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Mechanism of Metal Cutting.ppt

Dec 01, 2015

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Page 1: Mechanism of Metal Cutting.ppt

Mechanism of Metal Cutting

Page 2: Mechanism of Metal Cutting.ppt

• Deformation of metal during machining,• nomenclature of lathe, • milling tools, • mechanics of chip formation, • built-up edges, • mechanics of orthogonal and oblique cutting,• Merchant cutting force circle and shear angle relationship

in orthogonal cutting, • factors affecting tool forces. • Cutting speed, feed and depth of cut, surface finish.• Temperature distribution at tool chip interface. • Numerical on cutting forces and Merchant circle.

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Mechanism of chip formation in machining

• Machining is a semi-finishing or finishing process essentially done to impart required or stipulated dimensional and form accuracy and surface finish to enable the product to • fulfill its basic functional requirements • provide better or improved performance • render long service life.

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• Machining is a process of gradual removal of excess material from the preformed blanks in the form of chips.

• The form of the chips is an important index of machining because it directly or indirectly indicates : • Nature and behaviour of the work material under

machining condition • Specific energy requirement (amount of energy required to

remove unit volume of work material) in machining work • Nature and degree of interaction at the chip-tool

interfaces.

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• The form of machined chips depend mainly upon : • Work material • Material and geometry of the cutting tool • Levels of cutting velocity and feed and also to some extent

on depth of cut • Machining environment or cutting fluid that affects

temperature and friction at the chip-tool and work-tool interfaces.

• Knowledge of basic mechanism(s) of chip formation helps to understand the characteristics of chips and to attain favourable chip forms.

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Mechanism of chip formation in machining ductile materials

During continuous machining the uncut layer of the work material just ahead of the cutting tool (edge) is subjected to almost all sided compression as indicated in Fig. The force exerted by the tool on the chip arises out of the normal force, N and frictional force, F as indicated in Fig

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• Due to such compression, shear stress develops, within that compressed region, in different magnitude, in different directions and rapidly increases in magnitude.

• Whenever and wherever the value of the shear stress reaches or exceeds the shear strength of that work material in the deformation region, yielding or slip takes place resulting shear deformation in that region and the plane of maximum shear stress.

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• But the forces causing the shear stresses in the region of the chip quickly diminishes and finally disappears while that region moves along the tool rake surface towards and then goes beyond the point of chip-tool engagement.

• As a result the slip or shear stops propagating long before total separation takes place.

• In the mean time the succeeding portion of the chip starts undergoing compression followed by yielding and shear.

• This phenomenon repeats rapidly resulting in formation and removal of chips in thin layer by layer.

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• In actual machining chips also, such serrations are visible at their upper surface as indicated in Fig.

• The lower surface becomes smooth due to further plastic deformation due to intensive rubbing with the tool at high pressure and temperature.

• The pattern of shear deformation by lamellar sliding, indicated in the model, can also be seen in actual chips by proper mounting, etching and polishing the side surface of the machining chip and observing under microscope.

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• The pattern and extent of total deformation of the chips due to the primary and the secondary shear deformations of the chips ahead and along the tool face, as indicated in Fig, depend upon • work material • tool; material and geometry • the machining speed (VC) and feed (so) • cutting fluid application

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Primary and secondary deformation zones in the chip

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• The overall deformation process causing chip formation is quite complex and hence needs thorough experimental studies for clear understanding the phenomena and its dependence on the affecting parameters. The feasible and popular experimental methods [2] for this purpose are:

• Study of deformation of rectangular or circular grids marked on the side surface as shown in Fig.

• Microscopic study of chips frozen by drop tool or quick stop apparatus

• Study of running chips by high speed camera fitted with low magnification microscope.

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It has been established by several analytical and experimental methods including circular grid deformation that though the chips are initially compressed ahead of the tool tip, the final deformation is accomplished mostly by shear in machining ductile materials. However, machining of ductile materials generally produces flat, curved or coiled continuous chips.

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Mechanism of chip formation in machining brittle materials

• The basic two mechanisms involved in chip formation are • Yielding – generally for ductile materials • Brittle fracture – generally for brittle materials

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• During machining, first a small crack develops at the tool tip as shown in Fig. due to wedging action of the cutting edge.

• At the sharp crack-tip stress concentration takes place. In case of ductile materials immediately yielding takes place at the crack-tip and reduces the effect of stress concentration and prevents its propagation as crack.

• But in case of brittle materials the initiated crack quickly propagates, under stressing action, and total separation takes place from the parent workpiece through the minimum resistance path as indicated in Fig.

• Machining of brittle material produces discontinuous chips and mostly of irregular size and shape.

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Orthogonal and Oblique Cutting

• The two basic methods of metal cutting using a single point tool are the orthogonal (2 D) and oblique (3D).

• Orthogonal 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 inclined at an angle less than 90 degree to the line of action of the tool, the cutting action is known as oblique.

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Oblique Cutting

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Orthogonal Cutting

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Mechanics of orthogonal metal cutting

• During metal cutting, the metal is severely compressed in the area in front of the cutting tool.

• This causes high temperature shear, and plastic flow if the metal is ductile.

• When the stress in the workpiece 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.

• The outward or shearing movement of each successive element is arrested by work hardening and the movement transferred to the next element.

• The process is repetitive and a continuous chip is formed.• The plane along which the element shears, is called shear plane.

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Assumptions in orthogonal metal cutting

• No contact at the flank i.e. the tool is perfectly sharp.

• No side flow of chips i.e. width of the chips remains constant.

• Uniform cutting velocity.• A continuous chip is produced with no built up

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.

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Built-up-Edge (BUE) formation

• Causes of formation • In machining ductile metals like steels with long chip-tool

contact length, lot of stress and temperature develops in the secondary deformation zone at the chip-tool interface.

• Under such high stress and temperature in between two clean surfaces of metals, strong bonding may locally take place due to adhesion similar to welding.

• Such bonding will be encouraged and accelerated if the chip tool materials have mutual affinity or solubility.

• The weldment starts forming as an embryo at the most favourable location and thus gradually grows as schematically shown in Fig.

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Scheme of built-up-edge formation

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• With the growth of the BUE, the force, F also gradually increases due to wedging action of the tool tip along with the BUE formed on it.

• Whenever the force, F exceeds the bonding force of the BUE, the BUE is broken or sheared off and taken away by the flowing chip. Then again BUE starts forming and growing. This goes on repeatedly.

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Characteristics of BUE

• Built-up-edges are characterized by its shape, size and bond strength, which depend upon: • work tool materials • stress and temperature, i.e., cutting velocity and

feed • cutting fluid application governing cooling and

lubrication.

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Different forms of built-up-edge

• BUE may develop basically in three different shapes as schematically shown in Fig.

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Overgrowing and overflowing of BUE causing surface roughness

• In machining too soft and ductile metals by tools like high speed steel or uncoated carbide the BUE may grow larger and overflow towards the finished surface through the flank as shown in Fig.

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• While the major part of the detached BUE goes away along the flowing chip, a small part of the BUE may remain stuck on the machined surface and spoils the surface finish.

• BUE formation needs certain level of temperature at the interface depending upon the mutual affinity of the work-tool materials. With the increase in Vc

and so the

cutting temperature rises and favours BUE formation. • But if VC is raised too high beyond certain limit, BUE will

be squashed out by the flowing chip before the BUE grows.

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Role of cutting velocity and feed on BUE formation

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• Fig. 5.14 shows schematically the role of increasing VC and so

on BUE formation (size). • But sometime the BUE may adhere so strongly

that it remains strongly bonded at the tool tip and does not break or shear off even after reasonably long time of machining.

• Such detrimental situation occurs in case of certain tool-work materials and at speed-feed conditions which strongly favour adhesion and welding.

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Effects of BUE formation

• Formation of BUE causes several harmful effects, such as:

• It unfavorably changes the rake angle at the tool tip causing increase in cutting forces and power consumption

• Repeated formation and dislodgement of the BUE causes fluctuation in cutting forces and thus induces vibration which is harmful for the tool, job and the machine tool.

• Surface finish gets deteriorated • May reduce tool life by accelerating tool-wear at its rake surface

by adhesion and flaking

• Occasionally, formation of thin flat type stable BUE may reduce tool wear at the rake face.

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Types of chips and conditions for formation of those chips

• Different types of chips of various shape, size, colour etc. are produced by machining depending upon • type of cut, i.e., continuous (turning, boring etc.) or

intermittent cut (milling) • work material (brittle or ductile etc.) • cutting tool geometry (rake, cutting angles etc.) • levels of the cutting velocity and feed (low, medium or

high) • cutting fluid (type of fluid and method of application)

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• The basic major types of chips and the conditions generally under which such types of chips form are given below:

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• Often in machining ductile metals at high speed, the chips are deliberately broken into small segments of regular size and shape by using chip breakers mainly for convenience and reduction of chip-tool contact length.

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Metal cutting Terminologies

Schematic illustration of a two-dimensional cuttingprocess (also called orthogonal cutting).

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Chip thickness ratios

The outward flow of the metal causes the chip to be thicker after the separation from the parent metal. That is the chip produced is thicker than the depth of cut.

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Chip thickness ratio

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Velocity Relationship

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Velocity Relationship

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Benefit of knowing and purpose of determining cutting forces.

• The aspects of the cutting forces concerned : • Magnitude of the cutting forces and their

components • Directions and locations of action of those forces • Pattern of the forces : static and / or dynamic.

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• Knowing or determination of the cutting forces facilitate or are required for : • Estimation of cutting power consumption, which also

enables selection of the power source(s) during design of the machine tools

• Structural design of the machine – fixture – tool system • Evaluation of role of the various machining parameters

( process – VC, so, t, tool – material and geometry, environment – cutting fluid) on cutting forces

• Study of behaviour and machinability characterisation of the work materials

• Condition monitoring of the cutting tools and machine tools.

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Cutting forces

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Cutting forces

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Cutting Forces in Oblique Cutting

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Cutting Forces in Orthogonal Cutting

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Forces acting on Chip in two-dimensional cutting

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The forces acting on the chip in orthogonal cutting

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• Fs = Shear Force, which acts along the shear plane, is the resistance to shear of the metal in forming the chip.

• Fn = Force acting normal to the shear plane, is the backing up force on the chip provided by the workpiece.

• F = Frictional resistance of the tool acting against the motion of the chip as it moves upward along the tool.

• N = Normal to the chip force, is provided by the tool.

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Merchant’s Circle Diagram

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The procedure to construct a merchants circle diagram

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Thrust Force vs Rake Angle

FIGURE Thrust force as a function of rake angle and feed in orthogonal cutting of AISI 1112 cold rolled steel. Note that at high rakeangles, the thrust force is negative. A negative thrust force has important implications in the design of machine tools and in controlling the stability of the cutting processes.

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Shear and Normal Force

FIGURE Shear force and normal force as a function of the area of the shear plane and the rake angle for 85-15 brass. Note that the shear stress in the shear plane is constant, regardless of the magnitude of the normal stress. Thus, normal stress has no effect on the shear flow stress of the material.

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Shear and Normal Force

FIGURE: Schematic illustration of the distribution of normal and shear stresses at the tool-chip interface (rake face). Note that, whereas the normal stress increases continuously toward the tip of the tool, the shear stress reaches a maximum and remains at that value (a phenomenon know as sticking).

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Thermal Aspects of Chip Formation• Machining is inherently characterized by generation of heat and

high• cutting temperature. At such elevated temperature the cutting tool

if• not enough hot hard may lose their form stability quickly or wear

out• rapidly resulting in increased cutting forces, dimensional inaccuracy

of• the product and shorter tool life. The magnitude of this cutting• temperature increases, though in different degree, with the

increase of• cutting velocity, feed and depth of cut, as a result, high production• machining is constrained by rise in temperature.

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This problem increases further with the increase in strength and hardness of the work material. Knowledge of the cutting temperature rise in cutting is important, because increases in temperature:adversely affect the strength, hardness and wear

resistance of the cutting toolcause dimensional changes in the part being

machined, making control of dimensional accuracy difficult and

can induce thermal damage to the machined surface, adversely affecting its properties and service life.

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Temperature distribution at tool chip interface.

FIGURE: Typical temperature distribution in the cutting zone. Note that the maximum temperature is about halfway up the face of the tool and that there is a steep temperature gradient across the thickness of the chip. Some chips may become red hot, causing safety hazards to the operator and thus necessitating the use of safety guards.

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Temperature Distribution in Turning

FIGURE : Temperature distribution in turning: (a) flank temperature for tool shape (b) temperature of the tool chip interface. Note that the rake face temperature is higher than that at the flank surface.

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Numerical Problems 1

• If, in Orthogonal cutting a tool of the force components, Fx and Fz are

measured to be 400 N and 800 N respectively then what will be the value of the coefficient of friction at the tool interface at that condition.

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Numerical Problems 2

• Determine with out using MCD, the values of Fs (Shear force) and Fn using the following given values associated with a turning operation Fz=1000 N, Fx=400 N, Fy=200N,

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Numerical Problems 3

• During turning a ductile alloy by a tool of Fz= 400 N, Fx= 300 N and Fy = 2.5. Evaluate, using MCD, the values of F, N and μ as well as Fs and Fn for the above machining.

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Numerical Problems 4

• During turning a steel rod of diameter 160 mm at speed 560 rpm, feed 0.32 mm/rev. and depth of cut 4.0 mm by a ceramic insert of geometry 0o, —10o, 6o, 6o, 15o, 75o, 0 (mm)

The following were observed: Fz=1600 N, Fx=800 N and chip thickness = 1 mm.Determine with the help of MCD the possible

values of F, N, Fs, Fn, cutting power and specific energy consumption.

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