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Machining Cutting action involves shear deformation of work material to form a chip As chip is removed, new surface is exposed Figure 21.2 (a) A cross‑sectional view of the machining process, (b) tool with negative rake angle; compare with positive rake angle in (a).
Orthogonal Cutting Model Simplified 2-D model of machining that describes the mechanics of machining fairly accurately Figure 21.6 Orthogonal cutting: (a) as a three‑dimensional process.
to r tc where r chip thickness ratio; to thickness of the chip prior to chip formation; and tc chip thickness after separation Chip thickness after cut always greater than before, so chip ratio always less than 1.0
Based on the geometric parameters of the orthogonal model, the shear plane angle can be determined as: r cos tan 1 r sin where r chip ratio, and rake angle
Shear Strain in Chip Formation Figure 21.7 Shear strain during chip formation: (a) chip formation depicted as a series of parallel plates sliding relative to each other, (b) one of the plates isolated to show shear strain, and (c) shear strain triangle used to derive strain equation.
Shear strain in machining can be computed from the following equation, based on the preceding parallel plate model: tan( - ) cot where shear strain, shear plane angle, and rake angle of cutting tool
Chip Formation Figure 21.8 More realistic view of chip formation, showing shear zone rather than shear plane. Also shown is the secondary shear zone resulting from tool‑chip friction.
Discontinuous chip 2. Continuous chip 3. Continuous chip with Built-up Edge (BUE) 4. Serrated chip 1.
Discontinuous Chip Brittle work materials Low cutting speeds Large feed and depth of cut High tool‑chip friction Figure 21.9 Four types of chip formation in metal cutting: (a) discontinuous
Continuous Chip Ductile work materials High cutting speeds Small feeds and depths Sharp cutting edge Low tool‑chip friction Figure 21.9 (b) continuous
Continuous with BUE Ductile materials Low‑to‑medium cutting speeds Tool-chip friction causes portions of chip to adhere to rake face BUE forms, then breaks off, cyclically Figure 21.9 (c) continuous with built‑up edge
Serrated Chip Semicontinuous - saw-tooth appearance Cyclical chip forms with alternating high shear strain then low shear strain Associated with difficult-tomachine metals at high cutting speeds Figure 21.9 (d) serrated.
Forces Acting on Chip Friction force F and Normal force to friction N Shear force Fs and Normal force to shear Fn Figure 21.10 Forces in metal cutting: (a) forces acting on the chip in orthogonal cutting
Vector addition of F and N resultant R Vector addition of Fs and Fn resultant R' Forces acting on the chip must be in balance: › R' must be equal in magnitude to R › R’ must be opposite in direction to R › R’ must be collinear with R
Coefficient of friction between tool and chip: F N Friction angle related to coefficient of friction as follows: tan
Shear stress acting along the shear plane: S Fs As where As area of the shear plane t ow As sin Shear stress shear strength of work material during cutting
Cutting Force and Thrust Force F, N, Fs, and Fn cannot be directly measured Forces acting on the tool that can be measured: › Cutting force Fc and Thrust force Ft Figure 21.10 Forces in metal cutting: (b) forces acting on the tool that can be measured
Equations can be derived to relate the forces that cannot be measured to the forces that can be measured: F Fc sin Ft cos N Fc cos ‑ Ft sin Fs Fc cos ‑ Ft sin Fn Fc sin Ft cos Based on these calculated force, shear stress and coefficient of friction can be determined
Of all the possible angles at which shear deformation can occur, the work material will select a shear plane angle that minimizes energy, given by 45 2 2 Derived by Eugene Merchant Based on orthogonal cutting, but validity extends to 3-D machining
45 2 2 To increase shear plane angle › Increase the rake angle › Reduce the friction angle (or coefficient of friction)
ce relationships Merchant circle Ski edge angle shear angle Snow p Fs Fc Fn F R N - - Ft - Forces Fc centrifugal (cutting) Ft thrust Fs shear Fn normal to shear plane F friction on ski N normal to ski
The most important geometry’s to consider on a cutting tool are › Back Rake Angles › End Relief Angles › Side Relief Angles
Small to medium rake angles cause: › high compression › high tool forces › high friction › result Thick—highly deformed—hot chips
Larger positive rake angles › Reduce compression and less chance of a discontinuous chip › Reduce forces › Reduce friction › Result A thinner, less deformed, and cooler chip.
Problems .as we increase the angle: › Reduce strength of tool › Reduce the capacity of the tool to conduct heat away from the cutting edge. › To increase the strength of the tool and allow it to conduct heat better, in some tools, zero to negative rake angles are used.
Typical tool materials which utilize negative rakes are: Carbide Diamonds Ceramics These materials tend to be much more brittle than HSS but they hold superior hardness at high temperatures. The negative rake angles transfer the cutting forces to the tool which help to provide added support to the cutting edge.
Positive rake angles › Reduced cutting forces › Smaller deflection of work, tool holder, and › › › › machine Considered by some to be the most efficient way to cut metal Creates large shear angle, reduced friction and heat Allows chip to move freely up the chip-tool zone Generally used for continuous cuts on ductile materials which are not to hard or brittle
Negative rake angles › Initial shock of work to tool is on the face of the tool and not on the point or edge. This prolongs the life of the tool. › Higher cutting speeds/feeds can be employed
Factors to consider for tool angles › The hardness of the metal › Type of cutting operation › Material and shape of the cutting tool › The strength of the cutting edge
HIGH STRESSES & TEMPERATURES GRADUAL WEAR MANY VARIABLES MATERIAL CUTTING FLUIDS TOOL SHAPE SPEEDS & FEED RATE CHIPPING
1. 2. 3. 4. 5. Good cooling capacity 6. Good lubricating qualities 7. Resistance to 8. rancidity 9. Relatively low viscosity Stability (long life) Rust resistance Nontoxic Transparent Nonflammable 34
Most commonly used cutting fluids › Either aqueous based solutions or cutting oils Fall into three categories › Cutting oils › Emulsifiable oils › Chemical (synthetic) cutting fluids 35
Two classifications › Active › Inactive Terms relate to oil's chemical activity or ability to react with metal surface › Elevated temperatures › Improve cutting action › Protect surface 36
Those that will darken copper strip immersed for 3 hours at temperature of 212ºF Dark or transparent Better for heavy-duty jobs Three categories › Sulfurized mineral oils › Sulfochlorinated mineral oils › Sulfochlorinated fatty oil blends 37
Oils will not darken copper strip immersed in them for 3 hours at 212ºF 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 fattymineral oil blend 38
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) Good cooling and lubricating qualities Used at high cutting speeds, low cutting pressures 39
Also called synthetic fluids Introduced about 1945 Stable, preformed emulsions › Contain very little oil and mix easily with water Extreme-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 metal 40
Good rust control 2. Resistance to rancidity for long periods of time 3. Reduction of amount of heat generated during cutting 4. Excellent cooling qualities 1. 41
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 42
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. 43
Prime functions › Provide cooling › Provide lubrication Other functions › Prolong cutting-tool life › Provide rust control › Resist rancidity 44
Heat has definite bearing on cutting-tool wear › Small reduction will greatly extend tool life Two sources of heat during cutting action › Plastic deformation of metal Occurs immediately ahead of cutting tool Accounts for 2/3 to 3/4 of heat › Friction from chip sliding along cutting-tool face 45
Water most effective for reducing heat by will promote oxidation (rust) Decrease the temperature at the chiptool interface by 50 degrees F, and it will increase tool life by up to 5 times. 46
Reduces friction between chip and tool face › Shear plane becomes shorter › Area where plastic deformation occurs correspondingly smaller Extreme-pressure lubricants reduce amount of heat-producing friction EP chemicals of synthetic fluids combine chemically with sheared metal of chip to form solid compounds (allow chip to slide) 47
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 48
Heat and friction prime causes of cutting-tool breakdown Reduce temperature by as little as 50ºF, life of cutting tool increases fivefold Built-up edge › Pieces of metal weld themselves to tool face › Becomes large and flat along tool face, effective rake angle of cutting tool decreased 50
Built-up edge keeps breaking off and re-forming Result is poor surface finish, excessive flank wear, and cratering of tool face 51
1. 2. 3. 4. 5. Lowers heat created by plastic deformation of metal Friction at chip-tool interface decreased Less power is required for machining because of reduced friction Prevents built-up edge from forming Surface finish of work greatly improved 52
Water best and most economical coolant › Causes parts to rust Rust is oxidized iron Chemical cutting fluids contain rust inhibitors › Polar film › Passivating film 53
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 54
Cutting-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 ¾ width of cutting tool › Applied to where chip being formed 55
Another way to cool chip-tool interface Effective, inexpensive and readily available Used where dry machining is necessary Uses compressed air that enters vortex generation chamber › Cooled 100ºF below incoming air Air directed to interface and blow chips away 56
Hardness (Elevated temperatures) Toughness (Impact forces on tool in interrupted operations) Wear resistance (tool life to be considered) Chemical stability or inertness (to avoid adverse reactions)
Carbon & medium alloy steels High speed steels Cast-cobalt alloys Carbides Coated tools Alumina-based ceramics Cubic boron nitride Silicon-nitride-base ceramics Diamond Whisker-reinforced materials
Oldest of tool materials Used for drills taps,broaches,reamers Inexpensive ,easily shaped,sharpened No sufficient hardness and wear resistance Limited to low cutting speed operation
Hardened to various depths Good wear resistance Relatively Suitable for high positive rake angle tools
Molybdenum ( M-series) Tungsten ( T-series)
Contains 10% molybdenum, chromium, vanadium, tungsten, cobalt Higher, abrasion resistance H.S.S. are majorly made of M-series
12 % - 18 % tungsten, chromium, vanadium & cobalt undergoes less distortion during heat treating
H.S.S. available in wrought ,cast & sintered (Powder metallurgy) Coated for better performance Subjected to surface treatments such as casehardening for improved hardness and wear resistance or steam treatment at elevated temperatures High speed steels account for largest tonnage
Commonly known as stellite tools Composition ranges – 38% - 53 % cobalt 30%- 33% chromium 10%-20%tungsten Good wear resistance ( higher hardness) Less tough than high-speed steels and sensitive to impact forces Less suitable than high-speed steels for interrupted cutting operations Continuous roughing cuts – relatively high g feeds & speeds Finishing cuts are at lower feed and depth of cut
3-groups of materials Alloy steels High speed steels Cast alloys These carbides are also known as cemented or sintered carbides High elastic modulus,thermal conductivity Low thermal expansion 2-groups of carbides used for machining operations tungsten carbide titanium carbide
Composite material consisting of tungsten-carbide particles bonded together Alternate name is cemented carbides Manufactured with powder metallurgy techniques Particles 1-5 Mum in size are pressed & sintered to desired shape Amount of cobalt present affects properties of carbide tools As cobalt content increases – strength hardness & wear resistance increases
Titanium carbide has higher wear resistance than tungsten carbide Nickel-Molybdenum alloy as matrix – Tic suitable for machining hard materials Steels & cast irons Speeds higher than those for tungsten carbide
Individual cutting tool with severed cutting points Clamped on tool shanks with locking mechanisms Inserts also brazed to the tools Clamping is preferred method for securing an insert Carbide Inserts available in various shapes-Square, Triangle, Diamond and round Strength depends on the shape Inserts honed, chamfered or produced with negative land to improve edge strength
Fig : Methods of attaching inserts to toolholders : (a) Clamping and (b) Wing lockpins. (c) Examples of inserts attached to toolholders with threadless lockpins, which are secured with side screws.
Fig : Relative edge strength and tendency for chipping and breaking of inserts with various shapes. Strength refers to the cutting edge shown by the included angles. Fig : edge preparation of inserts to improve edge strength.
Purpose : Eliminating long chips Controlling chip flow during machining Reducing vibration & heat generated Selection depends on feed and depth of cut Work piece material,type of chip produced during cutting
- High strength and toughness but generally abrasive and chemically reactive with tool materials Unique Properties : Lower Friction High resistance to cracks and wear High Cutting speeds and low time & costs Longer tool life
Titanium nitride (TiN) Titanium carbide (Tic) Titanium Carbonitride (TicN) Aluminum oxide (Al2O3)thickness range – 2-15 µm (80600Mu.in) Techniques used : Chemical –vapor deposition (CVD) Plasma assisted CVD Physical-vapor deposition(PVD) Medium –temperature chemical- vapor deposition(MTCVD)
Fig : Ranges of properties for various groups of tool materials.
High hardness Chemical stability Low thermal conductivity Good bonding Little or no Porosity Titanium nitride (TiN) coating : Low friction coefficients High hardness Resistance to high temperatures Good adhesion to substrate High life of high speed-steel tools Titanium carbide (TiC) coating: Titanium carbide coatings on tungsten-carbide inserts have high flank wear resistance.
Low thermal conductivity ,resistance ,high temperature Resistance to flank wear and crater wear Ceramics are suitable materials for tools Al2O3 (most commonly used) Multi Phase Coatings : First layer –Should bond well with substrate Outer layer – Resist wear and have low thermal conductivity Intermediate layer – Bond well & compatible with both layers Coatings of alternating multipurpose layers are also formed.
Fig : Multiphase coatings on a tungsten-carbide substrate. Three alternating layers of aluminum oxide are separated by very thin layers of titanium nitride. Inserts with as many as thirteen layers of coatings have been made. Coating thick nesses are typically in the range of 2 to 10 µm.
Use of Polycrystalline diamond as a coating Difficult to adhere diamond film to substrate Thin-film diamond coated inserts now commercially available Thin films deposited on substrate with PVD & CVD techniques Thick films obtained by growing large sheet of pure diamond Diamond coated tools particularly effective in machining non-ferrous and abrasive materials
Titanium carbo nitride (TiCN) Titanium Aluminum Nitride(TiAlN) Chromium Based coatings Chromium carbide Zirconium Nitride (ZrN) Hafnium nitride (HfN) Recent developments gives nano coating & composite coating Ion Implementation : Ions placed into the surface of cutting tool No change in the dimensions of tool Nitrogen-ion Implanted carbide tools used for alloy steels & stainless steels Xeon – ion implantation of tools as under development
Cold-Pressed Into insert shapes under high pressure and sintered at high temperature High Abrasion resistance and hot hardness Chemically stable than high speed steels & carbides So less tendency to adhere to metals Good surface finish obtained in cutting cast iron and steels Negative rake-angle preferred to avoid chipping due to poor tensile strength Cermets, Black or Hot- Pressed : 70% aluminum oxide & 30 % titanium carbide cermets(ceramics & metal) Cermets contain molybdenum carbide, niobium carbide and tantalum carbide.
Made by bonding ( 0.5-1.0 mm ( 0.02-0.04-in) Layer of poly crystalline cubic boron nitride to a carbide substrate by sintering under pressure While carbide provides shock resistance CBN layer provides high resistance and cutting edge strength Cubic boron nitride tools are made in small sizes without substrate Fig : (a) Construction of a polycrystalline cubic boron nitride or a diamond layer on a tungsten-carbide insert. (b) Inserts with polycrystalline cubic boron nitride tips (top row) and solid polycrystalline CBN inserts (bottom row).
They consists various addition of Aluminum Oxide ythrium oxide, titanium carbide SiN have toughness, hot hardened & good thermal – shock resistance SiN base material is Silicon High thermal & shock resistance Recommended for machining cast iron and nickel based super alloys at intermediate cutting speeds
Hardest known substance Low friction, high wear resistance Ability to maintain sharp cutting edge Single crystal diamond of various carats used for special applications Machining copper—front precision optical mirrors for ( SDI) Diamond is brittle , tool shape & sharpened is important Low rake angle used for string cutting edge
Used for wire drawing of fine wires Small synthesis crystal fused by high pressure and temperature Bonded to a carbide substrate Diamond tools can be used fir any speed Suitable for light un-interrupted finishing cuts To avoid tool fracture single crystal diamond is to be re-sharpened as it becomes dull Also used as an abrasive in grinding and polishing operations
New tool materials with enhanced properties : High fracture toughness Resistance to thermal shock Cutting –edge strength Hot hardness
Examples: Silicon-nitride base tools reinforced with silicon-carbide( Sic) Aluminum oxide based tools reinforced with silicon-carbide with ferrous metals makes Sicreinforced tools Progress in nanomaterial has lead to the development of cutting tools Made of fine grained structures as (micro grain) carbides