metal cutting 2

CHAPTER METAL CUTTING

Presentation Prepared by Prof. Naman M. Dave Assistant Prof. (Mechanical Dept.) Gandhinagar Institute of Technology

• Definition of Manufacturing • The word manufacturing is derived from Latin:

manus = hand, factus = made

• Manufacturing is the economic term for making goods and services available to satisfy human wants.

• Manufacturing implies creating value to a raw material by applying useful mental and physical labour. Manufacturing converts the raw materials to finished products to be used for some purpose.

• Whether from nature or industry materials cannot be used in their raw forms for any useful purpose.

• The materials are then shaped and formed into different useful components through different manufacturing processes to fulfil the needs of day-to-day work.

INTRODUCTION

MANUFACTURING SYSTEM AND PRODUCTION SYSTEM

Manufacturing system:

• A collection of operations and processes used to obtain a desired product(s) or component(s) is called a manufacturing system.

• The manufacturing system is therefore the design or arrangement of the manufacturing processes..

Production system:

• A production system includes people, money, equipment, materials and supplies, markets, management and the manufacturing system.

Production System - The Big Picture

Raw materials

Manufacturing Process

Manufacturing

Process

Finished product

Manufacturing System

People, Money, Equipment, Materials and Supplies, Markets, Management

Material Removal Processes

A family of shaping operations, the common feature of which is removal of material from a starting work part so the remaining part has the desired geometry.

Machining – material removal by a sharp cutting tool, e.g., turning, milling, drilling.

Abrasive processes – material removal by hard, abrasive particles, e.g., grinding.

Nontraditional processes - various energy forms other than sharp cutting tool to remove material.

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.

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

Why Machining is Important

Variety of work materials can be machined ◦ Most frequently used to cut metals

Variety of part shapes and special geometric features possible, such as: ◦ Screw threads ◦ Accurate round holes ◦ Very straight edges and surfaces

Good dimensional accuracy and surface finish

Examples of Cutting Processes

Disadvantages with Machining

Wasteful of material ◦ Chips generated in machining are wasted material, at

least in the unit operation

Time consuming ◦ A machining operation generally takes more time to

shape a given part than alternative shaping processes, such as casting, powder metallurgy, or forming

Diagrammatic Representation of Material Removal Operations

Mechanism of Chip formation

The form of the chips is an important index of machining because it directly or indirectly indicates :

Nature and behavior 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.

Mechanism of Chip formation 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 mechanisms of chip formation helps to understand the characteristics of chips and to attain favourable chip forms.

A chip has two surfaces:

1. One that is in contact with the tool face (rake face). This surface is shiny, or burnished.

2. The other from the original surface of the work piece.

This surface does not come into contact with any solid body. It has a jagged, rough appearance, which is caused by the shearing mechanism.

Chip Formation

Figure. More realistic view of chip formation, showing shear zone rather than shear plane. Also shown is the secondary shear zone

resulting from tool-chip friction.

Primary & Secondary Shear Zone

Piispanen model of Card Analogy

Four Basic Types of Chip in Machining

1. Discontinuous chip 2. Continuous chip 3. Continuous chip with Built-up Edge (BUE) 4. Serrated chip

Figure: Shear strain during chip formation: (a) Chip formation depicted as a series of parallel plates sliding relative to each other, (b) One of the plates isolated to show shear strain, and (c) Shear strain triangle used to derive strain equation.

Shear Strain in Chip Formation

𝜺𝜺 = 𝑨𝑨𝑨𝑨𝑩𝑩𝑩𝑩

= 𝑨𝑨𝑩𝑩+𝑩𝑩𝑨𝑨𝑩𝑩𝑩𝑩

= 𝐂𝐂𝐂𝐂𝐂𝐂 ∅ + 𝑻𝑻𝑻𝑻𝑻𝑻 (∅ −∝)

Shear strain in machining can be computed from the following equation, based on the parallel plate model:

Where, ε =Shear strain, φ = Shear plane angle and α = Rake angle of cutting tool

Large shear strains are associated with low shear angles, or low or negative rake angles.

Shear strains of 5 or higher in actual cutting operations.

Deformation in cutting generally takes place within a very narrow deformation zone; that is, d = BD in Fig is very small. Therefore, the rate at which shearing takes place is high.

Shear angle influences force and power requirements, chip thickness, and temperature.

Consequently, much attention has been focused on determining the relationships between the shear angle and work piece material properties and cutting process variables.

Shear Strain in Chip Formation

Velocity Relationship in Orthogonal Cutting

Figure (a) Schematic illustration of the basic mechanism of chip formation by shearing. (b) Velocity diagram showing angular relationships among the three speeds in the cutting zone.

The tool has a rake angle of α, and relief (clearance) angle. The shearing process in chip formation is similar to the motion of cards in a deck sliding against each other.

Velocity Relationship

Using sine rule,

)90sin(sin))(90sin( αφαφ −==

−−sc vvv

αφαφ cossin)cos(sc vvv

==−

)cos(sin

αφφ

−=

vvc

rvvc ×=

=

)-( CosSin r

αφφ

)cos(cos

αφα

−=

vvs

cc

cc

ttrvv

tvt

0

0

r As,

vchip theup flowing material of Volume unit timeper material of Volume

=×=⇒

×=×⇒=

Velocity Relationship

Forces Acting on Chip

F, N, Fs, and Fn cannot be directly measured Forces acting on the tool that can be measured:

– Cutting force Fc and Thrust force Ft

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 work piece.

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.

NS

/

FFR

FNR

+=

+=

It is assumed that the resultant forces R & R’ are equal and opposite in magnitude and direction. Also they are Collinear. Therefore for the purpose of analysis the chip is regarded as an independent body held in mechanical equilibrium by the action of two equal and opposite forces R, which the workpiece exerts upon the chip and R’ which the tool exerts upon the chip.

Forces acting on chip in Orthogonal cutting

Resultant Forces Vector addition of F and N = resultant R

Vector addition of Fs and Fn = resultant R '

Forces acting on the chip must be in balance:

◦ R ' must be equal in magnitude to R

◦ R’ must be opposite in direction to R

◦ R’ must be collinear with R

Cutting Forces

Fig: (a) Forces acting on a cutting tool during 2-dimensional cutting. Note that the resultant force, R must be collinear to balance the forces. (b) Force circle to determine various forces acting in the cutting zone.

The following is a circle diagram. Known as Merchant’s circle diagram, which is convenient to determine the relation between the various forces and angles.

In the diagram two force triangles have been combined and R and R’ together have been replaced by R. the force R can be resolved into two components Fc and Ft.

Fc and Ft can be determined by force dynamometers.

tc FFR

+=

The rake angle (α) can be measured from the tool, and forces F and N can then be determined. The shear angle (φ) can be obtained from it’s relation with chip reduction coefficient. Now Fs & Fn can also be determined.

Merchant’s Circle Diagram

∅

Work

Tool Chip

Clearance Angle

Ft

Fc

F

N

Fn

Fs

α

α

β

(β - α)

R

Merchant’s Circle Diagram

Work

Tool Chip

Clearance Angle

Ft

Fc

F

N

Fn

Fs

α

α

β

∅

(β - α)

R

Clearance Angle

The procedure to construct a Merchant’s circle diagram

Work

Tool Chip

Ft

Fc

F

N

Fn

Fs

α

α

β

∅

R

Set up x-y axis labeled with forces, and the origin in the centre of the page. The cutting force (Fc) is drawn horizontally, and the tangential force (Ft) is drawn vertically. (Draw in the resultant (R) of Fc and Ft.

Locate the centre of R, and draw a circle that encloses vector R. If done correctly, the heads and tails of all 3 vectors will lie on this circle.

Draw in the cutting tool in the upper right hand quadrant, taking care to draw the correct rake angle (α) from the vertical axis.

Extend the line that is the cutting face of the tool (at the same rake angle) through the circle. This now gives the friction vector (F).

The procedure to construct a Merchant’s circle diagram

The procedure to construct a Merchant’s circle diagram A line can now be drawn from the head of the friction

vector, to the head of the resultant vector (R). This gives the normal vector (N). Also add a friction angle (β) between vectors R and N. Therefore, mathematically, R = Fc+Ft = F + N.

Draw a feed thickness line parallel to the horizontal axis. Next draw a chip thickness line parallel to the tool cutting face.

Draw a vector from the origin (tool point) towards the intersection of the two chip lines, stopping at the circle. The result will be a shear force vector (Fs). Also measure the shear force angle between Fs and Fc.

Finally add the shear force normal (Fn) from the head of Fs to the head of R.

Use a scale and protractor to measure off all distances (forces) and angles.

αα

αα

sincos

cossin

tC

tC

FFNGEODCDODABN

FFFGBEDGBCGCBOAF

−=⇒−=−==

+=⇒+=+===

Frictional Force System

angle Friction WhereNFtan

friction of tcoefficien The

=

==

β

βµ

Relationship of various forces acting on the chip with the horizontal and vertical cutting force from Merchant circle diagram

Ft

Fc

F

N

α

α

β

(β - α)

R

α α

α

(90-α)

(90-α) O

A

C

B

G

E

D

∅

Work

Tool Chip

Clearance Angle

Ft

Fc

F

N

Fn

Fs

α

α

β

(β - α)

R

Shear Force System

φφ

φαβφφ

cossin

(sincos

tCN

N

S

tCS

S

FFFDEBCDEADAEF

RCosFFFF

CDOBABOBOAF

+=⇒+=+==

)+−=−=⇒

−=−== Also: )tan( αβφ −+= SN FF


∅

Work

Tool Chip

Clearance Angle

Ft

Fc

F

N

Fn

Fs α

α

β

(β - α)

R

∅

Ft

Fc

A

O

Fn

Fs

α

α

(β - α)

R

B

C

D

E

∅

∅

(90-∅)

(90-∅)

)tan(cossinsincossincoscossin

αβφφφφφαααα

−+=+=−=−=+=

SN

tCN

tCS

tC

tC

FFFFFFFFFFNFFF


∅

Work

Tool Chip

Clearance Angle

Ft

Fc

F

N

Fn

Fs

α

α

β

(β - α)

R

Ft = R Sin (β-α) Fc = R Cos (β –α)

Knowledge of the cutting forces and power involved in machining operations is important for the following reasons:

a. Machine tools can be properly designed to minimize distortion of the machine components, maintain the desired dimensional accuracy of the machined part, and help select appropriate tool holders and work-holding devices.

b. The work piece is capable of withstanding these forces without excessive distortion.

c. Power requirements must be known in order to enable the selection of a machine tool with adequate electric power.

CUTTING FORCES and POWER

CUTTING FORCES and POWER Cutting force, Fc, acts in the direction of cutting speed, V, and supplies energy required for cutting. Thrust force, Ft , acts in a direction normal to cutting velocity, perpendicular to WP. The resultant force, R can be resolved into two components : Friction force: F, along the tool-chip interface Normal force: N, perpendicular to it.

F = R sin β N = R cos β

R is balanced by an equal and opposite force along the shear plane and is resolved into a shear force, Fs, and a normal force, Fn

Fs = Fc cos Ø – Ft sin Ø Fn = Fc sin Ø + Ft cos Ø

Coefficient of Friction Coefficient of friction between tool and chip:

Friction angle related to coefficient of friction as follows:

NF

=µ

βµ tan=The ratio of F to N is the coefficient of friction, μ, at the tool-chip interface, and the angle β is the friction angle.

The coefficient of friction in metal cutting generally ranges from about 0.5 to 2.

ααµ

tantan friction, oft Coefficien

tc

ct

FFFF

NF

−+

==

Shear Stress Shear stress acting along the shear plane:

φsinwtA o

s =

where As = area of the shear plane,

Shear stress = shear strength of work material during cutting

sAsF

= τ

References 1. Kalpakjian, Schmid, Manufacturing Processes for Engineering Materials, 4th

edition,, Prentice Hall 2003 2. DeGarmo, E. P., J. T. Black, and R. A. Kohser, Materials and processes in

Manufacturing, PHI. 3. P.N. Rao, Manufacturing Technology – Metal Cutting and Machine Tools, TMH. 4. George Schneider,Jr. CMfgE, Cutting Tool Applications 5. Amstead, B. H., P. F. Ostwald, and M. L. Begeman, Manufacturing

Processes, 8th ed., Wiley, New York, 1988. 6. Amitabha Battacharya , Metal Cutting Theory and Practice 7. Shaw, M. C., Metal Cutting Principles, Oxford University Press, Oxford, 1984. 8. Schey, J.A., Introduction to Manufacturing Processes, McGraw-Hill, NewYork,

1977. 9. Lindberg, R. A., Processes and Materials of Manufacture, 10.William J Patton, Machine tool Operations, Reston publishing company 11.O W Boston, Metal Processing, 2nd edition 1951, John Wiley and Sons 12.B.S.Raghuwanshi, A course in Workshop Technology-Dhanpat Rai & Sons. 13.Hajra Choudhury, Elements of Workshop Technology–Vol.-II, Media

Promoters and Publishers. 14.O P Khanna, Production Technology-(Vol. II) 15.R K Jain, Production Technology 16.HMT, Production Technology, HMT