Theory of Metal Cutting

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 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.  The differences between orthogonal and oblique cutting is given below

Orthogonal metal cutting Oblique metal cutting
Cutting edge of the tool is perpendicular to the direction of tool travel. The cutting edge is inclined at an angle less than 90o to the direction of tool travel.
The direction of chip flow is perpendicular to the cutting edge. The chip flows on the tool face making an angle.
The chip coils in a tight flat spiral The chip flows side ways in a long curl.
For same feed and depth of cut the force which shears the metal acts on a smaller areas.  So the life of the tool is less. The cutting force acts on larger area and so tool life is more.
Produces sharp corners. Produces a chamfer at the end of the cut
Smaller length of cutting edge is in contact with the work. For the same depth of cut greater length of cutting edge is in contact with the work.
Generally parting off in lathe, broaching and slotting operations are done in this method. This method of cutting is used in almost all machining operations.

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.

Different types of chips produced during machining process :

            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 :

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 :

Chips with built up edge :

Single point cutting tool:

Parts of a single point cutting tool:

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.

Important angles 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.  This slope is given from the nose along the length 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.

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 10o to 15o 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 200o C for carbon tools and 600o 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.

Operation Ratio
Turning 1:25
Milling 1:10
Drilling 1:25
Grinding 1:50

Oil based cutting fluids:

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

                    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.

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.

            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.

            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

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 ):

            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 ):

            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

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