Nano Manufacturing Technology Lab/ Chung-Ang University - Material Removal...

Department of Mechanical EngineeringChung-Ang University

Material Removal Processes(1)

Seok-min [email protected]


Classification of Material Removal Process


Single point cutting tool removes material from a rotating workpiece to form a cylindrical shape

Turning & Drilling

Used to create a round hole, usually by means of a rotating tool (drill bit) with two cutting edges


Rotating multiple-cutting-edge tool is moved across work to cut a plane or straight surface Two forms: peripheral milling and face milling

Milling


Advantages of Machining Process

Variety of work materials can be machined Most frequently used to cut metals

Variety of part shapes and special geometric features possible, such as: Screw threads Accurate round holes Very straight edges and surfaces

Good dimensional accuracy and surface finish


Disadvantages with Machining

Wasteful of material Chips generated in machining are wasted

material, at least in the unit operation Time consuming A machining operation generally takes more time

to shape a given part than alternative shaping processes, such as casting, powder metallurgy, or forming


Cutting Tool Classification1. Single-Point Tools One dominant cutting edge Point is usually rounded to form a nose

radius Turning uses single point tools

2. Multiple Cutting Edge Tools More than one cutting edge Motion relative to work achieved by rotating Drilling and milling use rotating multiple

cutting edge tools


Figure 21.4 (a) A single-point tool showing rake face, flank, and tool point; and (b) a helical milling cutter, representative of tools with multiple cutting edges.

Cutting Tool


Cutting Conditions in Machining Three dimensions of a machining process:

Cutting speed v – primary motion Feed f – secondary motion Depth of cut d – penetration of tool below original work surface

For certain operations, material removal rate can be computed as RMR = v f d

where v = cutting speed; f = feed; d = depth of cut


Mechanics of Chip Formation Cutting processes, such as turning on a lathe, drilling or milling remove

material from the surface of the workpiece by producing chips. The basic mechanics of chip formation is essentially the same for all

these operations, which we represent by the two-dimensional model in Fig. In this model a tool moves along the workpiece at a certain velocity V and depth of cut t0. A chip is produced ahead of the tool by shearing the material continuously along the shear plane.


Forces Acting on Chip(1)

tanNF

shearin stress yield :smaximum shear stress acts on the shear plane

cut of width is e, whersin

(AB) 0 wtwwA

AF

s

SSS

A

B

Cutting operator에관련된역학계산이중요한이유1. 작업에필요한동력계산 적정용량의모터설치

2. 공구설계에유효3. workpiece/tool 의변형해석

F : Friction force, N : Normal force to friction Fs : Shear force, Fn : Normal force to shear



A

B

Fc : cutting force, Ft : thrust force

F = Fc sin + Ft cosN = Fc cos - Ft sinFs= Fc cos - Ft sinFn= Fc sin + Ft cos

Force Diagram


Friction force F along the tool-chip interface a normal force N perpendicular to Ffriction ofcoffient

β = friction angle

cos R N sin RF

c

t

c

t

FF

FF

tantan1tantan)tan(

tantantan

tc

ct

FFFF

cttc FFFF tan)tantanβ(

tanNF



)cos()sin(F)sin(F

)cos()cos(F )cos(F

)cos(F

st

sc

s

R

R

R

0]sin)[cos(dd

find Φ such that is maximized

eqn.)(Merchant 224

)tan( cot() = tan(2

- )

maximize , maximize sin)cos( ∵

-(1)

in eqn.(1)

thus , in eqn.(2)

0

c sin)cos(

)cos(FwtA

F

S

S

0cos)cos(sin)sin( -(2)



Merchant Equation Assumption : is a constant,

unaffected by strain rate, temperature and other factors It defines the general relationship between rake angle, tool-chip friction

and share plane angle.

s


Example 21.1 In a machining operation that approximates orthogonal cutting, the

cutting tool has a rake angle = 10º, The chip thickness before the cut t0 = 0.5mm and the chip thickness after the cut tC =1.125 mm. Calculate the shear plane angle and the shear strain in the operation

444.0125.1

5.0

c

o

ttr

386.2 )104.25tan()4.25cot(

)tan()cot(

)tan()cot(

BDBDBD

BDDCAD

BDAC

o

rr

4.25

4738.010sin444.01

10cos444.0sin1

costan


Example 21.2 Fc = 1559 N, Ft = 1271 N, w=3mm Determine the shear strength of the work material

Fs= Fc cos - Ft sin = 863N

MPaNAF

twwA

AF

SSS

s

SSS

247mm/247/

3.497mm sin25.4

.5)(3)0( sin

(AB)

2

20


Example 21.3 Rake angle = 10º, Find friction angle β and coefficient of friction

o4.25

eqn.)(Merchant 224

16.1tan2.49

2210

41804.25

o

ooo

c

t

c

t

FF

FF

tantan1tantan)tan(


Total power input in cuttingPower = VFc

Total energy per unit volume of material removed (Specific energy)

o

c

o

ct wt

FVwtVFu , : projected area of the cutowt

Power required to overcome friction (total-chip interface)

o

tc

oo

cf wt

rFFwtFr

VwtFV

u)cossin(

Power required for shearing

VwtVF

uo

sss

sft uuu

Power Input In Cutting

0VttV cc )cos(

sin0

rtt

VV

c

c

sincos)cos(cs VVV


c

o

ttr

where r = chip thickness ratio; to = thickness of the chip prior

to chip formationtc = chip thickness after separation

)cos()( ABtc

sin)(0 ABt

Chip Ratio

B

A

sinsincoscossin

)cos(sin

r

sin1costan

sin1tancos

1 sincossincos

sin sinsincoscos

rr

rr

rr

rr


Chip Formation

Four Basic Types of Chip in Machining• Discontinuous chip• Continuous chip• Continuous chip with Built-up Edge (BUE)


Brittle work materials Low cutting speeds Large feed and depth of cut High tool-chip friction

Discontinuous Chip & Continuous Chip

Ductile work materials High cutting speeds Small feeds and depths Sharp cutting edge Low tool-chip friction


Ductile materials Low-to-medium cutting

speeds Tool-chip friction

causes portions of chip to adhere to rake face

BUE forms, then breaks off, cyclically

Continuous with BUE


Mechanics of oblique cutting

(a) Cutting with an oblique tool(b) Top view, showing the inclination angle, i

(c) Types of chips produced with different inclination angle


Temperature(1)

Why temperature rise in cutting important• Adversely affects the strength, hardness, and wear

resistance of the cutting tool• Cause dimensional changes in the part being machined• Thermal damage on the surface of the work piece

Main source of heat generation• Primary shear zone• Tool-chip interface

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Experimental methods can be used to measure temperatures in machining Most frequently used technique is the tool-chip

thermocouple Using this method, Ken Trigger determined the

speed-temperature relationship to be of the form: T = K vm

where T = measured tool-chip interface temperature, and v = cutting speed

Temperature(2)

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Tool Wear (1)

공구마멸유발의원인 – forces, temperature, sliding(friction)

Tool & workpiece material, 공구형상, 절삭유성질, … 등이 Tool wear (공구마멸)에영향을미친다.

Tool wear의분류• crater wear• flank wear• notch wear• nose radius wear

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-발생원인

i) 가공면을따른공구의미끄럼운동응착/연삭마멸유발

ii) 온도공구재료특성에영향

CVT n

V: cutting speed

T: time (flank 마멸폭이어떤값에도달할때걸리는시간)

n, c: constants (공작물, 공구재료, 절삭조건)

Tool Wear (2) –Taylor Tool Life Eq.

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Tool-life CurveToo-life의정의

: time(in minutes) required to reach a flank wear land of a specified dimension.

여러가지절삭공구재료들의공구수명곡선

기울기 n 값에주의

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절삭깊이, 이송속도의영향

CfdVT yxn

d: 절삭깊이 f: 이송속도 x, y: 지수값 (실험치)

예)

n = 0.15, x = 0.15, y = 0.6

중요순서 V, f, d

ny

nx

nn fdVCT

11

4177 fdVCTor

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혀용평균마멸폭

Allowable wear land. The allowable wear land (VB) for various conditions is given in Table. For improved dimensional accuracy and surface finish, the allowable wear land may be made smaller than the values given in Table. The recommended cutting speed for a high-speed-steel tool is generally the one that gives a tool life of 60-120 min and for carbide tools 30-60 min.

Optimum cutting speed. We have shown that as cutting speed increases, tool life is rapidly reduced. On the other hand, if cutting speeds are low, tool life is long but the rate at which material is removed is also low. Thus there is an optimum cutting speed.

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Cutting Fluids Any liquid or gas applied directly to machining operation to

improve cutting performance Two main problems addressed by cutting fluids:

1. Heat generation at shear and friction zones 2. Friction at tool-chip and tool-work interfaces

Other functions and benefits: Wash away chips (e.g., grinding and milling) Reduce temperature of workpart for easier handling Improve dimensional stability of workpart

Cutting fluids can be classified according to function: Coolants - designed to reduce effects of heat in machining Lubricants - designed to reduce tool-chip and tool-work

friction

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Cutting Fluid Contamination

Replace cutting fluid at regular and frequent intervals Use filtration system to continuously or periodically

clean the fluid Advantages of Fluid Filtration Prolong cutting fluid life between changes Reduce fluid disposal cost Cleaner fluids reduce health hazards Lower machine tool maintenance Longer tool life

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Rotational - cylindrical or disk-like shape Nonrotational (also called prismatic)

- block-like or plate-like

Classification of Machined Parts

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Each machining operation produces a characteristic part geometry due to two factors:

1. Relative motions between tool and workpart• Generating – part geometry determined by feed

trajectory (궤적) of cutting tool2. Shape of the cutting tool

• Forming – part geometry is created by the shape of the cutting tool

Machining Operations and Part Geometry

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Generating shape: (a) straight turning, (b) taper turning, (c) contour turning, (d) plain milling, (e) profile milling.

Generating Shape

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Forming to create shape: (a) form turning, (b) drilling, and (c) broaching.

Forming to Create Shape

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Combination of forming and generating to create shape: (a) thread cutting on a lathe, and (b) slot milling.

Forming and Generating

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Turning Single point cutting tool removes material from a

rotating workpiece to generate a cylinder Performed on a machine tool called a lathe Variations of turning performed on a lathe: Facing Contour turning Chamfering Cutoff Threading

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Turning Operation

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Facing & Contour Turning

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Chamfering, Cutoff, Threading

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Engine Lathe

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Methods of Holding the Work in a Lathe- Chuck & Collet

three-jaw chuck collet

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Face Plate

Face plate for non-cylindrical workparts

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Turret Lathe Tailstock replaced by “turret” that holds up to six tools Tools rapidly brought into action by indexing the turret Tool post replaced by four-sided turret to index four

tools Applications: high production work that requires a

sequence of cuts on the part

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http://blog.naver.com/garnetelf?Redirect=Log&logNo=20051697937


Multiple Spindle Bar Machines More than one spindle, so multiple parts machined

simultaneously by multiple tools Example: six spindle automatic bar machine works on

six parts at a time After each machining cycle, spindles (including collets and

workbars) are indexed (rotated) to next position

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Boring Difference between boring and turning: Boring is performed on the inside diameter of an

existing hole Turning is performed on the outside diameter of an

existing cylinder In effect, boring is internal turning operation Boring machines Horizontal or vertical

- refers to the orientation of the axis of rotation of

machine spindle

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Creates a round hole in a workpart

Compare to boring which can only enlarge an existing hole

Cutting tool called a drill or drill bit

Machine tool: drill press

Drilling

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Through-holes - drill exits opposite side of workBlind-holes – does not exit work opposite side

Figure 22.13 Two hole types: (a) through-hole, and (b) blind hole.

Through Holes vs. Blind Holes

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Upright drill press stands on the floor

Bench drill similar but smaller and mounted on a table or bench

Figure 22.15 Upright drill press

Drill Press

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Used to provide internal screw threads on an existing hole

Tool called a tap

Tapping

- 52 -


Provides a stepped hole, in which a larger diameter follows smaller diameter partially into the hole

Counterboring

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Milling Machining operation in which work is fed past a

rotating tool with multiple cutting edges Axis of tool rotation is perpendicular to feed Creates a planar surface Other geometries possible either by cutter path or

shape Other factors and terms: Interrupted cutting operation Cutting tool called a milling cutter, cutting edges

called "teeth" Machine tool called a milling machine

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Two Forms of Milling Peripheral milling

Cutter axis parallel to surface being machined Cutting edges on outside periphery of cutter

Face milling Cutter axis perpendicular to surface being milled Cutting edges on both the end and outside periphery of the cutter .

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Basic form of peripheral milling in which the cutter width extends beyond the workpiece on both sides

Slab Milling & Slotting Width of cutter is less than

workpiece width, creating a slot in the work

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Cutter overhangs work on both sides

Conventional Face Milling

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Cutter diameter is less than work width, so a slot is cut into part

End Milling / Profile Milling /Pocket Milling Form of end milling in

which the outside periphery of a flat part is cut

Another form of end milling used to mill shallow pockets into flat parts

End Milling Profile Milling Pocket Milling

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Ball-nose cutter fed back and forth across work along a curvilinear path at close intervals to create a three dimensional surface form

Surface Contouring

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Milling Machine

vertical knee-and-columnmilling machine

horizontal knee-and-column milling machine.

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Machining Centers Highly automated machine tool can perform multiple

machining operations under CNC control in one setup with minimal human attention Typical operations are milling and drilling Three, four, or five axes

Other features: Automatic tool-changing Pallet shuttles Automatic workpart positioning

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Similar operations Both use a single point cutting tool moved

linearly relative to the workpart

Shaping and Planing

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Shaping and Planing A straight, flat surface is created in both operations Low cutting speeds due to start-and-stop motion Typical tooling: single point high speed steel tools

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Requirements for High Speed Machining

Special bearings designed for high rpm High feed rate capability (e.g., 50 m/min) CNC motion controls with “look-ahead” features to

avoid “undershooting” or “overshooting” tool path Balanced cutting tools, toolholders, and spindles to

minimize vibration Coolant delivery systems that provide higher

pressures than conventional machining Chip control and removal systems to cope with much

larger metal removal rates

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High Speed Machining Applications

Aircraft industry, machining of large airframe components from large aluminum blocks Much metal removal, mostly by milling

Multiple machining operations on aluminum to produce automotive, computer, and medical components Quick tool changes and tool path control important

Die and mold industry Fabricating complex geometries from hard

materials

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Selection of Cutting Conditions

One of the tasks in process planning For each operation, decisions must be made about

machine tool, cutting tool(s), and cutting conditions Cutting conditions: depth of cut, feed, speed,

and cutting fluid These decisions must give due consideration to

workpart machinability, part geometry, surface finish, and so forth

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Selecting Depth of Cut

Depth of cut is often predetermined by workpiecegeometry and operation sequence In roughing, depth is made as large as possible

to maximize material removal rate, subject to limitations of horsepower, machine tool and setup rigidity, and strength of cutting tool

In finishing, depth is set to achieve final part dimensions

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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 determine optimal speed

Two alternative objectives in these formulas: 1. Maximum production rate 2. Minimum unit cost

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Maximum Production Rate Maximizing production rate = minimizing cutting time

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

Total time per unit product for operation: Tc = Th + Tm + Tt/np

Cycle time Tc is a function of cutting speed

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Cycle Time vs. Cutting Speed

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Minimizing Cost per Unit In turning, total production cycle cost for one part

consists of: 1. Cost of part handling time = CoTh ,

where Co = cost rate for operator and machine2. 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

Again, unit cost is a function of cutting speed,just as Tc is a function of v

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Unit Cost vs. Cutting Speed

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Comments on Machining Economics As C and n increase in Taylor tool life equation, optimum

cutting speed increases Cemented carbides and ceramic tools should be used at

speeds significantly higher than for HSS vmax is always greater than vmin

Reason: Ct/np term in unit cost equation pushes optimum speed to left in the plot of Cc vs. v

As tool change time Tt and/or tooling cost Ct increase, cutting speed should be reduced

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Product Design Guidelines Design parts that need no machining Use net shape 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 Reasons why machining may be required: Close tolerances Good surface finish Special geometric features such as threads, precision

holes, cylindrical sections with high degree of roundness Tolerances and surface finish should be specified to satisfy

functional requirements, but process capabilities should also be considered

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Design parts with features that can be produced in a minimum number of setups

Example: Design part with geometric features that can be accessed from one side of part

Figure 24.6 Two parts with similar hole features: holes that must be machined from two sides, requiring two setups, and holes that can all be machined from one side.

Product Design Guidelines

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