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ME 350 Lecture 5 Chapters 23 & 24
Ch 23 - CUTTING TOOL TECHNOLOGY
Ch 24 - ECONOMIC AND PRODUCT
DESIGN CONSIDERATIONS IN
MACHINING
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Three Modes of Tool Failure
1. Cutting force is excessive and/or dynamic,
leading to brittle fracture:
1. Cutting temperature is too high for the tool
material:
1. Preferred wearing of the cutting tool:
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Preferred Mode:
Longest possible tool life, wear locations:
Crater wear location:
Flank wear location:
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Tool wear as a function of cutting time. Flank wear (FW) is
used here as the measure of tool wear. Crater wear follows a
similar growth curve.
Tool Wear vs. Time
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Effect of cutting speed on tool flank wear (FW) for three cuttingspeeds, using a tool life criterion of 0.50 mm flank wear.
Effect of Cutting Speed
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Log log plot of cutting speed vs tool life.
Tool Life vs. Cutting Speed
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Taylor Tool Life Equation
CvTn=
where v= cutting speed;
T= tool life; and
n and Care parameters that depend on feed,
depth of cut, work material, and tooling material, butmostly on material (work and tool).
n is the
Cis the on the speed axis at one minute tool life
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Tool Near End of Life
Changes in sound emitted from operation
Chips become ribbon-like, stringy, and difficult to
dispose of
Degradation of surface finish
Increased power required to cut
Visual inspection of the cutting edge with magnifying
optics can determine if tool should be replaced
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Desired Tool Properties
Toughness to avoid fracture failure
Hot hardness ability to retain hardness at
high temperatures
Wear resistance hardness is the most
important property to resist abrasive wear
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Plain carbon steel shows a rapid loss of hardness as temperatureincreases. High speed steel is substantially better, while cemented
carbides and ceramics are significantly harder at elevatedtemperatures.
Hot Hardness
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Coated Carbide Tool
Photomicrograph
of cross section of
multiple coatings
on cemented
carbide tool (photo
courtesy of
Kennametal Inc.)
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Typical Values ofn and C
Tool material n C (m/min) C (ft/min)
High speed steel:
Non-steel work 0.125 120 350
Steel work 0.125 70 200
Cemented carbide
Non-steel work 0.25 900 2700
Steel work 0.25 500 1500
Ceramic
Steel work 0.6 3000 10,000
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Tool Geometry
Two categories:
Single point tools
Used for turning, boring, shaping, and planing
Multiple cutting edge tools
Used for drilling, reaming, tapping, milling,
broaching, and sawing
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(a) Seven elements
of single point tool
geometry; and (b)
the tool signature
convention that
defines the sevenelements.
Single-Point Tool Geometry
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Three ways of holding and presenting the cutting edge for
a single point tool: (a) solid tool (typically HSS); (b)
brazed cemented carbide insert, and (c) mechanically
clamped insert, used for cemented carbides, ceramics,and other very hard tool materials.
Holding the Tool
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Common insert shapes: (a) round, (b) square, (c) rhombus with
two 80 point angles, (d) hexagon with three 80 point angles, (e)
triangle (equilateral), (f) rhombus with two 55 point angles, (g)
rhombus with two 35 point angles.
Common Insert Shapes
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The most common cutting tool for hole making Usually made of high speed steel
Standard geometry of a twist drill.
Twist Drills
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Twist Drill Issues
Along radius of cutting edges cutting speed:
Relative velocity at drill point is , (no cutting takes
place) a large thrust force must deform the material
Problems:
Flutes must provide sufficient clearance to allow chips
to be extracted:
Rubbing between outside diameter of drill bit and
hole. Delivery of cutting fluid to drill point is difficult
because chips are flowing in opposite direction:
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Cutting Fluids (Lubricants and Coolants)
Function is to improve cutting performance:
1. Improvechip
2. Reduce
3. Improve surface
Types of cutting fluids:
1. Generally water based:
more effective at cutting speeds that are:
1. Generally oil based:
more effective at cutting speeds that are:
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Machinability Criteria in Production
Tool life longer tool life for the given workmaterial means better machinability
Forces and power lower forces and power
mean better machinability
Surface finish better finish means better
machinability Ease of chip disposal easier chip disposal
means better machinability
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Tolerances and Surface Finish
Tolerances Machining provides high accuracy relative to most
other shape-making processes
Closer tolerances usually mean higher costs
Surface roughness in machining determined by:
1. Geometric factors of the operation
2. Work material factors
3. Vibration and machine tool factors
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Effect of Cutting Conditions:
End CuttingNose Radius Feed Edge Angle
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Ideal Surface Roughness
where
Ri = theoretical arithmetic average
surface roughness;
f= feed;
NR= nose radius
NRfR
i
=
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Work Material Factors
Built up edge effects
Damage to surface caused by chip
Tearing of surface when machining ductile
materials
Cracks in surface when machining brittlematerials
Friction between tool flank and new work
surface
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Effect of Work Material Factors
To predict actual
surface roughness,
first compute ideal
surface roughness,
then multiply by the
ratio from the graph
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Vibration and Machine Tool Factors
Related to machine tool, tooling, and setup:
Chatter (vibration) in machine tool or cutting tool
Deflections of fixtures
Backlash in feed mechanism
If chatter can be eliminated, then surface
roughness is determined by geometric and
work material factors
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How To Avoid Chatter
Add stiffness and/or damping to setup
Operate at speeds that avoid frequencies
close to natural frequencyof machine tool
system
Reduce feed(and sometimes depth)
Change cutter design
Use a cutting fluid
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Determining Feed
Select feed first, speed second Determining feed rate depends on:
Tooling harder tool materials require lower feeds
Is the operations roughing or finishing?
Constraints on feed in roughing
Limits imposed by forces, setup rigidity, and sometimes
horsepower
Surface finish requirements in finishing
Select feed to produce desired finish
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Optimizing Cutting Speed
Select speed to achieve a balance between high
metal removal rate and suitably long tool life
Mathematical formulas available to determineoptimal speed
Two alternative objectives in these formulas:
1. Maximum
2. Minimum
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Maximum Production Rate
Maximizing production rate = minimizing cuttingtime per unit
In turning, total production cycle time for one
part consists of:
1. Part handling time per part = Th
2. Machining time per part = Tm
3. Tool change time per part = Tt/np, where np =
number of pieces cut in one tool life (round down)
Total time per unit product for operation:
Tc
= Th
+ Tm
+ Tt
/np
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Cycle Time vs. Cutting Speed
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Minimizing Cost per Unit
Inturning, total production cycle cost for onepart consists of:
1. Cost of part handling time = CoTh , where Co =
cost rate for operator and machine
2. Cost of machining time = CoTm
3. Cost of tool change time = CoTt/np
4. Tooling cost = Ct/np , where Ct= cost per cutting
edge
Total cost per unit product for operation:
Cc = CoTh + CoTm + CoTt/np + Ct/np
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Unit Cost vs. Cutting Speed
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Comments on Machining Economics
As Cand n increase in Taylor tool life equation,
optimum cutting speed
Cemented carbides and ceramic tools, compared to
HSS, should be used at speeds:
vmax is always greater than vmin
Reason: Ct/np term in unit cost equation pushes
optimum speed to left in the plot
As tool change time Tt and/or tooling cost Ct
increase, cutting speed should be reduced
Disposable inserts have an advantage over
regrindable tools if tool change time is significant
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Product Design Guidelines
Design parts that need no machining
Use netshape processes such as precision
casting, closed die forging, or plastic molding
If not possible, then minimize amount of
machining required
Use near net shape processes such as
impression die forging
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Product Design Guidelines
Machined features such as sharp corners,
edges, and points should be avoided
They are difficult to machine
Sharp internal corners require pointed cutting
tools that tend to break during machining
Sharp corners and edges tend to create burrs andare dangerous to handle