Mustafizur Rahman

Dept. of Mechanical Engineering

NUS

ME 3162

MANUFACTURING PROCESSES

(II)

Topics

1. Introduction to machining and machine tools

2. Tool materials

3. Tool life and tool wear

4. Introduction to rapid prototyping

5. Introduction to laser cutting

References:

1. Fundamentals of metal machining and machine tools by Geoffrey

Boothroyd, McGraw-Hill Book Company

2. Metal cutting theory and practice by A. Bhattacharya, Central

Book Publishers, India

Part 1:

Introduction to Machining and Machine Tools

ME 3162 (II)

Methods to shape a component

1. By putting materials together (+)

2. By moving material from one region to another (0)

3. By removing unnecessary material (-)

1. By putting materials together (+)

E.g. Welding, Rapid Prototyping

Methods to generate a required shape

Methods to generate a required shape

2. By moving material from one region to another (0)

E.g. Rolling, Forging

Forging

3. By removing unnecessary material (-)

E.g. Turning, Milling, drilling

Methods to generate a required shape

Material removal - metal cutting

Metal cutting is an important shaping process whereby the

component shape and size is generated by removing excess material

from the original workpiece by a cutting tool which interferes with,

and moves relative to, the workpiece.

tool

workpiece

Three basic elements:1. The cutting tool

2. The workpiece

3. The machine tool

E.g. Turning and lathe.

Axi-symmetrical parts are

generated on the workpiece

(Element 2) by the turning tool

(Element 1) using the lathe

(Element 3).

Material removal - metal cutting

(Turning)

Three basic elements required:1. The cutting tool for removal of excess material

Material removal - metal cutting (Milling)

2. The workpiece or component to be shaped

3. The machine tool which supports the tool and the workpiece and

provides relative motion, power and associated force to sustain

the interference (cut) and generate the component shape and size.

Three basic elements:1. The cutting tool

2. The workpiece

3. The machine tool

E.g. Milling and machine

Prismatic components are

generated on the workpiece

(Element 2) by the milling

tool (Element 1) using the

milling machine (Element 3)

Material removal - metal cutting

Peripheral Milling Operations

In peripheral milling, the rotation direction

of the cutter distinguishes two forms of

milling

• Upmilling

• Downmilling

Upmilling

• Also referred as conventional milling.

• The direction of motion of the cutter teeth is opposite to

the feed direction when the teeth cut into the work.

• The resultant cutting force acts upwards.

• Tendency is to lift the work piece off the table.

• The undeformed chip thickness is minimum at the start

of the cut and maximum at the end of the cut

• Undeformed chip thickness is the thickness of the layer of

material removed by one cutting edge in one pass.

• Reduced tool life and surface finish.

chip thickness

Downmilling

• Also referred as climb milling.

• The direction of motion of the cutter teeth is in the

feed direction when the teeth cut into the work.

• The resultant cutting force acts downwards.

• The undeformed chip thickness is maximum at the start

of the cut and minimum at the end of the cut

• Fixtures and holding devices are simpler and less costly.

• Less tool wear

• Chip disposal is easier and better surface finish

Material removal - metal cutting (Drilling)

Three basic elements:1. The cutting tool

2. The workpiece

3. The machine tool

E.g. Drilling and machine

A circular cylindrical hole is

generated in the workpiece

(Element 2) by the twist drill

(Element 1) using the drilling

machine (Element 3).

A CNC machine has a tool carousel to store tools which are

automatically located and picked by an automated mechanical arm for

each operation so that multiple operations can be performed on a single

workpiece in a single setup.

Material removal - metal cutting (Multiple operations)

Complex geometric features and multiple operations

Video Show

Machining is an important process because it:

1. Can produce a very wide variety of shapes and sizes of component

2. Can produce shapes and sizes with high dimensional accuracy and

very good surface finish

3. Can be computer controlled and automated

Material removal - metal cutting

If possible, avoid machining; otherwise, minimize the amount of

machining required on the parts, e.g. machining of reference surfaces

on cast parts.

Need for machining includes close tolerances, good surface finish,

special geometric features such as threads, precision holes, cylindrical

sections with high degree of roundness, etc.

Objectives of Machining Processes:

1. The process must be physically feasible, i.e. it should be

possible to remove material from the component by cutting

and arrive at the desired shape, size and surface finish.

2. The process must be technologically as efficient and

economical as possible in producing components

3. The process must be capable of competing with other

manufacturing (shaping) process in producing components.

Satisfying objectives 1 and 2 assist in making machining

more competitive. Objective 3 attempts to ensure that

machining is considered in the light of broader spectrum of

manufacturing processes.

Performance Measures and Criteria for

Assessing Machining Objectives:

1. Tool life, or time to tool failure, is required to be ‘infinite’

(ideally) or maximum (in practice).

2. Forces and power are required to be ‘nil’ or minimum.

3. Shape and size variation are required to be ‘nil’ or minimum.

4. Surface finish/roughness are required to be ‘nil’ or minimum

5. Component production rate are required to be ‘infinite’ or

maximum.

1. Tool material properties – chemical, physical, etc.

2. Tool geometry – rake angle, clearance angle, etc.

3. Cut geometry (thickness, width, shape).

4. Cutting speed.

• Machine tool variables – rigidity

• Cutting fluid used

• Cost and time variables, component dimensions

Variables affecting machining performance

1. Tool material properties – chemical, physical, etc.

2. Tool geometry – rake angle, clearance angle, etc.

3. Cut geometry (thickness, width, shape).

4. Cutting speed.

• Machine tool variables – rigidity

• Cutting fluid used

• Cost and time variables, component

dimensions

Variables affecting machining performance

kr

ap

ap : depth of cut

kr : major cutting edge angle

Cutting motions

Primary motion (-C)

Seconday feed motion (-Z)

Two directions of motions: rotation of workpiece about its axis and feed of tool parallel to its axis

Rotational motion of the workpiece at V relative to the tool:

dnVw

Feed of tool at Vf relative to the

workpiece.

Cutting motions

VeV

Vf

d

where nw, rotational speed of spindle

and d is diameter of the workpiece

t

Undeformed chip thickness

ac ap

Back engagement

(depth of cut)

f

Feed engagement

Major cutting edge

angle kr

Tool and cut geometries

ac = f sin k r

ac

f

kr

dnVw

2

mww

av

ddnV

where nw, rotational speed of spindle

dw, original diameter

dm, machined diameter

Tool and cut geometries

nw

Seconday feed motion (-Z)

Chip cross-section area Ac of the layer of the material being removed is

approximately given by

Ac = f ap

where f is the feed per revolution

f = Vf /nw

f

Tool and cut geometries

dw = dm + 2ap

Material removal rate:

f

Tool and cut geometries

nw

Secondary feed motion (-Z)

)(

2

pmwp

mwwp

avp

avcw

adnfa

ddnfa

Vfa

V AZ

Machining Time

tm= (lw / f.nw) [nw = (V /π .dw)]

= (lw / f ) x ( π x dw / V)

Removal rate and machining time

mwppmwpw dnafadnafZ ....).(...

Material removal rate

When the depth of cut ap is small compared to the diameter of the machined surface dm,

Power required

If energy required to remove unit volume of material, is ps; then power Pm required to perform any machining operation,

Pm= ps Zw

Electrical power consumed

If efficiency of the machine tool motor and drive systems is m, the electrical power Pe consumed by the machine tool,

Pe = Pm /m

Power required

Other turning operations

Vertical Milling

where Vf is the feed speed

nt is rotational frequency

N is number of teeth

t

f

n

Vf

Feed per revolution

N

fa f Feed per tooth

Vf

nt

ap

acma

x

dt

Vf

Vertical Milling

Vertical Milling

Vertical Milling

If tool axis is aligned to

that of the workpiece the

maximum undeformed

chip thickness

acmax = af

dt /2 dt /2lw

where is given by,

af

acmax

sin

sinmax

N

f

aa fc

t

e

t

et

d

a

d

ad

21

2/

2/cos

The maximum undeformed chip

thickness acmax (measured

normal to the direction of

primary motion

(dt/2

) -

ae

Vertical Milling

For small ae / dt

2

2

)/(/2

cos1sin

tete dada

)1(2

sinmax

t

e

t

e

t

f

c

d

a

d

a

Nn

V

N

fa

t

e

t

f

cd

a

Nn

Va

2max

t

f

n

Vf

af

acmax

(dt/2

) -

ae

Material removal rate

fpew VaaZ

where ae is the depth of cut and ap is the width of

the workpiece

Vertical Milling

t

e

d

a21cos

Travelling distance is given by

)(2 etew adal

f

etew

mV

adalt

)(2

(dt/2

) -

ae

Vertical Milling

Thus, the machining time

2)/(/2)2/(sin)2/( tetett dadadd

where, lw is the length of workpiece

Drilling

rc kf

a sin2

t

wm

nf

lt

.

Drilling

Most common is a twist drill which has two

cutting edges. On each edge,

where kr, is the major cutting edge angle

The machining time tm is given by

where lw is the length of the drilled hole and

nt is the rotational frequency of the tool

dm

kr

If an existing hole of diameter dw is

enlarged to dm

4

).( 22

twm

w

nddfZ

4

....

4

22 tm

fmw

ndfVdZ

Drilling

dm

kr

Metal removal rate Zw (cross-sectional area

of hole feed speed Vf)