2. 2. Introduction

3. 2.1 Background

4. 2.2 Hard turning, a viable process described

5. 2.3 Benefits from hard turning

6. 2.4 Machining requirements

7. 2.5 Concerns about hard turning

8. 3. Problem identification

9. 3.1 Motivation

10. 3.2 Problem statement

11. 3.3 Research Objective

12. 4. Literature review

13. 4.1 Previous studies

15. Tungsten2.3 Benefits from hard turning With hard turning, you'll reduce your costs in many ways:
Soft turn
and hard turn on the same machine Smaller floor space requirement Lower overall investment Metal removal rates of 4-6 times greater Can turn complex contours Multiple operations in a single setup Low micro finishes Easier configuration changes Lower cost tooling inventory Higher metal removal rates Easier waste management (chips vs.
swarf
) Hard Turning Advantages Figure 1.3 Advantages of hard turning process Machining Requirements Figure 1.4 Machining requirements for hard turning Grinding versus Hard Turning GRINDING HARD TURNING 1. Long set up times 1. Short set-up times 2. Multiple clamping 2. Single clamping 3. Long cycle times 3. Short cycle times 4. Low chip volume 4. High chip volume possible 5. Profiled grinding Wheel 5. Single point tool tip 6. Multiple dressing is non-productive 6. More effective cutting time 7. High investment cost 7. Low investment cost 8. Environmental unfriendly due to grinding sludge 8. Dry cutting, clean process 2.4 Machining requirements for hard turning 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. 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. 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. 2.5 Concerns about hard turning. 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. 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. 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. 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. 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. 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. Understanding of material science is critical 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. The key to success is optimizing the hard turning process to reduce overall cost(s)--purity of Material, Tool life, surface finish, and accuracy. Limitations Tooling 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. Ceramic does not perform well in the presence of high thermal shocks, so it is not generally a good candidate for coolant cutting. Figure 1.5 A graphical comparison of various cutting tool materials White Layer Hard turned surfaces frequently experience
white layer
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. 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. Machine Process Capability 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: Machine technology Process technology Materials and tooling technology Workholding technology 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. 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. 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. 3. PROBLEM IDENTIFICATION 3.1Motivation 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. 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. 3.2 Problem statement 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. 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. 3.3 Research Objectives 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. 2. To propose effective correlation between the chatter and tool wear, which inturn affects form, accuracy, surface integrity and economic considerations. 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. 4. LITERATURE REVIEW 4.1 Previous studies 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. 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. 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. 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. 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. 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. 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. 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. 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. 5. REFERENCES 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. 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. 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. 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. 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. 6. ‘Taking the hard out of hard turning’, Manufacturing Engineering, Mar 1997 by Thomas Sheehy, an online article. 7. ‘Using Hard Turning To Reduce Grinding Time’, Article from: Modern Machine Shop, Derek Korn, Senior Editor. 8. ‘All about hard turning’, an online article from Aston products limited. 9. ‘Report on Hard turning’, an online article from Hardinge machinery, posted on July 2004. 10. ‘Kummer precision hard turning: the key for success, Kummer machinery online article 11. ‘Hard Turning Might Not Be As Hard As You Think’, Article from: Modern Machine Shop, Derek Korn, Senior Editor. Draft copy: Full report not to be uploaded