1. Manufacturing Engineering Technology in SI Units, 6th Edition Chapter 25: Machining Centers, Machine Tool Structures and Machining Economics
Presentation slide for courses, classes, lectures et al.
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2. Chapter Outline Introduction Machining Centers Machine-tool Structures
Vibration and Chatter in Machining Operations
High-speed Machining
Hard Machining
Ultraprecision Machining
Machining Economics
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3. Introduction Computers improved the capabilities of machine tools
Have the capability of rapidly producing extremely complex part geometries
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4. Machining Centers Brief review:
Possibilities exist in net-shape or near-net shape production
Some form of machining is required and is more economical to finish machine parts to their final shapes
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5. 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
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6. Machining Centers
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7. 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)
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8. 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
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9. 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
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10. Major characteristics of machining centres:
Machining Centers: Characteristics and Capabilities of Machining Centers
Major characteristics of machining centres:
Handles a wide variety of part sizes and shapes efficiently
Versatile and quick changeover
Time required is reduced
Detection of tool breakage and wear
Inspection of machined work
Compact and highly automated
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11. Machining Centers: Selection of Machining Centers
Selection of type and size of machining centers depends on:
Type of products, their size, and their shape complexity
Type of machining operations to be performed and the type and number of cutting tools required
Dimensional accuracy required
Production rate required
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12. Machining Centers: Selection of Machining Centers
EXAMPLE 25.1
Machining Outer Bearing Races on a Turning Center
Machining of outer bearing races
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13. 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
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14. Machining Centers: Reconfigurable Machines and Systems
A five-axis machine can be reconfigured by assembling different modules
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15. Machine-tool Structures: Materials
A list of materials suitable for machine-tool structures:
Gray cast iron
Welded steel
Ceramic
Composites
Granite–epoxy composites
Polymer concrete
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16. Machine-tool Structures: Machine-tool Design Considerations
Important considerations in machine tools:
Design, materials, and construction
Spindle materials and construction
Thermal distortion of machine components
Error compensation and the control of moving components along slideways
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17. Machine-tool Structures: Machine-tool Design Considerations
Stiffness
It is a function of the:
Elastic modulus of the materials used
Geometry of the structural components
Enhanced by using diagonally arranged interior ribs
Thermal Distortion
2 sources of heat in machine tools:
Internal sources
External sources
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18. 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
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19. 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:
Simplicity and minimal maintenance
Smooth operation, better positioning accuracy, and repeatability
Wide range of linear speeds
Moving components encounter no wear
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20. Machine-tool Structures: Hexapod Machines
Goals in the developments of design and materials:
Machining flexibility to machine tools
Increasing their machining envelope
Making them lighter
Hexapods are parallel kinematic linked machines
They are loaded axially, bending stresses and deflections are minimal, resulting in stiff structure
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21. 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:
Poor surface finish
Loss of dimensional accuracy
Premature wear, chipping, and failure of the cutting tool
Damage to the machine-tool components
Objectionable noise
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22. 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
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23. 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:
Type of chips produced
Inhomogeneities in the workpiece material
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
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24. Vibration and Chatter in Machining Operations
Self-excited Vibration
Self-excited vibrations can be controlled by:
Increasing the stiffness and dynamic stiffness of the system
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
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25. 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
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26. 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
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27. Vibration and Chatter in Machining Operations
Damping
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28. Vibration and Chatter in Machining Operations
Guidelines for Reducing Vibration and Chatter
Basic guidelines:
Minimize tool overhang
Improve the stiffness of work-holding devices and support workpieces
Modify tool and cutter geometry to minimize forces or make them uniform
Change process parameters
Increase stiffness of the machine tool and its components
Improve the damping capacity of the machine tool
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29. 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
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30. High-speed Machining Machine-tool characteristics:
Spindle design for stiffness, accuracy, and balance at very high rotational speeds
Bearing characteristics
Inertia of the machine-tool components
Fast feed drives
Selection of appropriate cutting tools
Processing parameters and their computer control
Work-holding devices that can withstand high centrifugal forces
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31. 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
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32. 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
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33. Ultraprecision Machining
General Considerations for Precision Machining
Important factors in precision and ultraprecision machining and machine tools:
Machine-tool design, construction, and assembly
Motion control of various components
Spindle technology
Thermal growth of the machine tool
Cutting-tool selection and application
Machining parameters
Real-time performance and control of the machine tool
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34. Machining Economics Limitations of machining operations include
Longer time required
Need to reduce non-cutting time
Wasted material
The costs involved are:
Machine tools, work-holding devices, fixtures and cutting tools
Labor and overhead
Setting up time
Material handling and movement
Dimensional accuracy and surface finish
Cutting times and non-cutting time
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35. 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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36. 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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37. 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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38. 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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39. Machining Economics Minimizing Machining Cost per Piece
The optimum tool life is
For max production, we differentiate Tp with respect to V and set the result to zero,
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40. Machining Economics Minimizing Machining Cost per Piece
The optimum cutting speed is
The optimum tool life is
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