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MODULE-1 THEORY OF METAL CUTTING
CUTTING TOOL MATERIALS
Cutting tools are frequently used in our every day’s life
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Despite their
widespread applications in modern lives, not too many questions have been raised about the origin and history of
these tools
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In order to achieve successful cutting, cutting tools
must be mechanically harderthan the material to be machined
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Wear Resistance — wear resistance means the attainment of acceptable tool life before tools needto be replaced
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1
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8 to 1
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1 to 0
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1 to 0
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 It is used for machining soft metals like free cutting steels and brass and used as chiselsetc
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 Hardness of tool is about Rc = 65
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2
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S
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18- Tungsten is used to increase hot hardness and stability
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1- Vanadium is used to maintain keenness of cutting edge
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5% to 10% cobalt is used to increase red hot hardness
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S
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It looses hardness above 600°C
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Molybdenum based H
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S is cheaper than Tungsten based H
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S and also slightlygreater
toughness but less water resistance
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The major difference between HSS and
plain high carbon steel is the addition of alloying elements to harden and strengthen the steel and make it more
resistant to heat (hot hardness)
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While each of these elements will add certain specific
desirable characteristics, it can be generally state that they add deep hardening capability, high hot hardness,
resistance to abrasive wear, and strength, to HSS
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High speed steels (HSS)
 Hardened to various depths
 Good wear resistance
 Relatively
 Suitable for high positive rake angle tools
The most common HSS used primarily as cutting tools are divided into the M and T series
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M-series - Contains 10% molybdenum, chromium, vanadium, tungsten, cobalt
 Higher, abrasion resistance
 H
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S
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Non – ferrous cast alloys
It is an alloy of
Cobalt – 40 to 50%,
Chromium – 27 to 32%,
Tungsten – 14 to 29%,
Carbon – 2 to 4%





It can not heat treated and are used as cast form
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S
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They are weak in tension and like all cast materials tend to shatter when subjected toshock load or
when not properly supported
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Cemented carbides
 Produced by powder metallurgy technique with sintering at 1000°C
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S
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Can withstand up to 1000°C
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They are very stiff and their young’s modulus is about 3 times that of the steel
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High modulus of elasticity
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High thermal conductivity, low specific heat, low thermal expansion
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Used anywhere good wear resistance is required
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• Cutting Tool Grades — the cutting tool grades of cemented carbides are divided into two groups, depending on
their primary application
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If it is to be used to cut steel, a ductile material, it is graded as a steel grade carbide
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Ceramics and sintered oxides
 Ceramics and sintered oxides are basically made of Al2O3, These are made by powdermetallurgy technique
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 Used for continuous cutting only
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 Have very abrasion resistance
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 Has less tendency to weld metals during machining
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 Another ceramic tool material is silicon nitride which is mainly used for CI
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Cermets
 Cermets is the combination of ceramics and metals and produced by PowderMetallurgy process
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For cutting tools usual combination as Al2O3 + W + Mo + boron + Ti etc
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Increase in % of metals reduces brittleness some extent and reduces wear resistance
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Diamond
 Diamond has
1
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Low thermal expansion
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High thermal conductivity
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Very low coefficient of friction
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Cubic Boron Nitride (CBN)
 The trade name is Borozone
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 Used as a substitute for diamond during machining of steel
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S
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 Excellent surface finish is obtained
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UCON
 UCON is developed by union carbide in USA
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 This is refractory metal alloy which is cast, rolled into sheets and slit into blanks
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It is not used because of its higher costs
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Sialon (Si-Al-O-N)
 Sialon is made by powder metallurgy with milled powders of Silicon, Nitrogen, Aluminium and oxygen by
sintering at 1800°C
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Cutting speeds are 2
to 3 times compared to ceramics
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CUTTING FLUIDS
Cutting fluids consist of those liquids and gases that are applied to the tool and the material being machined to
facilitate the cutting operation
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e
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 To keep the work cool, preventing machining that results in inaccurate final dimensions
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This investigation wishes to establish a relationship between the surface chemistry of the
lubricants involved and how they can accomplish reducing the contact length on the rake face of the tool
where most of the heat during cutting is produced
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 To aid in providing a satisfactory chip formation (related to contact length)
 To wash away the chips/clear the swarf from the cutting area
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The desirable properties of cutting fluids are
 High thermal conductivity for cooling
 Good lubricating qualities
 High flash point, should not entail a fire hazard
 Must not produce a gummy or solid precipitate at ordinary working temperatures
 Be stable against oxidation
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 Must afford some corrosion protection to newly formed surfaces
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 No unpleasant odour must develop from continued use
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 A viscosity that will permit free flow from the work and dripping from the chips
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Such a film
assists the chip in sliding readily over the tool
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Cutting Fluids as Coolants
If a cutting fluid performs its lubricating function satisfactorily the problem of heat removal from the cutting tool,
chip, and work is minimised
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To perform this function
effectively, a cutting fluid should possess high thermal conductivity so that maximum heat will be absorbed and
removed per unit of fluid volume
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Moreover, water rapidly corrodes machine parts and components
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Also, it is not effective in absorbing heat as it cannot spread well on metallic surfaces
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Oil is suspended in water in the form of tiny globules
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This medium of oil in water is known as an
"Emulsion"
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In a metal
cutting operation using an emulsion "oil" provides lubricity and "water" does the cooling
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Instead they contain some synthetic chemicals assubstitutes
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They combine the advantages of
synthetic coolants and at the same time the disadvantages are not as in the case of hundred percent synthetics
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Cutting oils are generally mixtures of mineral oil and animal,
vegetable or marine oils to improve the wetting and lubricating properties
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Straight Cutting Oils or Neat Oils are petroleum based mineral oils reinforced with "Extreme
pressure" additives (EP additives)
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Single point cutting tool- It is simplest from of cutting tool & it have only one cutting edge
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Multi point cutting tool- In this two or more single point cutting tools arranged together as a unit
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Example- milling cutter, drills, brooches, grinding wheels, abrasive sticks etc
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Linear motion tools – lathe tools, brooches
2
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Linear & rotary motion tools – drills, taps, etc
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The point of cutting tool is bounded by cutting face, end flank, side/ main flank, & base
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The side / main cutting edge ‘ab’ is formed by intersecting of face & side / main flank
The end cutting edge ‘ac’ is formed by the intersection of end flank & base
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Multi point cutting tool

Fig
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If
the slope face is downward toward the nose, it is negative back rake angle and if it is upward toward
nose, it is positive back rake angle
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(ii)
Side rake angle: Side rake angle is the angle by which the face of tool is inclined side ways
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Side rake angle of cutting tool determines the thickness of the tool
behind the cutting edge
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(iii)
End relief angle: end relief angle is defined as the angle between the portion of the end flank
immediately below the cutting edge and a line perpendicular to the base of the tool, measured at right
angles to the flank
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(iv)
Side relief angle: Side rake angle is the angle between the portion of the side flank immediately below
the side edge and a line perpendicular to the base of the tool measured at right angles to the side
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It is incorporated
on the tool to provide relief between its flank and the workpiece surface
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It provides clearance between tool cutting edge and workpiece
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It is responsible for turning the chip away from the finished surface
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Nomenclature of single point cutting tool
Tool Signature of Single Point Cutting Tool:
Convenient way to specify tool angles by use of a standardized abbreviated system is known as tool signature
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It specifies the active angles of the tool normal to the cutting
edge
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Tool signature/tool designation is a convenient way to describe the tool angles by using the standardized
abbreviated system
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Back rake angle (0°)
2
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End relief angle (6°)
4
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End cutting edge angle (15°)
6
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Nose radius (0
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8
Fig
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 But increased rake angle reduces the strength of the tool section and heat conduction capacity
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


Fig
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
 These materials tend to be brittle but their ability to hold their superior hardness at high temperature results
in their selection for high speed and continuous machining operation
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This
is particularly important in making intermittent cuts and in absorbing the impact during the initial
engagement of the tool and work
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

Fig
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
 Type of tool material
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At high speeds
rake angle has little influence on cutting pressure
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

Orthogonal cutting
In orthogonal cutting, the tool approaches the work piece with its cutting edge parallel to the uncut surface
and at right angle to the direction of cutting
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Here
only two component forces are acting cutting force Fc and thrust force Ft
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



Figure: Schematic illustration of a two-dimensional cutting process, also called orthogonal cutting: (a)
Orthogonal cutting with a well-defined shear plane, also known as the Merchant Model
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
Orthogonal cutting

Oblique Cutting

The cutting edge of the tool is perpendicular to
the direction of feed motion
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Chip flow is expected to in a direction
perpendicular to the cutting edge
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There are only two components of force; these
components are mutually perpendicular
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The cutting edge is larger than cutting width
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Chips are in the form of a spiral coil
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High heat concentration at cutting region
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For a given feed and depth of cutting, the force
acts on a small area as compared with oblique
cutting, so tool life is less
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Surface finish is poor
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Used in grooving, parting, slotting, pipe cutting
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








Fig: Schematic of chip formation

TYPES OF CHIPS PRODUCED IN METAL CUTTING
1
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Built-up Edge
3
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Discontinuous
Continuous
• The lower boundary is below the machined surface, subjecting the machined surface to distortion,
as depicted by the distorted vertical lines
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• It can produce poor surface finish and induce residual surface stresses
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Built-up Edge, BUE
• BUE, consisting of layers of material from the workpiece that are gradually deposited on the tool,
may form at the tip of the tool during cutting
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• Part of BUE material is carried away by the tool side of the chip; the rest is deposited randomly on
the workpiece surface
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• Because of work hardening and deposition of successive layers of material
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BUE is generally undesirable
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As cutting speed increases the size of BUE decreases
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• The chips have a saw-tooth-like appearance
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Brittle workpiece materials
2
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3
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4
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5
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6
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TOOL LIFE: WEAR & FAILURE
Gradual wear occurs at three principal location on a cutting tool
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Crater wear affects the mechanics of the process increasing the actual rake angle of the cutting tool and
consequently, making cutting easier
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In general, crater wear is of a relatively small concern
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Flank wear appears in the form of so-called wear land and is measured by the width of this wear
land
Corner wear: occurs on the tool corner
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We consider corner wear
as a separate wear type because of its importance for the precision of machining
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03~0
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Fig
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Taylor’s tool life equation

n = 0
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2 for HSS tool
= 0
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15 for Cast Alloys
= 0
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4 for carbide tool
= 0
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7 for ceramic tool

Extended or Modified Taylor’s equation
V Tn dn1 f n2 = C
 where V is the cutting speed, f is the feed, d is depth of cut, T is the tool life,
 n, n1 and n2 are Taylor exponent
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 Merchant circle diagram is used to analyze the forces acting in metal cutting
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Each system is a
triangle of forces
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Shear surface is a plane extending upwards from the cutting edge
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The tool is perfectly sharp and there is no contact along the clearance force
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The cutting edge is a straight line extending perpendicular to the direction of motion and generates
a plane surface as the work moves past it
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The chip doesn’t flow to either side, that is chip width is constant
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The depth of cut remains constant
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Width of the tool, is greater than that of the work
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Work moves with uniform velocity relative tool tip
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No built up edge is formed
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A triangle of forces for the cutting forces,
2
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A triangle of forces for the frictional forces
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Figure : Merchant’s circle diagram
Let R = resultant force
Then resultant force is given by the formula
R = (Fc2 + Ft2)0
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 Merchant’s circle is used for establishing relationship between measurable and actual forces
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Forces in Metal Cutting
Forces in the secondary deformation zone:
1
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2
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Friction coefficient: µ = F/N
Friction angle: b
Resultant force: R
Forces in the first deformation zone:
3
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4
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Forces on the cutting tool:
5
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6
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Shear angle relationship

or
Trigonometric relationships:
F = Fc sin  + Ft cos 
N = Fc cos  - Ft sin 

Fs = Fc cos  - Ft sin 
Fn = Fc sin  + Ft cos 
Shear stress:
t = Fs/ As
where: As = t0 w/ sin 
The Merchant Equation
 = Fs/ As … (7)
As = t0 w/ sin  … (8)
Fs = Fc cos  - Ft sin  … (5)
Combine Eqs
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However, unless the tool is severely
worn, the heat generated at this source will be small and hence could be neglected
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The major portion of the heat is taken away
by the chips
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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

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