wear cutting ppt

8/4/2019 Wear Cutting Ppt

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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

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