MatlRemovl-SinglePoint

7/28/2019 MatlRemovl-SinglePoint

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Material Removal Processes

Single Point Processes

ME350 - Design For

Manufacture (DFM)

Dr. Mike L. Philpott

Department of Mechanical & Industrial Engineering,University of Illinois at Urbana-Champaign.


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F t

FC

Fr

DIRECTION OF ROTATION

WORKPIECE

CUTTING TOOL

DIRECTION OF FEED

Velocity ofTool relative toworkpiece V

Longitudinal

'Thrust' Force (27%)

Radial

Force (6%)

Tangential 'Cutting' Force (67%)

Turning Forces For Orthogonal Model

End view section 'A'-'A'

Note: For the 2D Orthogonal MechanisticModel we will ignore the radial component

Ft

'A' 'A'

cF


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FL

FC

Fr

DIRECTION OF ROTATION

WORKPIECE

CUTTING TOOL

DIRECTION OF FEED

Velocity ofTool relative toworkpiece V

Longitudinal Force

Radial Force

Thrust Force

Tangential Force

'Cutting' Force

Facing Forces For Orthogonal Model

End view

Note: For the 2D Orthogonal MechanisticModel we will ignore the Longitudinalcomponent


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'Turning' Terminology

N is the speed in rpmD is the diameter of the

workpiece

f is the feed (lineardistance/rev)

d is the depth of cutV is the surface speed

= pDN

Standard Terms

Beware, for turning: In the generalized

orthogonal model depth of cut (to) is f (the feed),

and width of cut (w) is d (the depth of cut)

N

D

d mm

feed(mm/rev)

Tool

Workpiece

rpm


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Tool Terminology

Side reliefangle

Side cuttingedge angle(SCEA)

Clearance or end

relief angle

BackRake(BR),+

Side Rake

(SR), +

End Cuttingedge angle

(ECEA)

Nose

Radius

TurningCuttingedge

FacingCuttingedge


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Cutting Tools (2)

6. Coated Tools -have reduced cutting times by 4,typically Titanium nitride, carbide, or ceramicssometimes in multiple coatings on TC inserts(chemical inertness often important)

7. Ceramic Tools -typically aluminium oxide, havehigh abrasion resistance and hardness but lacktoughness (suitable for uninterrupted finishing cuts)

8. Cubic Boron Nitr ide (CBN)- 0.5 to 1mm CBN pieceis added to TC insert, very hard material suitable

for hardened ferrous and high-temperature alloys9. Diamond- hardest of all known materials, but very

brittle, used mainly for aluminium and copper-frontmirrors - excellent surface finish and accuracy

Tool cost

Wear resistance

Hardness

Cutting Speeds

Surface finish

Accuracy

Increase


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TurningEstimating Cutting Power, Torque,

and Cutting Force from Unit PowerFor a given material the average unit power (or specific cuttingenergy Ut ) may be used, :

and as Power = Torque T x Speed w(rads/sec)Torque T =

Pc2pN , and as Torque = Force x Distance

Cutting Power Pc = Ut pDavgNfd)

Ut =Cutting Power (P

c

)

Material Removal Rate (MRR) =P

cpDavgNfd

Cutting Force Fc=2T D avg


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FacingEstimating Cutting Power, Torque,

and Cutting Force from Unit Power

and as Power = Torque T x Speed w(rads/sec)Torque T =

Pc2pN , and as Torque = Force x Distance

Ut =Cutting Power (Pc)

Material Removal Rate (MRR)=

Pc

pDTNfd

Cutting Power at Time T: Pc = Ut pDTNfd

Cutting Force Fc = 2T DT

where DT is the diameter at time T;

350


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Temperature in Cutting

Knowledge of the temperature rise in cutting isimportant because it:

Adversely affects the strength, hardness, andwear resistance of the tool

Causes dimensional changes in the part beingmachined

Can induce thermal damage to the machinedsurface, adversely affecting its properties

350


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Tool

Workpiece

Chip

Heat Generation Zones

(Dependent on sharpness

of tool)

(Dependent onm)(Dependent on)

10%

30%

60%

350


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Cutting Temperature(Empirical)

Temperature Increaseswith increasing:l Cutting Speedl Depth of Cut (feed rate)l Strength of workpiece

material

Where V = Cutting Velocity, f = feed rate

Mean Temperature V afb

Temperature Decreaseswith increasing:l specific heatl thermal conductivity

350


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Cutting Fluids(also called lubricants or coolants)

Cutting Fluids are used extensively in machiningoperations to:

Cool the heat generation zones - particularly at highspeeds (tool properties degrade and workpiece distorts)

Reduce friction and wear, thus improving tool life andsurface finish

Reduce forces and energy consumption Wash away the chips Protect the newly machined surfaces from corrosion

Importance ofcoolingversus lubricationproperties aredependent on the process and surface speeds - effectschoice of lubricant.

350


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Tool Wear, Failure and Life

Tool wear is generally a gradual process and depends on:

Cutting Temperatures Tool geometry Process parameters (e.g. speed, feed, and depth of cut) Machine tool characteristics Tool and workpiece materials

Cutting Fluids

Tool Failure generally refers to the sudden loss of toolmaterial and shape (e.g. chipping) and is caused by: Mechanical shock - impact by interrupted cutting (e.g.

spline, hex. bar, sudden feed/speed change) Thermal Fatigue - cyclic variations in temperature ininterrupted cutting, often in the form of thermal cracks,perhaps where a defect already exists.

350


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Wear Mechanisms

1. Abrasion Wear- sliding of the chip on the tool, sliding ofthe tool on the workpiece2. Adhesion Wear- Plastic deformation and friction

associated with high temperatures cause a welding action,fractures of the weld cause tool degeneration

3. Di ffusion wear -Displacement of atoms in the metalliccrystal results in gradual deformation of the tool surface

4. Chemical and Electrolytic Wear- chemical reactionbetween the tool and the workpiece in the presence of thecutting fluid

5. Oxidation Wear- At high temperatures oxidation of the

carbide in the cutting tool decreases its strength

350


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Wear Regions

F lank Wear:Takes place on the relief face and

attributed to largely to Abrasion and Adhesion wearmechanismsCrater Wear:Takes place on the rake face face and duelargely to Abrasion, Adhesion and Diffusion wearmechanisms

p

p

Tool

Workpiece

Chip

Crater Wear

(Tool-Chip)

Flank Wear(Tool-Workpiece)

RakeFace

ReliefFace

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Wear Regions(Showing Flank Wear Land VB)

p Nose Radius Wear:Partially a continuation of theflank wear but includes grooves spaced at a distanceequal to the feed

p Outer Diameter Notch Wear:Groove notch usuallydeeper than flank wear but not as critical

Depthof Cut

r n

3-D view ofSingle PointTool

NoseWear

FlankWear

NotchWear

CraterWear

Wear Land-Wear Scar

FlankFace

RakeFace

NoseRadius

VB

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Taylor's Tool Life Model(Empirical)

Classical F. W. Taylor's Tool Life Model (circa. 1907):

VTn

= C

Where V = Cutting SpeedT = Time to develop a certain wear land (VB)

n = Taylor's ExponentC = Taylor's Constant (V for 1 min life)

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Extended Tool Life Model

Although cutting speed has been found to be the mostsignificant process variable in tool life, depth of cut and feedrate are also important:

where f = feed rated = depth of cutV = Cutting VelocityC, n, x, y = Constants found by experiment

VTndxfy= C

Tool Life, T = C1/nV-1/nd-x /nf-y /n

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Allowable Wear Land andRecommended Cutting Speed

The Al lowable Wear Land(VB) is the amount of wearbeyond which the quality of cut is unacceptable or theforces increase by too much: (Table 20.3, p. 621)Operation H igh-speed steels CarbidesTurning 1.5 mm 0.4 mm

Face Milling 1.5 mm 0.4 mmEnd Milling 0.3 mm 0.3 mmDrilling 0.4 mm 0.4 mmReaming 0.15 mm 0.15 mm

Recommended Cutting Speed(V) is generally onethat gives an acceptable tool life:Typically for HSS : Tool life = 60-120 min.For Carbide Tools: Tool life = 30-60 min. are chosen

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Vibration and Chatter

Vibration and chatter during cutting is a common cause of :

Poor surface finish

Loss of dimensional accuracy of the workpiece

Premature wear, chipping, and failure of the cutting tool(particularly with brittle tool materials such as ceramicsand diamonds)

Damage to machine-tool components from excessivevibrations

Objectionable noise generated, particularly if it is a highfrequency

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Reducing Vibration and Chatter

p Minimize tool Overhangp Support workpiece and tool rigidlyp Modify tool and cutter geometryp Change process parameters, such as V, f, d, and coolant

p Increase the stiffness of the machine tool and itscomponents (larger cross-sections and higher modulus)

p Improve the damping capacity of the machine

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Surface Finish in Turning

In turning, as in other cutting operations, the tool leaves a

spiral profile - feed marks- on the machined surface as itmoves across the workpiece:

The higher the feed f, and the smaller the noseradius , the more prominent these marks will be.

Finished

WorkpieceSurface

UncutWorkpiece

Surface

feed rate, f

Single Point Tool(with nose radius)

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Surface Characterization in General

A 3 D surface or surface profile may be decomposed intoseveral components:

p Surface Roughness- (amplitude < 0.2 mm) - produced bythe chip formation mechanism, non-roundness of toolnose radius, etc

p Waviness -(amplitude 0.2 mm to 25 mm) - formed by thegeometry of the cutting process, i.e., feed, nose radius,ECEA, and SCEA.

p Surface Er ror / Error of form- (amplitude 10 mm to 150mm) - due to compliance inherent with any cutting system

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Surface Finish Measures

Roughness Average Ra(or CLA)

Characterizes the average deviation of the profile from thecenter line (CL) - where by definition of equal areas:

CL =1L

h(x) dx0

L

Ra =1L

h(x) - CL dx0

L

and

xDistance

h(x)Height

L

area1area2

CL

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Surface Finish Measures

Peak-to-Valley (Rt)

The height differential between a peak and valley.Normally as an average but sometimes quoted as max.

Root-M ean-Square - RMS (Rq)

Also characterizes the average deviation of the profilefrom the center line (CL):

Rq =

1

L (h(x) - CL)

2

dx0

L

Rq is generally 10%-20% larger than the Ra as it tendsto penalize the large deviations from the center line

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Surface Finish Model for TurningZero Nose Radius

Distance between peaks equalsfeed f (distance in 1 rev.)

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Peak-To-Valley (Rt)

and

x (1 + tan(ECEA) tan(SCEA)) = f tan(SCEA) tan(ECEA)

SCEA

ECEA

f

y

xf-x

tan (ECEA ) =y

f-xtan (SCEA )=xy

y = Rt=f tan (ECEA )

(1+ tan (ECEA ) tan (SCEA ))

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Roughness Average R

For a triangular waved surface: CL = R /2

Ra =Area A + Area BSampling length

x

h(x)

A

B

t

Ra=1 2 f2 Rt 2 + 1 2 f2

Rt 2f

Ra =Rt4

a

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A mechanistic model procedure may be used for thescalloped surface left by a radiused tool, but it is morecomplex, and therefore often approximated with:

Boothroyd's classical empirical model:

Where f is the feed and r is the nose radius ( in inch units)n

Ra =0.0321 f

2

rn

Surface Finish Model for Turning

Non -Zero Nose Radius

- 350 -Mechanistic Nose


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Mechanistic NoseRadius Model

Using a coordinate transformation, we have:

x = rn sin y = rn - rncos

So, the center line average, c, is first calculated by integrating,

to yield:

rn

f

rn

c =

CLA C

f

f/2

CutterRt

Tool Profile

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M h i ti N


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where f= sin -1 (f / 2rn) and then:

where

c 1/ f rn0

f

rncos)d

c rn 1 (sin f / f)

Ra 1/ f (c rn 0

c

r cos)d (c

f

rn rncos c)d

Ra 1/ f 2cc 2Rc 2Rsin c Rf R sin f cf

c cos1(1 c / rn)


(cont.)

- 350 -Mechanistic Nose


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rn

f

f/2

Tool Profile

rnrn f /4-

2 2

rt

Rt rn rn2

f2

4


(cont.)

Peak-to-Valley

- 350 -Tool


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Planing andShaping Modeling(Use orthogonal Model)

p In Planingthe workpiece is mounted on a table thatreciprocates along a straight path. The tool is fixed to a head

that indexes a small distance (feed), normal to the direction oftravel, after each stroke.

p In Shapingthe tool reciprocates whilst the workpieceindexes normal to the direction of travel after each stroke

In both cases cutting is usually in only one direction with thetool tilted out of the way during the return stroke.

Workpiece

Tool

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Sh i d Pl i


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Shaping and PlaningTypical Parameters

Planing:Cutting Speeds: up to 2 m/sPower capacities: 110 Kw (150 hp)Feeds: 0.5 - 3 mm/strokeStrokes: as large as 25m

Shaping:Cutting speeds: 0.3 m/s (typical but some up to 2 m/s)Return speeds: usually twice as fast

Power capacities: less than 15 Kw (20 hp) typicallyFeeds: same as planingStrokes : up to 1.2 mRam reciprocations: up to 150 per min. typically

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Shaping & Planing Terminology

L

d

f

Tool

Workpiece

b

Chip

f = Incremental feed(distance/pass)

d = depth of cutb = part width

L = Part lengthL' = L + % overstrokeV = Cutting Velocity(distance/time)Vr = Speed of return

stroke (dist./time)Vf= Speed if indexingstroke (dist./time)

L' = L + % overstroke

Return Stroke

Cutting Stroke

Indexing Stroke

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Sh i d Pl i


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Shaping and PlaningCycleTimes

Cutting Time per pass = L'/V Non-cutting Time per pass = L'/Vr + f/Vf

Time per pass = L'/V + L'/Vr + f/Vf

time

MRR

0

Cutting Period

Non-cutting

Period

Time/Pass


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Sh i d Pl i


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Shaping and PlaningMRR and Total Power

MRRmax = Cutting Velocity x feed x depth

MRRmax = V f d

Power in Cutting = U t x MRRmax .

Total Motor Power =MRRmax U t

E

Where E = Efficiency

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