cutting

Fundamentals of Cutting

Introductions(1)Major independent variables-tool material and its condition-too1 shape , surface finish , and sharpness-workpiece material , condition , and temp.-cutting conditions , such as speed , depth of cut , and feed

-cutting fluid-quality of machine tool , such as stiffness , damping

Introductions

(2)Dependent variables-type of chip-force and energy-temp. rise in the workpiece , the chip , and the tool-wear and failure of the tool-surface finish

Mechanics of Chip Formation

(1)Orthogonal cutting (fig .20.3)-rake angle-relief or clearance angle-shear angle-depth of cut (undeformed chip thickness)-chip thickness-cutting ratio (fig .20.4)

( )αφφ−

===cos

sin0

VV

ttr c

c

Fundamentals of CuttingFigure 20.1 Examples of cutting processes.

Figure 20.3 Schematic illustration of a two-dimensional cutting process, also called orthogonal cutting. Note that the tool shape and its angles, depth of cut, to, and the cutting speed, V, are all independent variables.

Figure 20.2 Basic principle of the turning operations.


(2)Shear Strain

-large with low and low or negative - =5 or larger in actual cutting

operations

OCOB

OCAO

OCABr +==

( )αφφ −+= tancotr

r


Figure 20.4 (a) Schematic illustration of the basic mechanism of chip formation in metal cutting. (b) Velocity diagram in the cutting zone. See also Section 20.5.3. Source: M. E. Merchant.

Mechanics of Chip Formation (3)Shear angle

-friction angle

-rake angle decreases and/or friction increases ,shear angle decreases and chip is thicker shear strain is higher and more energy dissipates temp. rise is also higher

2245 βαφ −+=

βµ tan=


(4)Chip velocity , V

(5)Velocity diagram (fig .20.4b)

c

)cos(sin

αφφ−

=VVc

Types of Chips

(1)Continuous chips-formed at high cutting speeds and/or high rake angles (fig .20.5a,b)-primary shear zone and secondary shear zone

Types of Chips -wide primary shear zone：poor surface finishinducing residual stressmachining soft metals at low speed andlow rake angle-chip breakers-decreasing rake angle and increasing shear strain ,chip becomes stronger , harder , and less ductility

Types of Chips

(2)Built-up edge chips (BUE) (fig .20.5c)-mechanism of formation adhesion (affinity) of the workpiece and toolgrowth of the adhered metal layerslarge strain hardening exponent , n

Types of Chips

-increasing cutting speed , cutting temp. , rake angle decreasing depth of cut , tip radius , using cutting fluid , a cold-worked metal , the size of BUE decreases-poor surface finish

Types of Chips

(3)Serrated chips (fig .20.5e)-due to low and high shear strain-low thermal conductivity , ex. Ti-decreasing sharply strength with temp.

Types of Chips (4)Discontinuous chips (fig .20.5f)-due to brittle materials (low shears train) ,

materials containing hard inclusions , impurities ,very low or very high cutting speeds , large DOC ,low rake angle , low stiffness of machine too1 ,no effective cutting fluid-forces variation , chatter and vibration

Chips and Their Photomicrographs

(f)

(b)(a) (c)

(d) (e)

Figure 20.5 Basic types of chips and their photomicrographs produced in metal cutting: (a) continuous chip with narrow, straight primary shear zone; (b) secondary shear zone at the chip-tool interface; (c) continuous chip with large primary shear zone; (d) continuous chip with built-up edge; (e) segmented or nonhomogeneous chip and (f) discontinuous chip. Source: After M. C. Shaw, P. K. Wright, and S. Kalpakjian.

Types of Chips

(5)Chip types for cutting thermoplastics , thermosetting,polymer , ceramics

(6)Chip curl (fig.20.7)-decreasing DOC and friction , increasing rake angle ,curlier chip

Types of Chips

(7)Chip breaker (fig.20.7b,c )

Chip Breakers

Figure 20.7 (a) Schematic illustration of the action of a chip breaker. Note that the chip breaker decreases the radius of curvature of the chip. (b) Chip breaker clamped on the rake face of a cutting tool. (c) Grooves in cutting tools acting as chip breakers; see also Fig. 21.2.

Examples of Chips Produced in Turning

Figure 20.8 Various chips produced in turning: (a) tightly curled chip; (b) chip hits workpiece and breaks; (c) continuous chip moving away from workpiece; and (d) chip hits tool shank and breaks off. Source: G. Boothroyd, Fundamentals of Metal Machining and Machine Tools. Copyright ©1975; McGraw-Hill Publishing Company. Used with permission.

Mechanics of Oblique Cutting

(1)Oblique cutting (fig.20.9)-inclination angle , i-chip flow angle , inclination angle ,

-normal rake angle ,-effective rake angle ,

≈cαi

nα

eα

( )ne αιια sincossinsin 221 += −

Mechanics of Oblique Cutting

-increasing i , increasing the effective rake angle ,chip becomes thinner and longer (fig .20.9c)

(2)Terminology for cutting tool (fig.20.10) and shaving Tool (fig .20.11)

Cutting With an Oblique Tool

Figure 20.9 (a) Schematic illustration of cutting with an oblique tool. (b) Top view showing the inclination angle, i. (c) Types of chips produced with different inclination.

Cutting Forces , Stresses , and Power

(1)Orthogonal cutting (fig.20.12)-cutting force , Fc

-thrust force , Ft

-normal force , N-friction force , F-resultant force , R-shear force , Fs , at the shear plane-normal force , Fn ,at the shear plane

Forces in Two-Dimensional Cutting

Figure 20.11 Forces acting on a cutting tool in two-dimensional cutting. Note that the resultant force, R, must be colinear to balance the forces.

Right-Hand Cutting ToolFigure 20.10 (a) Schematic illustration of a right-hand cutting tool. Although these tools have traditionally been produced from solid tool-steel bars, they have been largely replaced by carbide or other inserts of various shapes and sizes, as shown in (b). The various angles on these tools and their effects on machining are described in Section 22.3.1.


(2)Force relationships

μ= 0.5 - 2.0 in metal cutting

βsinRF =βcosRN =

φφ sincos tcs FFF −=φφ conFFF tcn += sin

ααµ

tantan

tc

ct

FFFF

NF

−+

==


-cutting force = a few l00 Newtonlocal high stress and pressure on the tool due to very small contact area of cutting zone ,lead to wear , chipping , and fracture of tool


(3)Thrust force-deflection of tool holder and machine tool

(4)Measuring forces-dynamometer

( ) ( )αβαβ −=−= tansin ct FRF


(5)Power-power=FcVin the shear zone and the tool-chip interface

-power for shearing = FsVs

-specific energy for shearing , uVFu ss

s = Vwto


-power for friction = FVc

-specific energy for friction , u

-total specific energy , u

oo

cf wt

FrVwt

FVu ==

fst uuu +=


-power data for various metals (table 20.1)-example：relative energies in cutting

Approximate Energy Requirements in Cutting Operations

TABLE 20.2 Approximate Energy Requirements inCutting Operations (at drive motor,corrected for 80% efficiency; multiply by1.25 for dull tools).

Specific energyMaterial W-s/mm

3hp-min/in.

3

Aluminum alloysCast ironsCopper alloysHigh-temperature alloysMagnesium alloysNickel alloysRefractory alloysStainless steelsSteelsTitanium alloys

0.4–1.11.6–5.51.4–3.33.3–8.50.4–0.64.9–6.83.8–9.63.0–5.22.7–9.33.0–4.1

0.15–0.40.6–2.00.5–1.21.2–3.1

0.15–0.21.8–2.51.1–3.51.1–1.91.0–3.41.1–1.5

Temperature in Cutting

(1)Effects-strength , hardness , and wear resistance of the tool

-dimensional changes-thermal damage


(2)Heat sources-primary shear zone-tool-chip interface-tool tip-workpiece interface


(3)Mean temperature in turning (fig .20.13)-mean temp. α Va Fb

tool a bcarbide 0.2 0.125

HSS 0.5 0.375 -typical temprature distribution (fig .20.14)max. temp. at halfway up the face of the tool (fig .20.15b)

Temperature Distribution and Heat Generated

Figure 20.12 Typical temperature distribution the cutting zone. Note the steep temperature gradients within the tool and the chip. Source: G. Vieregge.

Figure 20.14 Percentage of the heat generated in cutting going into the workpiece, tool, and chip, as a function of cutting speed. Note that the chip carries away most of the heat.

Temperature Distributions

Figure 20.13 Temperatures developed n turning 52100 steel: (a) flank temperature distribution; and (b) tool-chip interface temperature distribution. Source: B. T. Chao and K. J. Trigger.


(4)Techniques for measuring temp.-thermocouple , infrared pyrometer

Tool wear and Failure

(1)wear rate-tool and workpiece materials-tool shape-cutting conditions-cutting fluids-machine tool


(2)Two basic wear type(i)Flank wear (fig .20.15 )-due to rubbing and high temp.-Taylor life form

n value (table 20.2) CVT n =

Flank and Crater Wear

(e)(d)

(a) (b) (c) Figure 20.15 (a) Flank and crater wear in a cutting tool. Tool moves to the left. (b) View of the rake face of a turning tool, showing nose radius R and crater wear pattern on the rake face of the tool. (c) View of the flank face of a turning tool, showing the average flank wear land VB and the depth-of-cut line (wear notch). See also Fig. 20.18. (d) Crater and (e) flank wear on a carbide tool. Source: J.C. Keefe, Lehigh University.

Tool Wear

T A B L E 2 0 .3 R an g e o f n V a lu es fo r E q .(2 0 .2 0 ) fo r V a rio u s T o o lM a te r ia ls

H ig h -sp eed s tee lsC as t a llo y sC arb id esC eram ics

0 .0 8 – 0 .20 .1 – 0 .1 50 .2 – 0 .50 .5 – 0 .7


-modified form

typical value n=0.15 , x=0.15 , y=0.6or

CfdVT yxn =

ny

nx

nn fdVCT−−−

=11 4177 −−−≅ fdVCT


-for a constant tool life , feed and DOC increase (increase in the volume of the material removed) ,cutting speed decreases

-tool life curves (fig.20.17)

Tool Life Figure 20.16 Effect of workpiece microstructure and hardness on tool life in turning ductile cast iron. Note the rapid decrease in tool life as the cutting speed increases. Tool materials have been developed that resist high temperatures such as carbides, ceramics, and cubic boron nitride, as described in Chapter 21.

Figure 20.17 Tool-life curves for a variety of cutting-tool materials. The negative inverse of the slope of these curves is the exponent n in the Taylor tool-life equations and C is the cutting speed at T = 1 min.


-allowable wear land (table 20.4)HSS life = 60 - 120 mincarbide life = 30 - 60 min

-optimum cutting speedV increases , tool life decreasesV decreases , tool life increases , but MRR decreases

Tool Wear

TABLE 20.4 Allowable Average Wear Land (VB)for Cutting Tools in Various Operations

Allowable wear land (mm)Operation High-speed Steels CarbidesTurningFace millingEnd millingDrillingReaming

1.51.50.30.4

0.15

0.40.40.30.4

0.15Note: 1 mm = 0.040 in.


(ii)Crater wear (fig.20.18)-due to temp. at tool-chip interfacechemical affinity between tool and workpiece and diffusion-max. crater wear at the location of max. temp.

Examples of Wear and Tool Failures

Figure 20.18 (a) Schematic illustrations of types of wear observed on various types of cutting tools. (b) Schematic illustrations of catastrophic tool failures. A study of the types and mechanisms of tool wear and failure is essential to the development of better tool materials.


(3)Chipping-due to mechanical shock or thermal fatigue-small fragements on the cutting edge：microchipping-relative large fragements：gross chipping or fracture-high positive rake angle or too brittle of tool


(4)Plastic deformation-due to high temp.(5)Trends in wear , BUE , and plastic

deformation - V and f


(6)Techniques for measuring and monitoring wear

-direct measurement toolmakers' microscope

-indirect measurementforces , power , temp. rise , surface finish, vibration , and acoustic emission

Surface Finish and Integrity

(1)Surface finish-Surface geometry-influencing factors：BUE , cutting conditions , tool (diamond and ceramic) tool geometry , chatter and vibration

Surface Finish and Integrity

(2)Surface integrity-properties such as fatigue life and corrosion-influencing factors：

temp. , residual stresses , tearing , cracking , metallurgical (phase) trasformation ,urface plastic deformation

Machinability

(1)Define-surface finish and integrity-tool life-force and power requirements-temp.-chip type

Machinability

(2)Machinability ratings-AISI1112 stee1 , tool life T = 60 min. ,cutting speed = l00 ft /min (30/min.) -- rating 100-3140 steel：55free-cutting brass：300PH17-7 stee1 , Inconel 30:20

Machinability

(3)Machinability of Various metals-leaded steels：Pb reduces cutting forces-resulfurized steels：MnS inclusions causes chips to break up easily

-carbon steel-stainless steel-Al , gray cast iron , Co-base alloys , wrought Cu , Mo , Ta , Ti , W , Zr

Cutting-Tool Materials

Requirement of Cutting Tools

(1)Hardness and hot hardness (fig .21.1)(2)Toughness(3)wear resistance(4)Chemical stability or inertness

Cutting-Tool Materials(1)Carbon and medium-alloy steels(2)High-speed steels (HSS)(3)Cast-cobalt alloys(4)Carbides(5)Coated tools(6)Ceramics(7)Cubic boron nitride (CBN)(8)Silicon nitride(9)Diamond

Typical Properties of Tool Materials

Table 21.1Carbides

PropertyHigh-speed

steels Cast alloys WC TiC CeramicsCubic boron

nitrideSingle-crystal

diamond*

Hardness 83– 86 HRA 82– 84 HRA 90– 95 HRA 91– 93 HRA 91– 95 HRA 4000– 5000 HK 7000– 8000 HK46– 62 HRC 1800– 2400 HK 1800– 3200 HK 2000– 3000 HK

Compressive strength MPa psi x10

34100– 4500600– 650

1500– 2300220– 335

4100– 5850600– 850

3100– 3850450– 560

2750– 4500400– 650

69001000

69001000

Transverse rupture strength MPa psi x10

32400– 4800350– 700

1380– 2050200– 300

1050– 2600150– 375

1380– 1900200– 275

345– 95050– 135

700105

1350200

Impact strength J in.- lb

1.35– 812– 70

0.34– 1.253– 11

0.34– 1.353– 12

0.79– 1.247– 11

< 0.1< 1

< 0.5< 5

< 0.2< 2

Modulus of elasticity GPa psi x10

620030

––

520– 69075– 100

310– 45045– 65

310– 41045– 60

850125

820– 1050120– 150

Density kg/m

3

lb/in.3

86000.31

8000– 87000.29– 0.31

10,000– 15,0000.36– 0.54

5500– 58000.2– 0.22

4000– 45000.14– 0.16

35000.13

35000.13

Volume of hard phase, % 7– 15 10– 20 70– 90 – 100 95 95Melting or decompositiontemperature °C °F

13002370

––

14002550

14002550

20003600

13002400

7001300

Thermal conductivity, W/m K

30– 50 – 42– 125 17 29 13 500– 2000

Coefficient of thermalexpansion, x10

–6 °C

12 – 4– 6.5 7.5– 9 6– 8.5 4.8 1.5– 4.8

*The values for polycrystalline diamond are generally lower, except impact strength, which is higher.

General Characteristics of Cutting-Tool Materials

TABLE 21.2 General Characteristics of Cutting- Tool Materials. These Tool Materials Have a Wide Range ofCompositions and Properties; Thus Overlapping Characteristics Exist in Many Categories of Tool Materials.

Carbon andlow- to

medium- alloysteels

High speedsteels

Cast- cobaltalloys

Uncoatedcarbides

Coatedcarbides Ceramics

Polycrystallinecubic boron

nitride DiamondHot hardness IncreasingToughness IncreasingImpact strength IncreasingWear resistance IncreasingChippingresistance

Increasing

Cutting speed IncreasingThermal-shockresistance

Increasing

Tool material cost IncreasingDepth of cut Light to

mediumLight toheavy

Light toheavy

Light toheavy

Light toheavy

Light toheavy

Light to heavy Very light forsingle crystaldiamond

Finish obtainable Rough Rough Rough Good Good Very good Very good ExcellentMethod ofprocessing

Wrought Wrought,cast, HIP

*

sintering

Cast andHIPsintering

Coldpressingandsintering

CVD orPVD

†Coldpressingandsinteringor HIPsintering

High-pressure,high-temperaturesintering

High-pressure,high-temperaturesintering

Fabrication Machiningand grinding

Machiningandgrinding

Grinding Grinding Grinding Grinding andpolishing

Grinding andpolishing

Source : R. Komanduri, Kirk- Othmer Encyclopedia of Chemical Technology , (3d ed.). New York: Wiley, 1978.* Hot- isostatic pressing.†

Chemical- vapor deposition, physical- vapor deposition.

Carbon and Medium-Alloy Steels

(1)Low-speed cutting operations

High-Speed Steels

(1)High toughness and resistance to fracture

-suitable for high positive rake angle and machine tools with low stiffness , vibration and chatter

High-Speed Steels

(2)Molybdenum (M series)-composition：>10% Mo , with Cr , V , W , Co

-higher abrasion resistance than T series

-less distortion than T series-less expensive than T series

High-Speed Steels

(3)Tungsten (T series)-composition：12-18 % W ,with Cr ,V ,Co(4)Complex tool shapes (table 21.3)-drills , reamers , taps , gear cutters-wrought , cast , P/M forms

Operating Characteristics of Cutting-Tool Materials

TABLE 21.3

Tool materials General characteristicsModes of tool wear or

failure LimitationsHigh-speed steels High toughness, resistance

to fracture, wide range ofroughing and finishingcuts, good for interruptedcuts

Flank wear, crater wear Low hot hardness, limitedhardenability, and limitedwear resistance

Uncoated carbides High hardness over a widerange of temperatures,toughness, wear resistance,versatile and wide range ofapplications

Flank wear, crater wear Cannot use at low speedbecause of cold welding ofchips and microchipping

Coated carbides Improved wear resistanceover uncoated carbides,better frictional andthermal properties

Flank wear, crater wear Cannot use at low speedbecause of cold welding ofchips and microchipping

Ceramics High hardness at elevatedtemperatures, high abrasivewear resistance

Depth-of-cut line notching,microchipping, grossfracture

Low strength, low thermo-mechanical fatigue strength

Polycrystalline cubicboron nitride (cBN)

High hot hardness,toughness, cutting-edgestrength

Depth-of-cut line notching,chipping, oxidation,graphitization

Low strength, lowchemical stability at highertemperature

Polycrystalline diamond Hardness and toughness,abrasive wear resistance

Chipping, oxidation,graphitization

Low strength, lowchemical stability at highertemperature

Source: After R. Komanduri and other sources.

Cast-Cobalt Alloys

(1)Good wear resistance and hot hardness (HRC58-64)(2)Composition-38-53 % Co , 30-33 % Cr , 10-20 % W(3)Satellite tools (cast and grind)-relatively high feeds and speeds

Carbides(1)High hot hardness , high elastic modulus

High thermal conductivityLow thermal expansion

(2)Tungsten carbide (WC) (table 21.4-5)-P type：WC + Co + TiC + TaC

improve hot hardness and crater-wear resistance cutting ferrous metals with long chips , steel and alloy steels

Classification of Tungsten Carbides

Table 21.4 Classification of Tungsten Carbide According to Machining Applications. See also Chapters 22 and 23 for Cutting ToolRecommendations

Characteristics ofISO Standard ANSIClassification

Number

Materials to bemachined

MachiningOperation

Type of carbide

Cut Carbide

K30-K40 C-1 RoughingK20 C-2 General purposeK10 C-3 Light finishingK01 C-4

Cast iron,nonferrous metalsand nonmetallicmaterials requiringabrasion resistance

Precisionmachining

Wear-resistantgrades; generallystraight WC-Cowith varyinggrain sizes

Increasing Cuttingspeed

Increasing Feedrate

Increasinghardness and wear

resistance

Increasingstrength and

binder contentP30-P50 C-5 RoughingP20 C-6 General purposeP10 C-7 Light purposeP01 C-8

Steels and steelalloys requiringcrater anddeformationresistance

Precision finishing

Crater-resistantgrades; variousWC-Cocompositionswith TiC and/orTaC alloys

Increasing Cuttingspeed

Increasing Feedrate

Increasinghardness and wear

resistance

Increasingstrength and

binder contentNote: The ISO and ANSI comparisons are approximate.

ISO Classification of Carbide Cutting Tools According to Use

TABLE 21.5

Symbol Workpiece material Color code

Designation in increasing orderof wear resistance and

decreasing order of toughness ineach category, in increments of 5

P Ferrous metals with long chips Blue P01, P05 through P50M Ferrous metals with long or short

chips; nonferrous metalsYellow M10 through M40

K Ferrous metals with short chips;nonferrous metals; nonmetallicmaterials

Red K01, K10 through K40

Carbides

-k type：WC + Co (1-5 µm powder )cutting nonferrous metals and ferrous metals with short chips , nonmetallic materials , cast iron

-M type：WC + Co + TiC + TaCcutting ferrous metals with long or short chips ,cast steels

Carbides

(3)Titanium carbide (TiC)-higher wear resistance and less toughness than WC-cutting steels , cast iron with higher cutting speed (fig .21.2)

Carbide Inserts

Figure 21.2 Typical carbide inserts with various shapes and chip-breaker features; round inserts are also available (Fig. 21.4). The holes in the inserts are standardized for interchangeability. Source: Courtesy of Kyocera Engineered Ceramics, Inc., and Manufacturing Engineering Magazine, Society of Manufacturing Engineers.

Carbides (4)Inserts (fig .21.3)-brazed (fig21.4d)-clamp and/or lockpin (fig.21.4ab)-shpae and its strength (fig.21.5)clearancetolerancetypesize of inscribed circlethicknessnose radius and edge (fig.21.6)

ex. SNMG120408

Figure 21.3 Methods of attaching inserts to toolholders: (a) Clamping, and (b) Wing lockpins. (c) Examples of inserts attached to toolholders with threadless lockpins, which are secured with side screws. Source: Courtesy of Valenite. (d) Insert brazed on a tool shank (see Section 30.2).

Edge Strength Figure 21.4 Relative edge

strength and tendency for chipping and breaking of insets with various shapes. Strength refers to the cutting edge shown by the included angles. Source: Kennametal, Inc.

Figure 21.5 Edge preparation of inserts to improve edge strength. See also Section 23.2. Source: Kennametal, Inc.

Effect of Coating Materials

Figure 21.6 Relative time required to machine with various cutting-tool materials, indicating the year the tool materials were introduced. Source: Sandvik Coromant.

Coated Tools(1)Coating materials-TiN , TiC , Al2O3-thickness：5-10 µm-general characteristics

(a)High hot hardness(b)Chemical stability and inertness(c )low thermal conductivity(d )good bonding to the substrate(e )little or no porosity

-cutting steels , alloy steels , cast iron

Coated Tools (2)TiN coating-low u , high hardness , resistance to high temp.good adhesion higher cutting speeds and feeds gold color lower flank wear (fig .21.8)(3)TiC coating-high flank wear resistance

Multiphase CoatingsFigure 21.7 Multiphase coatings on a tungsten-carbide substrate. Three alternating layers of aluminum oxide are separated by very thin layers ottitanium nitride. Inserts with as many as thirteen layers of coatings have been made. Coating thicknesses are typically in the range of 2 to 10 µm. Source: Courtesy of Kennametal, Inc., and Manufacturing Engineering Magazine, Society of Manufacturing Engineers.

Coated Tools

(4)Al2O3 coating-low thermal conductivity , chemical stability

-resistance to high temp. , flank wear and crater wear-finishing operations

Coated Tools (5)Multiple coatings (fig .21.9)-high speed , continuous cutting：TiC / A12O3

-heavy duty , continuous cutting：TiC / Al2O3 / TiN-light , interrupted cutting：TiC / TiC + TiN / TiN(6)Diamond coatings-cutting nonferrous alloys , nonmetallic materials

Properties for Groups of Tool Materials

Figure 21.8 Ranges of properties for various groups of tool materials. See also Tables 21.1 through 21.5.

Ceramics(1)High abrasion resistance

Hot hardness (fig .21.10)(2)White or cold pressed ceramics-high purity Al2O3-high cutting speed(3)Black or hot pressed ceramics (cermets)-70 % A12O3 + 30 % TiC-improve strength , toughness , and reliability(4)Finishing operations(5)Avoid vibration and reduce thermal shock

Cubic Boron Nitride (CBN)(1)High hot hardness , stiffness , and brittle(2)High resistance to oxidation(3)Chemical Inertness to Fe and Ni(4)Cutting hardened ferrous and high temp.

alloys(5)0.5 – l mm CBN layer on the carbide

substrate (fig.21.11)

Cubic Boron NitrideFigure 21.9 Construction of a polycrystalline cubic boron nitride or a diamond layer on a tungsten-carbide insert.

Figure 21.10 Inserts with polycrystalline cubic boron nitride tips (top row) and solid polycrystalline cBN inserts (bottom row). Source: Courtesy of Valenite.

Silicon-Nitride Base Too1s(1)High toughness , hot hardness(2)Good therma1-shock resistance(3)Sialon-Si , Al , O , N

-cutting cast iron , Ni -base superalloys

Diamond(1)Low friction , high wear resistance(2)Cutting soft nonferrous alloys (A1 , Cu

alloys) ,abrasive nonmetallic materials(3)No t recommended for cutting steel ,Ti ,

Ni , Co base alloys

Cutting processes(1)Turning(2)Facing(3)Form tool(4)Boring(5)Drilling(6)Parting (Cutting off)(7)Threading(8)knurling

Cutting Operations

Figure 22.1 Various cutting operations that can be performed on a late. Not that all parts have circular symmetry.

Cutting conditions(1)Roughing-speed：0.15 – 4 m/s-feed：0.2 – 2 mm/rev.-depth of cut：> 0.5 mm(2)Finishing-lower depth of cut and feed-higher cutting speed

Lathes(1)Lathe components-bed

gray or nodular cast iron-carriage (fig.22.3)-headstock-tailstock

-feed rod and lead screw

Right-Hand Cutting Tool

Figure 22.4 (a) Designations and symbols for a right-hand cutting tool; solid high-speed-steel tools have a similar designation. Right-hand means that the tool travels from right to left as shown in Fig. 22.1a. (continued)

Right-Hand Cutting Tool (cont.)

Figure 22.4 (continued) (b) Square insert in a right-hand toolholder for a turning operation. A wide variety of toolholdersare available for holding inserts at various angles. Source: Kennametal Inc.

Schematic Illustration of a Turning Operation

Figure 22.3 (a) Schematic illustration of a turning operation showing depth of cut, d, and feed, f. Cutting speed is the surface speed of the workpiece at the Fc, is the cutting force, Ft is the thrust or feed force (in the direction of feed, Fr is the radial force that tends to push the tool away from the workpiecebeing machined. Compare this figure with Fig. 20.11 for a two-dimensional cutting operation.

Lathes

(2)Lathe specifications-swing：max. diameter of workpiece (table 21.1)-size：360 mm (swing) * 760 mm (between centers) * l830 mm (bed length)

General Characteristics of Machining Processes

TABLE 22.1 General Characteristics of Machining Processes Described in Chapters 22 and 23Process Characteristics Commercial tolerances(±mm)Turning Turning and facing operations on all types of materials; uses single-point or form tools; requires skilled

labor; low production rate, but medium to high with turret lathes and automatic machines, requiring less-skilled labor.

Fine: 0.05–0.13Rough: 0.13Skiving: 0.025–0.05

Boring Internal surfaces or profiles, with characteristics similar to turning; stiffness of boring bar important to avoidchatter.

0.025

Drilling Round holes of various sizes and depths; requires boring and reaming for improved accuracy; highproduction rate; labor skill required depends on hole location and accuracy specified.

0.075

Milling Variety of shapes involving contours, flat surfaces, and slots; wide variety of tooling; versatile; low tomedium production rate; requires skilled labor.

0.13–0.25

Planing Flat surfaces and straight contour profiles on large surfaces; suitable for low-quantity production; labor skillrequired depends on part shape.

0.08–0.13

Shaping Flat surfaces and straight contour profiles on relatively small workpieces; suitable for low-quantityproduction; labor skill required depends on part shape.

0.05–0.13

Broaching External and internal flat surfaces, slots, and contours with good surface finish; costly tooling; highproduction rate; labor skill required depends on part shape.

0.025–0.15

Sawing Straight and contour cuts on flat or structural shapes; not suitable for hard materials unless saw has carbideteeth or is coated with diamond; low production rate; requires only low labor skill.

0.8

Lathes(3)Workholding devices-chuck (fig.22.4)three or four jaws-power chucks-collet (fig .22.5)-face platesclamping irregularly shaped workpiece-mandrels (fig .22.6)

Cutting Speeds for Various Tool Materials

Figure 22.5 The range of applicable cutting speeds and feeds for a variety of tool materials. Source: Valenite.

Components of a LatheFigure 22.2 Components of a lathe. Source: Courtesy of Heidenreich & Harbeck

ColletsFigure 22.6 (a) and (b) Schematic illustrations of a draw-in type collet. The workpiece is placed in the collet hole, and the conical surfaces of the collet are forced inward by pulling it with a draw bar into the sleeve. (c) A push-out type collet. (d) Workholding of a part on a face plate.

Lathes

(4)Lathe operations (fig .22.1)(5)Tracer lathes (fig.22.7)-follow the template

(6)Automatic lathes-vertical type

Mandrels

Figure 22.7 Various types of mandrels to hold workpieces for turning. These mandrels are usually mounted between centers on a lathe. Note that in (a), both the cylindrical and the end faces of the workpiece can be machined, whereas in (b) and (c), only the cylindrical surfaces can be machined.

Lathes

(7)Turret lathes (fig .22.8)-perform multiple cutting operations such as turning ,boring , drilling , thread turning , facing(8)Computer-controlled lathes (CNC)

(fig.22.l0-11)

Swiss-Type Automatic Screw Machine

Figure 22.8 Schematic illustration of a Swiss-type automatic screw machine. Source: George Gorton Machine Company.

Computer Numerical Control Lathe

Figure 22.10 A computer numerical control lathe. Note the two turrets on this machine. Source: Jones & Lamson, Textron, Inc.

Examples of Turrets

(a) (b)

Figure 22.11 (a) A turret with six different tools for inside-diameter and outside-diameter cutting and threading operations. (b) A turret with eight different cutting tools. Source: Monarch Machine Tool Company.

Examples of Parts Made on CNC Turning Machine Tools

Figure 22.12

Turning Parameters and Process Capabilities

(1)Tool geometry (fig.22.12)(2)Rake angles-controll the direction of chip flow andthe strength of the tool tip-positve angle：reducing cutting forces and temp.

low strength of the tip


(3)Relief angles-controll interface and rubbingat the tool-workpiece interface-too largel：chipping of tool

too small：excessive flank wear of tool


(4)Cutting edge angles-chi p formation , tool strength ,cutting forces to various degrees(5)Nose radius-surface finish , tool-tip strength-small：rougher surface finish and low strength

of tip-large：chatter , better surface finish


(6)Tool materials (table 22.3-4)(7)Cutting fluids (table 22.5)(8)Material removal rate (MRR)

f：mm/rev , N：rpm , (9)Cutting time

dfNDMRR avgπ=( )

2fo

avgDDD +

=

fNLt =

cutting

Documents