Fundamentals_of_Machining

8/7/2019 Fundamentals_of_Machining

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Fundamentals of Machining,

Cutting Tools and Cutting Fluids


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Lesson Outcomes

By the end of this lesson, students should be able

to understand: The concept of machining

Cutting mechanism

Chip types and characteristics

Cutting forces and their significance

Cutting tool materials and their properties

Inserts, their advantages and their shape

characteristics Cutting fluids: advantages, types and applications


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Lesson Outcomes (cont.)

Cutting tool materials and their properties

Inserts, their advantages and their shape

characteristics

Cutting fluids: advantages, types andapplications


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Machining definition:The process of material removal from the

surface of a workpiece by chip formation


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Machining and

finishing processes


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Dimensional tolerances of various

machining processes


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Orthogonal Cutting(2D model)

Oblique cutting(3D model)

Cutting Models

(a) Orthogonal cutting with a well-defined shear plane(b) Orthogonal cutting without a well-defined shear plane

In orthogonal cutting, the chip slides

directly up the face of the tool, in oblique

cutting, the chip is helical and at an angle

i, called the inclination angle


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Chip formation (2D Model)

1. Tool move to left at velocity V,

and depth of cut, to

2. Shearing occurs along shear

plane3. Chip is formed

4. Chip pushed up the rake face

by the chip forming below

5. Chip breaks

Fig (a) shows the 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.


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4 types of chips produced:

1. Continuous chip2. Built up edge

3. Serrated or segmented chip4. Discontinuous chip


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Type of chips produced1. Continuous chip

Continuous chips usually are formed

with ductile materials, machined athigh cutting speeds (V) and/or high

rake angles ().

Produce good surface finish Chips tend to tangle around tool

holder, fixtures

Change cutting parameters or usechip breakers to reduce chip length


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Type of chips produced2. Built up edge

Workpiece material deposited on tool tip

As it gets large, it BUE breaks away:

some carried by chip, some deposited on

workpiece

Reduces quality of surface finish and

dulls tool point A thin & stable BUE can protect rake

face and reduce tool wear

Can be reduced by: Increase cutting

speed, decrease depth of cut, increase

rake angle, use a sharp tool, use cutting

fluid, use cutting tool that has low

chemical affinity with workpiece material


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Type of chips produced3. Serrated or segmented

Semi-continuous chips

Occurs in metals withlow thermal conductivity and

strength that decrease sharply

with temperature (eg: titanium)


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Type of chips produced

4. Discontinuous chip Chip is segmented

May be due to: Brittle workpiece material, or

materials that contain hard

inclusions

Very low or very high cutting

speeds

Large depths of cuts

Low rake angles

No cutting fluid

Vibration/chatter due to low

stiffness


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Cutting Forces and Power

Why study cutting forces and power?

Machine tool design

Workpiece selection

Machine tool selection


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Cutting Forces and Power

The thrust force, acts in a direction normal to the cutting speed.

These two forces produce the resultant force, R, as can be seen from the

force circle.

The resultant force can be resolved into two components on the tool face:a friction force, F, along the tool-chip interface and a normal force, N,

perpendicular to it.

Note also that the resultant force is balanced by an equal and opposite

force along the shear plane and is resolved into a shear force, and anormal force.


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Tool wear and tool failure Adverse conditions (due to cutting) affect

tool wear lead to tool failure

Adverse conditions are: High temp along rake face

Contact stresses

Chip sliding

along rake

face

Rubbing along

workpiece

Where

is temp

highest?

Why?

Localizedstress at tip


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Effects of elevated temperatures

Effect on cutting tool: lowerstrength/hardness/stiffness/wear resistence,

plastic deformation change in shape

Effect on workpiece: dimensional change

part accuracy, material properties


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Tool wear and tool failure

Types of tool wear/failure:1.Flank wear

2. Crater wear

3. Nose wear

4.Notching5. Plastic deformation of the tool tip

6. Chipping, and gross fracture


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Flank wear and crater wear:


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1. Flank wear Due to : high temp and rubbing along workpiece that

leads to abrasive/adhesive wear

Described by the Taylor equation:

2. Crater wear

Due to: high temp and chemical affinity with workpiece

CfdVTyxn=

speed tool lifedepthof cut

feed A constant

The time required to

develop a certain wear

stage, VB (allowable wearland)

CfdVTyxn=

speed tool lifedepthof cut

feed A constant








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Equation for tool wear:

CfdVTyxn=

Tool life curves

nT

V 1

The recommended cutting speed

for a high-speed steel tool is

generally the one that yields a tool

life of 60 to 120 min, and for acarbide tool, it is 30 to 60 min.


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3. Nose wear is the rounding of a sharp tool,due to mechanical and thermal effects. Itdulls the tool, affects chip formation, andcauses rubbing of the tool over theworkpiece, raising its temperature and

possibly inducing residual stresses on themachined surface.

4. Notching.

Scale and oxide layers on a workpiecesurface contribute to notch wear, becausethese layers are hard and abrasive.


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Tool wear and tool failure6. Chipping, and gross fracture

-A small fragment from the cutting edge of

the tool breaks away. Small: Microchipping/macrochipping

Large: Gross chipping/fracture and

catastrophic failure-Due to : mechanical shock, thermal

fatigue*


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Machinability of materials

Factors that affect machinability of materials:

1. Surface finish and surface integrity of part

2. Tool life most important factor

3. Force and power requirements

4. Chip control

Geometric feature Material properties:

eg: Fatigue life,

Corrosion resistance


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Machinability of materials

How is tool life rating determined?

1. Machine at various cutting speeds untilobtain T=60 min

2. The cutting speed at T=60min is used as

the tool life rating


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Cutting tool properties


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Trends in cutting tool properties


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Cutting tool property trend


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Cutting Tools

High-speed steel tools are shaped inone piece and ground to impact various

geometric features such tools include

drill bits and milling and gear cutters.

Fig 22.2 shows the typical carbide

inserts with various shapes and chip-

breaker features. The holes in the

inserts are standardized for

interchangeability in toolholders.


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Inserts

Carbon steel &HSS tools

machined to

desired shape

Tool wear

Need to change

tool

Take tool from

tool room

Time consuming

Use insert instead:

-Multiple cutting

points

-Can easily change

to a different

cutting point


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Inserts


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Insert shapesThe figure below shows the relative edge strength and tendency forchipping of inserts with various shapes. Strength refers to the cutting

edge indicated by the included angles.

The figure below shows the edge preparation for inserts to improve

edge strength.


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Coated toolsAdvantages of coatings:1. Lower friction

2. Higher adhesion

3. Higher resistance to wear and cracking4. Acting as a diffusion barrier

5. Higher hot hardness and impact resistance

Types of coatings:

1. Titanium nitride

2. Titanium carbide

3. Ceramics

4. Multiphase coatings

5. Diamond coatings

6. Etc.


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Cutting Fluids

Advantages:

1. Reduce friction and wear, thus improving tool life and

the surface finish of the workpiece.2. Cool the cutting zone, thus improving tool life and

reducing the temperature and thermal distortion of theworkpiece.

3. Reduce forces and energy consumption.

4. Flush away the chips from the cutting zone, and thusprevent the chips from interfering with the cuttingprocess, particularly in operations such as drilling andtapping.

5. Protect the machined surface from environmental

corrosion.


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Cutting Fluids

Types of cutting fluids:1. Oils (also called straight oils) including mineral, animal, vegetable,

compounded, and synthetic oils typically are used for low-speedoperations where temperature rise is not significant.

2. Emulsions (also called soluble oils) are a mixture of oil and waterand additives, generally are used for high-speed operations because

temperature rise is significant. The presence of water makesemulsions very effective coolants.

3. Semisynthetics are chemical emulsions containing little mineral oil,diluted in water, and with additives that reduce the size of oil

particles, making them more effective.4. Synthetics are chemicals with additives, diluted in water, and

contain no oil.


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Methods of cutting fluid

applications

The need for a cutting fluid depends on the severity of theparticular machining operation, which may be defined as thelevel of temperatures and forces encountered, the tendencyfor built-up edge formation, the ease with which chips

produced can be removed from the cutting zone, and howeffectively the fluids can be applied to the proper region atthe toolchip interface.



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applications Depending on the type of machining operation, the

cutting fluid needed may be a coolant, a lubricant,or both.

The effectiveness of cutting fluids depends on anumber of factors, such as the type of machining

operation, tool and workpiece materials, cuttingspeed, and the method of application: Flooding

Mist High pressure systems

Through the cutting tool system


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Application of cutting fluids


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Summary

Machining: The process of material removal fromthe surface of a workpiece by chip formation

When tool moves along a workpiece at a certaindepth of cut plastic deformation and shearing

leads to chip formation. 4 major chip types are continuous chips, built up

edges, serrated chips and discontinuous chips

Studies of cutting forces are important in machine

tool design, workpiece selection and machine toolselection


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Summary (cont.)

A great variety of cutting tool materials exists withvarying degrees of hardness, toughness, wearresistance and chemical stability

Inserts allow easy change when worn out. Inserts

with larger included angles are stronger and lesslikely to break

Cutting fluids reduce friction & wear, reducecutting forces, remove chips and protect

workpiece against corrosion Cutting fluids must be applied correctly to harness

its advantages


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Whats next?

Conventional Lathe

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Fundamentals_of_Machining