metal cutting - marmara

Metal Cutting

ME 3013Manufacturing Processes I

Machining processes

• Cutting processes remove material

• from the various surfaces of a workpiece

• by producing chips.

• Machining widespread in industry

• can be used on a wide variety of materials

• including plastics, metals, wood, composites, and even some ceramics.

• it is often the process of choice for production of parts

• requiring tight tolerances and high-quality surfaces.

Material Removal Processes

• A family of shaping operations,

• the common feature is removal of material

• so the remaining part has the desired shape

• Categories:

• Machining – material removal by a sharp cutting tool,

• e.g., turning, milling, drilling

• Abrasive processes – material removal by hard, abrasive particles, e.g., grinding

• Nontraditional processes - various energy forms other than sharp cutting tool to remove material

(a)A cross-sectional view of the machining process, with positive rake angle(b)tool with negative rake angle; compare in (a)

Metal Cutting: features

•Closer dimensional accuracy

• Surface texture/finish

• Economical

•Complex shape

• Size

• Material loss (~50%)• Scarcity of materials• Special equipment• Skilled operators• Time required• All materials cannot be

machined

Machining or Metal Cutting

• Removing unwanted material

• Cutting/chipping/machining/

• metal cutting

• To get desired

• Shape

• Size

• Surface finish

Outline of Metal Cutting

• Introduction

• Machining

• Chip Formation

• Cutting Tool Materials

• Tool Wear

• Taylor Tool Life Model

• Cutting Fluids

• General Machining Operations

Machining in the Manufacturing Sequence

• Generally performed after other manufacturing processes,

• such as casting, forging, and bar drawing

• Other processes create the general shape of the starting

workpart

• Machining provides the final shape, dimensions, finish, and

special geometric details

• that other processes cannot create

• Turning: workpiece is rotated and a cutting tool removes a layer of material as the tool moves along its length

• Cutting off: the tool moves radially inward, and separates the piece on the right in figure

• Slab milling: a rotating cutting tool removes a layer of material from the surface of the workpiece

• End milling: rotating cutter travels along a certain depth in the workpiece and produces cavity

Turning

• Single point cutting tool removes material

• from a rotating workpiece to form a cylindrical shape

Drilling

• Used to create a round hole,

• usually by means of a rotating tool (drill bit)

• that has two cutting edges

Milling

• Rotating multiple-cutting-edge tool is moved slowly

• relative to work to generate plane or straight surface

• Two forms: peripheral milling and face milling

Some examples of common machining processes.

(a) straight turning, (b) taper turning, (c) contour turning, (d) plain milling, (e) profile milling

(a) thread cutting on a lathe, and (b) slot milling

Metal Cutting

• Metal cutting is basically the process of manufacturing features by physically/mechanically removing material in the form of chips to achieve a desired geometry.

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• Types:– Turning– Boring– Shaping– Planing– Milling– Drilling – Sawing– Broaching– Grinding– Honing– Lapping

Lathe

https://www.youtube.com/watch?v=8EsAxOnzEms

Cutting Conditions in Machining

• The 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 found as

• MRR = v f d

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

Basic Cutting (turning) Terminology

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MRR= Vc*F*D

F=Feed

D=Depth of cut

F=Vaxial/N=(20cm/min)/(300 rpm)=0,067 cm/rev

MRR=(1880 cm/min)*(0,067 cm/rev)*((2-1,9)/2 cm}=6,298 cm3/min

Vc=p*DoN=3,14*(2cm)*(300/60 rev/sec)

=0,314 m/sec =18,8 m/min

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https://www.youtube.com/watch?v=mRuSYQ5Npek

• machining is essentially controlled breakage of material

• the extreme deformations that occur in order to remove that material.

• the material looks soft, like butter, even though it's a metal.

• continuous plastic deformation and shear at the tool-material

interface.

• what limits the rate and quality of a machining process.

Machining or Metal Cutting

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• Force

• motion,

• vibration.

• Heating

• lead to deformation of the workpiece and wear of the tool.

• influence part quality

Machining or Metal Cutting

what limits the rate and quality

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The edge of the tool is perpendicular to the motion.

Orthogonal Cutting Model• A simplified 2-D model of machining • describes the mechanics of machining fairly accurately

• for our analysis, we're going to assume a 2D cutting path. • the edge of the tool, such as our turning tool, is perpendicular to the

motion of the workpiece. • 2D assumption provides good insight for our understanding, • it's important to recognize that the real case is much more

complicated. • because of the shape of the tool and its motion trajectory,• there is really a three-dimensional cutting path.

Machining or Metal Cutting

2D assumption

Orthogonal Cutting

• The chip is formed by a localized shear process

• takes place over very narrow regions (along shear plane).

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• Phi: shear angle

• Rake angle: talas acisi, kritik egim

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Rake: scratch or scrape with a long sweeping movement.

• the rake angle, alpha-- angle between the slope plane of the tool and the perpendicular to the workpiece.

• the shear angle, phi – between a virtual plane that we call the shear plane

• that relates to the deformation of the material

• as it's removed and the final plane of the workpiece after the cut-

• the relief angle, e -- behind the tool, there is some relief.

Machining or Metal Cutting

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Shear strain: the length of deformation divided by the perpendicular length in the plane of the force applied.

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• More realistic view of chip formation, • showing shear zone rather than shear plane. • Also shown is the secondary shear zone resulting from tool-chip friction

Four Basic Types of Chip in Machining

1. Discontinuous chip

2. Continuous chip

3. Continuous chip with Built-up Edge (BUE)

4. Serrated chip

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Continuous Chip with BUE

• Built up edge (BUE) occurs when the work material adheres or welds to the cutting edge of the tool due to the local high temperature and extreme pressure.

• Can be eliminated or minimized by reducing the depth of cut, increasing the cutting speed, using positive rake tools, or applying a coolant.

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Segmented Chip

• Brittle work materials

(e.g., cast irons)

• Low cutting speeds

• Large feed and depth of cut

• High tool-chip friction

Discontinuous Chip

•Machining brittle materials

• Small rake angle

• Large depth of cut

•Machining ductile materials at low cutting speeds and high feed rate

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Continuous Chip

• Ductile work materials

(e.g., low carbon steel)

• High cutting speeds

• Small feeds and depths

• Sharp cutting edge on the tool

• Low tool-chip friction

Continuous Chip

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• Occurs while cutting highly ductile materials.

Continuous with BUE

• Ductile materials

• Low-to-medium cutting speeds

• Tool-chip friction causes

portions of chip to adhere to rake face

• BUE formation is cyclical; it

forms, then breaks off

Serrated Chip

• Semicontinuous - sawtooth

appearance

• Cyclical chip formation

of alternating high shear

strain then low shear strain

• Most closely associated

with difficult-to-machine

metals at high cutting speeds

Forces Acting on Chip

• Friction force F and Normal force to friction N

• Shear force Fs and Normal force to shear Fn

Resultant Forces

• Vector addition of F and N = resultant R

• Vector addition of Fs and Fn = resultant R'

• Forces acting on the chip must be in balance:

R' must be equal in magnitude to R

R’ must be opposite in direction to R

R’ must be collinear with R

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VariablesIndependent

• Tool material and coatings• Tool shape, surface finish

and sharpness• Material• Cutting speed, feed and

depth of cut• Cutting fluids• Fixturing

Dependent• Type of chip• Force and energy

dissipated• Temperature rise • Tool wear• Surface finish

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feed

Effects of Work Material• Force requirements• Tool wear• Chip formation

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Single Point Cutting Tool Geometry

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Tool Performance Example

High-speed steel (HSS)

Carbide: a very hard material made of carbon and one or more heavy metals.

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Tool Wear

FIGURE 8.20 Examples of wear in cutting tools. (a) Flank wear; (b) crater wear; (c) chipped cutting edge; (d) thermal

cracking on rake face; (e) flank wear and built-up edge; (f) catastrophic failure (fracture). Source: Courtesy of

Kennametal, Inc.

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Cutting Tool Materials

•Requirements:

•Hot hardness

•Wear resistance

• Toughness

• Low friction

• Favorable cost

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• Classification:

• Carbon tool steels

• Medium alloy steels

• High speed steel

• Cast cobalt based alloys

• Cemented carbides

• Ceramics and ultra-hard materials

Tool Materials - Steel

• Tool Steel• Plain carbon steel of 0.9 to 1.3% carbon

• Hardened and tempered

• Looses its toughness above 200oC.

• High Speed Steel (HSS)• 18% Tungsten, 4% Cromium, 1% Vanadium

• Retains hardness up to 600oC

• Compared to tools steel, cutting speed could be doubled for an equal

life.

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Tool Materials – Cemented Carbide• Produced by Powder Metallurgy.

• Can be operated at cutting speeds 200 to 500%greater than those of HSS.

• Not as tough as HSS

• Sometimes reacts with iron and steel during cutting

• Lose hardness at high cutting speeds

• Made in the form of throwaway inserts.

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Tool Materials - Ceramic

• Made of pure aluminum oxide by P/M technology.

• Can be operated at 2-3 times the cutting speed of TiC w/ocoolant.

• Resist cratering, usually require no cooling

• Are in the form of disposable tips.

• Are as hard as carbides but are more brittle.

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Titanium carbide

Tool Materials - Diamond

• Hardest material known

• Diamond machining is done at high speeds with fine feeds

• Produces excellent surface finish

• More expensive than any other tool

• Diamond particles are also used in grinding operations

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Tool Materials – Cubic boron nitrade (CBN)

• Hardest man-made material.

• Retains its hardness at ~1000oC.

• Can be used to machine hard aero-space materials.

• Generally used as coating material for other tools.

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Tool Materials – Coated Carbide

• A tough-, shock-resistant tool is

coated with a thin, hard, crater-

wear resistant material.

• TiC-coated tools are 2-3 times

more wear resistant than an

uncoated one.

• yields 50-100% increase in cutting

speed for the same tool life.

• Ceramic (Al2O3) coating

permits 90% speed increase

in machining of steel.87

Tool Failure and Tool Life

• The failure of the cutting tool can be classified into two broad categories:• Slow-death mechanism

• Due to gradual wear

• Sudden-death mechanism• Due to unpredictable and catastrophic failures

• Tool life is the elapse time between the start and finish of the cut, in minutes• Volume of the material removed

• Number of holes drilled

• Number of pieces machined per tool

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