Module 1
MET 306
ADVANCED MANUFACTURING ENGINEERING
Traditional, non-traditional, micro & nano machining process
Traditional/Conventional machining
Examples of conventional machining processes are
Turning, boring, drilling, milling, shaping, broaching, slotting, grinding etc.
Major characteristics of conventional machining
Difficulties with conventional machining processes
Machining processes that involve chip formation have a number of limitations
satisfactory, economical, or impossible for the following reasons:
Unconventional machining
Non-traditional machining (NTM) processes have several advantages
The classification of NTM processes is carried out depending on the nature of energy used for material removal.
Differences between Conventional and Non conventional machining processes.
Sl. No | Conventional machining | Non-conventional machining |
1 | The cutting tool and work piece are always in physical contact and is in relative motion with each other, which results in friction and tool wear. | There is no physical contact between the tool and work piece. In some non -traditional process tool wear exists. |
2 | Material removal rate is limited by mechanical properties of work material. | NTM can machine difficult to cut and hard to cut materials like titanium, ceramics, nimonics, SST, composites, semiconducting materials . |
3 | Relative motion between the tool and work is typically rotary or reciprocating. Thus the shape of work is limited to circular or flat shapes. In spite of CNC systems, production of 3D surfaces is still a difficult task. | Many NTM are capable of producing complex 3D shapes and cavities |
Sl. No | Conventional machining | Non-conventional machining |
4 | Machining of small cavities, slits, and blind holes or through holes are difficult | Machining of small cavities, slits and production of non-circular, micro sized, large aspect ratio holes are easy using NTM |
5 | Use relative simple and inexpensive machinery and readily available cutting tools | Non traditional processes requires expensive tools and equipment as well as skilled labor, which increase the production cost significantly |
6 | Capital cost and maintenance cost is low. | Capital cost and maintenance cost is high |
7 | Traditional processes are well established and physics of process is well understood | Mechanics of Material removal of Some of the NTM process are still under research |
Differences between Conventional and Non conventional machining processes- cont.
Sl. No | Conventional machining | Non-conventional machining |
8 | Conventional process mostly uses mechanical energy | Most NTM uses energy in direct form. For example : laser, Electron beam in its direct forms are used in LBM and EBM respectively |
9 | Surface finish and tolerances are limited by machining inaccuracies | High surface finish (up to 0.1 micron) and tolerances (25 microns) can be achieved |
10 | High metal removal rate. | Low material removal rate. |
11 | Cutting tool is harder than workpiece. | There may not be a physical tool present |
12 | Tool life is less due to high surface contact and wear | Tool life is more |
13 | Noisy operation | Quiet operation mostly |
Micro and Nano machining process
Why Micro Machining?
Present day high-tech industry design requirements are stringent.
– Extraordinary Properties of Materials (High strength, High heat resistant,
High hardness, Corrosion resistant etc.)
few tens of microns as hole diameter and thousands in number)
– Nano level surface finish on Complex geometries (thousands of tubulated cooling holes in a turbine blade)
Why Micro Machining?
manufacturers worldwide to improve interchangeability
of
components, improve quality control and longer wear/fatigue life.
Micro and Nano machining processes
range of microns.
Different Micro/nano machining techniques
Introduction
The powder metallurgy (P/M) process, in which metal powders are compacted into desired and often complex shapes and sintered (heated without melting) to form a solid piece.
Introduction
Earliest use of iron powder dates back to 3000 BC. Egyptians used it for making tools
Modern era of P/M began when W lamp filaments were developed by Edison
Components can be made from pure metals, alloys, or mixture of metallic and non-metallic powders
Commonly used materials are iron, copper, aluminium, nickel, titanium, brass, bronze, steels and refractory metals
Used widely for manufacturing gears, cams, bushings, cutting tools, piston rings, connecting rods, impellers etc.
Motor Cycle Parts
�Vehicles Engine Parts�
Why Powder Metallurgy is Important
PM parts can be mass produced to net shape or near net shape, eliminating or reducing the need for subsequent machining
PM process wastes very little material - ~ 97% of starting powders are converted to product
PM parts can be made with a specified level of porosity, to produce porous metal parts
Filters, oil‑impregnated bearings and gears
More Reasons Why PM is Important
Certain metals that are difficult to fabricate by other methods can be shaped by powder metallurgy
Tungsten filaments for incandescent lamp bulbs are made by PM
Certain alloy combinations made by PM cannot be produced in other ways
PM compares favorably to most casting processes in dimensional control
PM production methods can be automated for economical production
POWDER METALLURGY:
Steps in Making Powder-Metallurgy Parts
Figure 17.2 Outline of processes and operations involved in making powder-metallurgy parts.
Methods of Metal-Powder Production by Atomization
Figure 17.5 Methods of metal-powder production by atomization: (a) gas atomization; (b) water atomization; (c) atomization with a rotating consumable electrode; and (d) centrifugal atomization with a spinning disk or cup.
Methods of Powder Production-Atomization
Atomization produces a liquid-metal stream by injection molten metal through a small orifice.
The stream is broken by jets of inert gas or air 🡪 gas atomization (Fig. 17.5a), or water 🡪 water atomization (Fig. 17.5b).
The size and shape of the particles formed depends on the temperature of the molten metal, rate of flow, nozzle size, and jet characteristics.
Use of water results in slurry of metal powder and liquid at the bottom of the atomization chamber. Although the powders must be dried before they can be used, the water allows for rapid cooling of the particles and higher production rates.
Gas atomization usually results in more spherical particles (See Fig. 17.3c).
Methods of Powder Production-Atomization
In centrifugal atomization, the molten-metal stream drops onto a rapid rotating disk or cup, so that centrifugal forces break up the molten-metal stream and generate particles (Fig. 17.5c).
Methods of Powder Production-Electrolytic deposition
Utilizes either aqueous solution or fused salts.
The powders produced are among the purest available.
Methods of Powder Production-Carbonyls.
Metal carbonyls, such as iron carbonyl [Fe(CO)5] and nickel carbonyl [Ni(CO)4], are formed by letting iron or nickel react with carbon monoxide.
The reaction products are then decomposed to iron and nickel, and they turn into small, dense, uniformly spherical particles of high purity.
Methods of Powder Production-Comminution.
Mechanical comminution (pulverization) involves crushing (Fig. 17.6), milling in a ball mill, or grinding of brittle or less ductile metals into small particles.
A ball mill (Fig. 17.6b) is a machine with a rotating hollow cylinder partly filled with steel or white cast-iron balls.
For brittle materials, the powder particles produced have angular shapes.
For ductile materials, particles are flaky and not suitable for P/M applications
Mechanical Comminution to Obtain Fine Particles
Figure 17.6 Methods of mechanical comminution to obtain fine particles: (a) roll crushing, (b) ball mill, and (c) hammer milling.
Methods of Powder Production-Mechanical alloying.
Powders of two or more metals are mixed in a ball mill (see fig. 17.7).
Under the impact of hard balls, the powders fracture and bond together by diffusion, forming alloy powders.
The dispersed phase can result in strengthening of the particles or can impart special electrical or magnetic properties of the powder.
Mechanical Alloying
Figure 17.7 Mechanical alloying of nickel particles with dispersed smaller particles. As nickel particles are flattened between the two balls, the second smaller phase is impresses into the nickel surface and eventually is dispersed throughout the particle due to successive flattening, fracture, and welding events.
Particle size, shape, and distribution
Particle shape: Particle shape is usually described in terms of aspect ratio.
Aspect ratio is the ratio of the largest dimension to the smallest dimension of the particle.
The ratio ranges from unity (spherical particles) to about 10 for flake-like or needle-like particles.
Shape factor (SF)
Shape factor (shape index) is a measure of the ratio of the surface area of the particle to its volume – normalized by reference to a spherical particle of equivalent volume.
Thus, the shape factor for a flake is higher than that for a sphere.
Size distribution
It is an important consideration because it affects the processing characteristics of the powder.
The distribution of a particle is given in terms of frequency-distribution plot The maximum is called the mode size.
Particle size, shape, and distribution
Other properties of metal powders that have an effect on their behavior in processing them are:
Flow properties when filled into dies.
Compressibility when being compacted.
Density, as defined in various terms such as theoretical density, apparent density, and the density when the powder is shaken or tapped in the die cavity.
Blending Metal Powders (2nd in P/M)
Proper mixing is essential to ensure uniformity of mechanical properties throughout the part
Powders of different metals can be mixed to impart special physical and mechanical properties to the P/M product.
Lubricants can be mixed with the powders to improve their flow characteristics. They reduce friction between metal particles, improve flow of the powder metals into the die, and improve die life. Lubricants are typically stearic acids or zinc stearate in a proportion from 0.25% - 5% by weight.
Other additives – binders are used to develop sufficient green strength, and additives also can be used to facilitate sintering.
Powders can be mixed in air, in inert atmosphere (to avoid oxidization), or in liquids (which act as lubricants and make the mix more uniform).
Several types of blending equipment are available (Fig. 17.8).
Bowl Geometries in Blending Metal Powders
(e)
Figure 17.8 (a) through (d) Some common bowl geometries for mixing or blending powders. (e) A mixer suitable for blending metal powders. Since metal powders are abrasive, mixers rely on the rotation or tumbling of enclosed geometries as opposed to using aggressive agitators. Source: Courtesy of Gardner Mixers, Inc.
Compaction of Metal Powders
In this step blended powders are pressed into various shapes in dies.
Purposes of compaction are to obtain the required shape, density, and particle-to-particle contact and to make part sufficiently strong for further processes.
Figure 17.9 shows a sequence of steps. The powder (feedstock) is feed into the die by a feed show; the upper punch descends into the die (single or double punches). The lower punch raises the part out of the dies.
The presses used are actuated either hydraulically or mechanically.
The process is carried out at room temperature, although it can be done at elevated temperature.
COMPACTING METHODS
Die Compaction
Figure 17.9 (a) Compaction of metal powder to form a bushing. The pressed-powder part is called green compact. (b) Typical tool and die set for compacting a spur gear. Source: Courtesy of Metal Powder Industries Federation.
Compaction of Metal Powders
The pressed powder is known as green compact. It has low strength, very fragile (like chalk) and can crumble very easily; this situation is worsened by poor pressing practice.
To obtain higher green strengths, the powder must be fed properly into the die cavity, and proper pressures must be developed through out the part.
The density of the green compact depends on the pressure applied .As the compacting pressure increased, the compact density approaches that of the metal in its bulk form.
Size distribution of the particles is an important factor in density.
If all of the particles are of the same size, there always will be some porosity when they are packed together (theoretically a porosity of at least 24% by volume).
Introducing small particles into the powder mix will fill the spaces between the larger powder particles and, thus, result in a higher density of the compact
Equipment -Compacting Pressures for Various Powders
Pressure required for pressing metal powders ranges from 70 MPa (for aluminum) to 800 MPa (for high-density iron parts)
The compacting pressure required depends on the characteristics and shape of the particles, on the method of blending, and on the lubricants.
Press capabilities are on the order of 1.8 to 2.7 MN (200 to 300 tons), although presses with higher much higher capabilities are used for special applications. Most applications require less than 100 tons.
For small tonnage, mechanical presses are used. Hydraulic presses (Fig. 17.12) with capacities as high as 45 MN (5000 tons) can be use for large parts.
However, the higher the pressing speed, the greater the tendency for the press to trap air in the die cavity, thus, preventing proper compaction
Compaction
Increased compaction pressure
Provides better packing of particles and leads to ↓ porosity
↑ localized deformation allowing new contacts to be formed between particles
Compaction
At higher pressures, the green density approaches density of the bulk metal
Pressed density greater than 90% of the bulk density is difficult to obtain
Compaction pressure used depends on desired density
Press for Compacting Metal Powder
Figure 17.12 A 7.3-mn (825-ton) mechanical press for compacting metal powder. Source: Courtesy of Cincinnati Incorporated.
Press selection depends on part size and its configuration, density requirements, and production rate
Isostatic Pressing
The main advantages of isostatic pressing are:
Because the pressure is uniform from all directions and no die-wall friction, fully dense compacts are produced with uniform grain structure and density (isotopic properties), irrespective of part shape.
Parts with high length-to-diameter ratios have been produced with very uniform density, strength, toughness, and good surface detail.
HIP is capable of handling much larger parts than those in other compacting processes.
The limitations of HIP are as follows:
Wider dimensional tolerance than those obtained in other compacting process.
Higher equipment cost and production time than are required by other processes.
Applicability only to relatively small production quantities, typically less than 10,000 parts per year.
Isostatic Pressing
Green compact may be subjected to hydrostatic pressure in order to achieve more uniform compaction
Cold isostatic pressing (CIP)
Metal powder is placed in a flexible rubber mold typically made of neoprene rubber, urethane, polyvinyl chloride, or another elastomer
The assembly then is pressurized hydrostatically in a chamber, usually using water.
Most common pressure is 400 MPa, although pressures up to 1000 MPa may be used.
Cold Isostatic Pressing
Metal powder placed in a flexible rubber mold
Assembly pressurized hydrostatically by water (400 – 1000 MPa)
Typical: Automotive cylinder liners →
Cold Isostatic Pressing
Figure 17.13 Schematic diagram of cold isostatic pressing, as applied to forming a tube. The powder is enclosed in a flexible container around a solid-core rod. Pressure is applied isostatically to the assembly inside a high-pressure chamber. Source: Reprinted with permission from R. M. German, Powder Metallurgy Science, Metal Powder Industries Federation, Princeton, NJ; 1984.
Hot isostatic pressing (HIP)
The container is generally made of high-melting-point steel, and the pressurizing medium is high-temperature inert gas or vitreous (glasslike fluid)
Common pressure is 100 MPa, (although it can be three times as high) and at a temperature of 1200o C.
HIP produces compacts having almost 100% density, good metallurgical bonding of the particles and good mechanical properties. Known for making high quality parts.
HIP is used mainly in making superalloy components for the aircraft and aerospace industries and in military, medical, and chemical applications.
It is used to close porosity; and as a final densification step for tungsten carbide cutting tool and P/M tool steels.
Hot Isostatic Pressing
Figure 17.15 Schematic illustration of hot isostatic pressing. The pressure and temperature variation versus time are shown in the diagram.
Powder Rolling
Figure 17.18 Schematic illustration of powder rolling.
Sintering
Green compact obtained after compaction is brittle and low in strength
Green compacts are heated in a controlled-atmosphere furnace to allow packed metal powders to bond together
Parts are heated to 0.7~0.9 Tm.
Transforms compacted mechanical bonds to much stronger metallic bonds.
Sintering – Three Stages
Carried out in three stages:
First stage: Temperature is slowly increased so that all volatile materials in the green compact that would interfere with good bonding is removed
Rapid heating in this stage may entrap gases and produce high internal pressure which may fracture the compact
Sintering: High temperature stage
Third stage: Sintered product is cooled in a controlled atmosphere
Prevents oxidation and thermal shock
Gases commonly used for sintering:
H2, N2, inert gases or vacuum
Sintering: High temperature stage
Liquid Phase Sintering
During sintering a liquid phase, from the lower MP component, may exist
Alloying may take place at the particle-particle interface
Molten component may surround the particle that has not melted
High compact density can be quickly attained
Important variables:
Nature of alloy, molten component/particle wetting, capillary action of the liquid
Finishing Operations
SIZING
COINING
MACHINING
IMPREGNATION
INFILTRATION
HEAT TREATMENT
PLATING
Impregnation and Infiltration �
• Porosity is a unique and inherent characteristic of PM technology
• It can be exploited to create special products by filling the available pore space with oils, polymers, or metals
• Two categories:
1. Impregnation
2. Infiltration
Impregnation�
The term used when oil or other fluid is permeated into the pores of a sintered PM part
• Common products are oil‑impregnated bearings, gears, and similar components
• Alternative application is when parts are impregnated with polymer resins that seep into the pore spaces in liquid form and then solidify to create a pressure tight part
Infiltration�
Operation in which the pores of the PM part are filled with a molten metal • The melting point of the filler metal must be below that of the PM part
• Heating the filler metal in contact with the sintered part so capillary action draws the filler into the pores
– Resulting structure is nonporous, and the infiltrated part has a more uniform density, as well as improved toughness and strength
Single point cutting tool
Single point cutting tool consists of
a)Sharpened cutting part:-point
b)Shank
Point is surrounded by the face, the side flank and the base
Side cutting edge is formed by the intersection of the face and the side flank
End cutting edge is formed by the intersection of the face and the end flank
The chips are cut from the w/p by the side cutting edge
The point where the end and side cutting edges meets is called the nose of the tool
Various tool elements
Shank | Main body of the tool. The part of the which is gripped in the tool holder |
Flank | The surface or surfaces below and adjacent to the cutting edge |
Face | The top surface of the tool along which the chip slides |
Nose | Point where side cutting edge and end cutting edge intersects |
Base | Bearing surface of the tool when it is held in a tool holder |
Heel | It is the intersection of the base and the flank. Curved portion at the bottom of the tool |
Cutting Edge | Edge on the face of the tool which removes the material from the work piece. Side cutting edge-major Cutting Edge, End cutting edge-minor Cutting Edge
|
Nose radius | Cutting tip, which carries a sharp cutting point. Nose provided with radius to enable greater strength, increase tool life & surface life |
Tool angles
Side Cutting edge angle�
Angle b/w the side cutting edge and side of the tool shank
Allows flank of the tool to approach the work piece first
Spreads the material over a greater distance on the cutting edge, thereby thinning out the chip.
Approximately 150
End Cutting edge angle
Angle b/w the end cutting edge and line normal to the tool shank
Allows the cutting tool to machine close to the work piece during turning operations
Usually 20 – 300
Rake angle�
Angle b/w the face of the tool and a plane parallel to its base. This angle is positive if the side cutting edge slopes downwards from the point towards the shank is called back rake angle
Angle b/w the face of the tool and the shank of the w/p is called side rake angle
-ve if the slope is towards the cutting edge
Relief angles/Clearance angle�
Angle b/w the front surface of the tool and a line normal to the base is called End relief angle
Angle b/w the side surface of the tool with a plane normal to its base of the tool is called side relief angle
Methods of machining
Orthogonal Cutting or two dimensional cutting
Oblique Cutting or three dimensional cutting
Orthogonal Cutting or two dimensional cutting�
In orthogonal cutting the cutting edge of the tool is perpendicular to the line of action of tool
Direction of chip flow velocity is normal to the cutting edge of the tool
Cutting edge is longer than width of cut
Cutting forces acts on a small area and therefore the life of the cutting tool is less
Oblique Cutting or three dimensional cutting�
In Oblique cutting the cutting edge of the tool is inclined to the direction of work feed or tool feed
The chip flows side ways in a long curl
The cutting edge may or may not be longer than the width of cut
Force which cuts and shears the metal acts on longer area
The heat developed per unit area is also less
The oblique tool will remove more metals in the same life as compared to the other
TYPES OF CHIPS
Discontinuous chips
Continuous chip
Continuous chip with Built-up Edge
Continuous chip�
Produced while machining ductile materials
Chips are in the form of long coil and having same thickness throughout the length
Good surface finish, improved tool life and less power consumption
Condition favors continuous chip formation
1.Using ductile materials
2.Smaller depth of cut
3.High cutting speed
4.Large rake angle
5.Sharp cutting edge
6.Proper cutting fluid
Discontinuous chips�
Produced while machining brittle materials at a low cutting speeds without fluids when the friction exists b/w tool and the chips
Conditions favours discontinuous chip formation
Machining of brittle substances,
small rake angle
too much depth of cut
low cutting speed
Less cutting fluid
Continuous chip with Built-up Edge�
During cutting process if
the interface temperature and pressure is high
friction b/w tool and chip interface is high
then it causes the chip material to weld itself to the tool
surface leading to the formation of Continuous chip
with Built up Edge
�
DISADVANTAGES
Factors favours
1.Abrasive wear
• Softer material sliding over the face of hard material may contain appreciable concentration of hard particles
• Hard particles act as small cutting edge like grinding wheel
• Hard particles result worn out of tool material
• Particles of hard material are intermittently turn out from the surface and dragged along the
2. Adhesive Wear�
When softer metal slide over hard metal , parts of soft metal adhere high spots on the metal due to:
Friction
High temperature
Pressure
•The spots result irregular flow of chip over the face and build up of more particles on the tool
•Finally the built up edges will torn from the surface result uneven structure on the tool surface
3. Diffusion
•When a metal is in sliding contact with another metal the temperature at the interface is high
•The high temperature allows the atoms of hard material to diffuse into softer material matrix
•hence the strength and abrasiveness of the softer material Increase
•Atoms of the softer metal may also diffuse into harder medium, thus weakening the surface of harder material medium
•Diffusion phenomenon is strongly dependent on temperature
5. Fatigue Wear
•Will occur when two surfaces slides in contact with other under high pressure
•Roughness of one surface interlocks with those of other.
•Due to friction, compressive force will be produced in one side and tensile on other side ,These phenomenon cause surface crack
Economics of Machining�
Cutting Time, Tool Changing Time, Idle Time
Machining or metal cutting is one important aspect of the production system. Ultimate objecting of machining is to give intended shape, size and finish by gradually removing material from workpiece. Relevant steps such as removal of material, setting the job and cutting tool, and dispatching the machined job consume substantial amount of time, which are at least not negligible.
Overall machining time and cost
In today’s competitive market, time is equivalent to cost. Basically overall or total machining time (Tm) is the summation of three different time elements closely associated with the machining or metal cutting process. These three elements include—actual cutting time (Tc), total tool changing time (Tct) and other handling or idle time (Ti). Beside these three time elements, cost of cutting tool is also required to incorporate for any optimization. All these time or cost elements, except handling time, are affected by the variation of cutting speed and feed rate as explained below. Mathematically, total time for machining (Tm) can be expressed as:
Tm = Tc + Tct + Ti
What is actual cutting time (Tc)?�
As the name suggests, cutting time is the time taken during actual material removal action, i.e., from the beginning of chip production to the end for uninterrupted machining. In case of any planned or unplanned stoppage in cutting, the pause duration will not come under this time element. Therefore, increase in cutting speed and feed rate will result in reduction of actual cutting time as material removal rate (MRR) will increase. Hence, cost associated with cutting time will decrease if speed or feed is increased. The adjacent diagram depicts how cost associated with the actual cutting time varies with speed or feed employed during machining
Mathematical expression of actual cutting time�
If, Lc is the total length of cut (mm),
N is the spindle speed (rpm) and
s is the feed rate (mm/rev),
then estimated uninterrupted cutting or machining time can be expressed as:
Actual Cutting Time (Tc) = Lc/N.s
In most of the cases, where either workpiece or cutting tool is rotating, the spindle speed (N) and cutting velocity (Vc) are interchangeable. However, cutting velocity also depends on the diameter of the job/cutter (D). Cutting velocity can be expressed, in terms of speed and diameter of job or cutter (whichever is rotating), as follows. For better understanding of this conversion, you may read: Cutting speed and cutting velocity in machining.
Cutting velocity (Vc) = πDN/1000
�
What is tool changing time (Tct)?
Every time a tool has certain life within which it can perform satisfactorily; and thus replacement or re-sharpening is required to perform once tool life exceeds. Cutting or machining action is also required to pause for certain time. Increase in cutting speed or feed rate will reduce tool life (evident from Taylor’s Modified Tool Life Equation) and thus frequent tool changing will be desired. This unplanned and frequent interruption in machining will impose loss to the industry. The adjacent diagram depicts how cost associated with the tool changing time varies with the speed or feed employed during machining.
If TCT is the average time required by the operator or automation system (such as automatic tool changer – ATC) to change the tool, then, mathematically, tool changing time (Tct) can be expressed as follows. Basically it is the multiplication of tool changing time for one tool change (TCT) and the number of times such tool changing is desired within the specified cutting time (Tc).
Tct=(Tc/TL)×TCT
What is part handling time or idle time (Ti)?
This time element depends on the workpiece and its configuration as well as material handling system employed. It takes care of loading and unloading of job and is independent of the cutting velocity or feed rate employed during machining. It is frequently termed as idle time as machine remains idle during loading and unloading.
Tooling cost – cost of the cutting tool
Although it is not directly associated with cutting velocity, tooling cost also contributes in overall machining economy. If higher speed or feed is employed, the result will be faster tool wear and reduced tool life, which will ultimately multiply expenditure as more tools are required for cutting same length.
Tooling cost can be calculated by multiplying the price of individual cutting tool (K2) with the quantity of tool required. Quantity of tool required can again be determined by dividing actual machining time by tool life. Mathematically, tooling cost can be determined by the formula:
Tooling cost = (Tc/TL)×K2
Overall machining time and cost�
Since every time elements pertinent to machining contributes towards machining cost, so some factors are required to convert time to cost. On the basis of these factors, time elements can be converted to cost elements and estimation of machining economy becomes easier. Such factors include:
K1 = Cost-time conversion factor for machining
K2 = Cost-time conversion factor for tool sharpening or price of new tool.
Therefore, overall machining cost per piece (Cp)
= (Actual cutting cost/piece) + (Tool changing cost/piece) + (Handling cost/piece) + (Tooling cost/piece)
= K1 {(Actual cutting time/piece) + (Tool changing time/piece) + (Handling time/piece)} + K2 {(Tooling cost/piece)}
= K1 {Tc + Tct + Ti} + (Tc/TL)×K2
Traditional material removal process :Machining
Cutting action involves shear deformation of work material to form a chip, and as chip is removed, new surface is exposed: (a) Positive and (b) Negative rake tools
Cutting Conditions in Machining
The three dimensions of a machining process:
Cutting speed (v) :Primary motion
Feed (f) :Secondary motion
Depth of cut (d) :Penetration of tool below original work surface
For certain operations, material removal rate can be found as:
MRR = v× f × d
Where,
v = Cutting speed; f = Feed; d = Depth of cut
Mechanics of chip formation
Plastic deformation along shear plane (Merchant)
The figure, where the work piece remains stationary and the tool advances in to the work piece towards left. Thus the metal gets compressed very severely, causing shear stress.
This stress is maximum along the plane is called shear plane.
If the material of the work-piece is ductile, the material flows plastically along the shear plane, forming chip, which flows upwards along the face of the tool.
The tool will cut or shear off the metal, provided by:
The tool is harder than the work metal
The tool is properly shaped so that its edge can be effective in cutting the metal.
Provided there is movement of tool relative to the material or vice-versa, so as to make cutting action possible
Mechanics of Cutting (Shear plane angle)