contents...therefore in actual cutting operation we should include the side cutting edge (principal...

Contents

1.1 Machine Tools and it’s Function ............................................................................................... 1.2

1.2 Classification of Machine Tools ............................................................................................... 1.2

1.3 Motions In Machine Tools ......................................................................................................... 1.3

1.4 Cutting Tools .............................................................................................................................. 1.4

1.5 Cutting Tool Material ................................................................................................................. 1.8

1.6 Cutting Fluid or oil ...................................................................................................................... 1.8

1.7 Mechanism of Chip Formation ................................................................................................ 1.10

1.8 Cutting Force Analysis ............................................................................................................. 1.11

1.9 Determination of shear Angle ................................................................................................. 1.11

1.10 Tool wear, Life and Economics of Machining ........................................................................ 1.13

1.11 Types of Methods of Cutting ................................................................................................... 1.15

1.12 Importance of Various Tool Angles. ....................................................................................... 1.16

1.13 Reference .................................................................................................................................. 1.17

1.2

Mr. Ankur N Tank, Department of Mechanical Engineering

Manufacturing Process (3141908)

Unit-1 Basic Machine Tools and Metal Cutting Principle

1.1 Machine Tools and it’s Function

A Machine tools is a power driven machine for making articles of a given shape, size and

accuracy(according to blue prints) by removing metal from work pieces in the form of chips. Functions of machine tools are..,

- Hold the job

- Hold the cutting tools

- Move one or both of these(rotary or reciprocating motion)

- Provide a feeding motion for one of these.

Fig.1.1 Function of Machine Tools

1.2 Classification of Machine Tools

BASED ON APPLICATION

GENERAL PURPOSE MACHINE TOOLS : Versatile in use like lathe, milling, drilling etc.

SINGLE PURPOSE MACHINE TOOLS: Single operation design like gear cutting, shaping , hobbing

etc.

LIMITED PURPOSE MACHINE TOOLS: Narrow range of operation like automatic cutting off

machine.

PRODUCTION MACHINE TOOLS: Batch and mass production like multi tool lathe, automates,

grinders etc.

SPECIALIZED MACHINE TOOLS: For machining articles similar in shape but different in size like

CNC.

SPECIAL MACHINE TOOLS: For definite operation like tool and cutter grinder.

ACCORDING TO ACCURACY OF MACHINE TOOLS:

Normal Accuracy

Higher Accuracy

Precision

High Precision

Super High Precision




1.3

ACCORDING TO WEIGHT

I. Light weight: (upto 1t)

II. Medium weight: (upto 10t)

III. Heavy weight: (over 10t)

- Large size: (10 TO 30 T)

- Heavy: (30 TO 100T)

- Extra heavy: (OVER 100T)

ACCORDING TO PROCESSING OPERATIONS:

Lathes:

Drilling And Boring Machines

Planers, Shapers, Slotters And Broaching Machines:

Milling Machines

Grinding And Micro-finishing Machines

Gear And Thread Cutting

Combination Machine Tools

Cutting Off Machines

Miscellaneous Machine Tools: like Balancing Machines, Straightening Machines Etc.

1.3 Motions In Machine Tools

To obtain a finished work piece on a machine tool, certain coordinated motions must be imparted to the

work and cutting tool. These motions are of two types:

(A) Primary or Working Motions &

(B) Auxiliary Motions

Primary or Working Motions

Principal or cutting motions and feed motions.

They serve the purpose of removing metal from the work piece.

Working motions are power driven. However certain small machines have hand feeds.

The speed of the principal motion depends on the optimum cutting speed, while the feed motion

depends on the required degree of surface finish.

Principal motions are of three types: (I) ROTARY, (II) RECIPROCATING & (III) COMBINED. These

motion can be imparted either to the workpiece or to the tool.

1.4




Auxiliary Motions

It help in the completion of the machining process and include such motions as:

handling and clamping the work in the machine, advance and withdrawal of the cutting tool

Engagement and disengagement of working motions and changing their speeds etc.

Auxiliary motions are generally hand operated in conventional machines and power driven in

automatic machine tools.

1.4 Cutting Tools

The cutting tools may be classified in different ways. Depending upon the number of cutting points on the

tool, the cutting tools are of two types:

1. Single-point cutting tools,

2. Multi-point cutting tools.

A single-point cutting tool has only one cutting point or edge. The tools used for turning, boring, shaping,

or planning operations, that is, tools used on lathes, boring machines, shaper, planer, etc. are single point

tools. A multi-point tool has two or more than two cutting point or edge [for example, tools used on drilling

machines, milling machines, broaching machines etc.] multi-point tool can be considered to be basically

a series of single-point tools.

Depending upon the construction of the cutting tool, it is classified as :

1. Solid tools,

2. Tipped cutting tools.

The solid cutting tools are made entirely of the same material, whereas, in a tipped cutting tool, an insert

of cutting tool material is brazed or held mechanically to the shank of another material.

1.4.1 Tool Terminology

Fig.1.2 Tool Terminology




1.5

Shank: Main body of tool, it is part of tool which is gripped in tool holder

Face: Top surface of tool b/w shank and point of tool. Chips flow along this surface

Flank: Portion tool which faces the work. It is surface adjacent to & below the cutting edge when tool lies

in a horizontal position.

Point: Wedge shaped portion where face & flank of tool meet.

Base: Bearing surface of tool on which it is held in a tool holder.

Nose radius: Cutting tip, which carries a sharp cutting point. Nose provided with radius to enable greater

strength, increase tool life & surface life.

- Typical Value: 0.4 mm – 1.6 mm

1.4.2 Tool Geometry and Various Tool Angles

Fig.1.3 Tool Geometry

1.6




Side Cutting Edge Angle (SCEA): It is also known as lead angle (Cs) and approach angle. It is the

angle between the side cutting edge and side of the tool shank.

End Cutting Edge Angle (ECEA): This is the angle between the end cutting edge and the normal to

the tool shank.

Side Relief Angle (SRA): It is the angle between the portion of the side flank immediately below

the side cutting edge and a line perpendicular to the base of the tool and measured at right to the

side flank.

End Relief Angle (ERA): It is the angle between the portion of the end flank immediately below the

end cutting edge and a line perpendicular to the base of the tool and measured at a right angle to

the end flank.

Back Rake Angle (BRA): It is the angle between the tool face and line parallel to the base of the

tool and measured in a plane (perpendicular) through the side cutting edge. This angle is positive.

Side Rake Angle (SRA): It is the angle between the tool face and a line parallel to the base of the

tool and measured in a plane perpendicular to the base and the side cutting.

1.4.3 Tool Signature

1.4.3.1 American Standard Accosiation System (ASA)

Fig.1.4 American Standard accosiation system (ASA)




1.7

Tool Designation under ASA System is given in order next:

Back rake, side rake, End Relief , side relief, End Cutting Edge Angle, Side Cutting Edge Angle and

nose radius.

b - S – θ e – θ s - Ce – Cs – R

If tool Designation is 8 – 14 – 6 – 6 – 6 - 15 - 1/8 means that

b = 8°, S = 14°, θ e =6°, θ s = 6°, Ce = 6°, Cs = 6°, R= 1/8”

In ASA system of tool angles the angles are specified independently of the position of the cutting

edge it, therefire does not give any indication of the behavior of the tool in practice.

Therefore in actual cutting operation we should include the side cutting edge (principal cutting

edge) in the scheme of reference planes. Such system is known as ortho gonal rake system (ORS).

1.4.3.2 Orthogonal Rack System

Fig.1.5 Orthogonal Rack System

The angles are also measured in the plane MM (known as auxiliary reference plane) which is

normal to the projection of the end cutting edge on the basic plane. These angle are end relief

angle γ1 and back rake angle α1 (also called auxiliary rake angle).

The plane angles are the approach angle or entering angle λ which is equal to (90° -Cs) and the end

cutting edge angle Ce.

Tool Designation under ORS is:

i – α – γ – γ1 – Ce – λ – R

1.8




1.5 Cutting Tool Material

Carbon steel: Carbon steels having carbon percentage as high as 1.5% are used as tool materials

however they are not able to with stand very high temperature and hence are operational at low

cutting speed.

High speed steel (HSS): These are special alloy steel which are obtained by alloying tungsten,

Chromium, Vanadium, Cobalt and molybdenum with steel. HSS has high hot hardness, wear

resistance and 3 to 4 times higher cutting speed as compare to carbon steel. Most commonly used

HSS have following compositions.

a. 18-4-1 HSS i.e. 18% tungsten, 4% chromium, 1% vanadium with a carbon content of 0.6 - 0.7%. If

vanadium is 2% it becomes 18-4-2 HSS.

b. Cobalt high speed steel: This is also referred to as super high speed steel. Cobalt is added 2 – 15%.

The most common composition is tungsten 20%, 4% chromium, 2% vanadium and 12% cobalt.

c. Molybdenum high speed steel: It contains 6% tungsten, 6% molybdenum, 4% chromium and 2%

vanadium.

Cemented carbide: These are basically carbon cemented together by a binder. It is a powder

metallurgy product and the binder mostly used is cobalt. The basic ingredient is tungsten carbide-

82%, titanium carbide-10% and cobalt-8%. These materials possess high hardness and wear

resistance and it has cutting speed 6 times higher than high speed steel (HSS).

Ceramics: It mainly consists of aluminum oxide (Al2O3) and silicon nitride (Si3N4). Ceramic

cutting tools are hard with high hot hardness and do not react with the workpiece. They can be

used at elevated temperature and cutting speed 4 times that of cemented carbide. These have low

heat conductivity.

Diamond: It is the hardest known material having cutting speed 15 times greater than that for high

speed tools.

Cubic boron nitride (CBN): It is the second hardest material after diamond and a economical

alternative to the later. It is manufactured through high temperature and pressure to bond boron

crystals in cubic form with a ceramic or metal binder to form polycrystalline structure with nitride

particles present. It is an excellent cutting tool material because it combines extreme high hot

hardness up to high temperatures of 2000°C.

1.6 Cutting Fluid or oil

1.6.1 Advantages of Cutting Fluids

Reduction of tool costs.

Reduce tool wear, tools last longer.

Increased speed of production.

Reduce heat and friction so higher cutting speeds.

Friction reduced so less power required by machining.

Tools last longer and require less regrinding, less downtime, reducing cost per part.

Reduction of power costs and labour cost.




1.9

1.6.2 Characteristics of Good Cutting Fluid

Good cooling capacity

Good lubricating qualities

Resistance to rancidity

Relatively low viscosity

Stability (long life)

Rust resistance

Nontoxic

Transparent and Nonflammable.

1.6.3 Types of Cutting Fluids

Most commonly used cutting fluids either aqueous based solutions or cutting oils.

Cutting Fluid Fall into three categories

Cutting oils

Emulsifiable oils

Chemical (synthetic) cutting fluids

Cutting Oils There are two classifications Active and Inactive.

Terms relate to oil's chemical activity or ability to react with metal surface

Elevated temperatures

Improve cutting action

Protect surface

Active Cutting Oils Those that will darken copper strip immersed for 3 hours at temperature of 212ºF Dark

or transparent Better for heavy-duty jobs

This kind of oil has three categories

Sulfurized mineral oils

Sulfochlorinated mineral oils

Sulfochlorinated fatty oil blends

Inactive Cutting Oils Oils will not darken copper strip immersed in them for 3 hours at 212ºF Contained sulfur

is natural Termed inactive because sulfur so firmly attached to oil – very little released.

This kind of oil have four general categories

Straight mineral oils,

fatty oils,

fatty and mineral oil blends,

sulfurized fatty-mineral oil blend

Emulsifiable (Water Soluble) Oils Mineral oils containing soap like material that makes them soluble in water and causes

Them to adhere to work piece. Emulsifiers break oil into minute particles and keep them

Separated in water.

Supplied in concentrated form (1-5 /100 water).Good cooling and lubricating qualities.

Used at high cutting speeds, low cutting pressures

1.10




Chemical Cutting Fluids Also called synthetic fluids Introduced about 1945 and have Stable, preformed emulsions.

It Contain very little oil and mix easily with water and Extreme-pressure (EP) lubricants

added, React with freshly machined metal under heat and pressure of a cut to form solid

Lubricant it is also Reduce heat of friction and heat caused by plastic deformation of metal.

1.7 Mechanism of Chip Formation

A typical Metal cutting process can be schematically represented in Figure in which a wedge

shaped tool is made to move relative to the work piece. As the tool makes contact with the metal;

it exerts a pressure on it resulting in the compression of the metal near to the tool tip.

This includes shear type deformation with in the metal and it starts moving upward along the top

face of the tool, as the tool advances, the material ahead of it is sheared continuously along a plane

called the shear plane.

Fig.1.6 Mechanism of Chip Formation

This shear plane actually narrow zone. (About 0.025 mm) and extends from the cutting edge of the

tool is formed by two intersecting surfaces. The surface along which the chip moves upwards is

called “Rake Surface” and the other surface which is relieved to avoid rubbing with the machined

surface is called “flank”.

The angle between rake surface and the normal is known as “Rake Angle” (which may be positive

or negative) and the angle between the flank and the horizontal machined surface is known as the

“relief or clearance angle”.




1.11

1.8 Cutting Force Analysis

Here the analysis is limited to two dimensional or orthogonal cutting which is simpler to understand as

compared to the complicated three dimensional cutting process when a cut is made. The force acting on

metal chip are,

Fig.1.7 Cutting force Analysis

Fs = which is resistance to shear of the metal is forming the chip its act along shear plane.

Fn = Which is Normal to the shear plane. This is backing up force on the chip provided by the work piece.

F = It is the frictional resistance of the tool acting on chip. It acts downward against the motion of the chip

as it glides upwards along the tool face.

N = It is the force subjected at the tool chip interface acting normal to the cutting face of the tool and is

provided by the tool.

1.9 Determination of shear Angle

In the simplified model of two dimensional cutting operations the cutting tool is completely defined by

the rake angle α and clearance angle γ. In additions the following assumptions are made

1. Tool is perfectly sharp and contacts the chip on its front or rake face.

2. The primary deformation takes place in a very thin zone adjacent to the shear plane AB.

3. There is no flow of chip that is plain strain condition.

Shear angle α is defined as the angle made by the shear plane with the direction of the tool travel

If t = uncut chip or undeformed chip thickness.

tc = chip thickness after the metal is cut.

r = 𝑡

𝑡𝑐 is called the cutting ratio or chip thickness ratio or chip compression factor.

ζ = 𝑡𝑐

𝑡 is called chip reduction factor.

1.12




Fig.1.8 Determination of Shear Angle

The shear angle can be determined in the following ways. t = uncut chip thickness. tc = cut chip thickness. ∅ = Shear Angle.

α = Rake Angle.

Now,

r = 𝑡

𝑡𝑐 … … … … … … … … … … … … … … … … (i)

Now finding the value of t and tc,

Consider Δ BAC and angle ∅

Sin ∅ = 𝑡

𝐴𝐵

t = AB Sin ∅ … … … … … … … … … … … … … … … … (ii)

For tc consider Δ DBA

Cos (∅ - α) = 𝑡𝑐

𝐴𝐵

tc = AB Cos (∅ - α) … … … … … … … … … … … … … … … … (iii)

Now putting t and tc value in equation (i)

r = 𝐴𝐵 sin

𝐴𝐵 𝐶𝑂𝑆 ( − 𝛼)

r = sin

𝐶𝑂𝑆 ( − 𝛼)




1.13

r Cos (∅ - α) = sin ∅

r cos ∅ cos α + r sin ∅ sin α = sin∅ [using cos (A-B)]

Divide above equation by cos ∅ we get

r cos α + r tan ∅ sin α = tan ∅

r cos α = tan ∅ - r tan ∅ sin α

r cos α = tan ∅ (1- r sin α)

tan ∅ = 𝑟 cos 𝛼

1−𝑟 sin 𝛼

where ∅ = shear angle.

1.10 Tool wear, Life and Economics of Machining

1.10.1 Tool Wear:

The life of a cutting tool can be terminated by a number of means, although they fall broadly into two main

categories:

Gradual wearing of certain regions of the face and flank of the cutting tool, and abrupt tool failure.

Considering the more desirable case the life of a cutting tool is therefore determined by the amount of

wear that has occurred on the tool profile and which reduces the efficiency of cutting to an unacceptable

level, or eventually causes tool failure. When the tool wear reaches an initially accepted amount, there are

two options, to resharpen the tool on a tool grinder, or to replace the tool with a new one. This second

possibility applies in two cases,

(i) When the resource for tool resharpening is exhausted.

(ii) The tool does not allow for resharpening, e.g. in case of the indexable carbide inserts. Wear zones

Gradual wear occurs at three principal locations on a cutting tool. Accordingly, three mainTypes of tool

wear can be distinguished,

1. Crater wear

2. Flank wear

3. Corner wear

Crater wear: It is consists of a concave section on the tool face formed by the action of the chip sliding

on the surface. Crater wear affects the mechanics of the process increasing the actual rake angle of the

cutting tool and consequently, making cutting easier. At the same time, the crater wear weakens the tool

wedge and increases the possibility for tool breakage. In general, crater wear is of a relatively small

concern.

Fig.1.9 Types of wear observed in single point cutting tool

1.14




Flank wear: It is occurs on the tool flank as a result of friction between the machined surface of the work

piece and the tool flank. Flank wear appears in the form of so-called wear land and is measured by the

width of this wear land, VB, Flank wear affects to the great extend the mechanics of cutting. Cutting forces

increase significantly with flank wear. If the amount of flank wear exceeds some critical value (VB >

0.5~0.6 mm), the excessive cutting force may cause tool failure.

Fig.1.10 Cross-section perpendicular to the major cutting edge of a worn cutting tool showing the effect of crater wear on the tool rake angle and the flank wear land.

Corner wear: It is occurs on the tool corner. Can be considered as a part of the wear land and

respectively flank wear since there is no distinguished boundary between the corner wear and flank wear

land. We consider corner wear as a separate wear type because of its importance for the precision of

machining. Corner wear actually shortens the cutting tool thus increasing gradually the dimension of

machined surface and introducing a significant dimensional error in machining, which can reach values of

about 0.03~0.05 mm.

1.10.2 Tool life

The tool life is the duration of actual cutting time after which the tool is no longer usable. There are many

ways of defining the tool life, and the common way of quantifying the end of a tool life is by a limit on the

maximum acceptable flank wear.

Parameters, which affect the rate of tool wear are cutting conditions ( cutting speed V, feed f, depth of

cut d), cutting tool geometry (tool orthogonal rake angle), properties of work material From these

parameters, cutting speed is the most important one. As cutting speed is increased, wear rate increases,

so the same wear criterion is reached in less time, i.e., tool life decreases with cutting speed:

If the tool life values for the three wear curves are plotted on a natural log-log graph of cutting speed versus

tool life as shown in the right figure, the resulting relationship is a straight line expressed in equation form

called the Taylor tool life equation:

V Tn = C

Where n and C are constants, whose values depend on cutting conditions, work and tool material

properties, and tool geometry. These constants are well tabulated and easily available.




1.15

Fig.1.11 Tool Life Versus Cutting speed

Fig. Effect of cutting speed on wear land width and tool life for three cutting speeds. Natural log-log plot of cutting speed versus tool life. An expanded version of Taylor equation can be formulated to include the effect of feed, depth of cut and

even work material properties.Tool life also depends to a great extent on the depth of cut d and feed rate

per revolution f. Assuming a logarithmic variation C with d the equation can be written as,

V Tn dm = C

It has been seen that decrease of life with increase speed is twice as a great as the decrease of life with

increased feed. By considering feed rate also, the general equation can be written as:

V Tn dm fx = C

1.11 Types of Methods of Cutting

Orthogonal Cutting Oblique Cutting

The cutting edge of the tool remains

normal to the direction of tool feed or work

feed.

The cutting edge of the tool remains

inclined at an acute angle to the direction

of tool feed or work feed.

The direction of the chip flow velocity is

normal to the cutting edge of the tool.

The direction of the chip flow velocity is at

an angle with the normal to the cutting

edge of the tool. The angle is known as chip

flow angle.

Here only two components of forces are

acting: Cutting Force and Thrust Force. So

the metal cutting may be considered as a

two dimensional cutting.

Here three components of forces are

acting: Cutting Force, Radial force and

Thrust Force or feed force. So the metal

cutting may be considered as a three

dimensional cutting.

1.16




The cutting edge being non oblique, the

shear force acts on a smaller area and thus

tool life is decreased.

The cutting edge being oblique, the shear

force acts on a larger area and thus tool life

is increased

1.12 Importance of Various Tool Angles.

The Rake Angle

The rake angle is always at the topside of the tool.

The basic tool geometry is determined by the rake angle of the tool.

Rake angle has two major effects during the metal cutting process.

One major effect of rake angle is its influence on tool strength. A tool with negative rake will

withstand far more loading than a tool with positive rake.

The other major effect of rake angle is its influence on cutting pressure. A tool with a positive rake

angle reduces cutting forces by allowing the chips to flow more freely across the rake surface.

he rake angle has the following function:

It allows the chip to flow in convenient direction.

It reduces the cutting force required to shear the metal and consequently helps to increase the tool

life and reduce the power consumption.

It provides keenness to the cutting edge andimproves the surface finish.

Relief angle:

Relief angles are provided to minimize physical interference or rubbing contact with machined

surface and the work piece.

Relief angles are for the purpose of helping to eliminate tool breakage and to increase tool life.

If the relief angle is too large, the cutting tool may chip or break. If the angle is too small, the tool

will rub against the work piece and generate excessive heat and this will in turn, cause premature

dulling of the cutting tool.

Small relief angles are essential when machining hard and strong materials and they should be

increased for the weaker and softer materials.

A smaller angle should be used for interrupted cuts or heavy feeds, and a larger angle for semi-

finish and finish cuts.

Side relief angle:

The Side relief angle prevents the side flank of the tool from rubbing against the work when

longitudinal feed is given.

Larger feed will require greater side relief angle.




1.17

End relief angle:

The End relief angle prevents the side flank of the tool from rubbing against the work.

A minimum relief angle is given to provide maximum support to the tool cutting edge by increasing

the lip angle.

The front clearance angle should be increased for large diameter works.

End cutting edge angle:

The function of end cutting edge angle is to prevent the trailing front cutting edge of the tool from

rubbing against the work. A large end cutting edge angle unnecessarily weakens the tool.

It varies from 8 to 15 degrees.

Side cutting edge angle: The following are the advantages of increasing this angle:

It increases tool life as, for the same depth of cut; the cutting force is distributed on a wider surface.

It diminishes the chip thickness for the same amount of feed and permits greater cutting speed.

It dissipates heat quickly for having wider cutting edge.

The side cutting edge angle of the tool has practically no effect on the value of cutting force or

power consumed for a given depth of cut & feed.

Large side cutting edge angles are likely to cause the tool to chatter.

Nose Radious

The nose of a tool is slightly rounded in all turning tools.

The function of nose radius is as follows:

Greater nose radius clears up the feed marks caused by the previous shearing action and provides

better surface finish.

All finish turning tool have greater nose radius than rough turning tools.

It increases the strength of the cutting edge, tends to minimize the wear taking place in a sharp

pointed tool with consequent increase in tool life.

Accumulation heat is less than that in a pointed tool which permits higher cutting speeds.

1.13 Reference

1) R.K.Rajput,”Manufacturing Technology”,Lakshmi publication (P) Ltd.

2) B.S.Raghuvanshi, “Workshop Technology (Vol.ll)”,Dhanpat Rai & Co.

3) P.C.Sharma, “Production Engineering”, S.Chand Publications

contents...therefore in actual cutting operation we should include the side cutting edge (principal...

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