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5. References</li></ul>1. ABSTRACT<br />Hard turning is a developing technology that offers many potential benefits compared to grinding, which remains the standard finishing process for critical hardened surfaces. To increase the implementation of this technology, questions about the ability of this process to produce surfaces that meet surface finish and integrity requirements must be answered. Additionally, the economics of the process must be justified, which requires a better understanding of tool wear patterns, life predictions, cause and effects of defects, also to formulate effective measures to counter the same.<br /> Due to the potential advantages of the hard turning process, this research proposes to investigate the wear behavior of cutting tools in hard turning applications with the hope that findings will lead to further implementation in industry. The experimental work in combination with the development of a wear model based on the fundamental wear mechanisms typical in metal cutting will help to identify the effect of cutting parameters and tool material on wear behavior and tool life. Additionally, this research will utilize response to optimize the hard turning process and determine ideal process conditions.<br />Because of the increasing demands imposed on high-performance materials and the disastrous costly results of component failures, the need for enhanced machining technology has gained significance and importance. Ceramics being one fastest growing high performance application material, manufacturing union has given immense response to provide newer technologies to cater the ever rising appetite for these materials.<br />2. INTRODUCTION<br />2.1 Background<br />Producers of machined components and manufactured goods are continually challenged to reduce cost, improve quality and minimize setup times in order to remain competitive. Frequently the answer is found with new technology solutions. Such is the case with grinding where the traditional operations involve expensive machinery and generally have long manufacturing cycles, costly support equipment, and lengthy setup times. However, the grinding process itself may require several machine tools and several setups to finish all component surfaces. Because grinding can be a slow process with low material-removal rates, there has been a determined search for replacement processes<br />The newer solution is a hard turning process, which is best performed with appropriately configured turning centers or lathes. Hard turning really started to develop at the beginning of the nineties. The reason for this was the availability of new tool materials and the capability of designing a turning machine that was rigid, stable and accurate enough to successfully finish hard turn. The result of these developments have made finish hard turning a viable alternative to grinding, as an accurate finishing operation.<br />2.2 Hard turning, a viable process described<br />Manufacturers around the world constantly strive for lower cost solutions in order to maintain their competitiveness, on machined components and manufactured goods. Globally, part quality has been found to be at acceptable levels and it continues to improve, while the pressure for part piece cost is enormous and is constantly being influenced downward by competition and buyer strategies. The trend is toward higher quality, lower cost and smaller batch sizes. In order to compete against producing countries with low wage structures, it is necessary to seek out appropriate new technological solutions that can help to level the business playing field.<br />Technology has played an enormous role in advancing the metal working industry and creating opportunities to reduce costs and improve quality. Consider the role technology has played in transforming routine metal cutting operations. At one time machining was very much an operator dependent, skill critical process. Today, CNC machine tools, which operate with mature technology and provide both consistency and reliability, have now become the biggest contributor to part quality and cost. Technology-based tools such as 3- D CAD systems, computer programming, simulation packages and of course the CNC machine tool, are now commonplace in many shops and in most countries of the world. A rapid adoption of these newer and more cost effective manufacturing techniques will be constantly required if manufacturing operations are to remain competitive. In a much smaller way, but no less significant, we begin to see a technology evolution occurring in the area of hard turning. <br />Hard turning is defined as the process of single point cutting of part pieces that have hardness values over 45 RC but more typically are in the 58- 68 RC range.<br />From an applications standpoint, hard turning is very much a partspecific process. It excels at cutting complex geometries that contain intricate arcs, angles, and blended radii. Instead of having to buy a form wheel for the grinder, you can program the lathe's single point much faster and cheaper.<br />Hard turning can eliminate several types of grinding, as well as lapping and other finishing operations. Not only is removing steps from the process money in the bank, but, for some, it can also mean bringing outsourced work back under their roofs and under their own control. Hard turning is a way to achieve high machining efficiency in an environmentally-acceptable manner and a new technology to machine hardened parts processed by forging or casting. Compared with grinding, hard turning can machine some complex workpieces in one step. The machining cycle time of hard turning can be up to three times faster than grinding. Hard turning also consumes about one-tenth of the energy per unit volume of metal removed than grinding and is more environmental friendly <br />Hard turning yields very little scrap and minimizes rejected parts. Because it uses the same coolant as soft turning, adding another coolant and its associated waste stream is unnecessary. Better still, hard turning can make cutting dry an option, letting you avoid the expense and waste products of coolant altogether. Hard turning also eliminates annoying grinding slag-and the labor-intensive, cost-laden tasks of recycling and disposing of it. <br />Hard turning is a developing technology that offers many potential benefits compared to grinding, which remains the standard finishing process for critical hardened steel surfaces. To increase the implementation of this technology, questions about the ability of this process to produce surfaces that meet surface finish and integrity requirements must be answered. Additionally, the economics of the process must be justified, which requires a better understanding of tool wear patterns and tool life predictions. <br />Hard turning is best accomplished with cutting inserts made from either CBN (Cubic Boron Nitride), Cermet or Ceramic. Since hard turning is single point cutting, a significant benefit of this process is the capability to produce contours and to generate complex forms with the inherent motion capability of modern machine tools. High quality hard turning applications do require a properly configured machine tool and the appropriate tooling. For many applications, CBN tooling will be the most dominant choice. However, Ceramic and Cermet also have roles with this process.<br />The range of applications for hard turning can vary widely, where at one end of the process spectrum hard turning serves as a grinding replacement process, and can also be quite effective for pre-grind preparation processes. The attractiveness of the process lies in the performance numbers. A properly configured hard turning cell would typically demonstrate the following:<br />Surface finishes of 0.00011
(.003 mm)<br />Roundness values of .000009
(.00025 mm)<br />Size control ranges of .00020
(.005mm)<br />Production rates of 4- 6 over comparable grinding operations <br />Hard turning is a technology-driven process that requires certain performance features of the machine tool, work holding, process and the tooling. <br /> <br />Figure 1.1 & 1.2 showing hard turning process and CBN inserts respectively.<br />Hard turning can be certainly considered for most pre-grind applications, which are followed by an abbreviated grinding cycle. In some cases the hard turned surface may complete the operation and will completely eliminate the grinding cycle.<br />Hard turning is an important technology because all manufacturers are continually seeking ways to manufacture their parts with lower cost, higher quality, rapid setups, lower investment, smaller tooling inventory and the elimination of non-valued added activities. The migration of processing from grinders to lathes can satisfy each and every one of these goals.<br />If one were to list the current applications of hard turning it would certainly be a voluminous document. On a daily basis, parts are being hard turned in the following industry segments; automotive, bearing, marine, punch and die, mold, hydraulics and pneumatics, machine tool and aerospace. While these industries are representative, this list is certainly not conclusive and new applications and industry segments are constantly being added.<br />Commonly processed hard turned materials would include:<br />Steel Alloys such as <br />Bearing steels <br />Hot and cold-work tool steels <br />High speed steels <br />Die steels <br />Case hardened steels <br />Unique hard materials and aircraft types that fall within the hardness range like<br /><ul><li>Ceramics
Tungsten</li></ul>2.3 Benefits from hard turning<br />With hard turning, you'll reduce your costs in many ways:<br />
and hard turn on the same machine <br />Smaller floor space requirement <br />Lower overall investment <br />Metal removal rates of 4-6 times greater <br />Can turn complex contours <br />Multiple operations in a single setup <br />Low micro finishes <br />Easier configuration changes <br />Lower cost tooling inventory <br />Higher metal removal rates <br />Easier waste management (chips vs.
) <br /> <br />Hard Turning Advantages <br /> <br />Figure 1.3 Advantages of hard turning process<br />Machining Requirements <br /> <br /> <br />Figure 1.4 Machining requirements for hard turning<br /> Grinding versus Hard Turning<br />GRINDING HARD TURNING<br />1. Long set up times 1. Short set-up times <br />2. Multiple clamping 2. Single clamping <br />3. Long cycle times 3. Short cycle times<br />4. Low chip volume 4. High chip volume possible <br />5. Profiled grinding Wheel 5. Single point tool tip <br />6. Multiple dressing is non-productive 6. More effective cutting time <br />7. High investment cost 7. Low investment cost <br />8. Environmental unfriendly due to grinding sludge 8. Dry cutting, clean process<br />2.4 Machining requirements for hard turning<br />To be competitive under hard turning the turning machine must have high static and dynamic stability. In terms of precision, a lathe of this type is comparable to a top-of-the-range grinder (motorised spindle). It is essential that the whole of the machining system be stable. This applies to work piece clamping and the tool holder. The hard turning process features small cutting depths and feed and high specific cutting loads.<br />To deliver the cutting performance required (accuracy, finish, extended tool life etc) turning centres need to be of a rigid construction and design. A polymer composite base combined with wide-spaced, heavy-duty linear guides; super-finished tracks and centrally-located, short-pitch ball screws help reduce vibration and minimise stick/slip movement.<br /> Part rigidity - there is little point attempting to hard turn a part that does not have sufficient inherent rigidity to withstand the cutting forces generated by the process. To understand whether parts are literally up to the job - consider the length (L): diameter (D) ratio of the part. As a rule of thumb a L:D ratio of 4:1 for unsupported components and 8:1 for supported ones - produce optimum results. If the ratio is exceeded (for longer parts supported with a tailstock), it is likely that chatter will result.<br />2.5 Concerns about hard turning.<br />Hard turning is a viable process that has real and measurable economic and quality benefits. This is particularly true with a machine tool that has a high level of dynamic stiffness and the necessary accuracy performance and tooling requirements. The more demanding the application in terms of finish, roundness and size control, the more emphasis must be placed upon the characteristics of the machine tool. <br />From the process standpoint there are several areas of consideration. With the correctly chosen cutting tools, the hard turning process can support either coolant cutting or dry cutting. If the processing choice is to cut dry then the temperature of the chips and the workpieces need to be taken into account from both a safety and operational standpoint. <br />The current tooling technology allows the user to be able to choose between “wet or dry” operations. Wet operations refer to processes under flood or high-pressure with a water-soluble coolant. <br />The decision to produce under wet or dry conditions is normally made at the individual factory level. Some facilities have a local philosophy or mandate regarding the preference to operate one way or the other and fortunately, either forms of hard turning can be accommodated. There are several key items when choosing to operate wet and the first of these is the type of fluid to be used. Generally, straight oils should be avoided because of the inherent fire hazard. This is particularly true if during a cut the coolant flow is disrupted and the unquenched, high temperature chips contact the oil. Under these conditions, oils with a low flash point could start and sustain a fire.<br />Another point for wet operations is the importance to properly direct the coolant flow by applying fluid to both the top and the bottom of the tool tip simultaneously. Generated chip strings will frequently shield the coolant from the tool until the chip breaks away. The result is thermal shock and a process of degradation of the cutting edge. Anticipate this when establishing the coolant nozzle locations from a slight sideward vantage point. High-pressure coolant at pressures of approximately 68-95 atmospheres seems to be beneficial in keeping the chips small and manageable and in making the overall process more robust. As previously stated, the shorter chip results in a reduced amount of coolant blockage and less thermal shock to the cutting edge. Another variable in coolant cutting operations that can easily sabotage a fine-tuned process is an improper coolant mixture. Concentration, cleanliness and pH levels cannot be ignored for a proper application.<br />The one possible exception to coolant cutting is on interrupted surfaces, which seem to perform better in a dry environment. Logically, this is due to the higher degree of thermal shock caused during the interruption when the coolant has a better access to the tool tip and then immediately is followed by a re-entry into the workpiece and the severe temperatures.<br />Understanding of material science is critical<br />Certainly, an understanding of material science is vital to the heat treat operator so that the correct process and hardness range is accomplished. Material not properly drawn back might crack prematurely because of the high hardness. The best hard turning results will be achieved when the hardness range is as small as possible (a spread of less then 2 point is ideal), and the case depth is maintained consistently.<br />The key to success is optimizing the hard turning process to reduce overall cost(s)--purity of Material, Tool life, surface finish, and accuracy.<br />Limitations<br />Tooling<br />When considering the tooling material, it's important to understand the application and critical attributes such as size and finish requirements. The typical brazed tip CBN insert has a cost structure 3-4 times that of carbide. Ceramic, on the other hand, has a cost structure more similar to carbide but would not be used for applications which had a tolerance range smaller then .001” (.02540mm). Parts requiring a greater accuracy would logically use CBN. <br />Ceramic does not perform well in the presence of high thermal shocks, so it is not generally a good candidate for coolant cutting. <br />Figure 1.5 A graphical comparison of various cutting tool materials<br />White Layer<br />Hard turned surfaces frequently experience
formation, which appears as a white layer at the surface of the material under metallographic examination. This layer depth can vary greatly but for general discussions it is in the area of 1 micro-meter thick. <br />The white layer cannot be seen visually but requires a metallographic examination to detect its presence. white layer can be caused by either 1) severe plastic deformation that causes rapid grain refinement or 2) phase transformations as a result of rapid heating and quenching. White layer formation is not limited to hard turning operations but is also routinely found in grinding applications. White layer is not desirable in products which have high contact stresses and where fatigue failures can occur. <br />Machine Process Capability<br />Rigidity is critical for successful hard turning: the rigidity of tooling, workholding, and the machine tool itself are all crucial elements that will affect your ability to successfully hard turn. Hard turning is a technology-driven process, dependent upon:<br />Machine technology <br />Process technology <br />Materials and tooling technology <br />Workholding technology <br />When considering hard turning, the question is not “can it be done” (because many machine tools can hard turn), but “how well can it be done?” Success in hard turning is largely a measure of the machine construction and design along with the workholding and tool holding. The level of rigidity and damping in a hard turning application cannot be minimized.<br />In the area of hard turning, it's well-established that the presence of vibration is not desirable from multiple standpoints. A machine which has improved damping will demonstrate improvements in lowering the amplitude of vibration and the time to decay, all while maintaining static stiffness. The real and measurable results are longer tool life, better surface finishes, improved accuracy, increased productivity and higher overall part quality. System rigidity is of utmost importance.<br />Hard turning offers many potential benefits when compared to grinding. To increase the use of this technology, questions about its ability to produce surfaces that meet surface finish and integrity requirements must be answered. In addition, the process must be justified economically, which requires a better understanding of tool wear patterns and life predictions.<br />3. PROBLEM IDENTIFICATION<br />3.1Motivation<br />Although hard turning process was essentially developed to machine hardened steel materials, but recent advances in tooling technology has led into a broader path to adopt hard turning to machine brittle materials like ceramics.<br />Ceramics is generally considered to be one of the so called hard to machine materials. With the advent of extensive use of ceramics in critical and high performance applications in industries like aerospace and automotive sectors had led manufacturing union to lookout for new technology in machining of ceramics, which can be cost effective, simple, less time consuming yet without compromising its surface integrity, form, accuracy and structural tolerances.<br />3.2 Problem statement<br />The development of more wear-resistant tool materials such as Polycrystalline Cubic Boron Nitride (PCBN) and ceramics have made hard turning a potential alternative to grinding operations in the finishing of hard materials. However, hard turning is more sensitive to chatter than conventional mild turning. The reasons include both a high precision requirement in finishing and the relatively brittle property of PCBN cutting inserts. Therefore, the ability to predict chatter-free cutting conditions is very important for hard turning in order to be an economically viable manufacturing process. Despite a large demand from industry, a realistic chatter modeling for hard turning has not been available due to the complexity of the problem, which is mainly caused by flank wear and nonlinearity in hard turning.<br />White layer can be formed on component surfaces during various machining processes. The white layer may have significant influence on product performance. However, the nature, especially microstructure, of white layer is not well understood. This research hopes to bridge the gap between machining conditions and formation of defects and its effective control. Since no machining data are available for hard turning of materials like ceramics, so recommending such data will be of great help to manufacturers.<br />3.3 Research Objectives<br />1. To develop a scientific, systematic and reliable methodology to predict the tool flank / crater wear rate based on cutting condition, tool geometry for CBN tool and ceramics as workpiece material.<br />2. To propose effective correlation between the chatter and tool wear, which inturn affects form, accuracy, surface integrity and economic considerations.<br />3. To explore cause and effects of formation of white layer. To formulate effective strategy to counter white layer formation by process control (varying machining parameters), by reducing non value added operations.<br />4. LITERATURE REVIEW<br />4.1 Previous studies<br />4.1.1 Ian. S. Harrison, professor, Georgia Institute of technology has studied comprehensively on detecting the white layer formation. The tests were performed on heat treated 52100 steel, by using CBN tool. To gather data several different testing conditions were carried out. He stated that since white layer may occur unexpectedly, and because the current method for detecting it is destructive, a manufacturer cannot confidently determine if a hard turned part has the condition. Therefore, additional finishing processes must be used to remove material from any parts that might have white layer, effectively removing a potential benefit of hard turning. If a reliable, non-destructive technique for detecting white layer could be found, then hard turning might be more applicable as a finishing process.<br /> He also stated that current methods for detecting white layer are not suitable for industry. The most reliable laboratory method for detecting white layer involves examining a cross section of the finished workpiece under a microscope. The cross section is first polished and etched so that the grain structure is visible on a micrograph. This technique is time consuming, labor intensive, and most importantly, it destroys the workpiece. Therefore, this method is unacceptable for production parts. The primary objective of his project was to determine if either the Barkhausen sensor or electrochemical impedance spectroscopy methods are effective at detecting white layer non- destructively. If there is a correlation between white layer thickness and the measurements from either sensor, then this implies that the sensor is responsive to white layer.<br />The experimental results show that output from the Barkhausen sensor is not strongly correlated with the presence of white layer. Although this sensor has been shown to be effective at detecting either residual stress or hardness individually, it is not effective at detecting the material properties of white layer. Even when the parts are made of the same type of steel and machined under the same conditions, there is no clear trend of white layer thickness.<br />4.1.2 G. Dawson and Dr. Thomas R. Kurfess1, professors of George W. Woodruff School of Mechanical Engineering has studied about wear trends in PCBN cutting tools in hard turning. Hard turning was performed on AISI 52100 steel for the entire life of seventeen different PCBN cutting inserts. All machining was done on a Hardinge Conquest T-42 SP lathe, which has a 7.5 kW (10 hp) spindle and maximum spindle speed of 6000 rpm.<br />The material hardened to 58 HRC was solid bar stock, but it was found that the hardness decreased to as low as 50 HRC when cut to smaller diameters. Thus it was decided to machine hardened tube stock, which allowed a heat treatment that provided consistent hardness at 62±2 HRC. The cutting insert geometry was an 80° diamond shape with a 20° edge chamfer 0.1 mm wide. The toolholder provided negative 5° side and back rake angles, and 5° side cutting-edge and end cutting-edge angles. The test procedure consisted of twenty cuts 2.54 cm in length, each at a decreasing diameter. Once the twenty cuts were made, the 2.54 cm length was removed by wire EDM, and the test cuts began again at the outer diameter. The oxidized layer from heat treatment was removed with a mixed alumina cutting insert so that all test passes were made on a clean surface.<br />Results were on wear trends in hard turning, flank wear was between 150 and 200 μm. They compared results with previously done experiments and found that, there are differences in the wear behavior between manufacturers. Even though they are marketed as comparable low or high CBN content tools, there can be substantial differences in the tool materials. For instance, the CBN itself can differ by processing conditions when transforming the BN from hexagonal to cubic form, grain size, shape, porosity, defects, inclusions, etc. Additionally, the binder materials can differ significantly. High CBN content tools are typically more than 90% CBN with a cobalt binder, while low CBN content tools are less than 70% CBN with TiC and/or TiN as the binder.<br />To understand both the wear behavior and life of different CBN grades, full tool life studies were performed for a test matrix consisting of seventeen cutting conditions with four different tool materials. Results of this study indicate that cutting speed has a more dramatic effect on tool life than feed or depth of cut. In general, increased feed rates were found to decrease tool life in minutes, but increase the amount of material removed with the tool. The crater and flank wear behavior before tool failure was also monitored throughout the life of each tool. Significant changes in cutting geometry were recorded, resulting from crater wear on the chamfered cutting edge. Flank wear behavior showed a repeating trend that is being investigated further to develop the ability to confidently predict tool life over a wide range of conditions.<br />4.1.3 Jong-Suh Park, professor at George W. Woodruff School of Mechanical Engineering has studies effects of chatter stability on hard turning, and its predictions. The objective of this research was to predict chatter stability limits in hard turning, especially emphasizing the effects of flank wear and nonlinearity. The objective is to be achieved through the steps as follows: (1) Development of a nonlinear chatter models with non-uniform load distribution. (2) Stability analysis for worn tool cases. (3) Theoretical chatter stability predictions based on characteristic parameters, which measured in experiments. (4) Model validation with experimental chatter stability limits for different feed directions and tool wear conditions.<br />This report presented a chatter model for the purpose of predicting stability limits in hard turning. In order to address completed research, this research was organized based on theoretical predictions from proposed models and model validation through experimental investigations for facing and straight turning.<br />5. REFERENCES<br />1. Jong-Suh Park, ‘THE PREDICTION OF CHATTER STABILITY IN HARD TURNING’, 2004, George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology. USA.<br />2. Ty G. Dawson and Dr. Thomas R. Kurfess, ‘ WEAR TRENDS OF PCBN TOOLING IN HARD TURNING’, 2002, The George Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, USA.<br />3. F. Klocke, P. Frank, ‘SIMULATION OF TOOL WEAR IN HARD TURNING’, Laboratory for Machine Tools and Production Engineering (WZL) of Aachen University of Technology (RWTH), Germany.<br />4. A. Latif Razak, M. Kessler, H. El-Mounayri, ‘Investigation of Hard Lathe Turning Process Using Rotary Tools’ Advanced Engineering and Manufacturing Lab (AEML) Department of Mechanical Engineering Purdue School of Engineering and Technology, USA.<br />5. Vishal S Sharma1, Suresh Dhiman, Rakesh Sehgal and Surinder Kumar Sharma,’ Assessment and Optimization of Cutting Parameters while Turning AISI 52100 Steel’, Department of Industrial Engineering, National Institute of Technology, Jalandhar, India.<br />6. ‘Taking the hard out of hard turning’, Manufacturing Engineering, Mar 1997 by Thomas Sheehy, an online article.<br />7. ‘Using Hard Turning To Reduce Grinding Time’, Article from: Modern Machine Shop, Derek Korn, Senior Editor.<br />8. ‘All about hard turning’, an online article from Aston products limited.<br />9. ‘Report on Hard turning’, an online article from Hardinge machinery, posted on July 2004.<br />10. ‘Kummer precision hard turning: the key for success, Kummer machinery online article<br />11. ‘Hard Turning Might Not Be As Hard As You Think’, Article from: Modern Machine Shop, Derek Korn, Senior Editor.<br />Draft copy: Full report not to be uploaded<br />