MODULE 4

OUTLINES

Mechanical cutting techniques

Tool materials

Non –conventional machining

Economics of metal cutting

Why Machining is Important ?

Variety of materials can be machined

Most frequently applied to metals

Variety of part shapes & special geometry features possible

Accurate round holes

Very straight edges and surfaces

Good dimensional accuracy and surface finish

Disadvantages of 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

Lot of Power (electrical energy) is required

Three basic categories of material removal processes are

1. Conventional machining,2. Abrasive processes, and3. Non-conventional processes

THEORY OF METAL CUTTING           

The metal cutting is done by a relative motion between the work piece & the cutting tool. 

Metal cutting could be done either by a single point cutting tool or a multi  point cutting tool. 

The two basic types of metal cutting by a single point cutting tool are orthogonal and oblique metal cutting. 

If the cutting face of the tool is at 90o to the direction of the tool travel the cutting action is called as orthogonal cutting. 

If the cutting face of the tool is inclined at less than 90o to the path of the tool then the cutting action is called as oblique cutting.

Elements of Metal Cutting :

Cutting speed : It is the distance traveled by work surface related to the cutting edge of toolv = (πdN / 1000 ) m/min

Feed (s) : The motion of cutting edge of tool with reference to one revolution of work piece.

Depth of cut (t) : It is measured perpendicular to axis of work piece and in straight turning in one pass.

Chip Formation

In any traditional machining process, chips are formed by a shearing process

Machining = Chip formation by a tool

When the ultimate stress of the metal is exceeded, separation of metal takes place. The plastic flow takes place in a localized area called as shear plane

C

α = rake angle

•Cutting action involves shear deformation of work material to form a chip. • As chip is removed, new surface is exposed

Chip Formation

Types of Chips

1. Continuous2. Continuous chips with Built up edge3. Discontinuous

Type of chips produced influences surface finish, integrity and machining operation.

• Ductile work materials

(e.g., low carbon steel, Al, Copper)

• Usually formed at high cutting speed or high rake angle

• Small feeds and depths

(1) Continuous Chip

Continuous chip• Sharp cutting edge on the tool

• Low tool chip friction‑• Suitable cutting fluid

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

• Usually considered the ideal condition for efficient cutting action.

Problems with Continuous chip

• Chips are difficult to handle and dispose off.

• The chips coil in a helix and curl around work and tool and may injure the operator when it is breaking.

• The tool face is in contact for a longer period resulting in more frictional heat. However this problem could be rectified by the use of chip breakers.

This is desirable because it produces good surface finish, low power consumption and longer tool life.

• Forms at the tip of the tool • Plastic deformation causes

adhesion between chip and tool face

• Tool-chip friction causes portions of chip to adhere to rake face

• BUE forms, then breaks off, cyclically

(2) Continuous with BUE

• Adversely affects the surface finish• Changes the geometry of cutting tool

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

Continuous with BUE

To avoid BUE : 1. Decrease Depth of cut, 2. Increase rake angle,

3. Reduce tool tip radius, 4.Use proper cutting fluid

Continuous with BUE

• Brittle work materials ( like bronze, hard brass and gray cast iron )

• Generally non metals like ceramics, polymers, composites, form discontinuous chips

• Low cutting speeds

• Large feed and depth of cut

(3) Discontinuous Chip

• Materials that contain hard inclusions & impurities

• Lack of proper cutting fluid

• Low stiffness of the m/c • These are convenient to handle and dispose off

Discontinuous Chip

If these chips are produced from brittle materials, then the surface finish is fair, power consumption is low and tool life is reasonable however with ductile materials the surface finish is poor and tool wear is excessive.

Part  Description

Shank It is the body of the tool which is ungrounded.

Face It is the surface over which the chip slides.

Base It is the bottom surface of the shank.

Flank It is the surface of the tool facing the work piece.  There are two flanks namely end flank and side flank.

Cutting edge

It is the junction of the face end the flanks.  There are two cutting edges namely side cutting edge and end cutting edge.

Nose  It is the junction of side and end cutting edges.

Single point cutting tool:Parts of a single point cutting tool:

Angle Details

Top rake angle It is also called as back rake angle.  It is the slope given to the face or the surface of the tool. 

Side rake angle It is the slope given to the face or top of the tool.  This slope is given from the nose along the width of the tool.  The rake angles help easy flow of chips

Relief angle These are the slopes ground downwards from the cutting edges.  These are two clearance angles namely, side clearance angle and end clearance angle.  This is given in a tool to avoid rubbing of the job on the tool.

Cutting edge angle

There are two cutting edge angles namely side cutting edge angle and end cutting edge angle.  Side cutting edge angle is the angle, the side cutting edge makes with the axis of the tool.  End cutting edge angle is the angle, the end cutting edge makes with the width of the tool.

Lip angle It is also called cutting angle.  It is the angle between the face and end surface of the tool.

Nose angle It is the angle between the side cutting edge and end cutting edge.

CUTTING TOOL MATERIALS CHARATERISITCS

• Selection of cutting tool materials is very important

What properties should cutting tools have ?– Hardness at elevated temperatures (hot

hardness)– Toughness so that impact forces on the tool

can be taken– Wear resistance– Chemical stability or inertness

Different tool materials1. Carbon and medium alloy steel

2. High speed steel (HSS)

3. Cast cobalt alloys

4. Carbides

5. Coated tools

6. Ceramics

7. Diamond

The List below shows some commercial tool materials

CBN - Cubic Boron Nitride

Ceramics

HSS - High Speed Steel

PCD - Polycrystalline Diamond

WC - Tungsten Carbide

Coated WC - Tools coated with Tungsten Carbide

Hardness of Cutting Tool Materials as a Function

of Temperature

Figure . The hardness of various cutting-tool materials as a function of temperature (hot hardness). The wide range in each group of materials is due to the variety of tool compositions and treatments available for that group.

Tool life:

Volume of material removed b/w two successive tool grind. Number of work piece machined b/w two successive tool grinds. Time of actual cutting b/w two successive tool grinds.

Following are the factors influencing tool life.

Cutting speed:

It has the greatest influence.  When the cutting speed increases, the cutting temperature increases.  Due to this, hardness of the tool decreases. 

Feed and depth of cut: For a given cutting speed if the feed or depth of cut is increased, tool life will be reduced.

The relation ship between tool life and cutting speed is given by the Taylor's formula which statesVTn = CV is the cutting speed in meters / minuteT is the tool life in minutes.n depends on the tool and work.C a constant.

The useful tool life of a HSS tool at 18 m/min is 3 hours. Calculate the tool life when the tool operates at 24 m/min. ( take n = 0.125 )

Solution: VTn = C V = 18 m/min T = 3 x 60 = 180 min Constant C = 18 x ( 180 ) 0.125 = 34.45 Now V = 24 m/min. T = ( 34.45 / 24 ) 1/0.125

= 18 minutes.

Tool geometries:

There are two distinct tool geometries.  The are positive and negative rake angles. 

Positive is suitable for machining soft, ductile materials (like aluminum) and negative is for cutting hard materials, where the cutting forces are high (Hard material, high speed and feed).

Cutting Temperatures are Important….

High cutting temperatures …1. Reduce tool life2. Produce hot chips that pose safety hazards

to the machine operator3. Can cause inaccuracies in part dimensions

due to thermal expansion of work material

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.

Power and Energy Relationships

• A machining operation requires power• The power to perform machining can be

computed from: P = Fc v

where P = cutting power; Fc = cutting force;

v = cutting speed

Why Nonconventional processes? To machine new (harder, stronger & tougher) materials

difficult or impossible to machine conventionally

For unusual & complex geometries that cannot easily

machined conventionally

To achieve stringent surface (finish & texture) requirements

not possible with conventional machining

Advantages of Non-conventional machining: 1) High accuracy and surface finish 2) No direct contact of tool and w/p, so there is less/no wear 3) Tool life is more 4 ) Quieter operation

Disadvantages of non-conventional machining: 1) High cost 2) Complex set-up 3) Skilled operator required

Classification

Thermal & Electro thermal

Chemical & Electro chemical

Mechanical

1. EDM2. LBM3. PAM4. EBM5. IBM

1. ECM2. CHM

1. USM2. AJM3. WJM

ELECTRICAL DISCHARGE MACHINING

Electrical discharge machining (EDM) Based on erosion of metals by spark discharges.

EDM system consist of a tool (electrode) and work piece,

connected to a dc power supply and placed in a dielectric

fluid.

When potential difference between tool and work piece is

high, a transient spark discharges through the fluid,

removing a small amount of metal from the work piece

surface.

This process is repeated with capacitor discharge rates of 50-

500 kHz.

Dielectric fluid – mineral oils, kerosene, distilled and deionized water etc.

Role of the dielectric fluid

Acts as a insulator until the potential is sufficiently high. Acts as a flushing medium and carries away the debris. Also acts as a cooling medium.

Electrodes – usually made of graphite.

WIRE EDM

Wire EDM

This process is similar to contour cutting with a band saw.

A slow moving wire travels along a prescribed path, cutting the work piece with discharge sparks.

Wire should have sufficient tensile strength and fracture toughness.

Wire is made of brass, copper or tungsten. (about 0.25mm in diameter).

Examples of part made using EDMsource: www.agie-charmilles.com

Parts with complex, precise and irregular shapes for forging, press tools, extrusion dies, difficult internal shapes for aerospace and medical applications can be made by EDM process. Some of the shapes made by EDM process are shown in figure.

Applications of EDM

Advantages of EDM

Materials of any hardness can be machined No burrs are left in machined surface Thin and fragile/brittle components can be machined

without distortion Complex internal shapes can be machined

Limitations of EDM

Suitable only for electrically conductive materials MRR is low and the process is slow compared to

conventional machining processes Unwanted erosion and over cutting of material can occur Rough surface finish at high rates of material removal

Laser beam machining

Laser-beam machining :

Utilizes a high-energy, coherent light beam to melt and vaporize particles on the surface of metallic and non-metallic w/p.

Lasers can be used to cut, drill, weld and mark. LBM is particularly suitable for making accurately placed holes.

Different types of lasers are available for manufacturing operations which are as follows:

1.CO2 Gas laser : It is a gas laser that emits light in the infrared region. It can provide up to 25 kW in continuous-wave mode.

2. Nd:YAG: Neodymium-doped Yttrium-Aluminum-Garnet (Y3Al5O12) laser is a solid-state laser which can deliver light through a fibre-optic cable. It can provide up to 50 kW power in pulsed mode and 1 kW in continuous-wave mode.

Applications of Laser in Mfg

Advantage of laser cutting

No limit to cutting path as the laser point can move any path. The process is stress less allowing very fragile materials to be laser

cut without any support. Very hard and abrasive material can be cut. Sticky materials are also can be cut by this process. It is a cost effective and flexible process. High accuracy parts can be machined. No cutting lubricants required No tool wear Narrow heat effected zone

Limitations of laser cutting Uneconomic on high volumes compared to stamping Limitations on thickness due to taper High capital cost High maintenance cost Assist or cover gas required

Examples of laser cutting using pulsed CO2 Laser

Applications

LBM can make very accurate holes as small as 0.005 mm in refractory metals, ceramics,

and composite material without warping the work pieces Used widely for drilling and cutting of metallic and non-metallic materials.

Laser beam machining is being used extensively in the electronic and automotive industries.

Plasma Arc Machining - PAM Plasma arc machining (PAM) is a material removal process in which the material is removed by directing a high velocity jet of high temperature

(11,000-30,000°C) ionized gas on the work piece.

Plasma: A mixture of free electrons, positively charged ions and neutral atoms.

Plasma can be obtained by heating a gas to a very high temperature so that it is partially ionized.

Advantages Disadvantages

Cuts any metal. 5 to 10 times faster

than oxy-fuel. 150 mm thickness

ability. Easy to automate.

Large heat affected zone.

Difficult to produce sharp corners.

Smoke and noise. Burr often results.

PAM….

ELCTRON BEAM MACHINING

Electron beam machining (EBM)

• Similar to LBM except laser beam is replaced by high

velocity electrons.

• When electron beam strikes the work piece surface,

heat is produced and metal is vaporized.

• Surface finish achieved is better than LBM.

• Used for very accurate cutting of a wide variety of

metals.

Applications of EBM :

1. To drill gas orifices for pressure differential devices (used in

nuclear reactors, rotors and aircraft engines, etc.)

2. To produce wire drawing dies, light-ray orifices and spinnerets

to produce synthetic fibers.

3. To produce metering holes, either round or profile shaped, to

be used as flow holes on sleeve valves, rocket fuel injectors or

injection nozzles on diesel engines.

4. To scribe thin films.

5. To remove small broken taps from holes.

Advantages of EBM:

1. Very small holes and slots of high precision in a short time in almost

any material can be made.

2. Different shapes of holes, slots and orifices can be machined.

3. There is no mechanical contact between the tool and the w/p.

Limitations of EBM4. High cost of equipment.

5. Limited applicability (maximum depth of cut is 4 mm).

6. Low material removal rate.

7. Non-uniformity of holes and slots like taper and the entrance of holes

and slots is cratered and bell shaped.

8. Requires skilled workmanship.

C

ELECTROCHEMICAL MACHINING

Electrochemical Machining

• Reverse of electroplating• An electrolyte acts as a current carrier and high electrolyte

movement in the tool-work-piece gap washes metal ions away from the work piece (anode) before they have a chance to plate on to the tool (cathode).

• Tool – generally made of bronze, copper, brass or stainless steel.

• Electrolyte – salt solutions like sodium chloride or sodium nitrate mixed in water.

• Power – DC supply of 5-25 V.

Advantages of ECM• Burr free surface.• No thermal damage to the parts.• Lack of tool force prevents distortion of parts.• Capable of machining complex parts and hard materials

Limitations of ECM• ECM is not suitable to produce sharp square corners or flat

bottoms because of the tendency for the electrolyte to erode away sharp profiles.

• ECM can be applied to most metals but, due to the high equipment costs, is usually used primarily for highly specialised applications.

Parts made by ECM

Due to low forces on tool,ECM can be used to makeholes at very large angle toa surface – an example is

shown in the turbine nozzleholes in the figure here.[source: www.barber-nichols.com]

Due to low forces on tool,ECM can be used to makeholes at very large angle toa surface – an example is

shown in the turbine nozzleholes in the figure here.[source: www.barber-nichols.com]

Water Jet Machining

Fig : (a) Schematic illustration of WJM

(b) A computer-controlled, WJM cutting a granite plate.

(c) Example of various nonmetallic parts

produced by the water-jet cutting process.

Water jet machining (WJT)

• Water jet acts like a saw and cuts a narrow groove in the material.

• Pressure level of the jet is about 400MPa. Advantages

- no heat produced- cut can be started anywhere without the need for

predrilled holes- burr produced is minimum- environmentally safe and friendly manufacturing

Application – used for cutting composites, plastics, fabrics, rubber, wood products etc. Also used in food processing industry.

Abrasive Jet Machining

Fig : Schematic illustration of Abrasive Jet Machining

Abrasive Jet Machining (AJM)

• In AJM a high velocity jet of dry air, nitrogen or CO2 containing abrasive particles is aimed at the work piece.

• The impact of the particles produce sufficient force to cut small hole or slots, deburring, trimming and removing oxides and other surface films.