Ch6-CuttingTools.ppt

8/10/2019 Ch6-CuttingTools.ppt

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ME 482 - Manufactur ing Systems

Cutting Tool

Technology

by

Ed Red


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ObjectivesIntroduce tool terminology

Review reasons for tool wear, including failure modes

Introduce cutting parameters and Taylors tool life equation

Consider a tool life example

Review tool materials

Review modern tool technologies (papers)


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Cutting tool related terms Single point versus multiple point

Crateringwear that forms a concave region on the tool

Tool lifelength of cutting time that a tool can be used

Toughnesscapacity to absorb energy without failing

Hot hardnesscapacity to retain hardness at high temperatures

Cermetcombination of TiC, TiN, TiCN (CN = carbonitride), with

nickel and/or molybdenum as binders

Chip breakergeometry designed into tool to break stringy chips

Cutting fluidAny liquid/gas applied to improve cutting performance


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Tool wearAbrasion - dominant cause of flank wear

Adhesionhigh pressure localized fusion

and rupturing

DiffusionLoss of hardening atoms at

tool-chip boundary (contributes to

crater wear)

Plastic deformationcontributes to

flank wear

Three pronounced wearing regions


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Tool lifeTool lifelength of cutting time that a tool can be used or acertain flank wear value has occurred (0.02)

Taylors tool life equation:

v Tn= C (exponential again!)

v = cutting speed

n = cutting exponentC = cutting constant

Note:n and C depend on speed, work material, tool material, etc. C has

units of fpm and is the speed at which the tool life lasts 1 min, i.e.,

v T

n

= C (1)

n

= C .

For turning at feed = 0.01/rev. and depth = 0.100


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Operators tool lifeTool life is measured by:

Visual inspection of tool edge

Tool breaks

Fingernail test

Changes in cutting sounds

Chips become ribbony, stringy

Surface finish degrades

Computer interface says

- power consumption up

- cumulative cutting time reaches certain level

- cumulative number of pieces cut reaches certain value


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Tool life exampleThe n and C values in Table 23.2 in the text are based on a feed rate of 0.01

in./rev and a depth of cut of 0.10 in. Determine and compare the cubic inches

of steel removed for each of the following tool materials if a 15 minute tool

life is required in each case: a) HSS b) ceramic

Solution:

Approach is to determine the MRR = v f d. We are given the feed rate and the

depth of cut; thus, need to calculate the cutting speed.

Given Taylors equation and the n and C values, we can solve for v and thus

determine the MRR.

Given the MRR, we multiply it by the cutting time to get the volume of

material removed.



Tool life exampleSolution for HSS:

From Table 23.2, n = 0.125 and C = 200 ft/min (for steel cutting)

From v Tn= C we solve for v:

v = 200/(15)0.125= 142.6 ft/min

Now, get the MRR:

MRR = (142.6) (12) (0.01) (0.10) = 1.71 in3/min

Volume removed in 15 min is (15) (1.71) = 25.66 in3



Tool life exampleSolution for ceramic:

From Table 23.2: n = 0.6 and C = 10,000 ft/min

From v Tn

= C we solve for v:

v = 10,000/(15)0.6= 1969.5 ft/min

Now, get the MRR:

MRR = (1969.5) (12) (0.01) (0.10) = 23.63 in3/min

Volume removed in 15 min is (15) (23.63) =354.5 in3

Ceramic about an order-of-magnitude more effective than HSS!



Cutting tool materials

Plain carbon and low alloy steelsrarely used today

High-speed steel (HSS) primary alloys are tungsten

(AISI T grade > 12%) or molybdenum (M grade, 5

8%).... sometimes coated with TiN to improve

performance, toughness good

Cast cobalt alloys cobalt (50%), chromium (30%), and

tungsten (20%), improved wear resistance, but toughness

< HSS+ Hardness




Cemented carbides, cermets, and coated carbides related

materials that are a composite of ceramic and metallic materials.

Cemented carbides use tungsten carbides.cermets use titaniumcarbides. Coated carbides use coatings of TiC or Al2O3 to improve

wear properties. Higher WC contents in cemented carbides

detrimental to steel cutting (affinity of steel with carbon in WC),

but ok for other metals. Alloying with TiC and TaC reduces this

problem.

CeramicsPrimarily Al2O3... not good in dynamic (higher speeds,

shock) cutting situations.

+ Hardness




Synthetic polycrystalline diamond (SPD) and cubic boron nitride

(CBN)typically used as coating on base tool material such as WC-

Cothere is an affinity of SPD with iron and nickel; CBN does nothave this affinityexpensive.

+ Hardness



Cutting tool materialsHSS alloying

Element Properties

Tungsten Increases hot hardness

Hard carbides formed, improving abrasion resistance

Molybdenum Increases hot hardness


Chromium Depth hardenability during heat treat


Some corrosion resistance

Vanadium Combines with carbon for wear resistance

Retards grain growth for better toughness

Cobalt Increases hot hardness, toughness

Carbon Hardening element

Forms carbides



Cutting tool materialsMost modern cutting tool materials are a matrix of materials designed to be very

hard. Important terms are toughness, hardness and hot hardness. Note that the

rake angle is chosen small (near 0 deg) for the harder, more brittle tool materials

to keep the tool in compression.

U (Specific Energy) is a measure of toughness, while the table shows typical

hardness values at room temperature for cutting tool materials. The figure shows

how hardness degrades with increasing temperature.



Cemented carbide classification

C1 Roughing C5

C2 General purpose C6

C3 Finishing C7

C4 Precision finishing C8

Non-steel cutting Steel cutting

Al, Cu, Brass, Ti

Cast iron

Abrasive wear resistance

Crater wear resistance

Wearresistance

Toughness

%TiCforsteelcuttingg

rades

%Cobalt

No TiC, TaC Contain TiC, TaC



Cutting tool geometry

7 elements of a single

point tool geometry



Milling tool geometry

Face cutter

Chamfering cutter



Cutting tool geometry

Mini-cutters

Coated tools



Cutting

fluids

Lubricantspurpose is to reduce frictionusually oil based

Coolantspurpose is to transport heatusually water based

Both lose their effectiveness at higher cutting speeds!


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High speed machining characteristicsQuestiondoes Taylors equation even apply for HSM?

> 500 linearin/min

spindle speeds > 10,000 rpm

surfacecutter speeds > 1200 ft/min

spindles in the 50 hp range

head tilt speeds > 1000 deg/min

balanced tool holders

problems with tool deflection

must operate within machine harmonics


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High speed machining at Remmele

Remmele's High Speed and High Velocity

technology provide distinct advantages in increasing

product performance.Weight Reduction (thin walls to 0.010"/0.25mm)

Time Savings

Reduced Distortions and Warping

High Speed Machining is high volume metal

removal within a range of high surface-cutting

speeds (feet per minute) and feeds (in/min).

High Velocity Machining exhibits significant reduction in

machining forces and power absorption, and dramatically shifts

the heat energy distribution from the cutter/workpiece to the

chip.


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Cutting tools review questions:

Of the foll owing cutting conditions, which one has the greatest

effect on tool wear? (a) cutting speed, (b) depth of cut, or

(c) feed.

As an al loying ingredient in high-speed steel, tungsten serves

which of the following functions (more than one answer)?

(a) forms hard carbides to resist abrasion, (b) improves

strength and hardness, (c) increases corrosion resistance, and

(d) increases hot hardness.

Cast cobalt al loys typically contain which of the foll owing main

ingredients (more than one answer)? (a) aluminum, (b) cobalt,

(c) chromium, (d) nickel, and (e) tungsten.

Answer: (a)

Answer: (a), (b), (d)

Answer: (b), (c), (e)


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Cutting tools review questions: Which of the following is not a common ingredient in cemented

carbide cutting tools (more than one answer)?

(a) Al203, (b) Co, (c) CrC, (d) TiC, and (e) WC.

An increase in cobalt content has which of the fol lowing effects

on WC-Co cemented carbides (one best answer)?

(a) decreases transverse rupture strength, (b) increases

hardness, (c) increases toughness.

Steel cutting grades of cemented carbide are typicall y not

character ized by which of the foll owing ingredients (more than

one answer)? (a) Co, (b) Ni, (c) TiC, (d) TaC, and (e) WC.

Answer: (a), (c)

Answer: (c)

Answer: (b)


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Cutting tools review questions:

Which of the foll owing processes are used to provide the thin

coatings on the surface of coated carbide inser ts (more than

one answer)? (a) chemical vapor deposition, (b) electroplating,

(c) physical vapor deposition, or (d) pressing and sinter ing.

Which of the following mater ials has the highest hardness?

(a) aluminum oxide, (b) cubic boron nitr ide, (c) high speed

steel, (d) ti tanium carbide, or (e) tungsten carbide.

I f you had to select a cemented carbide for an application

involving the f inish turning of steel, which of the following

grades would you select (one best answer)? (a) C1, (b) C3,

(c) C5, or (d) C7.

Answer: (a), (c)

Answer: (b)

Answer: (d)


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ME 482 Manufactur ing Systems

Cutting tools review questions: Which of the following are the two main functions of a cutting

f luid in machining (two answers only)? (a) improve sur face

f inish on the workpiece, (b) reduce forces and power, (c)

reduce fr iction at the tool-chip inter face, (d) remove heat fromthe process, and (e) wash away chips.

Answer: (c), (d)

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Ch6-CuttingTools.ppt