THEORY OF METAL CUTTING

            The metal cutting is done by a relative motion between the work piece and the hard edge of a cutting tool.  Metal cutting could be done either by a single point cutting tool or a multi  point cutting tool.  There are two basic types of metal cutting by a single point cutting tool.  They are orthogonal and oblique metal cutting.  If the cutting face of the tool is at 90^o 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 90^o to the path of the tool then the cutting action is called as oblique cutting.  The differences between orthogonal and oblique cutting is given below

Elements of Metal Cutting :

Cutting speed : It is the distance traveled by work surface related to the cutting edge of Tool

v = 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.  This can be estimated from the relation

t = ( D - d ) / 2 mm

Undeformed chip (Fc) : The cross sectional area of chip before it is removed from work piece.  it is equal to the product of feed and depth of cut.

Fc = s x t mm²

            All tools have a major and minor cutting edge.  The major cutting edge removes bulk of material.  Where as the minor cutting edge gives good surface finish.

            When the tool advances into the work piece, the metal in front of the tool is severely stressed.  The cutting tool produces internal shearing action in the metal.  The metal below the cutting edge yields and flows plastically in the form of chip.  Compression of the metal under the tool takes place.  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.  The chip moves upward on the face of the tool.  There are three different types of chips.  They are

1. Continuous chips,
2. Discontinuous chips and
3. Chips with built up edge.

Continuous chips :

• The conditions that favor the production of continuous chips is small chip thickness, high cutting speed, sharp cutting edge, large rake angle in cutting tool and fine feed, smooth tool face and efficient lubricating system.  
• Such chips are produced while machining ductile materials like mild steel, copper and aluminum.  Because of plastic deformation of ductile material long and continuous chips are produced.  
• This is desirable because it produces good surface finish, low power consumption and longer tool life.  
• These chips are difficult to handle and dispose off.  Further 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.

Chip breakers: 

            During machining, long and continuous chip will affect machining.  It will spoil tool, work and machine.  It will also be difficult to remove metal and also dangerous.  The chip should be broken into small pieces for easy removal, safety and to prevent damage to machine and work.  The function of chip breakers is to reduce the radius of curvature of chips and thus break it.  The upper side of continuous chips notches while the lower side which slides over the face tool is smooth and shiny.  The chips have the same thickness through.

Discontinuous chips :

• These chips are produced when cutting more brittle materials like bronze, hard brass and gray cast iron.
• Since there chips break up into small segments the friction between chip and tool reduces resulting in better surface finish.
• These are convenient to handle and dispose off.
• Discontinuous chips are produced in ductile materials under the conditions such as large chip thickness, low cutting speed, small rake angle of tool etc.
• Brittle materials lack the ductility necessary for appreciable plastic chip deformation.  The amount of deformation which the chip undergoes by deformation is limited by repeated fracturing.
• 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.

Chips with built up edge :

• This is nothing but a small built up edge sticking to the nose of the cutting tool.  These built up edge occurs with continuous chips.
• When machining ductile materials due to conditions of high local temperature and extreme pressure the cutting zone and also high friction in the tool chip interface, there are possibilities of work material to weld to the cutting edge of tool and thus forming built up edges.
• This weld metal is extremely hard and brittle.  This welding may affect the cutting action of tool.
• Successive layers are added to the build up edge.  When this edge becomes large and unstable it is broken and part of it is carried up the face of the tool along with chip while remaining is left in the surface being machined.  Thus contributing to the roughness of surface.
• Thus the size of the built up edge, varies during the machining operation.  It first increases, then decrease and again increases.
• this built up edge protects the cutting edge of tool, thus changing the geometry of the cutting tool.
• Low cutting speeds lead to the formation of built up edge, however with high cutting speeds associated with sintered carbide tools, the build up edge is negligible or does not exist.
• Conditions favoring the formation of build up edge are low cutting speed, low rake angle, high feed and large depth of cut.  This formation can be avoided by the use of coolants and taking light cuts at high speeds.  This leads to the formation of crater on the surface of the tool.

Single point cutting tool:

Parts of a single point cutting tool:

Important angles of a single point cutting tool:

Required properties of cutting tool material:

Hot hardness:

            This is the ability of the material to with stand very high temperature without loosing its cutting edge.  The hardness of the tool material can be improved by adding molybdenum, tungsten, vanadium, chromium etc which form hard carbides.  High hardness gives good wear resistance but poor mechanical shock resistance.

Wear resistance:

            The ability of the tool to withstand wear is called as wear resistance.  During the process of machining, the tool is affected because of the abrasive action of the work piece.  If the tool does not have sufficient wear resistance then there are possibilities of failure of cutting edge.  Lack of chemical affinity between the tool and work piece also improve wear resistance.

Toughness:

            This property posses limitation on the hardness of the tool because of very high hardness the material becomes brittle and weak.

Low friction:

            In order to have a low tool wear and better surface finish the co-efficient of friction between the tool and chip must be low.  The thermal conductivity must be high for quick removal of heat from chip tool interface.

            In addition to the above, it must posses the following mentioned properties.

1. Mechanical and thermal shock resistance,

2. Ability to maintain the above properties at the high operating temperatures.

3. Should be easy to regrind and easy to weld the tool.

            In addition to the above, high thermal shock resistance is also desirable.  But no single material fulfills all the above requirements.

Tool life:

            It is an important factor in cutting tool performance.  The tool can not cut effectively for an unlimited period of time.  It has a definite life.  Tool life is the time for which the tool will operate satisfactorily until it becomes blunt.  It is the time between two successive 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.  Hence the tool flank wear and crater wear also occurs easily.  The relation ship between tool life and cutting speed is given by the Taylor's formula which states

VTⁿ = C

V is the cutting speed in meters / minute
T is the tool life in minutes.
n depends on the tool and work.
C a constant.

Feed and depth of cut:

            The tool life depends upon the amount of material removed by the tool per minute.  For a given cutting speed if the feed or depth of cut is increased, tool life will be reduced.

Tool geometry:

            Large rake angle reduces the tool cross section.  Area of the tool which will absorb heat is reduced.  So the tool will become weak.  Hence correct rake angle must be used for longer tool life.  If the cutting angle increases, more power will be required for cutting.  Clearance angle of 10^o to 15^o is optimal.

            Other factors include the material of tool (Carbon steel, medium alloy steel, high speed steel, molybdenum high speed steel, cobalt high speed steel, stellites, carbides, ceramics and diamond are the commonly used tool materials.), use of cutting fluids and work material.

Functions of cutting fluids:

1. To cool the tool and work piece and carry away the heat generated from cutting zone.   It is essential to maintain a temperature of 200^o C for carbon tools and 600^o C for HSS.

2. At low speeds the surface finish obtained by using cutting fluids is better than what is obtained without using cutting fluids.

3. To wash away the chips and keep the cutting region free.

4. It helps to keep the freshly machined surface bright by giving a protective coating against atmospheric oxygen and thus protect the finished surface from corrosion.

5. Cutting fluids improves machinability and reduces machining forces.

6. To prevent the expansion of work piece and

7. To cause the chips to break into small parts rather than remain as long ribbons which are hot and sharp and difficult to remove from  work piece.

Requirements of cutting fluid:

            A cutting fluid should posses the following properties.

1. High heat absorption to remove the heat developed immediately,

2. Good lubricating properties to have a low coefficient of friction,

3. High flash point to avoid fire hazard,

4. Stability must be high to that it does not oxidize with air,

5. It must not react with chemical and must be neutral,

6. Odorless, so that at high temperatures, it does not give a bad smell,

7. Harmless to the skin of operators,

8. Harmless to the bearings,

9. Should not have a corrosive action on the machine or work piece,

10. Cutting tool must be transparent so that the cutting action could be observed,

11. Low viscosity to permit the free flow of the cutting tool and

12. It must be economic.

            Choice of a cutting fluid depends upon type of operation, material of tool and work piece, rate of metal removal and cost of cutting fluid.

Types of cutting fluids:

Water based cutting fluids:

            In this water is mixed with soluble oil and soaps.  Following are the important characteristic features.

• It is a excellent cooling medium having maximum amount of specific heat,

• The disadvantage in using this is that it causes rust and corrosion,

• But a mixture of water and oil provides the best lubricating properties

• The ratio of oil to water is different for different machining process.  The usual ratio are 

Oil based cutting fluids:

            These are fixed oil and mineral oil.  Fixed oil has greater oiliness to become gummy and decompose when heated.

• To combine stability of mineral oil with lubricating properties of fixed oils they are often mixed.

• There are different types of oil based cutting fluids.  They are soluble oils, straight fatty cutting oils, sulphurised and aqueous solution.

• Following are the different types of cutting fluids based on different operating conditions.

                    Straight mineral oils for light duty and high speed work.
                    Mineral oil for light and medium duty.
                    Mineral oil with extreme pressure additives, such that they are suitable for heavy duty and
                    Mineral oil and extreme pressure additives for the heaviest duty.

Effect of cutting fluid on cutting speed, tool life and chip concentration:

Cutting speed:

            These are not only used to carry away the heat generated by also because of the lubricating effect of the fluid on the working surface of the tool.  When a cutting fluid is sued for machining touch material the productivity may be increased from 15% to 30% more when compared with dry operation.  But using cutting fluids, high speeds may be used.

Tool life:

            By using cutting fluids effectively during machining operations the tool life increases.  Carbon steel rods have less heat resistant have maximum increase in tool life for HSS it is around 25%.

Chip concentration:

            Without the use of cutting fluid chips are accumulated near the work tool interface and are difficult to remove because of its high temperature.  By the use of cutting fluid the temperature of the chip is reduced and also the chips are washed away from the work tool interface.

Application of cutting fluids:

            The cutting fluids may be applied to the cutting tool in the following ways.

1. By hand, using brush,

2. By means of drip tank and

3. By means of a pump.

            For effective use of cutting fluid and for heavy and continuous cutting the fluid should penetrate into the cutting zone.  The following are the famous methods of cutting fluid application.

1. Flood application (Hi-jet application):

            Here there is a continuous stream of cutting fluid is directed to the cutting zone with the help of nozzle.   The used cutting fluid drops into a tank at the bottom.  Before it is re-circulated by the pump, it passes through many filters to remove chips and dirt.  In some applications the cutting fluid is supplied through the tool itself and directed along the flank face of the tool.  Though economic it is not adopted universally because the high pressure jet may be dangerous to the operation.

2. Mist method of application:

            In this the cutting fluid is atomized the order of 10 - 25 mm.  The mist is sprayed on cutting zone at high velocities of about 300 mpm and more under high pressure.  This method is used in all cutting operation, but is generally more useful with high hardness work materials.  The benefits of this process are listed below.

• Due to high velocity the heat is dispersed immediately and maintains desired temperature gradient near tool surface.

• The surface area of coolant is greater when compared to flood application and hence increases the cooling capacity.

• Due to expansion of the mist in the issuing nozzle, it temperature falls down considerably.

The basic components of the system are

1. Air pump with air storage,

2. Cutting fluid container

3. Piping and

4. Spray nozzle.

Benefits of cutting fluids:

Cooling:

            By flowing over a tool, chip and job a cutting fluid can remove heat and reduce temperature at he cutting zone.   This reduction in temperature leads in increase in tool life and decrease in tool wear.  The cooling effect is also important in reducing thermal expansion and distortion of work piece.  The cooling action also bring about good surface finish, increase chip curl and reduces BUE formation.

Friction reduction:

            A fluid passing through the cutting zone  may be subjected to any one of the following conditions.

• High temperature approaching melting point,

• Clean freshly produced surface and

• High local pressure approaching the hardness of the metal cut.

            Under these conditions the chip may be made to react wit the fluid fro form a low shear strength solid lubricant.  This thin layer prevents the formation of the weld between the chip and the tool and hence reduces the co-efficient of friction between chip and tool.

Reduce shear strength:

            When the co-efficient of friction is reduced there is also a decrease in shear work, sue to the resulting increase in shear angle.  An increase in shear angle results in a decrease in shear strain giving rise to smaller shear stress and hence the net result is a decrease of shear energy per unit volume when cutting with an increased shear angle.

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).

Forces on a single point cutting tool :

            Following are the three forces acting on a tool

1. Axial force

2. Tangential force and

3. Radial force.

            In the above figure (a) is for orthogonal cutting and figure (b) is for oblique cutting.  Wattmeter is a indirect method for measuring cutting force.  More exact method is the use of dynamometer.  Of the total heat generated during machining process, given below is the rough heat distribution.

Chip carries 70 % of heat.
Work piece carries 15 % of heat and
Tool carries the remaining 15 % of heat generated.

Tool life :

            It could be defined from any of the below mentioned criteria.

• Volume of material removed between two successive tool grind.

• Number of work piece machined between two successive tool grinds.

• time of actual cutting between 2 successive tool grinds.

            Tool failure occurs by  chipping or breakage or wear ( Takes place by crater formation or by flank wear ) or deformation. 

Machinability : It could be evaluated by using

• Tool life

• mm³ of stock removed

• Cutting force required.

• Temperature of tool and chip.

Machinability Index ( % ) = ( Cutting speed of work piece for 20 mm Tool life ) / ( Cutting speed of SAE 1112 steel for 20 mm min tool life ) X 100.

TOOL FAILURE:

            A tool is said to fail when it losses its usefulness though wear, breakage, chipping and deformation.  During the machining operation high temperatures are reached and leads to the softening of tool point.  At a high temperature localized phase transformation occurs. This gives rise in residual stress due to which cracks appear on tool point and it is more prone to failure.  In some cases tool point may even melt and is frequently accompanied by sparking and hence can be easily  recognized.

            Thermal cracking occurs when there is a steep temperature gradient due to intermittent cutting.  Failure can be reduced by the proper selection of cutting parameters.

Wear of cutting tools:

Flank wear ( or edge wear ):

• This type of wear takes place when machining materials like cast iron or when the feed is less than 0.15 mm / rev.  The worn region at the flank is called as wear land.  This wear land is measured with the help of brinell microscope.

• The work and the tool are in contact at the cutting edge only.  Usually wear appears on the clearance face of the tool and is mainly the result of friction and abrasion.

• Flank wear is a flat portion worn behind the cutting edge, which eliminates some clearance on relief.

• Flank wear is a progressive form of detoriotion and will result in failure in spite of best precautions.

• There are three stages in flank wear.  They are primary, secondary and tertiary stage.  In the primary stage wear is rapid due to high stress at tool point.  In secondary stage, wear is less and linear.  In the third and final stage called as the tertiary stage the wear increases leading to catastrophic failure.

            Abrasion by hard particles and inclusions in the work piece, shearing of micro welds between tool and work material and abrasion by fragments of build up edge plowing against the clearance face of the tool are some of the causes of this wear.

Crater wear ( or face wear ):

• This is caused by the pressure of the chip as it slides up the face of the cutting tool.  Due to the pressure of the sliding chips the cool face wears out gradually.

• On the faces of the tool there is a direct contact of tool with the chip.  Wear takes place in the form of cavity or crater, which as its origin above the cutting edge.

• The crater occurs on the rake face and does not actually reach the cutting edge by ends near the nose.

• This type of wear takes place when cutting ductile material.  This wear weakens the tool.  Cutting temperature is increased.  Friction and cutting force will also increase.  When the crater becomes large the tool will totally fail.

            Severe abrasion between chip and tool interface and high temperature in the tool-chip interface reaching the softening (or melting temperature) of tool resulting in increased rate of wear.  These are the two causes of crater wear.  

            To combat crater wear, tool manufacturers can increase the chemical stability of the tool material, as when they added titanium carbide (TiC) to tungsten carbide (WC) in the first successful steel-cutting carbide tool. Applying a hard coating to put a hard, inert barrier between tool and work piece at high cutting speeds will also minimize crater wear. Tool geometry can also make a difference. A positive-rake tool will reduce tool pressure and decrease contact between the chip and the insert, and the reduction in pressure and contact can reduce crater wear.

Nose wear:

            This is similar to flank wear in certain operations like finish turning.  It takes place at the nose of the tool.  When the nose of the tool is rough, abrasion and friction between the tool and work piece will be high.  Due to this, too much heat is generated.  Also more cutting force is required.  As a result the nose of the tool wears quickly.  This is more pre-dominant than flank wear.  

Breakage:

            Because of high pressure acting on cutting edge of a tool there ay be immediate failure.  Breakage is usually attributed to mechanical shock, thermal shock, thermal cracks and fatigue.

Chipping:

            The cutting edge may crumble due to improper relief angle, excess clearance and insufficient support of the tool.  This could also happen if the work piece is very hard.  It is a microscopic form of breakage due to loss of many small particles caused due to unhoned carbide edges, excessive vibration and chatter.

Deformation:

            When a heavy load is applied close to the cutting edge of tool the surface becomes indented while the adjacent face shows a bulge.  Because of which crack occurs on periphery of indentation and finally leads to failure.

NUMERICAL PROBLEMS

1. 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.

Solution:

            VTⁿ = C
            
            V = 18 m/min
            T = 3 x 60 = 180 min

            Constant C = 18 x ( 180 ) ^0.125 = 34.45             ( Here n = 0.125 )

            Now V = 24 m/min.

            T = ( 34.45 / 24 ) ^1/0.125    

            = 18 minutes.

Last updated on Wednesday, November 26, 2003 , 07:35 PM