Lecture No 10 1

ME Course 3370Lecture 10

Material Removal or Machining

Dr. Ramon E. GoforthAdjunct Professor of Mechanical

EngineeringSouthern Methodist University

Lecture No 10 2Overview slide for Shaping and Forming Metals

Ass

embl

y

Ingotcasting

MoltenMaterial

Powders

CastingShapes

RollingForging/

Press forming

Stamping

Pressing

Sheet metalforming

ContinuousCasting/Rolling

InjectionMolding

Mac

hini

ng

Fin

ishi

ng

Raw

Mat

eria

l

Special

Extruding

Single crystalpulling

Firing/Sintering

SLS

Increasing level of detail

Blowmolding

Lecture No 10 3

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

Lecture No 10 4Categories of Material Removal/Machining Processes

Lecture No 10 5Why is Material Removal Important

• Significant proportion of all goods involve machined surfaces

• $70 - 100 billion industry• The only way to achieve high precision• The only way to create sharp corners, flat

surfaces and internal and external profiles

Lecture No 10 6Why 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

Lecture No 10 7

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

Lecture No 10 8

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

Lecture No 10 9Stationary Part- Milling, Drilling, Sawing, Etc

Lecture No 10 10

Rotating Part - Turning

Lecture No 10 11

Basic Processes

Kalpakjian p 595/535

Milling, drilling, etc

Turning

• Cutting process the same in both

Lecture No 10 12

Material Removal as a System

Raw Material

Finished part

Drawings, CAD model, Tolerances, Surface finish, Machine code

Cutting fluid Operator

Machine Tool

Workpiece Tool

Cutting tool

Cutting fluid

Lecture No 10 13Factors Affecting Machining/Cutting Processes

• Workpiece – Material, condition, temperature, (Machina

bility)– Temperature rise

Lecture No 10 14Factors Affecting Machining/Cutting Processes

• Tool (The cutting edge)– Material, condition

/sharpness, coatings, shape, surface finish– Cutting parameters (How much material remov

ed)• feed, speed, depth of cut

– Tool angles– Type of chip created– Tool wear– Temperature rise

Lecture No 10 15Factors 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

Lecture No 10 16

Independent variables in cutting

• Workpiece material - "machinability"• Cutting tools• Cutting parameters• Presence or absence of fluid• Characteristics of the machine tool• Fixture design

Lecture No 10 17

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

Lecture No 10 18

Independent variables in cutting• Workpiece material - "machinability"

Lecture No 10 19

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

Link is to the ASM Materials data base which includes an Machinability index

Lecture No 10 20

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

Lecture No 10 21

Machinability of MaterialsSteels Leaded Easy (lead acts as lubricant) Sulfurized Relatively easy Rephosphorized Relatively easy Calcium de-oxidized Relatively easyStainless Steels Austenitic General difficult: Ferritic SS Easy Martensic AbrasiveAluminum Easy to machine but softer

alloys give poor surface finish

Lecture No 10 22

Machinability of Materials

Magnesium Easy: danger from fireGrey Cast Iron Machinable but abrasiveWrought Copper Difficult to machine because

of ductilityBrass Easy to machineCobalt based Alloys Difficult and abrasive:

required low feeds and speeds

Nickel-based Alloys Difficult and abrasiveTitanium Difficult because of poor

thermal conductivity

Lecture No 10 23

Independent variables in cutting

• Workpiece material - "machinability"• Cutting tools

Lecture No 10 24

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)

Lecture No 10 25

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– http://www.indexable.com/product.html– http://www.sct-usa.com/index.html

Lecture No 10 26

Tool Design Parameters

• Material Properties– Hardness– Toughness to resist impact forces– Wear Resistance – Chemical Stability– Coating material

Lecture No 10 27Properties of various cutting tool materials

Kalpakjian p 660/582

Lecture No 10 28Hardness Vs Temperature for Cutting Tool Material

Lecture No 10 29Impact of tool material on cutting time

Kalpakjian Page 646/579

Lecture No 10 30

Tool Design Parameters• Shape

– Edge strength• Circles are stronger than triangles

– Edge design• Sharp vs rounded

Lecture No 10 31

Tool Design Parameters• Shape

– Influence on workpiece surface finish

Lecture No 10 32

Cutting Tool Material CostsMaterial 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 75-100• Whisker-reinforced materials

– See page 637/571 for properties

Lecture No 10 33

Tool Designs • Solid vs inserts

Lecture No 10 34

Independent variables in cutting

• Workpiece material - "machinability"• Cutting tools• Cutting parameters

Lecture No 10 35

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

Lecture No 10 36

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