Turning is the machining operation that produces cylindrical parts. In its basic form, it can be defined as the machining of an external surface, with the workpiece rotating, using a single-point cutting tool, and with the cutting tool feeding parallel to the axis of the workpiece and at a distance that will remove the outer surface of the work. Even though a single-point tool is specified, this does not exclude multiple-tool setups, which are often employed in turning. In such setups, each tool operates independently as a single-point cutter.
Turning is the most commonly used operation in Lathe. Normally the work piece is rotated on a spindle and the tool is fed into it radially, axially, or both ways simultaneously, to give the required surface. The term 'turning', in the general sense, refers to the generation of any cylindrical surface with a single point tool. By turning operation excess material from the work piece is removed to produce a cylindrical or cone shaped surface.
Types of Turning
Turning operation can be classified into two types in terms of the shape generated or the angle the cutter makes with the axis of the job.
Straight turning
Taper turning
Straight Turning
In this operation the work is held in the spindle and is rotated while the tool is fed past the work piece in a direction parallel to the axis of rotation. The surface generated is a cylindrical surface.
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Fig. 2
Taper Turning
A taper may be defined as a uniform increase or decrease in diameter of a work piece measured along its length. In a Lathe taper turning is an operation to produce a conical surface by gradual reduction in diameter from a cylindrical job. Taper turning can be done by the following ways;
By a form tool.
By setting over the tailstock.
By swiveling the compound rest.
By taper turning attachment.
By compound feed.
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Holding the Workpiece
One of the most important things in a turning operation is proper fixing or holding of the job.
In lathe work the three most common work holding methods are:
Held in a chuck
Held between centers
1.1.2.1 Chucks
A chuck is one of the most important devices for holding and rotating workpieces in a lathe. Workpiece of short length and large diameter or of irregular shape which cannot be conveniently mounted between centers are held quickly and rigidly in a chuck. A chuck is attached to the lathe spindle by means of bolts with the back plate or screwed n the spindle nose.
There are different kinds of chucks:
Three jaw self centering chuck
Four jaw independent chuck
Combination chuck: It is a combination of self centering and independent chuck.
Magnetic chuck: The workpieces are held in this chuck by means of powerful electro- magnets.
Air or hydraulic operated chuck: The workpieces are held in this chuck by means of fluid pressure.
Collet chuck:
Fig. 3 Fig. 4
Fig. 5
1.1.2.2 Holding between Centers
Lathe centers support the workpiece between the headstock and the tailstock. The center used in the headstock spindle is called the 'live' center. It rotates with the headstock spindle. The 'dead' center is located in the tailstock spindle. This center usually does not rotate and must be hardened and lubricated to withstand the wear of the revolving work. The workpiece must have perfectly drilled and countersunk holes to receive the centers. The center must have a 60-degree point.
Fig. 6
Adjustable Factors in Turning Operation
The three primary factors in any basic turning operation are speed, feed, and depth of cut. Other factors such as kind of material and type of tool have a large influence, of course, but these three are the ones the operator can change by adjusting the controls, right at the machine. Feed, speed, and depth of cut have a direct effect on productivity, tool life, and machine requirements. Therefore these elements must be carefully chosen for each operation. Whether the objective is rough cutting or finishing will have a great influence on the cutting conditions selected.
1.1.3.1 Speed
Speed, always refers to the spindle and the Workpiece. When it is stated in revolutions per minute (rpm) it tells their rotating speed. But the important figure for a particular turning operation is the surface speed, or the speed at which the workpiece material is moving past the cutting tool. It is simply the product of the rotating speed times the circumference (in feet) of the workpiece before the cut is started. It is expressed in surface feet per minute (s.f.p.m), and it refers only to the workpiece. Every different diameter on a workpiece will have a different cutting speed, even though the rotating speed remains the same.
Here, v is the cutting speed in turning, D is the initial diameter of the work piece in mm, and N is the spindle speed in RPM.
1.1.3.2 Feed
Feed, always refers to the cutting tool, and it is the rate at which the tool advances along its cutting path. On most power-fed lathes, the feed rate is directly related to the spindle speed and is expressed in inches (of tool advance) per revolution (of the spindle), or i.p.r. The figure, by the way, is usually much less than an inch and is shown as decimal amount. A greater feed rate reduces the rate of production while a lesser feed rate gives better tool life and surface finish.
Fm= fxN mm. min-1
Here, Fm is the feed in mm per minute, f is the feed in mm/rev and N is the spindle speed in RPM.
1.1.3.3 Depth of Cut
Depth of Cut is practically self explanatory. It is the thickness of the layer being removed from the workpiece or the distance from the uncut surface of the work to the cut surface, expressed in inches. It is important to note, though, that the diameter of the workpiece is reduced by two times the depth of cut because this layer is being removed from both sides of the work.
Here, D and d represent initial and final diameter (in mm) of the job respectively.
Fig .7
The selection of cutting speed and feed is based on the following parameters:
Workpiece material
Tool Material
Tool geometry and dimensions
Size of chip cross-section
Types of finish desired
Rigidity of the machine
Types of coolant used
1.1.4 Dynamics of Turning Operation
The relative forces in a turning operation are important in the design of machine tools. The machine tool and its components must be able to withstand these forces without causing significant deflections, vibrations, or chatter during the operation. There are three principal forces during a turning process: cutting force, thrust force and radial force.
The cutting force acts downward on the tool tip allowing deflection of the workpiece upward. It supplies the energy required for the cutting operation.
The thrust force acts in the longitudinal direction. It is also called the feed force because it is in the feed direction of the tool. This force tends to push the tool away from the chuck.
The radial force acts in the radial direction and tends to push the tool away from the workpiece.
Although it requires less-skilled labor, the engine lathes do need skilled labor and the production is somewhat slow. Moreover, it can be accelerated by using a turret lathe (In a turret lathe, a longitudinally feedable, hexagon turret replaces the tailstock. The turret, on which six tools can be mounted, can be rotated about a vertical axis to bring each tool into operating position, and the entire unit can be moved longitudinally, either manually or by power, to provide feed for the tools) and automated machines.
Fig xx
1.2 CUTTING TOOLS
Cutting tool is a device, used to remove the unwanted material from given workpiece. For carrying out the machining process, cutting tool is fundamental and essential requirement.
1.2.1 Cutting Tool Characteristics
A cutting tool must have the following characteristics:
Hardness: The tool material must be harder than the work piece material. Higher the hardness, easier it is for the tool to penetrate the work material. Hot hardness: Hot Hardness is the ability of the cutting tool must to maintain its Hardness and strength at elevated temperatures. This property is more important when the tool is used at higher cutting speeds, for increased productivity.
Toughness: In spite of the tool being tough, it should have enough toughness to withstand the impact loads that come in the start of the cut to force fluctuations due to imperfections in the work material. Toughness of cutting tools is needed so that tools don't chip or fracture, especially during interrupted cutting operations like milling.
Wear Resistance: The tool-chip and chip-work interface are exposed to severe conditions that adhesive and abrasion wear is very common. Wear resistance means the attainment of acceptable tool life before tools need to be replaced.
Low friction: The coefficient of friction between the tool and chip should be low. This would lower wear rates and allow better chip flow.
Thermal characteristics: Since a lot of heat is generated at the cutting zone, the tool material should have higher thermal conductivity to dissipate the heat in shortest possible time, otherwise the tool temperature would become high, reducing its life.
1.2.2 Cutting Tool Materials
Carbon and Medium alloy steels: These are the oldest of the tool materials dating back hundreds of years. In simple terms it is a high carbon steel (steel which contains about 0.9 to 1.3% carbon). Inexpensive, easily shaped, sharpened. No sufficient hardness and wear resistance. Limited to low cutting speed operation
High Speed Steel (1900): The major difference between high speed tool steel and plain high carbon steel is the addition of alloying elements (manganese, chromium, tungsten, vanadium, molybdenum, cobalt, and niobium) to harden and strengthen the steel and make it more resistant to heat (hot hardness). They are of two types:
Tungsten HSS (denoted by T), Molybdenum HSS (denoted by M).
• Cemented Carbides or Sintered Carbides (1926-30): These tools are produced by powder metallurgy. Carbide tools are basically of three types:
tungsten carbide (WC), tantalum carbide (TaC), and titanium carbide (TiC).
The carbides or combined carbides are mixed with a binder of cobalt. They are able to retain hardness to a temperature of about 10000C. So they can be used at high speeds. Carbide tool are available as brazed tip tools (carbide tip is brazed to steel tool) and inserts (inserts are of various shapes- triangular, square diamond and round).
Coated cemented carbide (1960): Tool life to about 200 to 300 % or more. A thin, chemically stable, hard refractory coating of TiC, TiN or Al2O3 is used. The bulk of the tool is tough, shock resistant carbide that can withstand high temperatures. Because of its wear resistance, coated tool can be used at still higher speeds.
Cast cobalt alloys or Stellites (1915): It is a non-ferrous alloy consisting mainly of cobalt, tungsten and chromium (38% to 53% Cobalt, 30% to 33% Chromium, and 4% to 20% Tungsten). Other elements added in varying proportions are molybdenum, manganese, silicon and carbon. It has good shock and wear resistance properties and retains its harness up to 9000 C. Stellite tools can operate at speed about 25% higher than that of HSS tools .
Cemented oxides or Ceramic Cutting Tools (1950s): Non-metallic materials made of pure Aluminum oxide by powder metallurgy. The application ceramic cutting tools are limited because of their extreme brittleness. The transverse rupture strength (TRS) is very low. This means that they will fracture more easily when making heavy interrupted cuts. However, the strength of ceramics under compression is much higher than HSS and carbide tools. It has high hot hardness (up to 1200 degree C), so capable of running at high speeds. Cutting Tool Materials
Cermets: Cermets are ceramic material in metal binders. TiC, nickel, TiN, and other carbides are used as binders. Cermets have higher hot hardness and oxidation resistance than cemented carbides but less toughness. They are used for finishing operation. The main problem with cermets is that due to thermal shock the inserts crack.
Diamond: They are of two types - industrial grade natural diamonds, and synthetic polycrystalline diamonds. Because diamonds are pure carbon, they have an affinity for the carbon of ferrous metals. Therefore, they can only be used on non-ferrous metals. Feeds should be very light and high speeds Rigidity in the machine tool and the setup is very critical because of the extreme hardness and brittleness of diamond.
Cubic Boron Nitride (1962): Cubic boron nitride (CBN) is similar to diamond in its polycrystalline structure and is also bonded to a carbide base. With the exception of titanium, or titanium-alloyed materials, CBN will work effectively as a cutting tool on most common work materials. However, the use of CBN should be reserved for very hard and difficult-to-machine materials.
Fig8
Fig 9
1.2.3 Cutting Tool Geometry
1.2.3.1 The rake angle:
The rake angle is always at the topside of the tool. The side rake angle and the back rake angle combine to form the effective rake angle. This is also called true rake angle or resultant rake angle of the tool. The basic tool geometry is determined by the rake angle of the tool. Rake angle has two major effects during the metal cutting process. One major effect of rake angle is its influence on tool strength. A tool with negative rake will withstand far more loading than a tool with positive rake. The other major effect of rake angle is its influence on cutting pressure. A tool with a positive rake angle reduces cutting forces by allowing the chips to flow more freely across the rake surface.
Back rake angle: The back rake angle is the angle between the face of the tool and a line parallel to the base of the shank in a plane parallel to the side cutting edge. •The back rake angle affects the ability of the tool to shear the work material and form chip.
Side Rake Angles: It is the angle by which the face of the tool is inclined side ways.
The rake angle has the following function:
It allows the chip to flow in convenient direction.
It reduces the cutting force required to shear the metal and consequently
helps to increase the tool life and reduce the power consumption.
It provides keenness to the cutting edge.
It improves the surface finish.
The rake angle of a tool depends on:
Type of tool material: Tool material like cemented carbide permits turning at very high speed. At high speeds rake angle has little influence on cutting pressure. Under such condition the rake angle can minimum or even negative rake angle is provided to increase the tool strength.
Depth of cut: In rough turning, high depth of cut is given to remove maximum amount of material. This means that the tool has to withstand severe cutting pressure. So the rake angle should be decreased to increase the lip angle that provides the strength to the cutting edge.
Rigidity of the tool holder and machine: An improperly supported tool on old or worn out machine cannot take up high cutting pressure. So while machining under the above condition, the tool used should have larger rake angle.
1.2.3.2 Relief angles
Relief angles are provided to minimize physical interference or rubbing contact with machined surface and the work piece. • Relief angles are for the purpose of helping to eliminate tool breakage and to increase tool life. If the relief angle is too large, the cutting tool may chip or break. If the angle is too small, the tool will rub against the workpiece and generate excessive heat and this will in turn, cause premature dulling of the cutting tool. Small relief angles are essential when machining hard and strong materials and they should be increased for the weaker and softer materials. A smaller angle should be used for interrupted cuts or heavy feeds, and a larger angle for semi-finish and finish cuts.
Side relief angle: The Side relief angle prevents the side flank of the tool from rubbing against the work when longitudinal feed is given. Larger feed will require greater side relief angle.
End relief angle: The End relief angle prevents the side flank of the tool from rubbing against the work. A minimum relief angle is given to provide maximum support to the tool cutting edge by increasing the lip angle. The front clearance angle should be increased for large diameter works.
Side cutting edge angle: It increases tool life as, for the same depth of cut; the cutting force is
distributed on a wider surface. It diminishes the chip thickness for the same amount of feed and permits greater cutting speed. It dissipates heat quickly for having wider cutting edge. The side cutting edge angle of the tool has practically no effect on the value of the cutting force or power consumed for a given depth of cut and feed. Large side cutting edge angles are lightly to cause the tool to chatter.
End cutting edge angle: The function of end cutting edge angle is to prevent the trailing front cutting edge of the tool from rubbing against the work. A large end cutting edge angle unnecessarily weakens the tool. It varies from 8 to 15 degrees.
1.2.3.3 Nose radius: The nose of a tool is slightly rounded in all turning tools. The function of nose radius is as follows:
Greater nose radius clears up the feed marks caused by the previous shearing action and provides better surface finish.
All finish turning tool have greater nose radius than rough turning tools.
It increases the strength of the cutting edge, tends to minimize the wear taking place in a sharp pointed tool with consequent increase in tool life.
Accumulation heat is less than that in a pointed tool which permits higher cutting speeds.
Fig 10
1.3 TURNING MACHINES
Fig 11
A lathe is a machine tool used principally for shaping pieces of metal, wood, or other materials by causing the workpiece to be held and rotated by the lathe while a tool bit is advanced into the work causing the cutting action. Lathes can be divided into three types for easy identification: Engine Lathe, Turret Lathe, and Special Purpose Lathes.
1.3.1 Types of Lathe
1.3.1.1 Engine Lathe
The basic engine lathe, which is one of the most widely used machine tools, is very versatile when used by a skilled machinist. However, it is not particularly efficient when many identical parts must be machined as rapidly as possible. The standard engine lathe is not a high production machine, but it can be readily tooled up for many one-piece or short-run jobs. It is also possible to modify the basic machine for many higher production applications. The modern engine lathe provides a wide range of speeds and feeds which allow optimum settings for almost any operation. There have been advances in headstock design to provide greater strength and rigidity. This allows the use of high horse power motors so that heavy cuts with carbide tools are practical. To utilize this high power without losing accuracy, new lathes incorporate heavier beds, wider hardened ways, and deeper-sectioned carriages.
Fig 12
1.3.1.2 Turret Lathe
It is a type of Semi-automatic machine. Generally used for high production work. In this type of lathe the tail stock is replaced by a hexagonal turret, on the face of which multiple tools can be fitted and fed into the work piece in proper sequence. The specialty of this type of lathe is that machining of more than one surface can be done at the same time.
Fig 13
1.3.1.3 Special Purpose Lathe
There are lathes that are used only for specific purposes. There configuration and design is different and customized according to the requirement.
Tool room lathe
Automatic bar machine
Copy lathe
CNC lathe
Retrofitting lathe
High speed lathe
All geared lathe
1.3.2 Parts of a Lathe
Apron: Front part of the carriage assembly on which the carriage hand wheel is mounted
Bed : Main supporting casting running the length of the lathe
Carriage: Assembly that moves the tool post and cutting tool along the ways
Carriage Hand wheel: A wheel with a handle used to move the carriage by hand by means of a rack and pinion drive
Center: A precision ground tapered cylinder with a 60° pointed tip and a Morse Taper shaft. Used in the tailstock to support the end of a long work piece. It may also be used in the headstock spindle to support work between centers at both ends.
Centerline : An imaginary line extending from the center of the spindle through the center of the tailstock ram, representing the central axis of the lathe around which the work rotates. Chuck: A clamping device for holding work in the lathe or for holding drills in the tailstock.
Compound: Movable platform on which the tool post is mounted; can be set at an angle to the work piece. Also known as the compound slide and compound rest.
Compound Hand wheel: A wheel with a handle used to move the compound slide in and out. Also known as the compound feed.
Cross slide: Platform that moves perpendicular to the lathe axis under control of the cross-slide hand wheel
Cross-slide Hand wheel: A wheel with a handle used to move the cross-slide in and out. Also known as the cross feed.
Faceplate: A metal plate with a flat face that is mounted on the lathe spindle to hold irregularly shaped work.
Half-nut : A nut formed from two halves which clamp around the lead screw under control of the half nut lever to move the carriage under power driven from the lead screw. Half nut Lever: Lever to engage the carriage with the lead screw to move the carriage under power
Headstock: The main casting mounted on the left end of the bed, in which the spindle is mounted. It houses the spindle speed change gears.
Lead screw: Precision screw that runs the length of the bed. Used to drive the carriage under power for turning and thread cutting operations. Smaller lead screws are used within the cross-slide and compound to move those parts by precise amounts.
Saddle: A casting, shaped like an "H" when viewed from above, which rides along the ways. Along with the apron, it is one of the two main components that make up the carriage.
Spindle: Main rotating shaft on which the chuck or other work holding device is mounted. It is mounted in precision bearings and passes through the headstock.
Tailstock: Cast iron assembly that can slide along the ways and be locked in place. It is used to hold long work in place or to mount a drill chuck for drilling into the end of the work.
Tailstock Hand wheel: A wheel with a handle used to move the tailstock ram in and out of the tailstock casting.
Tailstock Ram: A piston-type shaft that can be moved in and out of the tailstock by turning the
Tool post: A holding device mounted on the compound into which the cutting tool is clamped
1.4 SURFACE STRUCTURE AND PROPERTIES
Surface roughness is an important measure of product quality since it greatly influences the erformance of mechanical parts as well as production cost. Surface roughness has an impact on the mechanical properties like fatigue behavior, corrosion resistance, creep life, etc. It also effects other functional attributes of parts like friction, wear, light reflection, heat transmission, lubrication, electrical conductivity, etc. Before surface roughness, it is also necessary to discuss about surface structure and properties, as they are closely related.
Upon close examination of the surface of a piece of metal, it can be found that it generally consists of several layers. The characteristics of these layers are briefly outlined here:
The bulk metal, also known as the metal substrate, has a structure that depends on the composition and processing history of the metal.
Above this bulk metal, there is a layer that usually has been plastically deformed and work-hardened to a greater extent during the manufacturing process. The depth and properties of the work-hardened layer (the Surface Structure) depend on such factors as the processing method used and how much frictional sliding the surface undergoes. The use of sharp tools and the selection of appropriate processing parameters result in surfaces with little or no disturbance. For example, if the surface is produced by machining using a dull and worn tool, or which takes place under poor cutting conditions, or if the surface is ground with a dull grinding wheel, the surface structure layer will be relatively thick. Also, non-uniform surface deformation or severe temperature gradients during manufacturing operations usually cause residual stresses in the work-hardened layer.
Unless the metal is processed and kept in an inert (oxygen-free) environment, or is a noble metal such as gold or platinum, an oxide layer forms over the work-hardened layer.
Under normal environmental conditions, surface oxide layers are generally covered with absorbed layers of gas and moisture. Finally, the outermost surface of the metal may be covered with contaminants such as dirt, dust, grease, lubricant residues, cleaning-compound residues, and pollutants from the environment.
Thus, surfaces have properties that generally are very difficult from those of the
substrate. The oxide on a metal surface is generally much harder than the base metal.
Consequently, oxides tend to be brittle and abrasive. This surface characteristic has
several important effects on friction, wear, and lubrication in materials processing, and on
products.
1.4.1 Surface Integrity
Surface integrity is the sum of all the elements that describes all the conditions exiting on or at the surface of a work piece. Surface integrity has two aspects. The first is surface topography which describes the roughness, 'lay' or texture of this outermost layer of the work piece, i.e., its interface with the environment. The second is surface metallurgy which describes the nature of the altered layers below the surface with respect to the base of the matrix material. This term assesses the effect of manufacturing processes on the properties of the work piece material. Surface integrity describes not only the topological (geometric) features of surfaces and their physical and chemical properties, but their mechanical and metallurgical properties and characteristics as well. Surface integrity is an important consideration in manufacturing operations because it influences properties, such as fatigue strength, resistance to corrosion, and service life.
1.4.2 Surface Topography
Outermost layers of all machined surfaces display a great number of both macro-geometrical and micro-geometrical deviations from the ideal geometrical surface. Surface roughness refers to deviation from the nominal surface of the third up to sixth order. Order of deviation is defined in international standards. 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 inhomogenities. Third and 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 physical-chemical 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.
The principal elements of surfaces are discussed below:
Surface: The surface of an object is the boundary which separates that object from another substance. Its shape and extent are usually defined by a drawing or descriptive specifications.
Profile: It is the contour of any specified section through a surface.
Roughness: It is defined as closely spaced, irregular deviations on a scale smaller than that of waviness. Roughness may be superimposed on waviness. Roughness is expressed in terms of its height, its width, and its distance on the surface along which it is measured.
Waviness: It is a recurrent deviation from a flat surface, much like waves on the surface of water. It is measured and described in terms of the space between adjacent crests of the waves (waviness width) and height between the crests and valleys of the waves (waviness height). Waviness can be caused by,
Deflections of tools, dies, or the work piece,
Forces or temperature sufficient to cause warping,
Uneven lubrication,
Vibration
Any periodic mechanical or thermal variations in the system during manufacturing operations.
Flaws: Flaws, or defects, are random irregularities, such as scratches, cracks, holes, depressions, seams, tears, or inclusions.
Lay: Lay, or directionality, is the direction of the predominant surface pattern and is usually visible to the naked eye.
1.4.3 Surface Finish in Machining
The resultant roughness produced by a machining process can be thought of as the combination of two independent quantities:
Ideal roughness
Natural roughness.
1.4.3.1 Ideal roughness:
Ideal surface roughness is a function of feed and geometry of the tool. It represents the best oossible finish which can be obtained for a given tool shape and feed. It can be achieved only if the built-up-edge, chatter and inaccuracies in the machine tool movements are eliminated completely. For a sharp tool without nose radius, the maximum height of unevenness is given by:
Here f is feed rate, φ is major cutting edge angle and β is the minor cutting edge angle.
The surface roughness value is given by, Ra = Rmax/4
Figure 1.20: Idealized model of surface roughness
Practical cutting tools are usually provided with a rounded corner, and figure below shows the surface produced by such a tool under ideal conditions. It can be shown that the roughness value is closely related to the feed and corner radius by the following expression:
where r is the corner radius.
1.4.3.2 Natural roughness:
In practice, it is not usually possible to achieve conditions such as those described above,
and normally the natural surface roughness forms a large proportion of the actual roughness. One of the main factors contributing to natural roughness is the occurrence of a built-up edge and vibration of the machine tool. Thus, larger the built up edge, the rougher would be the surface produced, and factors tending to reduce chip-tool friction and to eliminate or reduce the built-up edge would give improved surface finish.
1.4.4 Factors Affecting the Surface Finish
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:
Machining Variables
Cutting speed
Feed, and
Depth of cut.
The Tool Geometry
Some geometric factors which affect achieved surface finish include:
Nose radius
Rake angle
Side cutting edge angle, and
Cutting edge.
Work piece and tool material combination and their mechanical properties
Quality and type of the machine tool used,
Auxiliary tooling, and lubricant used, and
Vibrations between the work piece, machine tool and cutting tool.
1.4.5 Roughness Parameters
Each of the roughness parameters is calculated using a formula for describing the surface. There are many different roughness parameters in use, but Ra is the most common. Other common parameters include Rz, Rq, and Rsk. Some parameters are used only in certain industries or within certain countries. For example, the Rk family of parameters is used mainly for cylinder bore linings.
Since these parameters reduce all of the information in a profile to a single number, great care must be taken in applying and interpreting them. Small changes in how the raw profile data is filtered, how the mean line is calculated, and the physics of the measurement can greatly affect the calculated parameter.
By convention every 2D roughness parameter is a capital R followed by additional characters in the subscript. The subscript identifies the formula that was used, and the R means that the formula was applied to a 2D roughness profile. Different capital letters imply that the formula was applied to a different profile. For example, Ra is the arithmetic average of the roughness profile.
Each of the formulas listed in the Table 2 assumes that the roughness profile has been filtered from the raw profile data and the mean line has been calculated. The roughness profile contains n ordered, equally spaced points along the trace, and yi is the vertical distance from the mean line to the ith data point. Height is assumed to be positive in the up direction, away from the bulk material.
1.5.6 Amplitude Parameters
Amplitude parameters characterize the surface based on the vertical deviations of the roughness profile from the mean line. Many of them are closely related to the parameters found in statistics for characterizing population samples. For example, Ra is the arithmetic average of the absolute values.
The amplitude parameters are by far the most common surface roughness parameters found in the United States on mechanical engineering drawings and in technical literature. Part of the reason for their popularity is that they are straightforward to calculate using a digital computer.
1.4.7 MEASUREMENT OF SURFACE ROUGHNESS
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:
Direct measurement methods
Comparison based techniques
Non contact methods
On-process measurement
1.4.7.1 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.
1.4.7.2 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 known surface finish. Because of the subjective judgment involved, this method is useful for surface roughness Rq >1.6 micron.
1.4.7.3 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.
1.4.7.4 On-Process Measurement
Many methods have been used to measure surface roughness in process. For example:
Machine vision: In this technique, a light source is used to illuminate the surface with a digital system to 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.
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.
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.
1.4.8 Factors Influencing Surface Roughness in Turning
Generally, it is found that the factors influencing surface roughness in turning are:
Depth of cut: Increasing the depth of cut increases the cutting resistance and the amplitude of vibrations. As a result, cutting temperature also rises. Therefore, it is expected that surface quality will deteriorate.
Feed: Experiments show that as feed rate increases surface roughness also increases due to the increase in cutting force and vibration.
Cutting speed: It is found that an increase of cutting speed generally improves surface quality.
Engagement of the cutting tool: This factor acts in the same way as the depth of cut.
Cutting tool wears: The irregularities of the cutting edge due to wear are reproduced on the machined surface. Apart from that, as tool wear increases, other dynamic phenomena such as excessive vibrations will occur, thus further deteriorating surface quality.
Use of cutting fluid: The cutting fluid is generally advantageous in regard to surface roughness because it affects the cutting process in three different ways. Firstly, it absorbs the heat that is generated during cutting by cooling mainly the tool point and the work surface. In addition to this, the cutting fluid is able to reduce the friction between the rake face and the chip as well as between the flank and the machined surface. Lastly, the washing action of the cutting fluid is considerable, as it consists in removing chip fragments and wear particles. Therefore, the quality of a surface machined with the presence of cutting fluid is expected to be better than that obtained from dry cutting.
Three components of the cutting force: It should be noted that force values cannot be set a priori, but are related to other factors of the experiment as well as to factors possibly not included in the experiment, i.e. force is not an input factor and is used as an indicator of the dynamic characteristics of the work piece-cutting tool-machine system.
Finally, the set of parameters including the above mentioned parameters that are thought to influence surface roughness, have been investigated from the various researchers.
Chapter2
Literature Review
