Manufacturing Engineering Technology in SI Units, 6 th Edition Chapter 25: Machining Centers,...

Manufacturing Engineering Technology in SI Units, Manufacturing Engineering Technology in SI Units,

66thth Edition Edition Chapter 25: Chapter 25:

Machining Centers, Machine Tool Machining Centers, Machine Tool Structures and Machining Economics 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


Introduction

Computers improved the capabilities of machine tools Have the capability of rapidly producing extremely

complex part geometries


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


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


Machining Centers


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)


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


Machining Centers:Types of Machining CentersVertical-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 anumber of their surfaces


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


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


Machining Centers:Selection of Machining Centers

EXAMPLE 25.1

Machining Outer Bearing Races on a Turning Center Machining of outer bearing races


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


Machining Centers:Reconfigurable Machines and Systems A five-axis machine can be reconfigured by assembling

different modules


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


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



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



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



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


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


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



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



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 produced2. 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



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



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



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



Damping



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


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


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


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


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


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


Machining Economics

Limitations of machining operations include 1. Longer time required2. Need to reduce non-cutting time3. Wasted material The costs involved are:1. Machine tools, work-holding devices, fixtures and

cutting tools2. Labor and overhead3. Setting up time 4. Material handling and movement5. Dimensional accuracy and surface finish6. Cutting times and non-cutting time


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


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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


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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


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Machining Economics

Minimizing Machining Cost per Piece Combination of equations is given by

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

The optimum cutting speed is


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Machining Economics

Minimizing Machining Cost per Piece The optimum tool life is

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Machining Economics

Minimizing Machining Cost per Piece The optimum cutting speed is

The optimum tool life is


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Manufacturing Engineering Technology in SI Units, 6 th Edition Chapter 25: Machining Centers,...

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