Metal Cutting Theory Notes PDF

UNIT 1: THEORY OF METAL CUTTING  Mechanism of metal cutting  Types  cutting force  chip formation  Merchant's circle diagram.  Calculations  tool geometry  machinability  tool wear  tool life  cutting tool materials  cutting fluids  types MECHANIS...

UNIT 1: THEORY OF METAL CUTTING  Mechanism of metal cutting  Types  cutting force  chip formation  Merchant's circle diagram.  Calculations  tool geometry  machinability  tool wear  tool life  cutting tool materials  cutting fluids  types MECHANISM OF METAL CUTTING  The mechanism of metal cutting involves using a wedge-shaped tool to remove material from a workpiece in the form of a chip. The process involves two motions: primary motion, such as the rotation of a workpiece in a lathe, and secondary motion, such as the feed of a lathe tool.  The two basic methods of metal cutting using a single point tool are orthogonal (2D) and oblique (3D). Orthogonal cutting occurs when the cutting face of the tool is 90 degrees to the line of action of the tool. Oblique cutting occurs when the cutting edge is inclined at an angle less than 90 degrees to the direction of tool travel. Orthogonal Cutting :  Orthogonal Cutting Is a type of Cuttings in Which the Cutting Tool Is Perpendicular to the Direction of Motion.  This Type of Cutting Has a Lower of the Tool. Oblique Cutting:  Oblique cutting is a type of cuttings in which the cutting tool is at an oblique angle in the direction of the tool’s motion.  The tool has a longer cutting life than orthogonal cutting. TYPES  Metal cutting processes remove material from a workpiece. The choice of process depends on the job's requirements, such as the material type, part size and shape, and desired surface finish. Here are some types of metal cutting: Orthogonal cutting The cutting tool moves directly into the workpiece at a right angle. The cutting edge of the tool is perpendicular to the direction of tool travel. Oblique cutting The cutting tool engages with the workpiece material at an angle other than 90 degrees. Waterjet cutting Uses the force of highly pressurized water for metal particle erosion. It is a cold cutting process that doesn't require physical contact of the waterjet cutting head with the workpiece. Plasma cutting Uses an electric arc that passes through the gas which spouts out of a constricted opening. The gases used in plasma cutting can be air, nitrogen, oxygen, and argon.  Other types of metal cutting include: Laser cutting, Flame cutting, Metal turning.  Different types of metal cutting process video link https://youtu.be/kuPGLUVVkZU?si=p9E1eBn5jtDWnYJo CUTTING FORCE  Cutting force is the resistance of a material to a cutting tool's intrusion. It is a key parameter in machining operations, as the power used by the machine to perform the process is always a factor to be optimized.   Cutting force video link https://youtu.be/x_4Feo_ETWk?si=NdasjnLD8Gq-l53n CHIP FORMATION  Chip formation is a process that occurs when material is cut mechanically using tools like saws, milling cutters, and lathes. It happens during machining, when the material is sheared and plastically deformed in the shear plane. The length or shortness of the chip depends on the material of the workpiece.  Types of chip formation – 1. Continuous chips 2. Discontinuous chips 3. Continuous chips with built up edges (BUE) Continuous chip formation  If the metal chips are formed during machining is without segments i.e. without breakage, then it is called as continuous types of chips.  Continuous chips are formed when the ductile material is machined with high cutting speed and minimum friction between the chip and tool face. While machining the soft metals at low speed and low rake angles, distortion occurs on the machined surface. So it gives unfavorable results while machining the soft metals. Overall, the formation of a continuous chip results in a good surface finish and smooth cutting. It also increases the tool life and decreases power consumption. Chip breakers are equipped along with the cutting tool, to avoid the chip tangling about the tool. This usually happens in the turning process. Discontinuous chips formation  If the chip is formed during machining is not continuous i.e. formed with breakage is called discontinuous chips. Discontinuous types of chips are formed when hard and brittle metals like brass, bronze and cast iron is machined. This type of chip is produced mainly due to high friction in tool-chip interface, large feed, and depth of cut. When machining ductile materials, discontinuous chips results in poor surface finish and reduce the tool life. Stiffness of the cutting tool and holding devices plays a major role in discontinuous chip formation because improper stiffness may cause vibration, dimensional inaccuracy, poor surface finish, and even damage the cutting tool. Continuous chips with built up edges (BUE) formation  Continuous chips with built up edge is formed by machining ductile material with high friction at the chip-tool interface.  It is similar to the continuous types of chips but it is of less smoothness due to the built up edge. The formation of Build up edge (BUE) can be reduced by following factors such as,  By using a sharp tool  Increasing the cutting velocities  Reducing the chip thickness  Increasing the rake angle  Using capable cutting fluid.  Type of chip formation video link https://youtu.be/huKlcD6N6pc?si=ESEM51fcOOG6dIos MERCHANT'S CIRCLE DIAGRAM  A merchant circle diagram is made as a graphical representation of the number of forces acting on a workpiece when it is subjected to orthogonal cutting. CALCULATIONS  ASSUMPTIONS FOR MERCHANT'S CIRCLE DIAGRAM  Tool edge is sharp.  The work material undergoes deformation across a thin shear plane.  There is uniform distribution of normal and shear stress on shear plane.  The work material is rigid and perfectly plastic.  The shear angle adjusts itself to minimum work.  The friction angle remains constant and is independent.  The chip width remains constant.  The chip does not flow to side, or there is no side spread. FORCES INCLUDED IN METAL CUTTING  F(s), Resistance to shear of the metal in forming the chip. It acts along the shear plane.  F(n), Backing up' force on the chip provided by the workpiece. Acts normal to the shear plane.  N, It is force at the tool chip interface normal to the cutting face of the tool and is provided by the tool.  F, It is the frictional resistance of the tool acting on the chip. It acts downward against the motion of the chip as it slides upwards along the tool face.  ADVANTAGES OF MERCHANT'S CIRCLE  Proper use of MCD enables the followings:-  Easy, quick and reasonably accurate determination of several other forces from a few forces involved in machining.  Friction at chip-tool interface and dynamic yield shear strength can be easily determined.  Equations relating the different forces are easily developed. LIMITATIONS OF MERCHANT'S CIRCLE  Some limitations of use of MCD are :-  Merchant's Circle Diagram (MCD) is valid only for orthogonal cutting.  By the ratio, F/N, the MCD gives apparent (not actual) coefficient of friction.  It is based on single shear plane theory.  Merchant's Circle Diagram video link https://youtu.be/YCLZMx_nhsM TOOL GEOMETRY  Tool geometry has been around since ancient times as it helps to understand how machines work by describing their functions, sizes, shapes and features. Tool geometry video link- https://youtu.be/BHEYrGrvp6U?si=gNZdQ9aKJmevzKED GEOMETRY OF CUTTING TOOL – ASA and ORS – SINGLE POINT TURNING TOOLS  Cutting tools may be classified according to the number of major cutting edges (points) involved as follows:  Single point: e.g., turning tools, shaping, planning and slotting tools and boring tools  Double (two) point: e.g., drills  Multipoint (more than two): e.g., milling cutters, broaching tools, hobs, gear shaping cutters etc. Concept of rake and clearance angles of cutting tools.   The word tool geometry is basically referred to some specific angles or slope of the salient faces and edges of the tools at their cutting point. Rake angle and clearance angle are the most significant for all the cutting tools. The concept of rake angle and clearance angle will be clear from some simple operations shown in Fig. below : Fig. Rake and clearance angles of cutting tools. Definition –  Rake angle (): Angle of inclination of rake surface from reference plane  clearance angle (a): Angle of inclination of clearance or flank surface from the finished surface  Rake angle is provided for ease of chip flow and overall machining. Rake angle may be positive, or negative or even zero as shown in Fig. below : Relative advantages of such rake angles are:  Positive rake – helps reduce cutting force and thus cutting power requirement.  Negative rake – to increase edge-strength and life of the tool  Zero rake – to simplify design and manufacture of the form tools. Clearance angle is essentially provided to avoid rubbing of the tool (flank) with the machined surface which causes loss of energy and damages of both the tool and the job surface. Hence, clearance angle is a must and must be positive (30 ~ 150 depending upon tool-work materials and type of the machining operations like turning, drilling, boring etc.) Systems of description of tool geometry  Tool-in-Hand System – where only the salient features of the cutting tool point are identified or visualized  Machine Reference System – ASA system  Tool Reference Systems * Orthogonal Rake System – ORS system * Normal Rake System – NRS system  Work Reference System – WRS system Demonstration (expression) of tool geometry in : Machine Reference System - This system is also called ASA system; ASA stands for American Standards Association. Geometry of a cutting tool refers mainly to its several angles or slope of its salient working surfaces and cutting edges.  Those angles are expressed w.r.t. some planes of reference.  In Machine Reference System (ASA), the three planes of reference and the coordinates are chosen based on the configuration and axes of the machine tool concerned.  The planes and axes used for expressing tool geometry in ASA system for turning operation are shown in Fig. below - Basic features of single point tool (turning) in Tool-in-hand system  Planes and axes of reference in ASA system The planes of reference and the coordinates used in ASA system for tool geometry are : Where R= Reference plane; plane perpendicular to the velocity vector (shown in Fig. above) X= Machine longitudinal plane; plane perpendicular to R and taken in the direction of assumed longitudinal feed Y= Machine Transverse plane; plane perpendicular to both R and X [This plane is taken in the direction of assumed cross feed] The axes Xm, Ym and Zm are in the direction of longitudinal feed, cross feed and cutting velocity (vector) respectively. The main geometrical features and angles of single point tools in ASA systems and their definitions will be clear from Fig. below  Tool angles in ASA system Definition of: Rake angles: [Fig. above] in ASA system X = side (axial rake: angle of inclination of the rake surface from the reference plane (R) and measured on Machine Ref. Plane,( X) Y = back rake: angle of inclination of the rake surface from the reference plane and measured on Machine Transverse plane (Y) Clearance angles: [Fig. above] X = side clearance: angle of inclination of the principal flank from the machined surface (or VC ) and measured on X plane Y = back clearance: same as X, but measured on Y plane. Cutting angles: [Fig. above] S= approach angle: angle between the principal cutting edge (its projection on R) and Y and measured on R e = end cutting edge angle: angle between the end cutting edge (its projection on R) from X and measured on R Nose radius, r (in inch), r = nose radius : curvature of the tool tip. It provides strengthening of the tool nose and better surface finish Tool Reference Systems Orthogonal Rake System – ORS This system is also known as ISO – old. The planes of reference and the co-ordinate axes used for expressing the tool angles in ORS are: which are taken in respect of the tool configuration as indicated in Fig. below Planes and axes of reference in ORS where, R = Reference plane perpendicular to the cutting velocity vector, V C C = cutting plane; plane perpendicular to R and taken along the principal cutting edge O = Orthogonal plane; plane perpendicular to both R and C and the axes; Xo = along the line of intersection of R and O Yo = along the line of intersection of R and C Zo = along the velocity vector, i.e., normal to both XO and YO axes. The main geometrical angles used to express tool geometry in Orthogonal Rake System (ORS) and their definitions will be clear from Fig. below/next pg.  Tool angles in ORS system Tool signature provides various static geometrical details of that particular cutting tool, especially various angles and nose radius. Commonly, it provides values of rake angles, clearance angles, cutting edge angles, nose radius, etc. Systems for specifying cutting tool signature Single point cutting tools are such cutters that contain only one main cutting edge that can participate in cutting action in a single pass. Examples include turning, boring, shaping, planing, and slotting tools. There exist three conventional systems for designation of such cutting tools, as enlisted below. A particular tool can be specified by all three systems and correspondingly tool signatures will also be different. Different tool signature does not indicate the tool angles are different; instead, it indicates inclination of a surface in different directions.  American Standards Association (ASA) system  Orthogonal Rake System (ORS)  Normal Rake System (NRS) Tool signature in ASA system American Standards Association (ASA) system utilizes three mutually perpendicular planes for reference purpose namely Machine longitudinal plane, Machine transverse plane and Reference plane. Tool signature in ASA system consists of two rake angles, two clearance angles, two cutting edge angles and the nose radius of a single point cutting tool. The sequence of writing tool signature in ASA system along with the name of various angles is depicted below. It is to be noted that different persons may use different symbol but the sequence must be maintained. A typical example is also provided below. Planes used as reference in ASA system of tool designation American Standards Association (ASA) system utilizes three mutually perpendicular planes as reference for measuring various angles of a single point turning tool (SPTT). These three planes and their basic characteristics are enlisted below.  Reference Plane (πR)—It is a plane perpendicular to the cutting velocity vector (Vc).  Machine Longitudinal Plane (πX)—It is a plane perpendicular to reference plane (πR) and along the direction of longitudinal feed for external straight turning operation.  Machine Transverse Plane (πY)—It is a plane perpendicular to reference plane (πR) and along the direction of transverse feed for external straight turning operation. So all three planes are mutually perpendicular. Representation of tool angles in ASA system of tool designation. Various features displayed in ASA system of tool designation ASA system of tool designation specifies two different rake angles, two different clearance angles, two different cutting edge angles, and the nose radius value in inch. Various features of a single point turning tool (SPTT) that ASA system displays are provided below.  Side Rake Angle (γX)—It is the angle of orientation of tool’s rake surface from the reference plane (πR) and measured on machine longitudinal plane (πX).  Back Rake Angle (γY)—It is the angle of orientation of tool’s rake surface from the reference plane (πR) and measured on machine transverse plane (πY).  Side Clearance Angle (αX)—It is the angle of orientation of tool’s principal flank surface from the cutting velocity vector (Vc) and measured on machine longitudinal plane (πX).  Back Clearance Angle (αY)—It is the angle of orientation of tool’s principal flank surface from the cutting velocity vector (Vc) and measured on machine transverse plane (πY).  Approach Angle (Φs)—It is the angle between principal cutting edge and the machine transverse plane (πY), measured on reference plane (πR).  End Cutting Edge Angle (Φe)—It is the angle between auxiliary cutting edge and the machine longitudinal plane (πX), measured on reference plane (πR).  Nose Radius (r)—This is nothing but the curvature at the tool tip. It is to be noted that in ASA system, nose radius value is expressed in inch. Tool nomenclature in ASA system  All of the above mentioned seven features of the turning tool are specified in a particular sequence as shown below. Such specification is also called tool nomenclature or tool signature. The sequence of designation should be followed strictly. However, different people may use different notations for various angles maintaining the original sequence unchanged. Example for ASA system of tool designation Few points should be considered for giving examples of tool nomenclature. First and foremost one is the value of clearance angles. Clearance angles are always positive—it cannot be zero or negative. Usually it ranges from 3º – 15º. Rake angle can have a positive, negative or even zero value. One example of ASA system of tool designation and interpretation of tool angles from such nomenclature is illustrated in the following figure. Let us consider another example. Say, a typical turning tool can be specified in ASA system as: –8º, 6º, 5º, 10º, 15º, 30º, 1/8 (inch) Therefore, upon interpretation, we may write:  Back Rake Angle (γY) = –8º  Side Rake Angle (γX) = 6º  Back Clearance Angle (αY) = 5º  Side Clearance Angle (αX) = 10º  End Cutting Edge Angle (Φe) = 15º  Approach Angle (Φs) = 30º  Nose Radius (r) = 1/8 inch Tool signature in ORS system Orthogonal Rake System (ORS) also utilizes three mutually perpendicular planes for reference purpose namely Cutting plane, Orthogonal plane and Reference plane. Similar to the ASA system, tool signature in ORS system consists of two rake angles, two clearance angles, two cutting edge angles and the nose radius of a single point cutting tool. Note that in ASA system, nose radius is measured in inch unit; whereas, in ORS system it is measured in mm. The sequence of writing tool signature in ORS system along with the name of various angles is depicted below. As usual, the sequence cannot be altered but alternative notation can be used. A typical example is also provided below. Planes used as reference in ORS system of tool designation  Orthogonal Rake System (ORS) utilizes three mutually perpendicular planes as reference for measuring various tool angles. These three planes are enlisted below.  Reference Plane (πR)—It is a plane perpendicular to the cutting velocity vector (Vc).  Cutting Plane (πC)—It is a plane perpendicular to reference plane (πR) and contains the principal cutting edge of the tool.  Orthogonal Plane (πO)—It is a plane perpendicular to reference plane (πR) and also perpendicular to the cutting plane. So all three planes are mutually perpendicular (πO is perpendicular to both πR and πC). Representation of tool angles in ORS system of tool designation.  Representation of auxiliary plane angles in ORS system of tool designation. Various features displayed in ORS system of tool designation This system of tool designation specifies two different rake angles, two different clearance angles, two different cutting edge angles, and the nose radius value in mm. Various features of the single point turning tool (SPTT) that are specified in ORS system are enlisted below.  Inclination Angle (λ)—It is the angle of inclination of tool’s principal cutting edge from the reference plane (πR) and measured on cutting plane (πC).  Orthogonal Rake Angle (γO)—It is the angle of orientation of tool’s rake surface from the reference plane (πR) and measured on orthogonal plane (πO).  Orthogonal Clearance Angle (αO)—It is the angle of orientation of tool’s principal flank surface from the cutting plane (πC) and measured on orthogonal plane (πO).  Auxiliary Orthogonal Clearance Angle (αo’)—It is the angle of orientation of tool’s auxiliary flank surface from the auxiliary cutting plane (πC’) and measured on auxiliary orthogonal plane (πO’).   Principal Cutting Edge Angle (Φ)—It is the angle between cutting plane (πC) (which contains principal cutting edge) and the longitudinal feed direction, measured on reference plane (πR).  Auxiliary Cutting Edge Angle (Φ1)—It is the angle between auxiliary cutting plane (πC’) (which contains auxiliary cutting edge) and the longitudinal feed line, measured on reference plane (πR).  Nose Radius (r)—This is nothing but the curvature at the tool tip. It is to be noted that in ORS system, nose radius value is expressed in mm. Tool nomenclature in ORS system  All the above mentioned seven features of the turning tool are specified in sequence as shown below. It is also called tool nomenclature. The sequence of designation should be followed strictly. However, different people may use different notations for various angles maintaining the original sequence unchanged. Example for ORS system of tool designation Few points should be considered for giving examples of tool nomenclature. First and foremost one is the value of clearance angles. Clearance angles are always positive — it cannot be zero or negative. Usually it ranges from 3º – 15º. Also auxiliary cutting edge angle is usually lower than principal cutting edge angle; however, for thread cutting tool, they can be same. A zero inclination angle indicates ORS and NRS system are same. One example of ORS system of tool designation and interpretation of tool angles from such nomenclature is illustrated in the following figure. Let us consider another example. Say, a typical turning tool can be specified in ORS system as: –6º, –6º, 10º, 15º, 15º, 45º, 1.2 (mm) Therefore, upon interpretation, we may write:  Inclination Angle (λ) = –6º  Orthogonal Rake Angle (γO) = –6º  Orthogonal Clearance Angle (αO) = 10º  Auxiliary Orthogonal Clearance Angle (αo’) = 15º  Auxiliary Cutting Edge Angle (Φ1) = 15º  Principal Cutting Edge Angle (Φ) = 45º  Nose Radius (r) = 1.2 mm. Tool signature in NRS system Normal Rake System (NRS) utilizes three planes (not necessarily mutually perpendicular) for reference purpose namely Cutting plane, Normal plane and Reference plane. Similar to the ORS system, tool signature in NRS system also consists of two rake angles, two clearance angles, two cutting edge angles and the nose radius of a single point cutting tool. The following images show the tool signature in NRS system and the way to retrieve values of various angle from the given tool signature. MACHINABILITY  Machinability is a measure of how easy it is to cut a material with a cutting tool. It's a crucial factor in machining processes because it defines how easily material can be removed with moderate force.  Machinability video link https://youtu.be/-IvhmjpxM-s?si=qxCIuhG4qJFumQTo TOOL WEAR  Definition of Tool Wear Tool wear is the gradual breakdown of machine tools as a result of cutting operation, eventually leading to tool failure. Because tools and workpieces are in constant contact with severe friction and rubbing, tools become stressed over time. Tool Wear Introduction to Tool Wear: Tool Wear is a term that describes the gradual failure of a cutting tool due to its operation. A cutting tool is ground with various angles to perform cutting operation efficiently & effectively on different materials & in different situations of varying speed, depth & feed of cut Under regular operation, the tool wears out gradually leading to changes in the angles ground on the cutting tool, which in turn ceases to tool to function satisfactorily A very short tool life is not economical, as tool grinding & tool replacement increases the cot of machining and in-turn increases the cost of the product Tool wear cannot be avoided, but under suitable operating conditions it can be minimized Conditions of Cutting Tool a) high localized stresses at the tip of the tool b) high temperatures, especially along the rake face c) sliding of the chip along the rake face d) sliding of the tool along the newly cut workpiece surface These conditions induce tool wear, which is a major consideration inall machining operations. Tool wear adversely affects tool life, thequality of the machined surface and its dimensional accuracy, and,consequently, the economics of cutting operations. Wear is a gradual process. The rate of tool wear depends on tool and workpiecematerials, tool geometry, process parameters such as speed, feed and depth of cut, cutting fluids, and the characteristics of the machine tool.  Modes of Tool Wear There are 3 possible ways a cutting tool can fail in machining: Fracture Failure: This mode of failure occurs when the cutting force at the tool point becomes excessive, causing it to fail suddenly by brittle fracture (Mechanical Chipping) Temperature Failure: This failure occurs when the cutting temperature is too high for the tool material, causing the material at the tool point to soften, which leads to plastic deformation and loss of the sharp edge Gradual Wear: Gradual wearing of the cutting edge causes loss of tool shape, reduction in cutting efficiency, an acceleration of wearing as the tool becomes heavily worn, and finally tool failure in a manner similar to a temperature failure Tool wear is gradual process; created due to: 1- High localized stresses at the tip of the tool 2- High temperatures (especially along rake face) 3- Sliding of the chip along the rake face 4- Sliding of the tool along the newly cut workpiece surface The rate of tool wear depends on - tool and workpiece materials - tool geometry - process parameters - cutting fluids - characteristics of the machine tool Tool Life: Wear and Failure Tool wear and the changes in tool geometry are classified as: a) Flank wear b) Crater wear c) Nose wear d) Notching (plastic deformation of the tool tip) e) Chipping f) Gross fracture Types of Tool Wear Flank Wear Flank Wear is the wear on the portion of the tool in contact with the finished part. It’s the most common type of Tool Wear and the most predictable. It occurs due to abrasion of the tool by the workpiece. Harder workpiece materials will be more abrasive. As flank wear increases, cutting forces will increase as well. Crater Wear Crater Wear occurs when chips strike and erode the rake face. It takes quite a lot of crater wear to degrade the effectiveness of a tool. Built Up Edge Built Up Edge, often abbreviated as BUE, occurs when the material being machined builds up on the cutting edge. Materials like aluminium and copper have a tendency to weld themselves to the cutting edge of a tool. It can be prevented by increasing cutting speeds and using lubricant (coolant). Notch Wear Notch Wear is wear that appears right at the depth of the cut line. It is caused by adhesion (pressure welding of chips) and a deformation-hardened surface. It is common when machining stainless steels. Thermal Cracks Thermal Cracks are tiny cracks along the cutting edge caused by shock cooling. They’re related to interrupted cuts and are aggravated by coolant. Edge Chipping Edge Chipping is caused by an overload of mechanical tensile stresses. TOOL LIFE  Tool life T is the period of time, expressed in minutes, for which the cutting edge, affected by the cutting procedure, retains its cutting capacity between sharpening operations.  Factors that affect tool life include: Cutting speed Feed and depth of cut Tool geometry Tool material  Tool wear is the incremental destruction of cutting-edge tools. It can be caused by: Localized pressures and tensions at the tip of the tool High temperatures Gliding the chip along the rake face Moving smoothly along the recently cut workpiece  Tool wear is not uniform through the life of the tool. The wear is initially rapid, then settles down to a uniform rate, and finally accelerates at a very high rate till catastrophic failure occurs.  Once a tool is worn to the point that the parts being created are out of spec, it's life is effectively over and the tool should be replaced.  You can monitor tool life by collecting data from sensors that monitor the tool group when a machine changes to the next tool. This data is sent to a software program or cloud-based machine data platform, where it's analyzed to predict the condition and lifespan of a tool. Taylor’s Tool Life Equation Taylor's tool life equation As per F.W. Taylor, the relationship between Cutting Speed and Tool Life can be expressed as VTn= C If V1, T1 initial condition and V2, T2 second condition then can be written as, V 1 T1 n = V 2 × T2n (V1/V2) = (T2/T1)n Where, V = Cutting speed (m/min) T= Tool life (minutes) n = a constant whose value depends upon the material of the cutting tool & job, called tool life Index. (Commonly, n=0.08 to 0.02 for H.S.S tools, n=0.2 to 0.4 for cemented carbide tools, n= 0.5 to 0.7 for Ceramic Tools, n = 0.1 to 0.15 for cast alloys) C = a constant, called machining constant SOLVED PROBLEMS ON TOOL LIFE 1. During a tool life-cutting test on HSS tool material, the following data were recorded. Calculate the values of n and C of Taylor's equation. Given data: T1 = 40 min V1 = 25 m/min T2 = 5 min V2 = 75 m/min Solution: By Taylor's equation 2. A cutting tool made of HSS gave a tool life of 50 min when operated at a speed of 200m/min. At what speed should the tool have to be operated in order to have a tool life of 2hrs 15 min. Assume n = 0.185. Given data: T1 = 50 min V1 = 200 m/min T2 = 2 hrs 15 min = 135 min n = 0.185 Solution: From Taylor's equation 3. The following equation for tool life was obtained for HSS tool VT0.13 f0.6 d0.3 = C A 60 min tool life was obtained using the following cutting condition. V = 40 m/min, f = 0.25 mm, d = 2 mm Calculate the effect on tool life if speed, feed and depth of cut are together increased by 25% and also if they are increased individually by 25%. Where ƒ = feed, d = depth and V = speed. Solution: The given tool life equation is given by V T0.13 ƒ0.6 d0.3 = C Substituting V, T, ƒ and d values in above equation 40 × 600.13 × 0.250.6 × 20.3 = C C = 36.5 When the speed, feed and depth of cut are together increased by 25% V = 50 m/min, f = 0.3125 mm, d = 2.5 mm Substituting above vales in given equation, 4. A cutting tool at 35 m/min gives a life of one hour twenty minutes when operating on roughening cuts. What will be the probable life when engaged on light finishing cuts? Take, n = 0.125 for rough cut & 0.1 for finishing cut. Solution: Taylor's equation is VTn = C For rough cut n = 0.125 V = 35 m/min and T = 1 hr and 20 min = 80 min 35 × 800.125 = C C = 60.53 For finish cut, n = 0.1 V = 35 m/min and C = 60.53 5. The lives of two tools, A and B are governed by equations VT 0.125 = 2.5 and VT0.25 = 7 - respectively in a certain machining operation, where V is the cutting speed in m/sec and T is tool life in seconds. (a) Find out the speed V at which both the tool will have the same tool life. Also calculate the corresponding tool life T. (b) If you have to machine at a cutting speed of 1m/s, which one these tools will you choose in order to have less frequent tool changes? Given data: Solution: (a) From equation (1.8), Since both tool having same life, substituting this T value in equation (1.10) (b) V = 1 m/s For tool A, from equation (1.9), 1 × T0.125 = 2.5 ⸫ T = 1525.88 seconds For tool B, from equation (1.10), 1 × T0.25 = 7 ⸫ T = 2401 seconds. Since tool B has greater tool life for the same cutting speed of 1m/s, it has less frequent tool changes and hence it is chosen. Ans. 6. Tool life testing on a lathe under dry cutting conditions gave n and C of Taylor's tool life equation as 0.12 and 130 m/min respectively. When a coolant was used, C increased by 10%. Find the percentage increase in tool life with the use of coolant at a cutting speed of 90 m/min. Given data: n =0.12 C = 130 m/min V = 90 m/min Solution: Taylor's tool life equation is given by VTn = C 90 × T0.12 = 130 When coolant was supplied, C increases by 10% 7. Cylindrical bars of 100 mm diameter and 576 mm length are turned in a single pass operation. The spindle speed used is 144 rpm, and the total feed is 0.2 mm/rev. Taylor's tool life relationship is VT 0.75 = 75. Where "V" is the cutting speed in m/min and T is tool life in min. calculate the: (i) _time required for turning one piece. (ii) average tool change time per piece given that it takes 3 min to change the tool each time, and (iii) time required to produce one piece given that the handling time is 4 min. Given data: D = 100 mm L = 576 mm N =144 rpm f = 0:2 mm/rev VT0.75 = 75 Solution: (i) Time required to turn one piece is given by (ii) Average total change time per piece 45.24 × T0.75 = 75 ⸫ T = 1.962 min Hence a break of 3 min to be given after 1.962 min for changing the tool, the number of times the tool needed to be changed during machining for 20 min. (iii) If handling time is 4 min, the total time required to produce one piece = handling time + production time Production time = machining time + total change time= 20 + 33 = 53 min 8. If under a given condition of plain turning, the life of the cutting tool decreased by 50% due to increase in the cutting velocity by 20%, by what percentage will the life of that tool increase due to reduction in the cutting velocity by 20% from its original value? Given data: T decreases by 50% V increased by 20% Solution: From Taylor's tool life equation, 9. If the relationship for HSS tools is VT1/8 = C1 and for tungsten carbide tools is VT1/5 = C2 and assuming that at a speed of 25 m/min, the tool life was 3 hours in each case, compare their cutting lives at 32 m/min. Given data: V = 25 m/min T = 3 hrs = 180 min V' = 32 m/min Solution: From equation (1.14), VT1/8 = C1 25 × (180)1/8 = C1 C1 = 47.846 From equation (1.15), VT1/5 = C2 25 × (180)1/5 = C2 C2 = 70.63 For V = 32 m/min From equation (1.14), 32 × T1/8 = 47.846 ⸫ T = 24.97 min From equation (1.15), 32 × T1/5 = 70.63 (⸫ C1 = 47.846) ⸫ T = 52.38 min (⸫ C2 = 70.63) For 32 m/min cutting speed, second equation i.e. VT1/5 = C2 gives better life. Ans. 10. In a tool wear test with high-speed steel cutting tool, the following data were recorded. Compute the Taylor's equation. Given data: T1 = 30 min V1 = 25 m/min T2 = 2 min V2 = 70 m/min Solution: According Taylor's equation, VTn = C CUTTING TOOL MATERIALS There are different cutting processes done on varying conditions. Depending on the cutting conditions and the requirements of the respective cutting tool, it is important that they are of the right properties. The type of material selected for a specific application depends on what is being machined. Here is a classification of these materials. Carbon Tool Steel This is one of the inexpensive metal cutting tools common in low-speed machining operations. These carbon steel cutting tools are constructed with a composition of 0.6%-1.5% carbon and small amounts, less than 0.5%, of Si and Mn. To enhance the hardness, other materials such as V and Cr could also be added. Carbon tool steels are preferred because they are abrasion resistant and can maintain the cutting edge for a long period. However, they lose their hardness when temperatures reach 250 °C. This means that they are not good for high-temperature operations. Common applications that use carbon steel tool include milling tools, twist drills, and forming tools. High-Speed Steel (HSS) This is another high carbon steel featuring a significant quantity of alloys like chromium and tungsten to increase their hardness and wear resistance. HSS loses its hardness when temperatures hit 650 °C. It is, therefore, advisable to use coolants to increase tool life. The following surface treatment is also used on HSS to improve the properties.  Super-finishing to lower friction   Chromium electroplating to lower friction  Nitriding to increase wear resistance  Oxidation to reduce friction High-speed steel tools are common in broaches, single point lathe tools, and milling cutters. Cemented Carbide and Cerment The cemented carbide cutting tool is created using powder metallurgy method. It is made from tungsten, titanium carbide and tantalum with cobalt as a binder. The most notable thing about the cemented carbide tools is that they are very hard and can be used for cutting at high speed and temperatures. For example, you can use them for cutting at temperatures of 1000 °C without losing their properties. For rough cuts, it is better to use high cobalt tool while low combat tools are ideal for finishing applications. Ceramics The common ceramic materials used in cutting tools are silicon nitride and aluminum oxide. When the ceramic material powder is compacted and inserted at very high temperatures, the resulting tools are inert and resistant to corrosion. Therefore, they have high compressive strength. The ceramics are stable when operating even in temperatures of up to 1800°C and are about 10 times faster than HSS. Because the friction between the chip and surface is low and heat conductivity is also low, you do not need an additional coolant. Cubic Boron Nitride (CBN) CBN is the second hardest material and is commonly used in hand machines. They provide high abrasion resistance and utilize abrasive in grinding wheels. They are ideal at speeds of 600-800m/min. Diamond This is the hardest material used in tools. It features a high melting point and thermal conductivity. Therefore, it provides excellent abrasion resistance, low thermal expansion, and low friction coefficient. It is considered ideal for machining hard materials like glass, nitrides, and carbides. Note that diamond is not ideal for machining steel. Video link- https://youtu.be/79YEVl21to0?si=hrHwW5zPhKZRf2bg CUTTING FLUIDS  Cutting fluids provide lubrication to the cutting tool and workpiece, which helps to improve the movement of the tool and reduce wear and tear on the machinery.  Cutting fluids can be used as coolants, lubricants, and flushing fluids, making them a versatile solution for a range of metal cutting operations.  Video link- https://youtu.be/GAysUOEU6Yg?si=XYOPSE4fRJchA1rD TYPES  Cutting fluids, also known as coolants or cutting oil, are multi-purpose formulations. They are typically used in metalworking processes such as machining, grinding, and milling. 1. Straight Oil Stroth oils are non-emulsifying. These oils are used without diluting them with water. The compositions of this type of oil are base minerals or petroleum oil. Examples of straight oil are paraffin oil, Naphthenic oil, vegetable oil. In systems where environmentally friendly oil is required, vegetable oil is used because it is biodegradable. Straight oils are best for lubricating, but they cannot serve as a good coolant because they have very cool properties. 2. Soluble Oil Soluble oils are made by mixing mineral oil, water, & coupling agents. It provides good lubrication between water-inaccurate liquids. Soluble oils are used in the machining of both ferrous and non-ferrous metals when high cooling quality is required & chip bearing capacity is not very high. 3. Mineral Oil Mineral oils are typically found in high production machines that have high metal removal rates. They are used in heavy cutting operations as they have very good lubricating properties. A disadvantage of these oils is that they are corrosives and therefore are not used for copper or its alloys. 4.Synthetic Liquids As these are synthetic liquids, they do not contain mineral oil or petroleum. These are water-based liquids, and water provides very good cooling properties. The problem with synthetic fluids is that it is not a good lubricant and also causes corrosion. Corrosion or corrosion can be prevented by adding corrosion inhibitors to synthetic liquids. Typically, synthetic fluids are used in grinding liquids. 5.Semi-Synthetic Fluids Semi-synthetic fluid is made from a combination of synthetic fluid and soluble oils. For semi-synthetic liquids, approximately 5 to 20% of mineral oil is emitted with water to produce micro abrasion. The size of microalgae particles varies from 0.01 to 0.1 mm, which can easily transmit all light. 6. Solid and Paste Lubricants These lubricants are in the solids phase or as a paste. These lubricants are placed directly on the workpiece or tool. Some examples of this are molybdenum disulfide, graphite, wax stick, etc. 7. Cutting Oil Cutting oil is made by mixing minerals oil & fatty oil. It is used as both a coolant and a lubricant. UNIT II: AUTOMATS, SHAPING AND PLANING MACHINES  Capstan and turret lathes construction  Indexing mechanism operations  Working principle of single and multi-spindle automats  Shaping and planning machines -types -construction -Mechanism -and -principle of different types of shaping operations  Work holding devices. CAPSTAN AND TURRET LATHES CONSTRUCTION What is Lathe Machine?  A lathe is a machine tool that rotates a workpiece around an axis to perform various operations. Lathes are used to shape wooden or metallic products. DESCRIPTION OF LATHE Lathe is a machine which has several parts in it. They are 1. Bed It is the base of the machine. On its left side, the head stock is mounted and on its right it has movable casting known as tailstock. Its legs have holes to bolt down on the ground. 2. Head stock It consists of spindles, gears and speed changing levers. It is used to transmit the motion to the job. It has two types one is the headstock driven by belt and the other one is the gear driven. 3. Carriage Carriage is used to carry a tool to bring in contact with the rotating work piece or to with draw from such a contact. It operates on bed ways between the headstock and tail stock. 4. Saddle It is an ‘H’ shaped part fitted on the lathe bed. There is a hand wheel to move it on the bed way. Cross slide, compound rest, tool post is fitted on this saddle. a) Cross slide It is on the upper slide of saddle in the form of dove tail. A hand wheel is provided to drive the cross slide. It permits the cross wise movement of the tool (i.e.) movement of tool towards or away from the operator b) Compound rest It is fitted over the cross slide on a turn table. It permits both parallel and angular movements to cutting tool. c) Tool post It is fitted on the top most part of the compound rest. Tool is mounted on this tool post. Cutting tool is fixed in it with the help of screws. 5. Apron It is the hanging part in front of the carriage. It accommodates the mechanism of hand and power feed to the cutting tool for carrying out different operations. 6. Lead screw It is a long screw with ACME threads. It is used for transmitting power for automatic feed or feed for thread cutting operation. 7. Tail stock It is located at the right end of the lathe bed and it can be positioned anywhere in the bed. It is used for supporting lengthy jobs and also carries tool to carry out operations such as tapping, drilling, reaming.