Machining Economics

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Manufacturing Engineering Technology in SI Units, 6
th

Edition



Chapter 25:

Machining Centers, Machine Tool Structures and
Machining Economics


Copyright © 2010 Pearson Education South Asia Pte Ltd

Chapter Outline


Introduction


Machining Centers


Machine
-
tool Structures


Vibration and Chatter in Machining Operations


High
-
speed Machining


Hard Machining


Ultraprecision Machining


Machining Economics


Copyright © 2010 Pearson Education South Asia Pte Ltd

Introduction


Computers
improved the capabilities of machine tools


Have the capability of rapidly producing extremely
complex part geometries


Copyright © 2010 Pearson Education South Asia Pte Ltd

Machining Centers


Brief review:

1.
Possibilities exist in
net
-
shape
or
near
-
net shape
production

2.
Some form of machining is required and is more
economical to finish machine parts to their final shapes


Copyright © 2010 Pearson Education South Asia Pte Ltd

Machining Centers

The Concept of Machining Centers


Machining parts
can be highly automated to increase
productivity


Transfer lines
are

used in
high
-
volume
or
mass
production,
consist of several specific machine tools
arranged in a logical sequence


Workpiece is moved from station to station, with a
specific machining operation performed at each station


A machining center is an advanced computer
-
controlled machine tool that perform machining
operations without removing


Copyright © 2010 Pearson Education South Asia Pte Ltd

Machining Centers


Copyright © 2010 Pearson Education South Asia Pte Ltd

Machining Centers

Components of a Machining Center


The workpiece in a machining center is placed on a
pallet
, or
module


Can be moved and swiveled in various directions


New pallet is brought in by an
automatic pallet
changer


A machining center is equipped with a programmable
automatic tool changer
(ATC)


Copyright © 2010 Pearson Education South Asia Pte Ltd

Machining Centers

Components of a Machining Center


The
tool
-
exchange arm
swings around to pick up a
particular tool and places it in the spindle


Tool
-
checking
and/or
part
-
checking station
would
feeds information to the machine control system


Touch probes
select the tool settings and inspect parts
being machined


Copyright © 2010 Pearson Education South Asia Pte Ltd

Machining Centers:

Types of Machining Centers

Vertical
-
spindle Machining Centers


Performing various machining

operations on parts with deep

cavities, as in mold and die making


Horizontal
-
spindle Machining Centers


Suitable for large and

tall workpieces that

require machining on a

number of their surfaces


Copyright © 2010 Pearson Education South Asia Pte Ltd

Machining Centers:

Characteristics and Capabilities of Machining Centers


Major characteristics of machining centres:

1.
Handles a wide variety of part sizes and shapes
efficiently

2.
Versatile and quick changeover

3.
Time required is reduced

4.
Detection of tool breakage and wear

5.
Inspection of machined work

6.
Compact and highly automated


Copyright © 2010 Pearson Education South Asia Pte Ltd

Machining Centers:

Selection of Machining Centers


Selection of type and size of machining centers
depends on:

1.
Type of products, their size, and their shape complexity

2.
Type of machining operations to be performed and the
type and number of cutting tools required

3.
Dimensional accuracy required

4.
Production rate required


Copyright © 2010 Pearson Education South Asia Pte Ltd

Machining Centers:

Selection of Machining Centers

EXAMPLE 25.1


Machining Outer Bearing Races on a Turning Center


Machining of outer bearing races


Copyright © 2010 Pearson Education South Asia Pte Ltd

Machining Centers:

Reconfigurable Machines and Systems


There is a need for the flexibility of manufacturing
which involve
concept of
reconfigurable machines,
consisting of various modules


3 axis machining center can perform different
machining operations while accommodating various
workpiece sizes and part geometries


Copyright © 2010 Pearson Education South Asia Pte Ltd

Machining Centers:

Reconfigurable Machines and Systems


A five
-
axis machine can be reconfigured by assembling
different modules


Copyright © 2010 Pearson Education South Asia Pte Ltd

Machine
-
tool Structures:

Materials


A list of materials suitable for machine
-
tool structures:

1.
Gray cast iron

2.
Welded steel

3.
Ceramic

4.
Composites

5.
Granite

epoxy composites

6.
Polymer concrete


Copyright © 2010 Pearson Education South Asia Pte Ltd

Machine
-
tool Structures:

Machine
-
tool Design Considerations


Important considerations in machine tools:

1.
Design, materials, and construction

2.
Spindle materials and construction

3.
Thermal distortion of machine components

4.
Error compensation and the control of moving
components along slideways


Copyright © 2010 Pearson Education South Asia Pte Ltd

Machine
-
tool Structures:

Machine
-
tool Design Considerations

Stiffness


It is
a function of the:

1.
Elastic modulus of the materials used

2.
Geometry of the structural components


Enhanced by using diagonally arranged interior ribs


Thermal Distortion


2 sources of heat in machine tools:

1.
Internal sources

2.
External sources


Copyright © 2010 Pearson Education South Asia Pte Ltd

Machine
-
tool Structures:

Machine
-
tool Design Considerations

Assembly Techniques for Machine
-
tool Components


Traditionally components have been assembled using
threaded fasteners and welding


Advanced assembly techniques include integral casting
and resin bonding


Guideways


Plain cast
-
iron ways in machines require much care to
achieve the required precision and service life


Copyright © 2010 Pearson Education South Asia Pte Ltd

Machine
-
tool Structures:

Machine
-
tool Design Considerations

Linear Motor Drives


A
linear motor
is a typical rotary electric motor that has
been rolled out (opened) flat


Sliding surfaces in drives are separated by an air gap
and have very low friction


Some advantages:

1.
Simplicity and minimal maintenance

2.
Smooth operation, better positioning accuracy, and
repeatability

3.
Wide range of linear speeds

4.
Moving components encounter no wear


Copyright © 2010 Pearson Education South Asia Pte Ltd

Machine
-
tool Structures:

Hexapod Machines


Goals in the developments of design and materials:

1.
Machining flexibility to machine tools

2.
Increasing their
machining envelope

3.
Making them lighter


Hexapods
are
parallel kinematic linked machines


They are loaded axially,

bending stresses and

deflections are minimal,

resulting in stiff structure


Copyright © 2010 Pearson Education South Asia Pte Ltd

Vibration and Chatter in Machining Operations


Low stiffness can cause
vibration and chatter
of the
cutting tools and the machine components, causing
adverse effects on product quality


Chatter results in:

1.
Poor surface finish

2.
Loss of dimensional accuracy

3.
Premature wear, chipping, and failure of the cutting tool

4.
Damage to the machine
-
tool components

5.
Objectionable noise


Copyright © 2010 Pearson Education South Asia Pte Ltd

Vibration and Chatter in Machining Operations

Forced Vibration


Caused by some
periodic
applied force present in the
machine tool


The basic solution to forced vibration is to isolate or
remove the forcing element


Vibrations can be minimized by changing the
configuration of the machine
-
tool components


Due to driving forces that are close to the
center of
gravity


Copyright © 2010 Pearson Education South Asia Pte Ltd

Vibration and Chatter in Machining Operations

Self
-
excited Vibration


Caused by the interaction of the chip
-
removal process
with the structure of the machine tool, they have high
amplitude


Possible causes are:

1.
Type of chips produced

2.
Inhomogeneities in the workpiece material

3.
Variations in the frictional conditions at the tool

chip
interface


Regenerative chatter
is when a tool cutting a surface
that has a roughness or geometric disturbances
developed


Copyright © 2010 Pearson Education South Asia Pte Ltd

Vibration and Chatter in Machining Operations

Self
-
excited Vibration


Self
-
excited vibrations can be controlled by:

1.
Increasing the
stiffness
and
dynamic stiffness
of the
system

2.
Damping


Dynamic stiffness is defined as the ratio of the applied
-
force amplitude to the vibration amplitude


Operation will likely lead to chatter, beginning with
torsional vibration around the spindle axis and twisting
of the arm during turning


Copyright © 2010 Pearson Education South Asia Pte Ltd

Vibration and Chatter in Machining Operations

Factors Influencing Chatter


Tendency for chatter during machining is proportional to
the cutting forces and the depth and width of the cut


Cutting forces increase with strength and the tendency
to chatter increases as hardness increases


Continuous chips involve steady cutting forces and do
not cause chatter


Discontinuous chips and serrated chips cause chatter


Copyright © 2010 Pearson Education South Asia Pte Ltd

Vibration and Chatter in Machining Operations

Damping


Damping
is defined as the rate at which vibrations
decay


A major factor in controlling machine
-
tool vibration and
chatter


Internal damping
results from the energy loss in
materials during vibration


External damping
is accomplished with external
dampers that are similar to shock absorbers on
automobiles or machines


Copyright © 2010 Pearson Education South Asia Pte Ltd

Vibration and Chatter in Machining Operations

Damping


Copyright © 2010 Pearson Education South Asia Pte Ltd

Vibration and Chatter in Machining Operations

Guidelines for Reducing Vibration and Chatter


Basic guidelines:

1.
Minimize tool overhang

2.
Improve the stiffness of work
-
holding devices and
support workpieces

3.
Modify tool and cutter geometry to minimize forces or
make them uniform

4.
Change process parameters

5.
Increase stiffness of the machine tool and its
components

6.
Improve the damping capacity of the machine tool


Copyright © 2010 Pearson Education South Asia Pte Ltd

High
-
speed Machining


Spindle designs for high speeds require
high stiffness
and
accuracy


Due to
inertia effects
during the acceleration and
decelaration of machine
-
tool components, there is a
use of lightweight materials consideration


High
-
speed machining should take
cutting time
as a
cosideration


High
-
speed machining is economical for certain
specific applications


As cutting speed increases, more heat is generated,
while the tool and workpiece should remain close to
ambient temperature


Copyright © 2010 Pearson Education South Asia Pte Ltd

High
-
speed Machining


Machine
-
tool characteristics:

1.
Spindle design for stiffness, accuracy, and balance at
very high rotational speeds

2.
Bearing characteristics

3.
Inertia of the machine
-
tool components

4.
Fast feed drives

5.
Selection of appropriate cutting tools

6.
Processing parameters and their computer control

7.
Work
-
holding devices that can withstand high
centrifugal forces


Copyright © 2010 Pearson Education South Asia Pte Ltd

Hard Machining


As the hardness of the workpiece increases, its
machinability decreases, and tool wear and fracture,
surface finish, and surface integrity are problems


Hard machining
or
hard turning
produces machined
parts with good dimensional accuracy and surface
finish


Hard turning can compete successfully with the
grinding
proces


Copyright © 2010 Pearson Education South Asia Pte Ltd

Ultraprecision Machining


Modern
ultraprecision machine tools
with advanced
computer controls can have an accuracy approaching 1
nm


Ultraprecision machines are located in a dust
-
free
environment


Copyright © 2010 Pearson Education South Asia Pte Ltd

Ultraprecision Machining

General Considerations for Precision Machining


Important factors in precision and ultraprecision
machining and machine tools:

1.
Machine
-
tool design, construction, and assembly

2.
Motion control of various components

3.
Spindle technology

4.
Thermal growth of the machine tool

5.
Cutting
-
tool selection and application

6.
Machining parameters

7.
Real
-
time performance and control of the machine tool


Copyright © 2010 Pearson Education South Asia Pte Ltd

Machining Economics


Limitations of machining operations include

1.
Longer time required

2.
Need to reduce non
-
cutting time

3.
Wasted material


The costs involved are:

1.
Machine tools, work
-
holding devices, fixtures and
cutting tools

2.
Labor and overhead

3.
Setting up time

4.
Material handling and movement

5.
Dimensional accuracy and surface finish

6.
Cutting times and non
-
cutting time


Copyright © 2010 Pearson Education South Asia Pte Ltd

Machining Economics

Minimizing Machining Cost per Piece


Machining cost per piece
and
machining time per piece

can be minimized


It is important that input data is accurate and up to date


Total machining cost per piece is


Copyright © 2010 Pearson Education South Asia Pte Ltd

t
l
s
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Machining Economics

Minimizing Machining Cost per Piece


The
machining cost
is given



The
loading
,
unloading
, and
machine
-
handling cost
is



The
tooling cost
is



The time required to produce one part is


Copyright © 2010 Pearson Education South Asia Pte Ltd



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Machining Economics

Minimizing Machining Cost per Piece


For a turning operation; the machining time is




From the Taylor tool
-
life equation,



The number of pieces per insert face is



Number of pieces per insert is given by


Copyright © 2010 Pearson Education South Asia Pte Ltd

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Machining Economics

Minimizing Machining Cost per Piece


Combination of equations is given by



For min cost, we differentiate
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with respect to
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and
set it to zero,




The optimum cutting speed is


Copyright © 2010 Pearson Education South Asia Pte Ltd

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Machining Economics

Minimizing Machining Cost per Piece


The optimum tool life is





For max production, we differentiate
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with respect to
V
and set the result to zero,


Copyright © 2010 Pearson Education South Asia Pte Ltd

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Machining Economics

Minimizing Machining Cost per Piece


The
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is






The
optimum tool life
is


Copyright © 2010 Pearson Education South Asia Pte Ltd
















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