical (synthetic) cutting fluids
Fall into three categories
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Cutting Oils
Two classifications
Active Inactive
Terms relate to oil's chemical activity or ability to react with metal surface
Elevated temperatures Improve cutting action Protect surface51
Active Cutting Oils
Those that will darken copper strip immersed for 3 hours at temperature of 212F 212F Dark or transparent Better for heavy-duty jobs heavyThree categories
Sulfurized mineral oils Sulfochlorinated mineral oils Sulfochlorinated fatty oil blends52
Inactive Cutting Oils
Oils will not darken copper strip immersed in them for 3 hours at 212F 212F Contained sulfur is natural Termed inactive because sulfur so firmly attached to oil very little released Four general categories Straight mineral oils, fatty oils, fatty and mineral oil blends, sulfurized fatty-mineral oil blend fatty-
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Emulsifiable (Water Soluble) Oils
Mineral oils containing soaplike material that makes them soluble in water and causes them to adhere to workpiece Emulsifiers break oil into minute particles and keep them separated in water
Supplied in concentrated form (1-5 /100 water) (1-
Good cooling and lubricating qualities Used at high cutting speeds, low cutting pressures54
Chemical Cutting Fluids
Also called synthetic fluids Introduced about 1945 Stable, preformed emulsions Contain very little oil and mix easily with water ExtremeExtreme-pressure (EP) lubricants added React with freshly machined metal under heat and pressure of a cut to form solid lubricant Reduce heat of friction and heat caused by plastic deformation of metal55
Advantages of Synthetic Fluids1. 2.
3.
4.
Good rust control Resistance to rancidity for long periods of time Reduction of amount of heat generated during cutting Excellent cooling qualities
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5. 6. 7. 8. 9.
10.
11.
Longer durability than cutting or soluble oils Nonflammable - nonsmoking Nontoxic?????? Easy separation from work and chips Quick settling of grit and fine chips so they are not recirculated in cooling system No clogging of machine cooling system due to detergent action of fluid Can leave a residue on parts and tools
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Caution
Chemical cutting fluids widely accepted and generally used on ferrous metals. They are not recommended for use on alloys of magnesium, zinc, cadmium, or lead. They can mar machine's appearance and dissolve paint on the surface.
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Functions of a Cutting Fluid
Prime functions Provide cooling Provide lubrication Other functions Prolong cutting-tool life cutting Provide rust control Resist rancidity
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Functions of a Cutting Fluid: Cooling
Heat has definite bearing on cutting-tool wear cutting
Small reduction will greatly extend tool life Plastic deformation of metal
Two sources of heat during cutting action
Occurs immediately ahead of cutting tool Accounts for 2/3 to 3/4 of heat
Friction from chip sliding along cutting-tool face cutting-
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Functions of a Cutting Fluid: Cooling
Water most effective for reducing heat by will promote oxidation (rust) Decrease the temperature at the chip-tool chipinterface by 50 degrees F, and it will increase tool life by up to 5 times.
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Functions of a Cutting Fluid: Lubrication
Reduces friction between chip and tool face Shear plane becomes shorter Area where plastic deformation occurs correspondingly smaller ExtremeExtreme-pressure lubricants reduce amount of heatheatproducing friction EP chemicals of synthetic fluids combine chemically with sheared metal of chip to form solid compounds (allow chip to slide)
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Cutting fluid reduces friction and produces a shorter shear plane.
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Cutting fluid reduces friction and produces a shorter shear plane.
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Applications of cutting fluidsMaterialAluminum
MillingSoluble oil (96% water) or mineral oil
DrillingSoluble oil (70-90% water)
Tapping25% sulfur-based oil mixed with mineral oil 10-20% lard oil with mineral oil 30% lard with mineral oil 30% lard oil with 70% mineral oil
TurningMineral oil with 10% fat (or) soluble oil
Brass
Soluble oil (96% water)
Soluble oil
Mineral oil with 10% fat
Bronze Alloy Steels
Soluble oil 10% lard oil with 90% mineral oil
Soluble oil Soluble oil
Soluble oil 25% sulfur base oil with 75% mineral oil
Cast Iron
Dry
Dry
Dry or 25% lard oil with 80% mineral oil
Dry
Malleable Iron Copper Low Carbon and Tool Steels
Soluble oil Soluble oil Soluble oil
Soluble oil Soluble oil Soluble oil
Soluble oil Soluble oil 25-40% lard oil with mineral oil
Soluble oil Soluble oil 25% lard oil with 75% mineral oil
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CuttingCutting-Tool Life
Heat and friction prime causes of cutting-tool cuttingbreakdown Reduce temperature by as little as 50F, life of 50F, cutting tool increases fivefold BuiltBuilt-up edge Pieces of metal weld themselves to tool face Becomes large and flat along tool face, effective rake angle of cutting tool decreased
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BuiltBuilt-up EdgeBuilt-up edge keeps breaking off and re-forming Result is poor surface finish, excessive flank wear, and cratering of tool face67
Cutting Fluid's Effect on Cutting Tool Action1.
2. 3.
4. 5.
Lowers heat created by plastic deformation of metal Friction at chip-tool interface decreased chipLess power is required for machining because of reduced friction Prevents built-up edge from forming builtSurface finish of work greatly improved68
Rust Control
Water best and most economical coolant
Causes parts to rust
Rust is oxidized iron Chemical cutting fluids contain rust inhibitors
Polar film Passivating film
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Rancidity Control
Rancidity caused by bacteria and other microscopic organisms, growing and eventually causing bad odors to form Most cutting fluids contain bactericides that control growth of bacteria and make fluids more resistant to rancidity
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Application of Cutting Fluids
CuttingCutting-tool life and machining operations influenced by way cutting fluid applied Copious stream under low pressure so work and tool well covered
Inside diameter of supply nozzle width of cutting tool Applied to where chip being formed
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Refrigerated Air System
Another way to cool chip-tool interface chipEffective, inexpensive and readily available Used where dry machining is necessary Uses compressed air that enters vortex generation chamber
Cooled 100F below incoming air 100F
Air directed to interface and blow chips away72
Tool Wear
Loss of materials due to rubbing of two sliding surfaces accompanying friction is called wear. In case of machining loss of cutting tool material is called tool wear. The cutting tool is subjected to (a) high localised stresses (b) high temperature (c) sliding of chip along the rake face (d) rubbing of flank surface with freshly machined workpiece surface and (e) vibration & shock due to improper machining. Due to above factors the loss of material from the tool body accelerates and it loses sharp cutting edge which further hinders effective and efficient machining.73
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Types of Tool Wear1. Flank wear 2. Crater wear 3. Nose or corner wear 4. Chipping of cutting edge
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Flank Wear
Flank Wear: This wear appears as wear land of non-uniform Wear: nonwidth on side and end flank surfaces of the cutting tool. The reasons for this wear can be attributed to (a) rubbing of the tool flank along the freshly machined workpiece surface (b) high temperature which affects tool material properties as well as the work piece surface. The wear land is larger near the two ends of the active portion of the principal cutting edge. The width of wear land is usually maximum at rear end which ultimately results in groove or notch. At the nose chip flow is complicated and the wearing conditions severe. ISO has recommended the maximum width of wear land for different tool materials both for roughing and finishing operation.77
VB = 0.3 mm and VB Max. = 0.6 mm
Cutting forces increase significantly with flank wear. If the amount of flank wear exceeds some critical value, (VB > 0.5-0.6 mm), the excessive cutting force may cause tool failure.78
Crater Wear
It appears as a pit or crater on the rake surface of the cutting tool near the principal cutting edge. The most significant factors influencing the formation of crater wear are (a) temperature at the tool-chip interface tool(b) the chemical affinity between the tool and workpiece materials (c) rubbing between chip and tool rake face accompanying friction. Crater wear affects the mechanics of the process increasing the actual rake angle of the cutting tool and consequently, making cutting easier. At the same time, the crater wear weakens the tool wedge and increases the possibility for tool breakage. In general, crater wear is of a relatively small concern. ISO recommends the following value of crater depth as the tool failure criterion.79
Crater Wear
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Crater Wear
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Crater Wear
KT = 0.06 + 0.3f ; where f= feed per revolution KT / KM = 0.2 to 0.4 (Opitz and Weber) Location of the maximum depth of crater coincides with the location of the maximum temperature at the chip tool interface.
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Nose or Corner Wear
It occurs on the tool corner which may be considered as a part of the wear land since there is no distinguished boundary between the corner wear and flank wear land. We consider corner wear as a separate wear type because of its importance for the precision of machining. Corner wear actually shortens the cutting tool thus increasing gradually the dimension of machined surface and introducing a significant dimensional error in machining, which can reach values of about 0.03 to 0.05 mm.83
Nose or Corner Wear
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Chipping of the cutting edge
Chipping is the term used to describe the breaking away of a small piece from the cutting edge of the tool. The chipped tool material may be very small or relatively large in size. Unlike above mentioned wears, chipping results in sudden loss of tool material and change in tool geometry which affects machining performance. The reasons for chipping can be attributed to (a) mechanical shock ( impact due to interrupted cutting like milling), (b) thermal fatigue ( cyclic variation in temp. of the tool ), (c) transient thermal stresses,
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(d) excessive plastic deformation of the cutting edge, (e) localized cooling, (f) vibration and chatter, (g) excessive crater and flank wears.
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Mechanisms of wear
Shearing at high temperature: The strength of tool material decreases at high temperature which is encountered during machining. Due to work hardening the chip becomes harder and is sufficiently hardened to exert frictional stress sufficient to cause yielding by shear of the hard tool material.
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Mechanisms of wear
Abrasion: Abrasion: Loss of material from rake and flank surfaces of cutting tool may occur due to abrading action of hard particles formed during machining. The workpiece and the chip may contain hard particles like inclusions, sand particles etc. which act as small cutting edges like those of a grinding wheel on the surface of cutting tool. Also fragments of built up edge which are very hard, stick to the underside of the chip and plough the rake surface producing grooves. This action is relatively more severe on the flank surface because of nature of contact and the hard backing provided by the workpiece as compared to the ribbon like chip on the rake surface.
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Mechanisms of wear
Adhesion: Adhesion: During machining the freshly cut chip at very high temperature and pressure gets welded to the high spots on the rake surface. The strength of the bonding at the points of adhesion is often so great that while attempting to free the surfaces, separation takes place not along the interface but either in chip or tool material.91
Mechanisms of wear
Diffusion: Diffusion: With progressive increase in cutting speed the temperature produced at the tool chip interface becomes higher. At this condition the alloying atoms from the hard tool surface may diffuse into softer material of the chip due to concentration gradient. Similarly certain atoms from the chip may also diffuse into the surface layer of the tool, thus weakening it. The particles from this layer can be easily dislodged or sheared off during further machining. Diffusion phenomenon is strongly dependent on temperature. This wear will take place both at flank and rake surfaces and depending on the magnitude and nature of temperature distribution, the predominant wear may shift to crater zone.
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Mechanisms of wear
Fatigue: Fatigue: During sliding of chip over the rake surface of the tool the asperities on both surfaces get interlocked under high pressure. Due to friction compressive stress is developed on one side of the asperities while tensile stress is induced on the other side. After the asperities of a given pair have moved over, the above stresses are relieved. New pairs of asperities are continuously formed and the stress cycle is repeated. This phenomenon causes surface cracks and the rake surface ultimately loses material in the form wear debris. The other factors that contribute to loss of material in this mode are variable thermal stresses owing to temperature variation brought about by cutting fluid, discontinuous chip formation, variable dimensions of cut etc.
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Mechanisms of wear
Electrochemical effect: Since very high effect: temperature is produced at the tool chip interface therefore thermo-electric emf may be thermoproduced between hot and cold junctions which aides traditional wear on the rake surface. For example the current produced due to thermo-electric emf may increase the thermodiffusion of carbon ions from the carbide tools to the flowing chip (galvanic corrosion).94
Mechanisms of wear
Oxidation effect: The freshly cut chips and the effect: machined surfaces may react with atmospheric oxygen to form hard oxides which in turn may damage both rake and flank surfaces of the cutting tool. Chemical decomposition: Some localised chemical decomposition: reactions may occur that weaken the tool material through formation of weak compounds or dissolution of the bond between materials of the tool. If the cutting fluid used in machining is active to the tool material then wear may be greatly accelerated.95
Growth of wear
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In the beginning, the tool is sharp and the wear land on the flank surface has zero width. With further machining the wear land develops and grows in size on account of abrasive wear, shear etc. The magnitude of temperature at the interfaces may cause a shift from abrasion to adhesion or from adhesion to diffusion wear process. In the above figure AB represents initial formation of wear land while BC shows steady growth of wear land due to abrasion. The portion CD represents rapid growth of wear land due to onset of diffusion process. If a cutting tool is operated in this zone for a longer period of time then catastrophic tool failure may occur.
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Orthogonal and Oblique cuttingThe two basic methods of metal cutting using a single point tool are the orthogonal (2 D) and oblique (3D). Orthogonal D). cutting takes place when the cutting face of the tool is 90 degree to the line of action of the tool. If the cutting face is tool. inclined at an angle less than 90 degree to the line of action of the tool, the cutting action is known as oblique. oblique.
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Orthogonal and Oblique cutting
Work Work Feed Feed Tool Tool
Orthogonal cutting Orthogonal Cutting: The cutting edge of the tool remains normal to the direction of tool feed or work feed. The direction of the chip flow velocity is normal to the cutting edge of the tool. Here only two components of forces are acting: Cutting Force and Thrust Force. So the metal cutting may be considered as a two dimensional cutting.
Oblique cutting Oblique Cutting: The cutting edge of the tool remains inclined at an acute angle to the direction of tool feed or work feed. The direction of the chip flow velocity is at an angle with the normal to the cutting edge of the tool. The angle is known as chip flow angle. Here three components of forces are acting: Cutting Force, Radial force and Thrust Force or feed force. So the metal cutting may be considered as a three dimensional cutting. The cutting edge being oblique, the shear force acts on a larger area and thus tool life is increased.
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Mechanics of orthogonal metal cuttingDuring metal cutting ,the metal is severely compressed in the area in front of the cutting tool. tool. This causes high temperature shear, and plastic flow if the metal is ductile. ductile. When the stress in the work piece just ahead of the cutting tool reaches a value exceeding the ultimate strength of the metal, particles will shear to form a chip element, which moves up along the face of the work. work. The outward or shearing movement of each successive element is arrested by work hardening and the movement transferred to the next element. element. The process is repetitive and a continuous chip is formed. formed. The plane along which the element shears, is called shear plane. plane.100
Assumptions in orthogonal metal cutting
No contact at the flank i.e. the tool is perfectly sharp. sharp. No side flow of chips i.e. width of the chips remains constant. constant. Uniform cutting velocity. velocity. A continuous chip is produced with no built up edge. edge. The chip is considered to be held in equilibrium by the action of the two equal and opposite resultant forces R and R/ and assume that the resultant is collinear. collinear.101
Oblique Cutting
The cutting edge of the tool remains inclined at an acute angle to the direction of feed (of the work or tool) The direction of the chip flow is not normal to the cutting edge. Rather it is at an angle to the normal to the cutting edge. The cutting edge is inclined at an angle to the normal to the feed. This angle is called inclination angle. angle. The chip flows at an angle to the normal to the cutting edge. This angle is called chip flow angle.
The shear force acts on a larger area, hence the shear force per area is smaller The tool life is higher than obtained in orthogonal cutting There are only three mutually perpendicular components of cutting forces on the tool The cutting edge is smaller than the width of cut.102
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