1. ME Course 3370 Lecture 10 Material Removal or Machining
Lecture No 10
ME Course 3370 Lecture 10 Material Removal or Machining
Dr. Ramon E. Goforth
Adjunct Professor of Mechanical Engineering
Southern Methodist University
This lecture cover the removal of metal by various machining processes. Material removal of some type is possible for all materials, but machining is typically only possible on metals and plastics. Ceramics cannot be machined by single point cutting tools, but can be abrasively shaped.

2. Overview slide for Shaping and Forming Metals
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Overview slide for Shaping and Forming Metals
Assembly
SLS
Powders
Special
Pressing
Firing/
Sintering
Injection
Molding
Stamping
Sheet metal
forming
Raw Material
Continuous
Casting/Rolling
Rolling
Forging/
Press forming
Finishing
Ingot
casting
Molten
Material
Extruding
Casting
Shapes
Machining
Single crystal
pulling
Blow
molding
Increasing level of detail

3. Outline of Lecture Basic information on material removal
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Outline of Lecture
Basic information on material removal
Factors involved in material removal
Independent variables
Dependent variables
Machining Processes
Machining Economics
Machines
Lecture 10
Lecture 11
Lecture 12

4. Categories of Material Removal/Machining Processes
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Categories of Material Removal/Machining Processes
Material removal processes can be classified into three categories as shown. All of the processes chown can be automated by machines to vary degrees and all depend on knowledge of materials science and mechanics

5. Why is Material Removal Important
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Why is Material Removal Important
Significant proportion of all goods involve machined surfaces
$ billion industry
The only way to achieve high precision
The only way to create sharp corners, flat surfaces and internal and external profiles
Material removal and specifically machining is a very large segment of the economy. This slide shows why it is important

6. Why is Material Removal Important
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Why is Material Removal Important
The only way to shape hardened or brittle material
Economics (for small part volumes (e.g. prototypes)
Can achieve special surface finishes
Indispensable for creating complex shapes with good dimensional accuracy and surface finish

7. Limitations of Material Removal
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Limitations of Material Removal
Generate lots of scrap (high buy to fly ratio)
Takes longer to remove material than to form it
Can mess up the properties and surface finish if not done properly
It does have some limitations, the high buy to fly ratio being the most significant.

8. Useful Web Sites on Machining
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Useful Web Sites on Machining
The Machine Tool Agile Manufacturing Research Institute
Software tools for
Fixture design
Machining force analysis
Part distortion
Institute for Advanced Manufacturing Sciences
Software for
Feed and speed selection
Process planning
University of Minnesota
Active models of cutting
These are some links to interesting web pages. The MTAMRI allows you to simulate cutting and look at forces, stresses and deflections on a milled piece.
IAMS is a source for software which computes feeds speeds and does process planning. The software Costs. The last had some nice simulations of how chips size varies with changes in the cutting parameters but does not seem to be available anymore. It does provide a detailed discussion of the details of the cutting model used in this lecture.

9. Stationary Part- Milling, Drilling, Sawing, Etc
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Stationary Part- Milling, Drilling, Sawing, Etc

10. Rotating Part - Turning
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Rotating Part - Turning

11. Cutting process the same in both
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Basic Processes
Cutting process the same in both
Milling, drilling, etc
This shows the various components of the cut. The tool shears off a thin layer called the chip. The various geometric relationships among tool angles, chip angles, etc are shown.
Turning
Kalpakjian p 595/535

12. Material Removal as a System
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Material Removal as a System
Drawings, CAD model, Tolerances, Surface finish, Machine code
Cutting fluid
Workpiece
Tool
Raw
Material
Finished part
Machine Tool
It is very important to think of machining as a system with various components, all of which interact.
The tool is supported by the machine so the machine dynamics, for example can cause the tool to move on the workpiece.
The forces generated by the tool/workpiece interaction can cause deformation and vibrations in the machine which subsequently leads to error in the tool position.
And so one
Details are provided later.
Cutting tool
Cutting fluid
Operator

13. Factors Affecting Machining/Cutting Processes
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Factors Affecting Machining/Cutting Processes
Workpiece
Material, condition, temperature, (Machinability)
Temperature rise
This slide the next provide some more details of the factors involved in the cutting process.

14. Factors Affecting Machining/Cutting Processes
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Factors Affecting Machining/Cutting Processes
Tool (The cutting edge)
Material, condition/sharpness, coatings, shape, surface finish
Cutting parameters (How much material removed)
feed, speed, depth of cut
Tool angles
Type of chip created
Tool wear
Temperature rise

15. Factors Affecting Machining/Cutting Processes
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Factors Affecting Machining/Cutting Processes
Presence or absence of cutting fluid (How process is cooled and lubricated)
Machine tool parameters (To achieve tolerances)
Machine design
Force and power availability
Stiffness, damping, backlash)
Fixture design (How the workpiece is held while shaped)
Also dependent on other variables
Some are independent and some are dependent variables
This slide continues with more detail on thesse two

16. Independent variables in cutting
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Independent variables in cutting
Workpiece material - "machinability"
Cutting tools
Cutting parameters
Presence or absence of fluid
Characteristics of the machine tool
Fixture design

17. Dependent Variables Material removal rate
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Dependent Variables
Material removal rate
Surface finish of the workpiece
Force and energy dissipated
Type of chip produced
Temperature rise in workpiece, tool and the chip
Wear and failure of the tool

18. Independent variables in cutting
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Independent variables in cutting
Workpiece material - "machinability"
All these factors can be classified into two groups: independent and dependent variables.
The independent variables are those you, the operator control.
The dependent variable are dependent on the independent variables in complex ways which are discussed later.
Note that cutting tool selection is an independent variable, while tool life is dependent. You choose the tool that best suites the conditions of workpiece material, machine, coolant, and its life depends on the specific operating conditions.

19. Lecture No 10
Machinability
Machinability depends on the strength, toughness and hardness of the workpiece material
Machinability can be improved by the addition of certain elements
Lead and sulfur added to steels gives free machining steels
The first independeent variable is the machinability of the workpiece. It is independent in the sense that is is chosen by the operator. However, a dsienger may specific a material with poor machinability. Then, the other independent variable become important.
While theoretically it is possible to derived fundamental relationships among the independent and dependent variables, there exists an empirical value called machinability. Machinability depends mainly on the material properties.
Link is to the ASM Materials data base which includes an Machinability index

20. Machinability Good Machinability indicates
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Machinability
Good Machinability indicates
Good surface finish and part integrity
no tearing
Long tool life
Low power and force requirements
Good chips
No long thin chips

21. Machinability of Materials
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Machinability of Materials
Steels
Leaded Easy (lead acts as lubricant)
Sulfurized Relatively easy
Rephosphorized Relatively easy
Calcium de-oxidized Relatively easy
Stainless Steels
Austenitic General difficult:
Ferritic SS Easy
Martensic Abrasive
Aluminum Easy to machine but softer alloys give poor surface finish
Note how different properties of material can cause poor machinability. For example titanium has poor machinability because its low thermal conductivity does not allow the heat generated to dissipate causing excessive temperatures at the tool workpiece interface. Vry ductile materials like copper have low machinability contrary to normal expectations. The abrasiveness of the workpiece also prays a role.

22. Machinability of Materials
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Machinability of Materials
Magnesium Easy: danger from fire
Grey Cast Iron Machinable but abrasive
Wrought Copper Difficult to machine because of ductility
Brass Easy to machine
Cobalt based Alloys Difficult and abrasive: required low feeds and speeds
Nickel-based Alloys Difficult and abrasive
Titanium Difficult because of poor thermal conductivity

23. Independent variables in cutting
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Independent variables in cutting
Workpiece material - "machinability"
Cutting tools
All these factors can be classified into two groups: independent and dependent variables.
The independent variables are those you, the operator control.
The dependent variable are dependent on the independent variables in complex ways which are discussed later.
Note that cutting tool selection is an independent variable, while tool life is dependent. You choose the tool that best suites the conditions of workpiece material, machine, coolant, and its life depends on the specific operating conditions.

24. Tool Selection and Design
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Tool Selection and Design
Tool selection is a very complex process involving many parameters:
Workpiece Machinability
Type of cut - continuous, interrupted
Tool material type
Process parameters
Feed
Speed
Depth of cut
Shape
Cost
Tool life critical to economics (A dependent variable, see later)
The second independent variable is the tool selection.
The selection of the proper cutting tool is a critical decision in any machining and many parameters. The cutting tool is the actual device that shears off the metal. There are a wide variety of materials which can be used and an even wider variety of shapes. Choosing the correct one is difficult but there are new data bases which make it easier.
The catalogues are not specifically recommended by Dr Mills. They are simply examples of the kind of on-line help that is available for selecting tools

25. Lecture No 10
Tool Selection Guides
See page 637/571 for general properties of tool materials
See page 644/578 of Kalpakjian for guide to selecting Tungsten Carbide tools
A couple of on-line catalogues

26. Tool Design Parameters
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Tool Design Parameters
Material Properties
Hardness
Toughness to resist impact forces
Wear Resistance
Chemical Stability
Coating material
These are some of the properties of the tool material which have to be considered when choose a cutting tool.

27. Properties of various cutting tool materials
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Properties of various cutting tool materials
This figure shows how hot hardness and strength and toughness ( which are interrelated in a complex way) of various material vary. Although diamond and cubic boron nitride have low strength and toughness ( they are brittle materials) they make good tools because of the hot hardness and wear resistance as long as there are no shocks in the process ( I.e no interruptions in the cut.
Kalpakjian p 660/582

28. Hardness Vs Temperature for Cutting Tool Material
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Hardness Vs Temperature for Cutting Tool Material
Since hardness of the tool material is an important parameter and high temperatures are generated the the tool/workpiece interface the variation of hardness with temperature is an important consideration. Temperatures at this interface increase with cutting speed, so materials such as ceramics allow higher cutting speeds, which improve the productivity and cost pictures. They are not universally applicable, however. For instance, interrupted cuts, where the tool is sometimes cutting the workpiece and sometimes not, usually have impact shocks associated with them. The brittle nature of ceramics leads to fracture caused by these shocks.

29. Impact of tool material on cutting time
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Impact of tool material on cutting time
This shows how cutting speed has increased (I.e. cutting time decrease) with time as newer material have come on the market. Cutting speeds over 100 times faster than possible 90 years ago are now quite common place. Ever higher cutting speeds are being achieved with "high speed machining"
Kalpakjian Page 646/579

30. Tool Design Parameters
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Tool Design Parameters
Shape
Edge strength
Circles are stronger than triangles
Edge design
Sharp vs rounded

31. Tool Design Parameters
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Tool Design Parameters
Shape
Influence on workpiece surface finish

32. Cutting Tool Material Costs
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Cutting Tool Material Costs
Material Cost ($)
Carbon and medium alloy steels
High-speed steels 1-7
Cast-cobalt alloys
Carbides 2-5
Coated tools
Alumina Based Ceramics 5-8
Cubic Boron Nitride 60-80
Silicon Nitride-base ceramics
Diamond
Whisker-reinforced materials
See page 637/571 for properties
The cost of tool materials varies over two orders of magnitude. The higher costs can often be justified, however by higher metal removal rates and longer life both of which lead to higher productivity and lower costs..

33. Tool Designs Solid vs inserts Lecture No 10
There are basically two designs: the solid tool typically ground out a single piece of high speed steel and the insert which is clamped on to a holder. The insert holder and the single piece cutting tool are both clamped onto a tool holder on the machine. The designs shown are for turning. The principle applies to other forms of machining.
Indexable cutters would appear to be dominant nowadays.

34. Independent variables in cutting
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Independent variables in cutting
Workpiece material - "machinability"
Cutting tools
Cutting parameters
All these factors can be classified into two groups: independent and dependent variables.
The independent variables are those you, the operator control.
The dependent variable are dependent on the independent variables in complex ways which are discussed later.
Note that cutting tool selection is an independent variable, while tool life is dependent. You choose the tool that best suites the conditions of workpiece material, machine, coolant, and its life depends on the specific operating conditions.

35. Cutting Process Parameters
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Cutting Process Parameters
Depend on the process (turning, milling, drilling, etc.)
Determines tool life for a specific tool material and design
Will discuss later when discussing individual processes
The cutting process parameters are the settings on the machine tool which determine how much and how fast material is sheared off. They are collectively referred to as feeds and speeds and, since they are determined mostly by the geometry of the cutting tool, we discuss them when we talk about specific processes.

36. Summary Machining is The independent variables include
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Summary
Machining is
a cost effective way of making low volume parts
The only way of making certain shapes
Involves shear fracture
uses the same basic cutting process for all processes
The independent variables include
material machinability, cutting tools, cutting parameters, presence or absence of fluid, the machine tool, fixture design
The dependent variables include
Material removal rate, surface finish of the workpiece, cutting force, energy dissipated, type of chip produced, temperature rise in workpiece, tool and the chip, wear and failure of the tool