13513_metal cutting theory and mechanism

8/6/2019 13513_Metal Cutting Theory and Mechanism

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Prepared by : Er. MANISHA YADAV


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IntroductionyMachining is probably the most expensive

among all the manufacturing process

available (energy and material loss) yet it isbasically adopted to get higher surfacefinish, close tolerance and complex

geometry shapes which are otherwisedifficult to obtain.

yHowever all component undergo machining

process at some stages.


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Introduction


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Material Removal Process


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Various Cutting Operations

y Turing produces straight, conical, curved, orgrooved work pieces

y Facing produces a flat surface at the end of thepart

y Boring to enlarge a hole

y Drilling - to produce a hole

y Cutting off to cut off a work piece

y Threading to produce threads

y

Knurling produces a regularly shaped roughness


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Cutting Operations

Fig : Various cuttingoperations that

can be performedon a late. Not that

all parts havecircular symmetry


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Machine ToolyMachine tool is one which while holding

the cutting tool is able to remove the

metal from a work piece in order togenerate the requisite part of the givensize, configuration and finish.

y It is different from a machine, which isessentially a means of converting thesource of power from one form to other.


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Functions of machine tool

yTo hold and support the job or work tobe machined.

yTo hold and support the cutting tool inposition.

yTo move the cutting tool, work or both of

them in a desire direction.yTo generate the cutting speed and feed.


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TypesofMachineTools


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Classification of cutting tool

ySingle Point cutting tool :

Those having only one cutting edge.

Ex: lathe tools, shaper tools , planner tools etc.

yMulti-point cutting tool :

Those having more than one cutting edges.

Ex: milling cutters, drills, broaches, grinding wheelsetc.


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CuttingConditions


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Singlepointcuttingtool geometry


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Important terms

Shank : portion of the tool which is gripped in thetool holder.

Face: The top surface of the tool between theshank and the tool point. Chip flows along thissurface only.

Flank:

Base:

Heel:

Nose radius:


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single point cutting tool anglesy Rake angle: angle formed between the face of the

tool and a plane parallel to its base.

y

Back rake angle: If the inclination is towards theshank.

y Side rake angle: If the inclination is towards theside of the tool.

These rake angles guide the chip away from thecutting edge, thereby reducing the chip pressureon the face hence less power consumption.Strength of the tool decreases with increase inrake angle.


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Tool angles

Lip angle: angle between the face and flank of the tool.

Larger the lip angle stronger will be the cutting edge.

Clearance angle: angle formed by the front or side

surfaces of the tool with the plane normal to the base

of the tool when tool is in horizontal position.

Cutting angle: The angle formed between the tool face

and a line through the point which is a tangent to the

machined surface.


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Tool signaturesy Back rake angle (y)

y Side rake angle (x )

y End relief angle (y)y Side relief angle (x)

y End cutting edge angle (e)

y Side cutting edge angle (s)

y Nose radius


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Machine Tool Classification


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Machine Tool Classification


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Chip Formation


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Chip

form

ation


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Typesofchips


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Discontinuouschip


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Continuouschip


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Continuouschip with BUE


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Chip with builtupedge


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Thermal Aspects


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Thermal Aspects


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Chip Thickness Ratio (r)

Let

t= Chip thic nessprior to

deformation.tc = Chip thic nessafter deformationor cut chipthic ness.


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Velocity Relationship


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Where :V = Velocity of tool relative to worVC = Velocity of chip flow relative to toolVS = Velocity of displacement of the chip alongshear plane relative to wor or Velocity of shear

VS = V .[ cos / cos (-) ]VC = V.[ Sin / cos (-) ]

VC =V x r


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Cylindrical Turning on an EngineLathe


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Resultant Cutting Motion inCylindrical Turning


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Single-point Tool Operation


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Important Formulas

Length along shear plane ls = ac / sin

Area of chip cross section A0 = a0 awArea of unchip cross section Ac = ac aw

Shear Area of chip cross section

As = ls aw


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If mass of chip is given as mc and density then :

a0 = mc / lc aw


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Q 1. In an orthogonal cutting test on mild steel, the followingresults were obtained:

Cutting force Fc = 900 N

Thrust force Ft = 450 N

Uncut chip thic ness ac = 0.25 mm

Chip thic ness a0 = 0.75 mm

Width of cut aw = 2.50 mm, ra e angle zero degrees.

Length of contact between chip and tool lf = 0.5 mm

Calculate mean angle of friction on the tool face.Calculate mean shear strength of the wor material s.

Calculate the mean frictional stress f.


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TheoryofErnst and Merchant

In their analysis chip is assumed to behave as arigid body held in equilibrium by the action of

forces transmitted across chip tool interfaceand across the shear plane.

It is assumed that whole of the resultant tool

force is transmitted across the tool chipinterface and that no force acts on the tool edgeor f lan .


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TheoryofErnst and Merchanty The basis of Ernst and Merchant theory was the

suggestion that the shear angle would ta e up such avalue as to reduce the wor done in cutting tominimum


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Q2. In an experimental turning operation where low-carbon

steel was being machined using a carbide tool, thefollowing data were obtained:

Cutting force Fc = 1 kN

Thrust force Ft = 0.5 kN

Working normal rake ne

= 20 deg.

Feed f= 0.141 mm

Working major cutting-edge angle r = 45 deg.

Depth of cut (back engagement) ap = 5 mm

Cutting speed v= 2 m/s

Cutting ratio rc = 0.2Work piece diameterdw = 100 mm

Work piece machined length lw = 300 mm.


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Estimate from the above data:

a. The specific cutting energy of the work material ps

.

b. The power required for machining Pm.

c. The undeformed chip thickness ac.

d. The width of cut aw.

e. The shear angle .

f. The mean angle of friction on the tool face .

g. The time taken to complete the machining operation.


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13513_metal cutting theory and mechanism

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