CHAPTER 1 INTRODUCTION

Transcription

1 1 CHAPTER 1 INTRODUCTION In the current Engineering scenario, the production based industry continually focuses on achieving the increased product performance, quality and efficiency in order to improve a time to market and customer satisfaction to meet the current demand. The heart of any manufacturing organization lays on its production operations. Only with the right application of manpower, equipment and materials, manufacturing industry could transform purchased raw materials into a finished product. 1.1 METAL CUTTING Parts produced by casting, forming etc. requires further processing in order to convert into a finished product. Before the product is ready to use, it requires dimensional accuracy, surface finish. This further processing is called metal removal or metal cutting process. Metal cutting is the process of producing a job by removing a layer of unwanted material from a given work piece as shown in Figure 1.1. Edged tool is set to a certain depth of cut and moves relative to the work piece. Under the action of force, pressure is exerted on the workpiece metal causes its compression near the tip of the tool. The metal undergoes shear type deformation and a piece or layer of metal gets repeated in the form of a chip. If the tool is continued to move relative to the workpiece, there is continuous shearing of the metal ahead of the tool. The shear occurs along a plane called the shear plane. All machining processes involve the formation of chips; this occurs by deforming the work material on

2 2 the surface of job with the help of a cutting tool. Metal cutting process is shown in Figure 1.1. Depending upon the tool geometry, cutting conditions and work material, chips are produced in different shapes and sizes. Figure1.1 Metal cutting process 1.2 BASIC PRINCIPLES OF METAL CUTTING Manufacturing processes can be broadly divided into four categories, viz., primary (casting, forging, moulding), secondary (machining, finishing), tertiary (fabricating processes like welding, brazing, riveting), and fourth level processes (painting, electroplating). Secondary manufacturing processes are most important as any other level processes. In case of the practical machining, metal cutting is divided into i) Orthogonal cutting and ii) Oblique cutting. Orthogonal cutting operation is the simplest type of cutting operation, in which the cutting edge is straight, parallel to the original plane surface of the workpiece and perpendicular to the direction of cutting, and in which the length of the cutting edge is greater than the width of the chip removed.the

3 3 Figure 1.2 Orthogonal cutting Figure 1.3 Oblique cutting operation operation cutting angle is 90 in pure orthogonal cutting. This orthogonal cutting is also known as Two Dimensional (2-D) Cutting. In this model wedge shaped tool is used as shown in Figure 1.2. In oblique cutting, thecutting edge of the tool is inclined or arranged to make an angle with the line which is perpendicular to the direction of relative work-tool motion to the line normal to the cutting direction, and this angle is known as angle of obliquity. This angle of obliquity is also called the inclination angle. In actual machining, majority of the cutting operations are three dimensional (3-D) in nature and are called as oblique cutting. The Figure 1.3 shows the oblique cutting process. Variables which are influencing the metal cutting process may be dependent variables and independent variables. Independent variables: The variables that can change at the time of cutting operation and it can be directly controlled. Tool materials, Tool nomenclature, surface finish and sharpness of the tool.

4 4 Cutting condition such as cutting speed, feed rate, axial depth of cut and radial depth of cut. Work piece material properties such as roughness, hardness, ductility, brittle, etc. Type of cutting fluid used at the time of operation. Vibration of the machine tool. Work holding devices and type of fixtures used. Torque, Spindle Motor current, Cutting Time, Clearance angle, Feed Drive Current, Type of lubricants used for machine tool. Dependent variables: The variables that are influenced by changes in the independent variables are known as dependent variables and listed below: Tool wear and its failure Type of chip produced Cutting force, energy and temperature rise in workpiece, chip and tool at the time of cutting operation. Surface finish and integrity of the molecular structure of the workpiece after machining. The chip is formed by the deformation of the metal lying ahead of the cutting edge by a process of shear. Four main categories of chips are: Discontinuous Chips Continuous or Ribbon Type Chips Continuous Chip

5 5 Built-up-Edge (BUE) Serrated Chips Discontinuous Chips: Discontinuous chips are small segments, which are either firmly or loosely to each other. They are formed when the amount of deformation to which chips undergo and is limited by repeated fracturing. These types of chips usually formed under the following conditions. i) The work piece materials are brittle in nature like bronze, brass and cast iron etc, because they do not have the capacity to undergo the high shear stress strains in cutting. ii) iii) iv) The work piece materials contain hard inclusions and impurities. When the cutting speed is very low or high, depth of cut is too high, rake angle is very low. Poor damping of machine structure v) Lack of cutting fluid. Continuous or Ribbon Type Chips: In continuous chip formation, the pressure of the work piece built up until the material fails by slip along the plane. The inside of the chip displays steps produced by the intermittent slip, but the outside is very smooth. It has its elements bonded together in the form of long coils and is formed by the continuous plastic deformation of material without fracture ahead of the cutting edge of the tool and is followed by the smooth flow of chip up the tool face. These types of chips usually form under the following conditions. i) The work piece materials are ductile in nature like Mild steel, Aluminium, copper etc.

6 6 ii) iii) High cutting speed High rake angle Continuous Chip Built Up Edge: This type of chip is very similar to that of continuous type, with the difference is that it is not as smooth as the previous one. This type of chip is associated with poor surface finish, but protects the cutting edge from wear due to movement of chips and the action of heat causing the increase in tool life. These types of chips usually form under the following conditions. i) Decreasing depth of cut ii) iii) iv) Increasing the rake angle Using a tool with a small tip radius Using an effective cutting fluid Serrated Chips: These chips are semi continuous in the sense that they possess a saw-tooth appearance that is produced by a cyclical chip formation of alternating high shear strain followed by low shear strain. This chip is most closely associated with certain difficult-to-machine metals such as titanium alloys, nickel-base super alloys, and austenitic stainless they are machined at higher cutting speeds. However, the phenomenon is also found with more common with work metals (e.g., steels), when they are cut at higher speeds. 1.3 END MILLING OPERATION Milling is the process of cutting away material by feeding a work piece into a rotating multiple tooth cutter. The cutter at the end milling process generally rotates on an axis vertical to the work piece. Cutting teeth

7 7 are located on both of the end face of the cutter and the periphery of the cutting body. End mills are used for producing precision shapes and holes on a Milling. End mills are available in a variety of design styles and materials. The successful application of end milling depends on how well the tool is supported by the tool holder. To achieve best results an end mill must be mounted concentric in a tool holder. The end mill can be selected for the following basic processes: Plain Milling- Plain milling is the milling of a flat surface with the axis of the cutter parallel to the machining surface. It can be carried out either on a horizontal machine or a vertical machine. of the part. Face Milling- Face milling primarily is used to mill the top surface Keyway Production- A rectangular slot or groove that is machined down the length of a hole. Woodruff Keyways- Normally produced with a single cutter, in a straight plunge operation. Specialty Cutting- Milling of tapered surfaces, T shaped slots and dovetail production. Finish Profiling- To finish the inside/outside shape on a work piece with a parallel side wall.

8 End Mill Cutter Terminology Milling is the process of cutting away material by feeding a work piece past a rotating multiple tooth cutter. The cutting action of the many teeth around the milling cutter provides a fast method of machining. The machined surface may be flat, angular, or curved. The surface may also be milled to any combination of shapes. The cutter in end milling generally rotates on an axis vertical to the work piece. Cutting teeth are located on both the end face of the cutter and the periphery of the cutter body. The Figure 1.4 shows the End mill cutter terminology and they are explained as given below. Flute Length- The effective axial length of the peripheral cutting edge which has been relieved to cut. Flute - The flutes of the milling bit are the deep helical grooves running up the cutter, while the sharp blade along the edge of the flute is known as the tooth. Shank - Projecting portion of cutter which locates and drives the cutter from the machine spindle or adapter. Rake - The angular relationship between the tooth face or a tangent to the tooth face at a given point and a reference plane or line. An angular feature ground onto the surface of an end mill. i) Axial rake - The angle formed by a plane passing through the axis and a line coinciding with or tangent to the tooth face. ii) Effective rake - The rake angle influencing chip formation and almost measured normal to the cutting edge. The effective rake angle is greatly affected by the

9 9 radial and axial rakes only when corner angles are involved. iii) Helical rake - For most purposes, the terms helical and axial rake can be used interchangeably. It is the inclination of the tooth face with reference to a plane through the cutter axis. iv) Negative Rake - Exists when the initial contact between tool and workiece occurs at a point or line on the tooth other than the cutting edge. The rake surface leads the cutting edge. v) Positive Rake - Exists when the initial contact between the cutter and the workpiece occurs at the cutting edge. The cutting edge leads the rake surface. Figure 1.4 End Mill cutter Terminology

10 10 Radial Rake angle - The angle made by the rake face and a radius measured in a plane normal to the axis. Helix Angle - The angle formed by a line tangent to the helix and a plane through the axis of the cutter or the cutting edge angle which a helical cutting edge makes with a plane containing the axis of a cylindrical cutter Statement of Problem There are various parameters that effect the machine tool, work piece and cutting tool, causes vibration, poor surface finish and reduces the life of cutting tool. By optimizing the input parameters such as depth of cut, feed rate, spindle speed, axial / radial depth of cut, tool geometry (cutter diameter, number of teeth, side cutting edge angle, rake angle, shank diameter, helix angle, overall length of tool, nose radius., etc.) torque, spindle motor current, cutting time, clearance angle, feed drive current, type of lubricants used at the time of manufacturing, will have greater impact on equipment effectiveness. The optimized combination of the above input parameters will increase the surface finish, reduces the tool wear, increase the tool life by decreasing the vibration produced in the machine. Optimization of machining parameter is an imperative step to play down the machining time, failure rate, tool life, vibration and to obtain a better surface finish. Among various machining processes; end milling is one of the most fundamental and commonly encountered metal removal operations occurring in a real manufacturing environment. An end mill is one of the indispensable tools in the milling process.

11 Effect of Machining Parameters on Metal Cutting are as follows. The parameter that affect the machining at the time of metal cutting Spindle speed - The rotational speed of the spindle and tool in revolutions per minute (RPM). The spindle speed is equal to the cutting speed divided by the circumference of the tool. Feed rate - The speed of the cutting tool's movement relative to the work piece as the tool makes a cut. The feed rate is measured in inches per minute (IPM) or in mm. Axial depth of cut - The depth of the tool along its axis in the work piece as it makes a cut. A large axial depth of cut will require a low feed rate, or else it will result in a high load on the tool and reduce the tool life. Radial depth of cut - The depth of the tool along its radius in the work piece as it makes a cut. If the radial depth of cut is less than the tool radius, the tool is only partially engaged and is making a peripheral cut. If the radial depth of cut is equal to the tool diameter, the cutting tool is fully engaged and is making a slot cut. A large radial depth of cut will require a low feed rate, or else it will result in a high load on the tool and reduce the tool life. Cutting feed - The distance that the cutting tool or workpiece advances during one revolution of the spindle and tool, measured in inches per revolution (IPR). In some operations the tool feeds into the work piece and in others the work piece feeds into the tool. For a multi-point tool, the cutting feed is also equal to the feed per tooth, measured in inches per tooth (IPT), and multiplied by the number of teeth on the cutting tool.

12 12 Cutting speed - The speed of the workpiece surface relative to the edge of the cutting tool during a cut, measured in surface feet per minute (SFM). Cutting force, temperature, torque, spindle motor current, cutting Time, clearance angle, feed drive Current, type of lubricants used are also the important parameters that influence the metal cutting. 1.4 FACTORS INFLUENCING THE WORKPIECE AND TOOL LIFE Factors influencing the workpiece and tool life during the metal cutting process are divided into dependent variables and independent variables: Independent variables that can change at the time of cutting operation and it can be directly controlled. Tool materials, Tool nomenclature, surface finish and sharpness of the tool. Cutting condition such as cutting speed, feed rate, axial depth of cut, radial depth of cut etc. Work piece material properties such as roughness, hardness, ductile, brittle, etc. Type of cutting fluid used at the time of operation. Vibration of the machine tool. Work holding devices and type of fixtures used. Torque, spindle motor current, cutting time, clearance angle, feed drive current, Type of lubricants used for machine tool.

13 13 Cutting forces: The work piece must be able to withstand the cutting forces without excessive distortion in order to maintain dimensional tolerances. Dependent variables: The variables that are influenced by changes in the independent variables are Tool wear and its failure Type of chip produced Cutting force, energy and temperature rise in the workpiece, chip and tool at the time of cutting operation. Surface finish and integrity of the molecular structure of the workpiece after machining. Tool geometry: The tool geometry also affects the cutting operation such as cutter diameter, number of teeth, side cutting edge angle, rake angle, shank diameter, helix angle, overall length of tool, nose radius., etc. Clamping and positioning surfaces of the work piece and tool also affect the workpiece and tool. Type of Jigs and fixtures used at the time of machining operation. Work piece thickness is an important factor Work Piece Geometry and Work Piece Material In all machining processes, the work piece is a shape that can entirely cover the final part shape. Austenitic steels, Nickel and Titanium alloys have medium to high shear stresses and work hardening rates and low thermal diffusivities. They are likely to generate large tool stresses and

14 14 temperatures. Copper and aluminium alloys, although showing high work hardening rates, have relatively low shear stresses and high thermal diffusivities. They are likely to create low tool stresses and low temperature rises in machining. Due to the softness and sticky nature of aluminum, specific geometries and characteristics of an end mill are required for efficient machining. Many cutting tool manufacturers offer end mills specifically designed for aluminum for this reason. A sharp edge and high rake angles are needed to separate a chip from the parent material. Positive rake angles up to 25 degrees radial and 20 degrees axial are commonly used Cutting Tool Geometry and Holding Methodology The rake angle increases more, the strength on the cutting edge of the tool decreases. When cutting process on a positive rake angle, the normal force on the tool-chip interfaces causes bending of the tip of the cutting edge. The presence of the bending significantly reduces the strength of the cutting edge, causing its chipping. Moreover, the tool-chip contact area reduces with the rake angle so the point of application of the normal force shifts closer to the cutting edge. The tool cutting edge angle significantly effects the cutting process because, for a given feed and cutting depth, it defines the uncut chip thickness, width of cut, and thus tool life. The flank angle directly effects the tool life. When the inclination angle is positive, then the chip flows to the right and when it is negative the chip flows to the left. The direction of chip flow, however, is defined not only by the inclination angle but also from the cutting edge angle. The feed rate can be significantly increased by adjusting the tool cutting edge angle. The milling machine spindle may be self-holding or self-releasing. The spindle taper in most milling machines is self-releasing; tooling must be held in place by a draw bolt extending through the center of the spindle. Milling cutter is positioned on the arbor with sleeves so that it is as close as

15 15 practical to the milling machine spindle while maintaining sufficient clearance between the vise and the milling machine column. This practice reduces torque in the arbor and permits more rigid support for the cutter. End mills may be aligned centrally by first causing the the workpiece to contact the periphery of the cutter. This cutting usually requires to be completed in several steps - in each step, the part is located on a fixture, and the exposed portion can be accessed from the tool. Common fixtures include vise, clamps, 3-jaw or 4-jaw chucks, etc. Each position of holding the part is called a setup. One or more cutting operations may be performed, using one or more cutting tools, in each setup. To switch from one setup to the next, we must release the part from the previous fixture, by changing the fixture on the machine, clamp the part in the new position of the new fixture, set the coordinates of the machine tool with respect to the new location of the part, and finally start the machining operations for the mentioned setup Cutting Tool Material The cutting tool materials currently used for machining operations are high speed tool steel, cobalt-base alloys, cemented carbides, ceramic, polycrystalline cubic boron nitride and polycrystalline diamond. Different machining applications require six different cutting tool materials. The Ideal cutting tool material should have all of the following characteristics: ¾ Harder than the work it is cutting High temperature stability Resists wear and thermal shock Impact resistant Chemically inert to the work material and cutting fluid

16 16 To select an effective tool for the appropriate machining process, a machinist or engineer must know the specific information about: The starting and finished part shape The work piece hardness The material's tensile strength The material's abrasiveness The type of chip generated The work holding setup The power and speed capacity of the machine tool Some common cutting tool materials are: High Carbon Steel, High Speed Steel (HSS), Cast Alloys, Cemented carbides (sintered carbide), Coated Carbides, Ceramics, Diamonds High Carbon Steel: This material is one of the earliest cutting materials used in machining. It is however now virtually superseded by other materials used in engineering because it starts to temper at about 220 C. This softening process continues as the temperature rises. As a result cutting using this material for tools is limited to speeds up to 0.15 m/s for machining mild steel with lots of coolant. High Speed Steel (HSS) : This range of metals contains about 7% carbon, 4% chromium plus additions of tungsten, vanadium, molybdenum and cobalt. These metals maintain their hardness at temperature up to about 600º C, but soften rapidly at higher temperatures. These materials are suitable for cutting mild steel at speeds up maximum rates of 0.8 m/s to 1.8 m/s.

17 17 Cast Alloys: These cutting tools are made of various nonferrous metals in a cobalt base. They can withstand cutting temperatures of up to 760º C and are capable of cutting speeds about 60% higher than HSS. Cemented carbides (sintered carbide): This material usually consists of tungsten carbide or a mixture of tungsten carbide, titanium, or tantalum carbide in powder form, sintered in a matrix of cobalt or nickel. As this material is expensive and has low rupture strength, it is normally made in the form of tips which are brazed or clamped on a steel shank. The clamped tips are generally used as throw away inserts. Coated Carbides: The cutting system is based on providing a thin layer of high wear-resistant titanium carbide fused to a conventional tough grade carbide insert, thus achieving a tool combining the wear resistance of one material with the wear resistance of another. These systems provide a longer wear resistance and a higher cutting speed compared to conventional carbides. Ceramics: Ceramics are made by powder metallurgy from aluminium oxide with additions of titanium oxide and magnesium oxide to improve cutting properties. These have a very high heat resistance and wear resistance and can cut at very high speed. However they are brittle and have little resistance to shock. Their use is therefore limited to tips used for continuous high speed cutting on a vibration-free machines. Diamonds: Diamonds have limited application due to the high cost and the small size of the stones. They are used on very hard materials to produce a fine finish and on soft materials especially those inclined to clog other cutting materials. They are generally used at very high cutting speed with low feed and light cuts. Due to the brittleness of the diamonds the

18 18 machine has to be designed to be vibration free. The tools last for 10 (up to 400) times longer than carbide based tools. Most modern tools use steel shafts with smaller, cutting pieces called inserts. The inserts may be carbide, or coated carbide. The coating is a layer of 5-8 microns, of materials such as tungsten carbide, titanium carbide, titanium nitride, CBN or even diamond. 1.5 VIBRATION Introduction Mechanical vibrations are produced by the cyclic variations in the machine components and due to the dynamic interactions between the cutting tool and the work piece which results in reduced productivity, poor surface finish and reduced tool life and deteriorated quality. The Vibrations that occur in the machining of metals are ordinarily of two types, Forced vibrations and self-excited vibrations. Forced vibrations are those which occur under the action of a periodically varying force on the cutting tool arising out of mechanical causes. The frequency of such vibrations depends upon the frequency of force variations at the source, which may be quite different from the natural frequencies of the vibrating members and self-excited vibrations are those which occur because of dynamic instability of the vibrating member as shown in figure 1.5. Chatter is the unstable transient vibration in the cutting process and mainly caused by the regenerative and mode-coupling effects.

19 19 Figure 1.5 Cycles and Amplitude Amplitude of vibration: The amplitude of vibration is the peak magnitude of vibration. A machine with large vibration amplitude is one that experiences large, fast, or forceful vibratory movements. The larger the amplitude, the more movement or stress is experienced by the machine, and the more prone to damage of the machine. Vibration amplitude is thus an indication of the severity of vibration. In general, the severity or amplitude of vibration relates to: i) The size of the vibratory movement ii) iii) The speed of the movement The force associated with the movement In most situations, the speed or velocity amplitude of a machine that gives the most useful information about the condition of the machine. Velocity amplitude can be expressed in terms of peak value, it is known as its root-mean-square value (rms). The root-mean-square velocity amplitude of a vibrating machine provides the detail of vibration energy in the machine.

20 20 Higher the vibration, the higher will be the root-mean-square velocity amplitude. The unit of vibration amplitude is expressed as mm/s. Frequency: The rate at which a machine component oscillates is called its oscillation or vibration frequency. The unit of frequency is expressed as cycles per second (cps) or Hertz or 60 cycles per minute (60 cpm).the Figure 1.6 shows the frequency wave form. Figure 1.6 Frequency Waveform: The graphical display of electrical signals of vibratory motion are useful tools for analyzing the nature of vibration or A waveform is a graphical representation as shown in Figure 1.7 of how the vibration level changes with time. The amount of information a waveform contains depends on the duration and resolution of the waveform. The duration of a waveform is the total time period over which information may be obtained from the waveform.

21 21 Figure 1.7 Waveform Spectrum: A spectrum is a graphical display of the frequencies at which a machine component is vibrating, together with the amplitudes of the component at these frequencies as shown in Figure 1.8. Figure 1.8 Spectrum Vibration in Machine Tool Machine tool vibration reduces the performance of machining operations. It results in degraded quality on the machined parts, shorter tool life, and creates unpleasant noise.

22 22 Disturbances in the tool drives: Vibrations result from rotating unbalanced masses; gear, belt, and chain drives; bearing irregularities; unbalanced electromagnetic forces in electric motors; pressure oscillations in hydraulic drives; etc. Vibration caused by rotating unbalanced members: Forced vibration induced by rotation of some unbalanced member may affect both surface finish and tool life, especially when its rotational speed falls near one of the natural frequencies of the machine-tool structure. This vibration can be eliminated by careful balancing. The effect of vibration on the wheel properties: If vibration exists between wheel and work piece, normal forces are produced which react on the wheel and tend to alter the wheel shape and/or the wheel s cutting properties. Drives: Spindle and feed drives can be important sources of vibration caused by motors, power transmission elements (gears, traction drives, belts, screws, etc.), bearings, and guide ways Vibration in Cutting Tool Cutting speed and chip cross section vary during the vibration, it affect the tool life. Elimination of vibration may significantly improve the life of tool. The torsional vibration caused by dynamic instability of the cutting process, because of resonance caused by one of the harmonics of impact excitation from interrupted chip removal, by tool run out, etc. The forced vibration of the tool and the work piece may also significantly enhance tool life by reducing cutting forces, leading to enhanced dynamic stability. The vibration behavior of a machine tool can be improved by a reduction of the intensity of the sources of vibration, by enhancement of the effective static

23 23 stiffness and damping for the modes of vibration which result in relative displacements between tool and work piece, and by appropriate choice of cutting regimes, tool design, and work piece design. In addition, the application of vibration dampers and absorbers is an effective technique for the solution of machine vibration problems Causes for the Cutting Tool Vibration causes: Almost all machine vibration is due to one or more of the following Repeating forces: Most machine vibration is due to repeating forces. Repeating forces act on machine components which cause the machine to vibrate. Repeating forces on machine are mostly due to the rotation of imbalanced, misaligned, worn, or improperly driven machine components. Examples of these four types of repeating forces are shown below. i) Imbalanced Machine Components-Machining errors, nonuniform material density, variation in bolt sizes, air cavities in cast parts, missing balance weights, incorrect balancing, uneven electrical motor windings, broken, deformed, corroded or dirty fan blades. ii) iii) iv) Misalignment of Machine Components- inaccurate assembly, uneven floors, thermal expansion, distortions due to fastening torque, improper mounting of couplings. Worn-out of Machine Components- rubbing of uneven worn surface, wear in roller bearings, poor lubrication, manufacturing defects and over loading. Machine components are improperly driven by improper air supply in machinery components.

24 24 may: Forces generated within the machine causes vibration. These forces Change in direction with respect to time, such as the force generated by a rotating unbalance. Change in amplitude or intensity with respect to time, such as the unbalanced magnetic forces generated in an induction motor due to unequal air gap between the motor armature and stator (field). Result in friction between the rotating and stationary machine components in much the same way that friction from a rosined bow causes a violin string to vibrate. Cause impacts, such as gear tooth contacts or the impacts generated by the rolling elements of a bearing passing over flaws in the bearing raceways. Cause randomly generated forces such as flow turbulence in fluid-handling devices such as fans, blowers and pumps; or combustion turbulence in gas turbines or boilers. Looseness: Looseness of machine parts causes a machine to vibrate. If parts become unfastened, vibration that is normally of tolerable levels may become unrestrained and excessive. Misalignment is often due to excessive bearing clearances, unfastened mounting bolts, mismatched parts, corrosion and cracked structures. Resonance: The machine will vibrate greatly due to the repeating force encouraging the machine to vibrate at a rate it is most natural with. The machine will vibrate vigorously and excessively, not only because it is doing so at a rate it 'prefers' but also because it is receiving external aid to do so. A

25 25 machine vibrating in such a manner is said to be experiencing resonance. A repeating force causing resonance may be small and may originate from the motion of a good machine component. Such a mild repeating force would not be a problem until it begins to cause resonance. Resonance, however, should always be avoided as it causes rapid and severe damage to the machine components. Vibrations caused by chip formation: When a discontinuous type of chip is formed, the recurring fractures of the metal in the shear plane ahead of the tool produce periodic variations in the cutting force. Similarly, in the case of machining operations which produce a continuous chip with built up edge as fragments of the built- up edge pass off with the chip of the work piece, a variation on the force on the cutting tools Effect of Vibration in a workpiece When machining is done under certain conditions, it will result in discontinuous chip thrust. If the frequency of these fluctuations coincides with one of the natural frequencies of the structure, forced vibration of appreciable amplitude may be excited. The breaking away of a built-up edge from the tool face also imparts impulses to the cutting tool which result in vibration. However, marks left by the built-up edge on the machined surface are far more pronounced than those caused by the ensuing vibration; probably for this reason that the built-up edge has not been studied from the vibration point of view. The built-up edge frequently accompanies certain types of vibration (chatter), and instances have been known when it disappeared as soon as the vibration was reduced. Milling tests were used to validate the performance of the active control system. Three modes of testing were used are signature testing, surface finish testing and tool wear testing.

26 Vibration Reduction Techniques There are the basic methods exist for vibration control of industrial equipment, as detailed below; Out of balance of rotating or reciprocating machine components, faulty gears, belts, ball and roller bearings, the mechanisms which transfer energy in uniformly timed impulses. These vibrations can be eliminated by careful static and dynamic balancing of the faulty components. Force Reduction of excitation inputs due to, for example, unbalances or misalignment will decrease the corresponding vibration response of the system. Mass Addition will reduce the effect (system response) of a constant excitation force. Tuning (changing) the natural frequency of a system or component will reduce or eliminate amplification due to resonance. Isolation rearranges the excitation forces to achieve some reduction or cancellation. Damping is the conversion of mechanical energy (vibrations) into heat. Vibrations transmitted from other machines through foundations, such vibrations can be suppressed by applying vibration isolators. Common vibration isolators are steel springs, rubber pads or bellows. Structural Damping is a control technique reduces both impact-generated and steady-state noises at their source. It

27 27 dissipates vibrational energy in the structure before it can build up and radiate as sound Vibration Measuring Techniques Vibration analysis is widely accepted as a reliable technique to monitor the operating conditions of a machine as it is a non-destructive and a continuous monitoring method. Vibration analysis increases knowledge and provides necessary information for: Evaluation of machine condition Recognition of on-going machine damage symptoms Identification of the cause and the damaged components Prognosis of remaining service life Various types of vibration sensors are available, but a type called accelerometer is normally used as it offers advantages over other sensors. An accelerometer is a sensor that produces an electrical signal that is proportional to the acceleration of the vibrating component to which the accelerometer is attached. The acceleration signal produced by the accelerometer is passed onto the instrument that in turn converts the signal to a velocity signal. Depending on the user s choice, the signal can be displayed as either a velocity waveform or a velocity spectrum. A velocity spectrum is derived from a velocity waveform by means of a mathematical calculation known as the Fast Fourier Transform or FFT. Other vibration measuring instruments and data acquisition are:

28 28 Instrumentation Microphones Intensity probes Accelerometers Displacement devices Velocity devices Sound level meters Vibration meters 1.6 SURFACE ROUGHNESS Introduction Roughness consists of surface irregularities which result from the various machining process. These irregularities combine to form surface texture. Every components surface has some form of texture which varies according to its structure and the way it has been manufactured. The machining processes generate a wide variety of surface textures. Surface texture consists of the repetitive and/or random deviations from the ideal smooth surface Surface Texture and Terminology Order of deviation is defined in the international standards for surface roughness. Surface roughness refers to deviation from the nominal surface of the third up to sixth order. First and second-order deviations refer to form, i.e. flatness, circularity, etc. and to waviness, respectively, and are due to machine tool errors, deformation of the work piece, erroneous setups and clamping, vibration and work piece material in homogeneities. Third and

29 29 fourth-order deviations refer to periodic grooves, and to cracks and dilapidations, which are connected to the shape and condition of the cutting edges, chip formation and process kinematics. Fifth and sixth-order deviations refer to work piece material structure, which is connected to physicalchemical mechanisms acting on a grain and lattice scale (slip, diffusion, oxidation, residual stress, etc.). Different order deviations are superimposed and form the surface roughness profile and these surfaces as shown in Figure 1.9, 1.10 can be broken down into three main categories: Surface roughness, Waviness and Form. Roughness: Perfect smoothness is impossible, if not undesirable. Roughness is the single biggest determinant of the surface texture. Small, finely spaced surface irregularities are surface roughness Waviness: Waviness is a feature of a surface finish that arises due to structural reasons. Surface irregularities of grater spacing (macro irregularities) are waviness. Lay: Lays denote the patterns that arise due to the dominant direction of the machining process. Depending on how the machining is done, the lay patterns can either be vertical, horizontal or circular. Figure 1.9 Different terms related to Surface Texture

30 30 Figure 1.10 Deviations of surface roughness Flaws: Irregularities that occur occasionally on the surface Includes cracks, scratches, inclusions, and similar defects in the surface Although some flaws relate to surface texture, they also affect surface integrity. In order to predict components behavior during use or to control the manufacturing process, it is necessary to quantify these surface characteristics. This is done by using surface texture parameters. Roughness Height: It is the height of the irregularities with respect to a reference line. It is measured in millimeters or microns or micro inches. It is also known as the height of unevenness. Roughness Width: The roughness width is the distance parallel to the nominal surface between successive peaks or ridges which constitute the predominant pattern of the roughness. It is measured in millimeters. Roughness Width cut off: Roughness width cut off is the greatest spacing of respective surface irregularities to be included in the measurement

31 31 of the average roughness height. It should always be greater than the roughness width in order to obtain the total roughness height rating. Waviness Height: Waviness height is the peak to valley distance of the surface profile, measured in millimeters. Arithmetic Average (AA): A close approximation of the arithmetic average roughness-height can be calculated from the profile chart of the surface. Averaging from a mean centerline may also be automatically performed by electronic instruments using appropriate circuitry through a meter or chart recorder. If X is the measured value from the profilometer. Root Mean Square (rms): The rms value can be calculated as shown below. Its numerical value is about 11% higher than that of AA. Mean line of the Profile: It is the line that divides the effective profile such that, within sampling length the sum of squares of distances (y 1, y 2,.y n ) between effective points and mean line is minimum. Center line of the Profile: It is the line for which the area embraced by the profile above or below the line is equal. Average Surface roughness (Ra): The average roughness is the area between the roughness profile and its mean line, or the integral of the absolute value of the roughness profile height over the evaluation length Graphically, the average roughness is the area (shown below) between the roughness profile and its center line divided by the evaluation length (normally five sample lengths with each sample length equal to one evaluation length) This is the parameter that has been used universally for many years It is the arithmetic mean of the absolute departures of the roughness profile from the mean line.

32 32 Figure 1.11 Center Line Average Center Line Average (CLA), Ra is the Arithmetic Average of the distance of the filtered or unfiltered Roughness profile from its Mean Line. RMS Value, Root Mean Square of the distance of the filtered or unfiltered Roughness Profile from its mean line. Ten Point height of Irregularities: Mean Roughness Depth (R z ): R z is numerically the average height difference between the five highest peaks and the five lowest valleys within the assessment length. R p : Maximum profile peak height, mathematically, the largest peak deviation of the roughness profile from the mean line within a sampling length. RSm: The mean spacing between profile peaks at the mean line, measured within the sampling length. (A profile peak is the highest point of the profile between an upwards and downwards crossing of the mean line). Rmax measures the vertical distance from the highest peak to the lowest valley within five sampling lengths, and selects the largest of the five values.

33 Factors Influencing the Surface Roughness in Milling Whenever two machined surfaces come in contact with one another the quality of the mating parts plays an important role in the performance and wear of the mating parts. The height, shape, arrangement and direction of these surface irregularities on the work piece depend upon a number of factors such as: i) The machining variables which include Cutting speed Feed, and Depth of cut. ii) The tool geometry Some geometric factors which affect achieved surface finish include: Nose radius Rake angle Side cutting edge angle, and Cutting edge. iii) Work piece and tool material combination and their mechanical properties iv) Quality and type of the machine tool used, v) Auxiliary tooling, and lubricant used, and vi) Vibrations between the work piece, machine tool and cutting tool. Factors, influencing surface.

34 34 The process parameter improves surface finish is outlined below: Increasing the tool rake angle generally improves surface finish Higher work material hardness results in better surface finish Tool material has minor effect on surface finish. Cutting fluids affect the surface finish changing cutting temperature and as a result the built-up edge formation Surface Finish Prediction Techniques Surface roughness also affects several functional attributes of parts, such as contact causing surface friction, wearing, light reflection, heat transmission, ability of distributing and holding a lubricant, coating, or resisting fatigue. Therefore, the desired finish surface is usually specified and the appropriate processes are selected to reach the required quality. The final surface roughness might be considered as the sum of two independent effects: i) The ideal surface roughness is a result of the geometry of tool and feed rate ii) The natural surface roughness is a result of the irregularities in the cutting operation. Factors such as spindle speed, feed rate, and depth of cut that control the cutting operation can be setup in advance. However, factors such as tool geometry, tool wear, chip loads and chip formations, or the material properties of both tool and work piece are uncontrolled. Even in the occurrence of chatter or vibrations of the machine tool, defects in the structure of the work material, wear of the tool, or irregularities of chip formation contribute to the surface damage in practice during machining.

35 35 Surface measuring techniques: In the past, surface texture has been assessed by the judgment of the inspector either by eye or fingernail. In order to put a number on the surface texture, we need to use a more accurate means of measurement. A typical surface measuring instrument consists of a stylus with a small tip (fingernail), a gauge or transducer, a traverse datum and a processor. The surface is measured by moving the stylus across the surface. As the stylus moves up and down along the surface, the transducer converts the movement into a signal which is then exported to a processor which converts this into a number and usually a visual profile. For a collection of correct data, the gauge needs to pass over the surface in a straight line such that only the stylus tip follows the surface under test. This is done using a straightness datum. This can consist of some form of datum bar that is usually lapped or precision ground to a high straightness tolerance. On small portable instruments this is not always a good option and can add to the expense of the instrument. In these cases, it is possible to use an alternate means of datum. This is a skid and is shown in Figure Figure 1.12 Surface Roughness Tester

36 36 A skid or shoe is drawn slowly over the surface. A stylus move over the surface with the skid. An amplifying device for magnifying the stylus movement and indicator. A recording device to produce trace of the surface profile. A mean of analyzing the trace Inspection and assessment of surface roughness of machined work pieces can be carried out by means of different measurement techniques. These methods can be ranked into the following classes: i) Direct measurement methods ii) iii) iv) Comparison based techniques Non contact methods On-process measurement i) Direct measurement methods: Direct methods assess surface finish by means of stylus type devices. Measurements are obtained using a stylus drawn along the surface to be measured. The stylus motion perpendicular to the surface is registered. This registered profile is then used to calculate the roughness parameters. This method requires interruption of the machine process, and the sharp diamond stylus can make micro-scratches on surfaces.

37 37 Figure 1.13 Schematic diagram of surface roughness measurement technique by stylus equipment Stylus equipment Basically, this technique uses a stylus that tracks small changes in surface height, and a skid that follows large changes in surface height. The use of the two together reduces the effects of non-flat surfaces on the surface roughness measurement. The relative motion between the skid and the stylus is measured with a magnetic circuit and induction coils. Schematic diagram of surface roughness measurement technique by stylus equipment has been shown in Figure The actual apparatus uses the apparatus hooked to other instrumentation. The induction coils drive amplifiers, and other signal conditioning hardware. The then amplified signal is used to drive a recorder that shows stylus position, and a digital readout that displays the CLA/Ra value. The paper chart that is recorded is magnified in height by : 1, and in length by 82: 1 to make the scale suitable to the human eye.

38 38 The datum that the stylus position should be compared to can and it can be one of the three, i) Skid - can be used for regular frequency roughness (Figure 1.14) ii) Shoe - can be used for irregular frequency roughness (Figure 1.15) iii) Independent - can use an optical flat (Figure 1.16) Figure 1.14 Skid - used for regular frequencies Figure 1.15 Flat shoe - used for surfaces with irregular frequencies

39 39 Figure 1.16 Independent datum ii) Comparison based techniques: Comparison techniques use specimens of surface roughness produced by the same process, material and machining parameters as the surface to be compared. Visual and tactile sensors are used to compare a specimen with a surface of the known surface finish. Because of the subjective judgment is involved, this method is useful for surface roughness where Rq>1.6 micron. iii) Non-contact methods: There have been some works done to attempt to measure surface roughness using non-contact technique. Here is an electronic speckle correlation method given as an example. When coherent light illuminates a rough surface, the diffracted waves from each point of the surface mutually interfere to form a pattern which appears as a grain pattern of bright and dark regions. The spatial statistical properties of this speckle image can be related to the surface characteristics. The degree of correlation of two speckle patterns produced from the same surface by two different illumination beams can be used as a roughness parameter. A rough surface is illuminated by a monochromatic plane wave with an angle of incidence with respect to the normal to the surface; multiscattering and shadowing effects are neglected. The photo-sensor of a CCD

40 40 camera placed in the focal plane of a Fourier lens is used for recording speckle patterns. Assuming Cartesian coordinates x,y,z, a rough surface can be represented by its ordinates Z(x,y) with respect to an arbitrary datum plane having transverse coordinates (x,y). Then the rms value of surface roughness can be defined and calculated. iv) On-process measurement: Many methods have been used to measure surface roughness in the process. For example: a. Machine vision: In this technique, a light source is used to illuminate the surface with a digital system for viewing the surface and the data being sent to a computer for analysis. The digitized data is then used with a correlation chart to get actual roughness values. b. Inductance method: An inductance pickup is used to measure the distance between the surface and the pickup. This measurement gives a parametric value that may be used to give a comparative roughness. However, this method is limited to measuring magnetic materials. c. Ultrasound: A spherically focused ultrasonic sensor is positioned with a non normal incidence angle above the surface. The sensor sends out an ultrasonic pulse to the personal computer for analysis and calculation of roughness parameters Possible value of Surface Finishing for the different Materials Manufacturing process employed will reflect the surface finish level of the materials. Some processes are inherently capable of producing better surfaces than others. The processes recognized for good surface finish are honing, lapping, polishing and surface finishing. Tolerance and range of

41 41 surface roughness produced by different processes are given below in Table 1.1. Table 1.1 Possible Surface Roughness value Roughness Value Ra ( µm) Roughness Grade Number 50 N12 25 N N N9 3.2 N8 1.6 N7 0.8 N6 0.4 N5 0.2 N4 0.1 N N N1

42 42 Table 1.2 Surface-Roughness-Range of Production Techniques 1.7 PROBLEM IDENTIFICATION Rake angle is the angle between the leading edge of a cutting tool and perpendicular to the surface being cut. Rake angles for milling cutters are specified in two directions, axial and radial rake angle. Most cutters have two clearance angles viz. a primary (A) a secondary (B) and radial rake angle (C). All these three angles are shown in Figure 1.17.

43 43 Figure 1.17 End mill cutter While milling the aluminium material, careful consideration must be given to the effect of centrifugal force on the cutter. End mills for aluminium should have a sharp clearance angle with sufficient clearance to prevent heeling, according to Joseph Davis and Davis (1993). Experimental studies have been conducted to predict the effects of rake angle (-4, 0, 4, 8, 12, 16, 20) and helix angles on the cutting forces variations during milling of hard materials. Rake angle becomes higher; the cutting forces on components become lower as per the study by Bissey et al (2007). The optimization of cutting conditions to predict surface roughness (Ra) in end milling involving radial rake angle is still lacking, (Azlan Mohd Zain et al 2010a) conducted an experiment by using an end mill cutter to predict surface roughness by considering the input parameters such as radial rake angle (6.2, 7.0, 9.5,