Published on March 13, 2014
THEORY OF METAL CUTTING Thermal aspects of Machining, Tool materials, Tool wear Cutting fluids and Machinability. ME 010 803 PRODUCTION ENGINEERING Module II
Cutting Temperatures Of the total energy consumed in machining, nearly all of it is converted into heat. The heat generated can cause temperatures to be as high as 6000C at tool chip interface. Cutting temperature has a controlling influence on the rate of tool wear and friction between tool and chip. Elastic deformation- Energy required for the operation is stored in the material as strain energy and no heat is generated. Plastic deformation – Most of the energy used is converted as heat.
Cutting Temperatures Cutting temperatures are important because high temperatures, 1. Reduce tool life. 2. Produce hot chips that pose safety hazards to the machine operator. 3. Can cause inaccuracies in work part dimensions due to thermal expansion of work piece material.
Effect of cutting temperature The effect of cutting temperature, particularly when it is high is mostly detrimental to both the tool and the job. The major portion of the heat is taken away by the chips. But it does not matter because chips are thrown out. So attempts should be made such that the chips take away more and more amount of heat leaving small amount of heat to harm the tool and the job.
Effect of cutting temperature on tool The possible detrimental effects of the high cutting temperature on cutting tool (edge) are Rapid tool wear which reduces tool life plastic deformation of the cutting edges if the tool material is not enough hot-hard and hot-strong thermal flaking and fracturing of the cutting edges due to thermal shocks. Built up Edge formation.
Effect of cutting temperature on Job The possible detrimental effects of the high cutting temperature on machined job are: Dimensional inaccuracy of the job due to thermal distortion and expansion-contraction during and after machining surface damage by oxidation, rapid corrosion, burning etc. induction of tensile residual stresses and micro cracks at the surface / subsurface.
Effect of Cutting Temperature However, often the high cutting temperature helps in reducing the magnitude of the cutting forces and cutting power consumption to some extent by softening or reducing the shear strength, τs of the work material ahead the cutting edge. To attain or enhance such benefit the work material ahead the cutting zone is often additionally heated externally. This technique is known as Hot Machining and is beneficially applicable for the work materials which are very hard and hardenable like high manganese steel, Hadfield steel, Ni-hard, Nimonic etc.
Factors Affecting Temperature
Sources and Causes of heat generation in Machining During machining, heat is generated at the cutting point from three sources, as indicated in Fig. Those sources and causes of development of cutting temperature are: Primary shear zone (1) where the major part of the energy is converted into heat. Secondary deformation zone (2) at the chip – tool interface where further heat is generated due to rubbing and / or shear. At the worn out flanks (3) due to rubbing between the tool and the finished surfaces.
Sources of heat generation in Machining
Thermal Aspects of Machining The heat generated is shared by the chip, cutting tool and the blank. The apportionment of sharing the heat depends upon the configuration, size and thermal conductivity of the tool – work material and the cutting condition. The following figure visualizes that maximum amount of heat is carried away by the flowing chip. From 10 to 20% of the total heat goes into the tool and some heat is absorbed in the blank. With the increase in cutting velocity, the chip shares heat increasingly.
Thermal Aspects of Machining
Temperature distribution in Metal Cutting Fig. shows temperature distribution in work piece and chip during orthogonal cutting (obtained from an infrared photograph, for free- cutting mild steel where cutting speed is 0.38m/s, the width of cut is 6.35mm, the normal rake is 300, and work piece temperature is 6110C)
Temperature distribution in Metal Cutting
Analytical methods to compute Cutting Temperatures Cook’s Method Where, δT = Mean temperature rise at tool chip interface, C0 U = Specific Energy in the operation, N-m/mm3 V = Cutting Speed, m/s t0 = Chip thickness before the cut, m ρC = Volumetric Specific heat of work material, J/mm3-C0 K = Thermal diffusivity of the work material, m2/s 0.333 04.0 K Vt C U T
Measurement of tool-chip interface temperature
Tool work Thermocouple Technique In a thermocouple two dissimilar but electrically conductive metals are connected at two junctions. Whenever one of the junctions is heated, the difference in temperature at the hot and cold junctions produce a proportional current which is detected and measured by a milli-voltmeter. In machining like turning, the tool and the job constitute the two dissimilar metals and the cutting zone functions as the hot junction. Then the average cutting temperature is evaluated from the mV after thorough calibration for establishing the exact relation between mV and the cutting temperature.
Tool work thermocouple technique
Embedded thermocouple technique In operations like milling, grinding etc. where the previous methods are not applicable, embedded thermocouple can serve the purpose. Fig. shows the principle. The standard thermocouple monitors the job temperature at a certain depth, hi from the cutting zone. The temperature recorded in oscilloscope or strip chart recorder becomes maximum when the thermocouple bead comes nearest (slightly offset) to the grinding zone. With the progress of grinding the depth, hi gradually decreases after each grinding pass and the value of temperature, θm also rises as has been indicated in Fig. For getting the temperature exactly at the surface i.e., grinding zone, hi has to be zero, which is not possible. So the θm vs hi curve has to be extrapolated up to hi = 0 to get the actual grinding zone temperature. Log – log plot helps such extrapolation more easily and accurately.
Embedded thermocouple technique
Infra-red photographic technique This modern and powerful method is based on taking infra-red photograph of the hot surfaces of the tool, chip, and/or job and get temperature distribution at those surfaces. Proper calibration is to be done before that. This way the temperature profiles can be recorded as indicated in Fig. The fringe pattern readily changes with the change in any machining parameter which affect cutting temperature.
Infra-red photographic technique
Tool wear and failure The usefulness of tool cutting edge is lost through Wear Breakage Chipping Deformation Tool failure implies that the tool has reached a point beyond which it will not function satisfactorily until it is re-sharpened.
Three Modes of Tool Failure Fracture failure When the Cutting force at tool point becomes excessive, it leads to failure by brittle fracture. Temperature failure Cutting temperature is too high for the tool material, which makes the tool point to soften, and leads to plastic deformation along with a loss of sharp edge. Gradual wear Gradual wearing of the cutting edge causes loss of tool shape, reduction in cutting efficiency and finally tool failure.
Preferred Mode of Tool Failure: Gradual Wear Fracture and temperature failures are premature failures Gradual wear is preferred because it leads to the longest possible use of the tool Gradual wear occurs at two locations on a tool: Crater wear – occurs on top rake face Flank wear – occurs on flank (side of tool)
Figure: Diagram of worn cutting tool, showing the principal locations and types of wear that occur
Crater wear It consists of a concave section on the tool face formed by the action of the chip sliding on the surface. 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.
Flank wear It occurs on the tool flank as a result of friction between the machined surface of the work piece and the tool flank. Flank wear appears in the form of so-called wear land and is measured by the width of this wear land, VB, Flank wear affects to the great extend the mechanics of cutting. Cutting forces increase significantly with flank wear. If the amount of flank wear exceeds some critical value i.e. (VB > 0.5~0.6 mm), the excessive cutting force may cause tool failure.
Corner Wear It occurs on the tool corner. Can be considered as a part of the wear land and respectively flank wear 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 0.03~0.05 mm.
Figure : (a)Crater wear, and (b)flank wear on a cemented carbide tool, as seen through a toolmaker's microscope
Tool Wear: Mechanisms Adhesion wear: Fragments of the work-piece get welded to the tool surface at high temperatures; eventually, they break off, tearing small parts of the tool with them. Abrasion: Hard particles, microscopic variations on the bottom surface of the chips rub against the tool surface and break away a fraction of tool with them. Diffusion wear: At high temperatures, atoms from tool diffuse across to the chip; the rate of diffusion increases exponentially with temperature; this reduces the fracture strength of the crystals.
Figure: Tool wear as a function of cutting time Flank wear (FW) is used here as the measure of tool wear Crater wear follows a similar growth curve Tool Wear vs. Time
Factors affecting Tool life Tool material Hardness Work material Surface roughness of work piece Profile of cutting tool Type of machining operation Cutting speed, feed and depth of cut Cutting temperature
Tool life Criteria Actual cutting time to failure. Length of work cut to failure. Volume of metal removed to failure. Cutting speed for a given time to failure. Number of components produced.
Figure: Effect of cutting speed on tool flank wear (FW) for three cutting speeds, using a tool life criterion of 0.50 mm flank wear Effect of Cutting Speed
Figure: Natural log-log plot of cutting speed vs tool life
Tool Life Tool wear is a time dependent process. As cutting proceeds, the amount of tool wear increases gradually. Tool wear must not be allowed to go beyond a certain limit in order to avoid tool failure. Tool life is defined as the time interval for which tool works satisfactorily between two successive grinding or re-sharpening of the tool.
Taylor Tool Life Equation This relationship is credited to F. W. Taylor (~1900) CvT n Where, v = cutting speed; T = tool life; and n and C are parameters that depend on feed, depth of cut, work material, tooling material, and the tool life criterion used n is the slope of the plot C is the intercept on the speed axis
Tool Life vs. Cutting Speed As cutting speed is increased, wear rate increases, so the same wear criterion is reached in less time, i.e., tool life decreases with cutting speed
Typical Values of n and C in Taylor’s Tool Life Equation Tool material n C (m/min) C (ft/min) High speed steel: Non-steel work 0.125 120 350 Steel work 0.125 70 200 Cemented carbide Non-steel work 0.25 900 2700 Steel work 0.25 500 1500 Ceramic Steel work 0.6 3000 10,000
Tool life Volume of metal removed per minute Vm Vm = . . . 3/ in D = dia of workpiece, mm t = depth of cut, mm f = feed, mm/rev N = RPM
Tool Life If T be the time for tool failure in mins, The total volume removed up to Tool Failure = . . . . Cutting Speed, V = DN/ 000 m/min Volume of material removed up to tool failure = 000 V. . .
Tool Near End of Life Changes in sound emitted from operation. Chips become ribbon-like, stringy, and difficult to dispose off. Degradation of surface finish. Increased power required to cut. Visual inspection of the cutting edge with magnifying optics can determine if tool should be replaced.
Tool life is measured by: Visual inspection of tool edge Tool breaks Fingernail test Changes in cutting sounds Chips become ribbony, stringy Surface finish degrades Computer interface says - power consumption up - cumulative cutting time reaches certain level - cumulative number of pieces reaches certain value Operator’s Tool life
Wear Control The rate of tool wear strongly depends on the cutting temperature, therefore, any measures which could be applied to reduce the cutting temperature would reduce the tool wear as well. The figure shows the process parameters that influence the rate of tool wear: Additional measures to reduce the tool wear include the application of advanced cutting tool materials, such as coated carbides, ceramics, etc..
Cutting Tool Technology It has two principal aspects: 1. Tool material Developing materials that can withstand the forces, temperatures and wearing in machining process. 2. Tool geometry Optimizing the geometry of the cutting tool for the tool material and for a given operation.
The cutting tool materials must possess a number of important properties to avoid excessive wear, fracture failure and high temperatures in cutting. The following characteristics are essential for cutting materials to withstand the heavy conditions of the cutting process and to produce high quality and economical parts: Tool failure modes identify the important properties that a tool material should possess: Toughness - to avoid fracture failure. Hot hardness - ability to retain hardness at high temperatures. Wear resistance - hardness is the most important property to resist abrasive wear. CUTTING TOOL MATERIALS
CUTTING TOOL MATERIALS hardness at elevated temperatures (so-called hot hardness) so that hardness and strength of the tool edge are maintained in high cutting temperatures. Toughness: ability of the material to absorb energy without failing. Cutting is often accompanied by impact forces especially if cutting is interrupted, and cutting tool may fail very soon if it is not strong enough. wear resistance: although there is a strong correlation between hot hardness and wear resistance, latter depends on more than just hot hardness. Other important characteristics include surface finish on the tool, chemical inertness of the tool material with respect to the work material, and thermal conductivity of the tool material, which affects the maximum value of the cutting temperature at tool-chip interface.
Fig: Typical hot hardness relationships for selected tool materials. Plain carbon steel shows a rapid loss of hardness as temperature increases. High speed steel is substantially better, while cemented carbides and ceramics are significantly harder at elevated temperatures.
Carbon Steels It is the oldest of tool material. It is inexpensive, easily shaped, sharpened. The carbon content is 0.6~1.5% with small quantities of silicon, chromium, manganese, and vanadium to refine grain size. This material has low wear resistance and low hot hardness. Maximum hardness is about HRC 62. Used for drills taps, broaches, reamers. Limited to hand tools and low cutting speed operation. (Red hardness temp.: 200 C) The use of these materials now is very limited.
High Speed Steel (HSS) First produced in 1900s. They are highly alloyed with vanadium, cobalt, molybdenum, tungsten and chromium added to increase hot hardness and wear resistance. Can be hardened to various depths by appropriate heat treating up to cold hardness in the range of HRC 63-65. The cobalt component give the material a hot hardness value much greater than carbon steels.(Red hardness temp.: 6500 C) The high toughness and good wear resistance make HSS suitable for all type of cutting tools with complex shapes for relatively low to medium cutting speeds.
High Speed Steel (HSS) Highly alloyed tool steel capable of maintaining hardness at elevated temperatures better than high carbon and low alloy steels. One of the most important cutting tool materials Especially suited to applications involving complicated tool geometries, such as The most widely used tool material today for taps, drills, reamers, gear tools, end cutters, slitting, broaches, etc. Two basic types 1. Tungsten-type, designated T- grades 2. Molybdenum-type, designated M-grades
High Speed Steel Composition Two basic types of HSS M-series (6-6-4-2): Contains 6% molybdenum, 6% tungsten, 4% chromium, 2% vanadium & cobalt Higher, abrasion resistance H.S.S. are majorly made of M-series T-series (18-4-1): Contains 18 % tungsten, 4% chromium, 1% vanadium & cobalt undergoes less distortion during heat treating
Cemented Carbides Introduced in the 1930s. These are the most important tool materials today because of their high hot hardness and wear resistance. There may be other carbides in the mixture, such as titanium carbide (TiC) and/or tantalum carbide (TaC) in addition to WC. The main disadvantage of cemented carbides is their low toughness.
Cemented Carbides – General Properties High compressive strength, but low to moderate tensile strength High hardness (90 to 95 HRA) Good hot hardness Good wear resistance High thermal conductivity High elastic modulus - 600 x 103 MPa (90 x 106 lb/in2) Toughness lower than high speed steel
This hard tool material is produced by a powder metallurgy technique, sintering grains of tungsten carbide (WC) in a cobalt (Co) matrix (as the binder, it provides toughness). Particles 1-5 μm in size are pressed & sintered to desired shape in a H2 atmosphere furnace at 15500 C. Amount of cobalt present affects properties of carbide tools. As cobalt content increases – strength, hardness & wear resistance increases. Cemented Carbides
Insert Attachment In spite of more traditional tool materials, cemented carbides are available as inserts produced by powder metallurgy process. Inserts are available in various shapes, and are usually mechanically attached by means of clamps to the tool holder, or brazed to the tool holder. The clamping is preferred because after an cutting edge gets worn, the insert is indexed (rotated in the holder) for another cutting edge. When all cutting edges are worn, the insert is thrown away. The indexable carbide inserts are never reground. If the carbide insert is brazed to the tool holder, indexing is not available, and after reaching the wear criterion, the carbide insert is re-sharpened on a tool grinder.
Types of Cemented Carbides Two basic types: 1. Non-steel cutting grades - only WC-Co 2. Steel cutting grades - TiC & TaC added to WC-Co
Non-Steel Cutting Carbide Grades • Used for nonferrous metals and gray cast iron • Properties determined by grain size and cobalt content – As grain size increases, hardness and hot hardness decrease, but toughness increases. – As cobalt content increases, toughness improves at the expense of hardness and wear resistance.
Steel Cutting Carbide Grades Used for low carbon, stainless, and other alloy steels – For these grades, TiC and/or TaC are substituted for some of the WC. – This composition increases crater wear resistance for steel cutting, but adversely affects flank wear resistance for non-steel cutting applications.
Coated WC One advance in cutting tool materials involves the application of a very thin coating (~ 10 μm) to a K-grade substrate, which is the toughest of all carbide grades. Coating may consists of one or more thin layers of wear-resistant material, such as titanium carbide (TiC), titanium nitride (TiN), aluminum oxide (Al2O3), and/or other, more advanced materials. Coating allows to increase significantly the cutting speed for the same tool life. Structure of a multi-layer coated carbide insert
Coated Carbides Cemented carbide insert coated with one or more thin layers of wear resistant materials, such as TiC, TiN, and/orAl2O3 Coating is applied by chemical vapor deposition or physical vapor deposition. Coating thickness = 2.5 - 13 m (0.0001 to 0.0005 in) Applications: cast irons and steels in turning and milling operations. Best applied at high speeds where dynamic force and thermal shock are minimal.
Ceramics Primarily fine-grained Al2O3, pressed and sintered at high pressures and temperatures into insert form with no binder. • Applications: high speed turning of cast iron and steel • Not recommended for heavy interrupted cuts (e.g. rough milling) due to low toughness • There is no occurrence of built-up edge, and coolants are not required. • Al2O3 also widely used as an abrasive in grinding.
Ceramics Two types are available: White or cold-pressed ceramics, which consists of only Al2O3 cold pressed into inserts and sintered at high temperature. Black or hot-pressed ceramics, commonly known as cermet (from ceramics & metal). This material consists of 70% Al2O3 and 30% TiC. Both materials have very high wear resistance but low toughness, therefore they are suitable only for continuous operations such as finishing turning of cast iron and steel at very high speeds.
Cermets Combinations of TiC, TiN, and titanium carbonitride (TiCN), with nickel and/or molybdenum as binders. Some chemistries are more complex. Applications: high speed finishing and semi-finishing of steels, stainless steels and cast irons. – Higher speeds and lower feeds than steel-cutting carbide grades – Better finish achieved, often eliminating need for grinding.
Diamond • Diamond is the hardest substance ever known of all materials. • Low friction, high wear resistance. • Ability to maintain sharp cutting edge. • Use is limited because it gets converted into graphite at high temperature (700 C). Graphite diffuses into iron and make it unsuitable for machining steels. • It is used as a coating material in its polycrystalline form, or as a single- crystal diamond tool for special applications, such as mirror finishing of non-ferrous materials.
Synthetic Diamonds Sintered polycrystalline diamond (SPD) - fabricated by sintering very fine grained diamond crystals under high temperatures and pressures into desired shape with little or no binder. Usually applied as coating (0.5 mm thick) on WC-Co insert Applications: high speed machining of nonferrous metals and abrasive nonmetals such as fiberglass, graphite, and wood. - Not for steel cutting
Cubic Boron Nitride Next to diamond, cubic boron nitride (CBN) is hardest material known. Retain hardness up to 1000 C. By bonding 0.5 mm thick polycrystalline CBN onto a carbide substrate through sintering under pressure. CBN is used mainly as coating material because it is very brittle. In spite of diamond, CBN is suitable for cutting ferrous materials. Applications: machining steel and nickel-based alloys.
SPD and CBN tools are expensive. Made by bonding (0.5-1.0 mm) 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. Cubic Boron Nitride
Lubricants – purpose is to reduce friction… usually oil based Coolants – purpose is to transport heat… usually water based Both lose their effectiveness at higher cutting speeds! Cutting Fluids
Dry Machining • No cutting fluid is used • Avoids problems of cutting fluid contamination, disposal, and filtration • Problems with dry machining: – Overheating of the tool – Operating at lower cutting speeds and production rates to prolong tool life – Absence of chip removal benefits of cutting fluids in grinding and milling
77 Cutting Fluids • Essential in metal-cutting operations to reduce 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 1944
Cutting Fluids Any liquid or gas applied directly to machining operation to improve cutting performance Two main problems addressed by cutting fluids: 1. Heat generation at shear zone and friction zone 2. Friction at the tool - chip and tool - work interfaces Other functions and benefits: – Wash away chips (e.g., grinding and milling) – Reduce temperature of work part for easier handling – Improve dimensional stability of work part
79 Heat Generated During Machining • Heat finds its way into one of three places – Work piece, tool and chips Too much, work will expandToo much, cutting edge will break down rapidly, reducing tool life Act as disposable heat sink
80 Heat Dissipation • Ideally most heat taken off in chips • Indicated by change in chip colour as heat causes chips to oxidize. • Cutting fluids assist taking away heat – Can dissipate at least 50% of heat created during machining.
Characteristics of a Good Cutting Fluid
82 Characteristics of a Good Cutting Fluid 1. Good cooling capacity 2. Good lubricating qualities 3. Resistance to rancidity 4. Relatively low viscosity 5. Stability (long life) 6. Rust resistance 7. Nontoxic 8. Transparent 9. Non inflammable
83 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 machining
84 Functions of a Cutting Fluid Prime functions – Provide cooling – Provide lubrication Other functions – Prolong cutting-tool life – Provide rust control – Resist rancidity
85 Functions of a Cutting Fluid: Cooling • Most effective at high cutting speeds where heat generation and high temperatures are problems. • Most effective on tool materials that are most susceptible to temperature failures. (e.g., HSS) • 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.
86 Functions of a Cutting Fluid: Cooling • Water used as base in coolant - type cutting fluids. • Water most effective for reducing heat by will promote oxidation (rust). • Heat has definite bearing on cutting-tool wear – Small reduction will greatly extend tool life • Decrease the temperature at the chip-tool interface by 50 degrees F, and it will increase tool life by up to 5 times.
87 Functions of a Cutting Fluid: Lubrication • Usually oil based fluids and are most effective at lower cutting speeds. • Also reduces temperature in the operation. • Reduces friction between chip and tool face – Shear plane becomes shorter – i.e., the area where plastic deformation occurs is smaller • Extreme-pressure lubricants reduce amount of heat produced by friction. • EP chemicals of synthetic fluids combine chemically with sheared metal of chip to form solid compounds (allows chip to slide)
88 Cutting fluid reduces friction and produces a shorter shear plane.
89 Cutting-Tool Life • Heat and friction are prime causes of cutting-tool breakdown • Reduce temperature by as little as 500F, 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
90 Built-up Edge Built-up edge keeps breaking off and re-forming and result is poor surface finish, excessive flank wear, and cratering of tool face.
91 Cutting Fluid's Effect on Cutting Tool Action 1. Lowers heat created by plastic deformation of metal 2. Friction at chip-tool interface decreased 3. Less power is required for machining because of reduced friction 4. Prevents built-up edge from forming 5. Surface finish of work greatly improved
92 Rust Control • Water is the best and most economical coolant – Causes parts to rust • Rust is oxidized iron • Chemical cutting fluids contain rust inhibitors
93 Rancidity Control Rancidity caused by bacteria and other microscopic organisms, growing and eventually causing bad odours to form. Most cutting fluids contain bactericides that control growth of bacteria and make fluids more resistant to rancidity.
94 Application of Cutting Fluids • 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
95 Refrigerated Air System • 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 1000F below incoming air • Air directed to interface and blow chips away
96 Types of Cutting Fluids • Most commonly used cutting fluids – Either aqueous based solutions or cutting oils • Fall into three categories – Straight Cutting oils – Emulsifiable oils or Water Soluble oils – Chemical (synthetic) cutting fluids
97 Straight Cutting Oils • Derived from petroleum, animal, marine or vegetable substances and may be used straight or in combination. • Their main function is lubrication and rust prevention. • They are chemically stable and lower in cost. • Usually restricted to light duty machining on metals of high machinability, such as aluminium, magnesium, brass and leaded steels. – Two classifications » Active » Inactive
98 Active Cutting Oils • Those that will darken copper strip immersed for 3 hours at temperature of 2120F • Dark or transparent • Better for heavy-duty jobs • Three categories – Sulfurized mineral oils – Sulfochlorinated mineral oils – Sulfochlorinated fatty oil blends
99 Inactive Cutting Oils • Oils will not darken copper strip immersed in them for 3 hours at 2120F • 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
100 Emulsifiable (Water Soluble) Oils • About 90% of all metal cutting and grinding operations make use of emulsions due to their high sp. Heat, high thermal conductivity and high heat of vapourisation. • Mineral oils containing soap like material that makes them soluble in water and causes them to adhere to work piece. • Emulsifiers break oil into minute particles and keep them separated in water. – Water blend is in the ratio of 1 part oil to 15~20 parts water (for cutting) and 40 to 60 parts of water (for grinding) • Good cooling and lubricating qualities. • Used at high cutting speeds, low cutting pressures.
101 Chemical Cutting Fluids • 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
Merits & Demerits of Synthetic Fluids
103 Advantages of Synthetic Fluids Good rust control Resistance to rancidity for long periods of time Reduction of amount of heat generated during cutting due to Excellent cooling qualities 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 re- circulated in cooling system No clogging of machine cooling system due to detergent action of fluid Can leave a residue on parts and tools.
104 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.
105 • Ease or difficulty with which metal can be machined with satisfactory finish at low cost. • Measured by length of cutting-tool life in minutes or by rate of stock removal in relation to cutting speed employed. Machinability
Machinability Machinability is defined in terms of: 1. Surface finish and surface integrity of machined part 2. Tool life 3. Force and power required 4. The level of difficulty in chip control Good machinability indicates – good surface finish and surface integrity – a long tool life – and low force and power requirements Note, continuous chips should be avoided for good machinability. 106
107 Grain Structure • Machinability of metal affected by its microstructure. • Ductility and shear strength modified greatly by operations such as annealing, normalizing and stress relieving. • Certain chemical and physical modifications of steel improve Machinability. – Addition of sulfur, lead, or sodium sulfate – Cold working, which modifies ductility
Machinability: Machinability of Nonferrous Metals • Aluminum – very easy to machine – but softer grades: form BUE ⇒ poor surface finish – ⇒ recommend high cutting speeds, high rake and relief angles • Beryllium – requires machining in a controlled environment – this is due to toxicity of fine particles produced in machining • Cobalt-based alloys – abrasive and work hardening – require sharp, abrasion-resistant tool materials, and low feeds and speeds • Copper – can be difficult to machine because of BUE formation 109
Machinability: Machinability of Nonferrous Metals • Magnesium – very easy to machine, good surface finish, prolonged tool life – Caution: high rate of oxidation and fire danger • Titanium and its alloys – have very poor thermal conductivity – ⇒ high temp. rise and BUE ⇒ difficult to machine • Tungsten – brittle, strong, and very abrasive – ⇒ machinability is low • Zirconium – Good machinability – Requires cooling cutting fluid (danger of explosion, fire) 110
111 Aluminum • Pure aluminum generally more difficult to machine than aluminum alloys – Produces long stringy chips and harder on cutting tool • Aluminum alloys – Cut at high speeds, yield good surface finish – Hardened and tempered alloys easier to machine – Silicon in alloy makes it difficult to machine • Chips tear from work (poor surface)
112 Copper • Heavy, soft, reddish-colored metal refined from copper ore (copper sulfide) – High electrical and thermal conductivity – Good corrosion resistance and strength – Easily welded, brazed or soldered – Very ductile • Does not machine well: long chips clog flutes of cutting tool – Coolant should be used to minimize heat
113 Copper/Beryllium • Heavy, hard, reddish-colored copper metal with Beryllium added – High electrical and thermal conductivity. – Good corrosion resistance and strength. – Can be welded. – Somewhat ductile. – Withstands high temperature. • Machines well – Highly abrasive to HSS Tooling. – Coolant should be used to lubricate and minimize tool wear.
114 Copper-Based Alloys: Brass • Alloy of copper and zinc with good corrosion resistance, easily formed, machines, and cast. • Several forms of brass. – Alpha brasses: up to 36% zinc, suitable for cold working. – Alpha 1 beta brasses: Contain 54%-62% copper and used in hot working. • Small amounts of tin or antimony added to minimize pitting effect of salt water. • Used for water and gas line fittings, tubings, tanks, radiator cores, and rivets.
115 Copper-Based Alloys: Bronze • Alloys of copper and tin which contain up to 12% of principal alloying element – Exception: copper-zinc alloys • Phosphor-bronze – 90% copper, 10% tin, and very small amount of phosphorus – High strength, toughness, corrosion resistance – Used for lock washers, cotter pins, springs and clutch discs
116 Copper-Based Alloys: Bronze • Silicon-bronze (copper-silicon alloy) – Contains less than 5% silicon – Strongest of work-hardenable copper alloys – Mechanical properties of machine steel and corrosion resistance of copper – Used for tanks, pressure vessels, and hydraulic pressure lines
117 Copper-Based Alloys: Bronze • Aluminum-bronze (copper-aluminum alloy) – Contains between 4% and 11% aluminum – Other elements added Iron and nickel (both up to 5%) increases strength Silicon (up to 2%) improves machinability Manganese promotes soundness in casting – Good corrosion resistance and strength – Used for condenser tubes, pressure vessels, nuts and bolts
118 Effects of Temperature and Friction • Heat created – Plastic deformation occurring in metal during process of forming chip – Friction created by chips sliding along cutting-tool face • Cutting temperature varies with each metal and increases with cutting speed and rate of metal removal
119 Effects of Temperature and Friction • Temperature of metal immediately ahead of cutting tool comes close to melting temperature of metal being cut. • Greatest heat generated when ductile material of high tensile strength is cut. • Lowest heat generated when soft material of low tensile strength is cut. • Maximum temperature attained during cutting action. – affects cutting-tool life, quality of surface finish, rate of production and accuracy of work piece.
120 Friction • Kept low as possible for efficient cutting action • Increasing coefficient of friction gives greater possibility of built-up edge forming – Larger built-up edge, more friction – Results in breakdown of cutting edge and poor surface finish • Can reduce friction at chip-tool interface and help maintain efficient cutting temperatures if use good supply of cutting fluid.
121 Factors Affecting Surface Finish • Feed rate • Nose radius of tool • Cutting speed • Rigidity of machining operation • Temperature generated during machining process
122 Surface Finish • Direct relationship between temperature of work piece and quality of surface finish – High temperature yields rough surface finish – Metal particles tend to adhere to cutting tool and form built- up edge • Cooling work material reduces temperature of cutting-tool edge – Result in better surface finish
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