Theory of metal cutting

by

MECH DEPT

KIOT

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• Introduction

• Metal removing process

• Metal cutting process

• Angles of single point cutting tool

• Types of chips

• Chip breakers

TOPICS

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Metal removing process

• Non cutting process

• Cutting process

Generally cutting edge of tool is__________ then the metal which is going to cut

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Machining Process

• Cast, formed and shaped products may need further machining operations to give them the desired final shape, after removal of extra material in the form of chips.

• Machining processes remove material from a work piece by

• CUTTING ( As in case of machine tools like lathe, shaper etc)

• ABRASIVE ( As in case of a grinding wheel)

• NON TRADITIONAL ( Processes such as EDM, Etc.)

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Relative motions

• Rotations of work pcs against tool

• Rotations of tool against the work

• Linear motion of work against tool

• Linear movements of tool against the work

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Metal Cutting Processes

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Turning • High proportion of work machined in shop

turned on lathe – Turning tool set to given depth of cut, fed

parallel to axis of work (reduces diameter of work) • Chip forms and slides along cutting tool's upper

surface created by side rake

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Planing or Shaping

• Workpiece moved back and forth under cutting tool

– Fed sideways a set amount at end of each table reversal

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Drilling

• Multi-edge cutting tool that cuts on the point

• Drill's cutting edges (lips) provided with lip clearance to permit point to penetrate work piece as drill revolves

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What is a Cutting Tool • A cutting tool is any tool that is used to

remove metal from the work piece by means of shear deformation.

• It is one of most important components in machining process

• It must be made of a material harder than the material which is to be cut, and the tool must be able to withstand the heat generated in the metal cutting process.

• Two basic types – Single point

– Multiple point

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Single Point Cutting Tool

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Multi Point Cutting Tool

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Cutting-Tool Materials

• Cutting tool bits generally made – High-speed steel

– Cast alloys

– Cemented carbides

– Ceramics

– Cermets

– Cubic Boron Nitride

– Polycrystalline Diamond

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Cutting Tool Properties

• Hardness – Cutting tool material must be 1 1/2 times harder

than the material it is being used to machine.

• Capable of maintaining a red hardness during machining operation – Red hardness: ability of cutting tool to maintain

sharp cutting edge at elevated temp.

– It is also sometimes referred to as hot hardness or hot strength

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Cutting Tool Properties

• Wear Resistance – Able to maintain sharpened edge throughout

the cutting operation

– Same as abrasive resistance

• Shock Resistance – Able to take the cutting loads and forces

• Shape and Configuration – Must be available for use in different sizes and

shapes.

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Single Point Cutting Tool

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Single Point Cutting Tool

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Know the Single Point Cutting Tool

• Shank: Main body of tool, it is part of tool which is gripped in tool holder • Face: Top surface of tool b/w shank and point of tool. Chips flow along this surface • Flank: Portion tool which faces the work. It is surface adjacent

to & below the cutting edge when tool lies in a horizontal position.

• Point: Wedge shaped portion where face & flank of tool meet. • Base: Bearing surface of tool on which it is held in a tool

holder. • Nose radius: Cutting tip, which carries a sharp cutting point.

Nose provided with radius to enable greater strength, increase tool life & surface life.

Typical Value : 0.4 mm – 1.6 mm MECH-KIOT

SPC Tool

Geometry

SIDE RELIEF

SIDE CLEARANCE

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Nomenclature of Single Point Lathe Tool

The most significant terms in the geometry of a

cutting tool angles are:

–Relief or clearance angle

» Side relief

» End relief

–Rake angle

» Back Rake angle

» Side Rake angle

–Cutting edge angle

» Side Cutting edge angle

» End Cutting edge angl

» Nose Radius

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Cutting-Tool Terms Relief or Clearance angle:

– Ground on the end and side faces of a tool to prevent it from rubbing on the work piece.

– To enable only the cutting edge to touch the work piece.

Side Relief angle:

• Angle ground directly below the cutting edge on the flank of the tool

End Relief angle:

• Angle ground from the nose of the tool

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Cutting-Tool Terms

Cutting edge angle • Ground on a tool so that it can be mounted in the

correct position for various machining operations.

Side Cutting edge angle • Allows flank of the tool to

approach the work piece first • Spreads the material over a

greater distance on the cutting edge, thereby thinning out the chip.

• Approximately 150

End Cutting edge angle • Allows the cutting tool to

machine close to the work piece during turning operations

• Usually 20 – 300

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Cutting-Tool Terms Rake angle:

– Ground on a tool to provide a smooth flow of the chip over the tool so as to move it away from the work piece

Back Rake angle • Ground on the face of the

tool • Influences the angle at

which chip leaves the nose of the tool

• Generally 8 - 100

Side Rake angle • Ground on the tool face

away from the cutting edge • Influences the angle at

which the chip leaves the work piece

• A lathe tool has 140 side rake.

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Side Rake

• Large as possible to allow chips to escape

• Amount determined – Type and grade of cutting tool

– Type of material being cut

– Feed per revolution

• Angle of keenness – Formed by side rake and side

clearance

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Back Rake

• Angle formed between top face of tool and top of tool shank

– Positive

• Top face slopes downward away from point

– Negative

• Top face slopes upward away from point

– Neutral

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Rake Angles

• Small to medium rake angles cause:

– high compression

– high tool forces

– high friction

– result = Thick—highly deformed—hot chips

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Negative Rake Tools

• Typical tool materials which utilize negative rakes are:

• Carbide

• Diamonds

• Ceramics

• These materials tend to be much more brittle than HSS but they hold superior hardness at high temperatures. The negative rake angles transfer the cutting forces to the tool which help to provide added support to the cutting edge.

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Cutting-Tool Terms

Functions: • Strengthens finishing

point of tool • Improves surface

finish on work • Should be twice

amount of feed per revolution

• Too large – chatter; too small – weakens point

Nose Radius: • Rounded tip on the point of the tool

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Tool Angle Application

• Factors to consider for tool angles

– The hardness of the metal

– Type of cutting operation

– Material and shape of the cutting tool

– The strength of the cutting edge

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Basic Mechanics of Metal Cutting

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Fundamentals of cutting

• Fig 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 are all independent variables.

Fig 20.1 Examples of cutting process

Fig 20.2 Basic principle of turning operation

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Factors influencing cutting process

Parameter Influence and interrelationship

Cutting speed depth of

cut,feed,cutting fluids.

Tool angles

Continuous chip

Built-up-edge chip

Discontinuous chip

Temperature rise.

Tool wear

Machinability

Forces power,temperature rise,tool life,type of chips,surface

finish.

As above;influence on chip flow direction;resistance to tool

chipping.

Good surface finish;steady cutting forces;undesirable in

automated machinery.

Poor surface finish,thin stable edge can product tool surface.

Desirable for ease of chip disposal;fluctuating cutting

forces;can affect surface finish and cause vibration and

chatters.

Influences surface finish,dimensional accuracy,temperature

rise,forces and power.

Influences surface finish,dimensional accuracy,temperature

rise,forces and power.

Related to tool life,surface finish,forces and power

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Chip Formations

• During this process three basic types of chips are formed namely:

– Discontinuous

– Continuous

– Continuous with a Built-Up Edge (BUE)

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Types of chips

• Continuous

• Built up edge

• Serrated or segmented

• Discontinuous

Fig20.5 Basic types of chips and their photomicrographs produced in metal cutting (a) continuous ship with a 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 chips

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Continuous chips

Fig :20.6 (a) Hardness distribution in the cutting zone for 3115 steel.Note that some regions in the built-up edge are as mach as three times harder than the bulk metal

(b) Surface finish in turning 5130 steel with a built-up

edge

(c) Surface finish on 1018 steel in face milling

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• Continuous chips are usually formed at high rake angles and/or high cutting speeds.

• A good surface finish is generally produced.

• continuous chips are not always desirable, particularly in automated machine tools,

• tend to get tangled around the tool

• operation has to be stopped to clear away the chips.

Continuous chips

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Continuous

• Continuous “ribbon” of metal that flows up the chip/tool zone.

• Usually considered the ideal condition for efficient cutting action.

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Continuous

• Conditions which favor this type of chip:

– Ductile work

– Fine feeds

– Sharp cutting tools

– Larger rake angles

– High cutting speeds

– Proper coolants

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Discontinuous

• Typically associated with brittle metals like Cast Iron

• As tool contacts work, some compression takes place

• As the chip starts up the chip-tool interference zone, increased stress occurs until the metal reaches a saturation point and fractures off the work piece.

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Discontinuous

• Conditions which favor this type of chip

– Brittle work material

– Small rake angles on cutting tools

– Coarse machining feeds

– Low cutting speeds

– Major disadvantage—could result in poor surface finish

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Discontinuous chips

• Discontinuous chips consist of segments that may be firmly or loosely attached to each other

• These chips occur when machining hard brittle materials such as cast iron.

• Brittle failure takes place along the shear plane before any tangible plastic flow occurs

• Discontinuous chips will form in brittle materials at low rake angles (large depths of cut).

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Serrated chips

• Figure :20.5e

• Segmented chips or non-homogeneous chips

• Semi continuous chips with zones low and high shear strain

• Low thermal conductivity and strength metals exhibit this behavior

Fig 20.5 (e)segmented or

nonhomogeneous chip and

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Built-up edges chips • BUE consists of layers of material from the workpiece

that are gradually deposited on the tool.

• BUE then becomes unstable and eventually breaks up

• BUE material is carried away on the tool side of the chip

• the rest is deposited randomly on the workpiece surface.

• BUE results in poor surface finish

• reduced by increasing the rake angle and therefore decreasing the depth of cut.

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Continuous with a Built-up Edge(BUE)

• Same process as continuous, but as the metal begins to flow up the chip-tool zone, small particles of the metal begin to adhere or weld themselves to the edge of the cutting tool.

• As the particles continue to weld to the tool it affects the cutting action of the tool.

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Continuous with a built-up edge(BUE)

This type of chip is common in softer non-ferrous metals and low carbon steels.

Problems ◦ Welded edges break off and

can become embedded in workpiece

◦ Decreases tool life ◦ Can result in poor surface

finishes

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Chip Breakers • Long continuous chip

are undesirable

• Chip breaker is a piece of metal clamped to the rake surface of the tool which bends the chip and breaks it

• Chips can also be broken by changing the tool geometry,thereby controlling the chip flow

Fig 20.7 (a) Schematic illustration of the action of a chip breaker .(b) Chip breaker clamped on the rake of a cutting tool. (c) Grooves in cutting tools acting as chip breakers

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Chip Breakers

Fig: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

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Chip Formation in Nonmetallic Materials

Fig: a) cutting with an oblique tool b) Top view showing the inclination angle, i.

c) Types of chips produced with different inclination

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Temperature In Cutting

Fig:Typical temperature distribution in the

cutting zone.

Fig:Percentage of the heat generated in cutting

going into the workpiece,tool,and chip,as a

function of cutting speed.

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Tool Life: Wear and Failure

1. Flank wear :It occurs on the relief face of the tool and the side relief angle.

2. Crater wear:It occurs on the rake face of the tool.

3. Chipping :Breaking away of a small piece from the cutting edge of the tool .

Fig (a) Flank and crater wear in a cutting tool.tool moves to the left. (b) View of the rake 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,sowing the average flank wear land VB and the depth-of-cut line (wear notch)

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Heat & Temperature in Machining

• In metal cutting the power input into the process in largely converted to heat.

• This elevates the temperature of the chips, work-piece and tool.

• These elements along with the coolant act as heat sinks.

• So lets look at coolants…

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Coolants/Cutting fluids

• Cutting fluids are used extensively in metal removal processes and they

– Act as a coolant, lubricant, and assist in removal of chips.

– Primary mission of cutting fluids is to extend tool life by keeping keep temperatures down.

– Most effective coolant is water…

– However, it is hardly ever used by itself.

– Typically mixed with a water soluble oil to add corrosion resistance and add lubrication capabilities. MECH-KIOT

Issues Associated With Coolants

• Environmental Concerns

• Machine systems and Maintenance

• Operators Safety

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Machining Operations

• Machining Operations can be classified into two major categories:

– Single point = Turning on a Lathe

– Multiple tooth cutters = pocket milling on a vertical milling machine

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Tool Selection Factors

• Inputs

• Work material

• Type of Cut

• Part Geometry and Size

• Lot size

• Machinability data

• Quality needed

• Past experience of the decision maker MECH-KIOT

Constraints

• Manufacturing Practice

• Machine Condition

• Finish part Requirements

• Work holding devices/Gigs

• Required Process Time

Outputs

• Selected Tools

• Cutting parameters MECH-KIOT

Elements of an Effective Tool

• High Hardness

• Resistance to Abrasion and Wear

• Strength to resist bulk deformation

• Adequate thermal properties

• Consistent Tool life

• Correct Geometry

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Tool Materials

• Wide variety of materials and compositions are available to choose from when selecting a cutting tool

• We covered these in the previous chapter

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