High Performance Grinding - Science Direct

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ScienceDirect Procedia CIRP 46 (2016) 266 – 271

7th HPC 2016 – CIRP Conference on High Performance Cutting

High Performance Grinding Fritz Klockea, Sebastian Barth*a, Patrick Mattfelda a

Laboratory for Machine Tools and Production Engineering (WZL) of RWTH Aachen University, Steinbachstraße 19, 52074 Aachen, Germany

* Corresponding author. Tel.: +49-241-80-28183; fax: +49-241-80-22293. E-mail address: [email protected]

Abstract The efficient processing of innovative materials, the further development of grinding tools and machine concepts as well as an increasing economic and environmental pressure are current challenges in grinding technology. High Performance Grinding processes offer a huge potential to overcome these challenges and increase productivity. They are characterized by a significant increase in material removal rate and component quality plus a reduced need of resources. In this paper, possibilities are presented and discussed for extending the process boundaries of conventional grinding processes in order to create High Performance Grinding processes. Based on current research, potentials to increase productivity of conventional grinding processes are described. In addition to process-specific optimization of the machine and the grinding tools, opportunities are pointed out for the numerical prediction of the process boundaries as well as methods for mathematical interpretation and analysis of grinding wheel structures and topographies. Moreover, opportunities are presented for future increases in productivity by combining High Performance Grinding processes with other manufacturing technologies. © Published by Elsevier B.V This © 2016 2016The TheAuthors. Authors. Published by Elsevier B.V.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the International Scientific Committee of 7th HPC 2016 in the person of the Conference Chair Prof. under responsibility of the International Scientific Committee of 7th HPC 2016 in the person of the Conference Chair Peer-review Matthias Putz. Prof. Matthias Putz Keywords: Grinding, High Performance Process, Optimisation, Finishing, Machining

1. Introduction In order to maintain the competitiveness of high-wage countries in global markets, the manufacturing cost must be kept low with rising labor expenses. At the same time the range of products is increasing rapidly while the product lifecycles decrease. So, the competitiveness of manufacturing companies depends on their ability to adapt to swiftly changing global conditions [1]. High precision machining with geometrically undefined cutting edges represents a key technology to meet these challenges. Continuously increasing quality and cost reduction demands, especially in automotive, bearing and aerospace industries, require enhanced processes that provide optimal yield. Usually, the aim is to maximize the production rate while maintaining the specified product quality frame and to reduce the cost and time of the production simultaneously. High Performance Grinding (HPG) processes expand the field of grinding from traditional finishing operations to highly efficient and high-precision machining.

Current developments have led to new grinding challenges which refer to the configuration of improved processes with high performance capabilities [2]. Depending on the requirements, competitive grinding processes need to be allocated to highly efficient processes with maximized material removal rates or to high-precision processes for outstanding surface qualities. The present paper provides the fundamentals of grinding and lists the characteristics of High Performance Grinding processes from a scientific point of view. In the following, the driving factors for HPG processes are discussed. It is shown how to react to recent challenges in grinding by new tool and process designs. Furthermore, the high potential of a modelbased optimization of grinding processes is laid out. In addition, strategies how to identify the material behavior of innovative and unique materials are presented and the potential of a combination of grinding with other manufacturing processes is discussed.

2212-8271 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the International Scientific Committee of 7th HPC 2016 in the person of the Conference Chair Prof. Matthias Putz doi:10.1016/j.procir.2016.04.067

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Fritz Klocke et al. / Procedia CIRP 46 (2016) 266 – 271

Nomenclature Ac1b Ac1e ae ae, tot ae, min, crit Ani Ati df Epulse εlim fpulse Fn,S Ft,S hcu HPC HPG HSC Nkin PCD Q’w Ra rw SSD tpulse

T up/down

Tµ vc vf vs vw V’w vw,crit

Austenite start temperature Austenite finish temperature Depth of cut Total depth of cut Critical depth of cut Specific cutting edge area in normal direction Specific cutting edge area in tangential direction Diameter of the glass fiber Pulse energy Limiting angle of cutting edge offset Pulse frequency Normal cutting force Tangential cutting force Chip thickness High Performance Cutting High Performance Grinding High Speed Cutting Kinematic cutting edge count Polycrystalline diamond Specific material removal rate Average roughness height Workpiece radius Sub-surface-damage Pulse duration Heat-up/Heat down rate Grain cutting depth Cutting speed Axial feed rate Circumferential speed of the grinding wheel Workpiece speed Specific material removal Critical table speed

the workpiece material generate a thermo-mechanical load on the workpiece. This thermo-mechanical load determines the functionality and the application characteristics of the workpiece. Among other things, the thermo-mechanical load depends highly on the topography of the grinding wheel, which in turn depends on the grinding tool specification, the grinding conditions and the dressing conditions [5]. High performance grinding processes are often designed to maximize material removal rates or to optimize surface qualities. Therefore, many processes are designed by varying the “resistor” process parameters. One possibility to increase the efficiency of a grinding process is to increase the circumferential speed of the grinding wheel vs. An important advantage of increased productivity due to grinding at high cutting speeds (HSC) is the increased grinding wheel tool life because of decreasing single grain loads [5,6]. The most significant disadvantage of increasing the circumferential speed vs is the rising thermal load acting on the surface layer of the workpiece. Corresponding countermeasures are necessary to avoid a damage of the workpiece, when increasing vs. An appropriate strategy to reduce the thermal load on the workpiece surface layer lies in the increase of the workpiece speed vw (HPC). This way, the single grain load increases slightly, but the process temperature can be lowered significantly. Additionally, the related material removal rate can be increased without damaging the workpiece material. Thus, when varying one of the process parameters, the influence on the thermo-mechanical load acting on the workpiece must be considered. In most cases, this entails the adaptation of at least one other “resistor” such as another process parameter or the specification of the grinding tool. Only with a methodical approach it is possible to extend conventional grinding processes and to achieve reliable HPG processes. Grinding wheel

Fn,S

2. Fundamentals of grinding Knowledge of the fundamental principles of grinding is essential to tap the full potential of High Performance Grinding (HPG) processes. Considering the grinding process with all its components, such as grinding wheel, process parameters or cooling lubrication, as a series of “resistors”, it becomes evident that the system “grinding” can show its full potential only if all these resistors are optimized. Therefore, it is necessary to know the influence of each component on the ground workpiece. Due to the high complexity of the grinding process caused by machining with geometrically undefined and stochastically distributed cutting edges, the process design and process control of HPG processes is very challenging. During the engagement of only one grain with the workpiece material, the grain first deforms the ground material elastically and plastically before a chip is formed from the surface of the workpiece (Figure 1). This process is repeated countless times per second. This leads to a permanent superposition of engagements [3]. A direct observation and determination of these processes was recently monitored in detail by Denkena [4]. Beside the generation of a surface finish, the contact conditions between the grains and

Ft,S

Grain path

vS

Bond

Grain (Cutting edge)

Bulging Workpiece I

Elastic deformation

Chip

Tµ II Elastic and plastic deformation

hcu III Elastic and plastic deformation and chip formation

Figure 1: Zones of elastic deformation, plastic deformation and chip formation during grinding [3]

3. Characteristics of High Performance Grinding High Performance Grinding processes are defined by a significant improvement in comparison to conventional grinding processes of one or more of the four performance characteristics time, cost, quality and feasibility (Figure 2).

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According to this characterization, first HPG processes are defined by a significant raise of productivity which means an increased material removal rate or an improved flexibility. Furthermore, operations to achieve outstanding surface qualities can be defined as High Performance Grinding processes. Even the feasibility of machining novel or difficultto-machine materials is a High Performance Grinding process because the underlying process knowledge can be a competitive advantage for the manufacturer. [1]

CBNGrinding wheel with CFRP-base body

Chuck

Chuck

Quality

Time

Ze Zerspankräfte

HPG (HPC/HSC)

Cost

Grinding Grinding spindle 1 spindle 2 Tailstock

Grinding Grinding spindle 1 spindle 2 Tailstock

Feasibility

Figure 2: Performance characteristics of High Performance Grinding processes

4. Driving forces for HPG The main driving force for HPG processes is a steadily increasing economic pressure. Thus, an optimization of the whole process chain is desirable. The base for further increase in high performance machining are driving forces such as new machine concepts, powerful but energy-efficient components and effective cutting materials with high durability. Driven by the further development of the machined materials and the rising product spectrum an increase in tool life is favorable. Thus, new and advanced powerful tools provide an important driving force for high performance processes. To reach the full potential of HPG processes scientific findings need to be considered as a driving force, too. Also the combination of grinding processes with other manufacturing processes is one of the driving forces and will have a significant role in the future. Hereafter, different approaches for reaching High Performance Grinding processes are presented and discussed. 4.1. Design

their tools and develop grinding tools for specific requirements. Regardless of the quality of a grinding wheel, it only performs optimally if the relationship between the grinding wheel topography and the grinding results is known and the process parameters are adapted accordingly.

of grinding tools

Further development of conventional external cylindrical grinding processes to high performance processes is possible by implementing an optimized process design. One opportunity represents the reduction of the tool weight, so that it becomes possible to increase the number of tools on a tool spindle. This can be achieved by substituting a steel base body for aluminum or carbon fiber base body, as proposed by Diamant-Gesellschaft Tesch GmbH. This allows to rise the number of simultaneously machined surfaces, which results in an increased material removal rate and an increased productivity. In addition, a deflection of long components can be reduced, allowing good cylindricity and running qualities. Another advantage of such a process design is the reduced time-related tool wear, which means that set-up time is reduced (Figure 3). Abrasive tools play a central role in the grinding process. Therefore, manufacturers of abrasives continuously optimize

Improved productivity by reducing the weight of the grinding wheels Machining of Good Reduced up to 5 bearing roundness time-related + + positions and high wear of simultaneously running quality grinding wheel Figure 3: Example for an effective process optimization by reduction of the grinding wheel weight 4.2. Model-based

optimization of grinding processes

With more efficient computers the importance of simulation in grinding technology is continuously growing. Respectively, the number of approaches for the numerical simulation of grinding processes is continuously increasing [7]. The same is true for the simulation of the grinding wheel topography. The results of any of these simulations must be validated. Therefore firstly, the topography of real grinding wheels must be measured, analyzed and described in detail. Weiß et al. presented an innovative approach for the description of the functional properties of a grinding wheel. Based on the theoretical approach for the kinematic contact conditions by the limiting angle of cutting edge offset εlim [8] the Topotool was developed. This tool enables the user to describe the contact conditions between the grinding wheel topography and the workpiece by means of the parameters kinematic cutting edge count Nkin (Figure 4, I), the specific cutting edge area in normal direction Ani and tangential direction Ati (Figure 4, II). Additionally, the tool analyzes the shape of each grain which interacts with the workpiece. The shape of the grains significantly influences the chip formation during grinding. The precise knowledge of the grain shapes allows to describe the influence of changing grinding wheel specifications on the grinding wheel topography. Thus, it is possible to analyze the contact conditions between the grinding wheel and the workpiece in great detail. This in turn makes it conceivable to adapt grinding wheels more application-specific. [9]

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Measured grinding wheel topography I

3

1

4

5

2 At3

At1

II

At2 An5

At5

At4

Ati

An3 An1 An4

An2

Ani

Residual stress VA [MPa]

Fritz Klocke et al. / Procedia CIRP 46 (2016) 266 – 271 600 400 200 0 -200 -400 -600 -800

vw = 50 m/min

vw = 100 m/min

vw = 200 m/min

Ti3Al TiAl 0 100 200 300 0 100 200 300 0 100 200 300 Distance from surface d [µm]

Material: Ti-45Al-2Mn-2Nb + 0.8 %TiB2

Grinding wheel: D151 L6 V600

Grinding parameters: Q‘w = 40 mm³/mms; vc = 160 m/s; = 500 mm³/mm V‘w Down grinding

Coolant: Emulsion

Grinding machine: Blohm Profimat MT 408 HTS

Figure 5: Change from residual tensile to residual compressive stresses with increasing table speed

4.4. Investigation of material behaviour Figure 4: Detailed analysis of the grinding wheel topography with the number of kinematic cutting edges (I) and specific cutting edge areas in normal direction Ani and tangential direction Ati (II)

4.3. Influencing workpiece properties Products with increasing demand towards mechanical load require hard materials with high strength values. Machining hard materials requires, in addition to the technological and economic needs, energy-efficient machining. Thus, the economic machining of difficult-to-machine materials, as Inasaki and Takao [10, 11] investigated for advanced ceramics, is another driver for High Performance Grinding processes. Due to its low specific weight and a similar hightemperature strength like nickel-base alloys, J-titanium aluminides become more and more important, especially for the aerospace industry. J-titanium aluminides can be considered difficult-to-machine materials as a consequence of their low heat conductivity and their brittle character. Therefore machining strategies with low thermal loads are required. Speed stroke grinding represents a promising alternative to conventional surface grinding strategies. To exploit this potential the chip formation and tool wear mechanisms must be known and the process design must be adapted to the material characteristics. In order to consider tool wear mechanisms, energy aspects and crack, Zeppenfeld developed a process model. He determined that increasing table speeds result in decreasing tool wear up to critical table speeds vw,crit or critical depth of cut ae,min,crit. The increase of tool wear while exceeding this critical table speed vw,crit can be assigned to the process dynamics and grain load. He also determined that as a consequence of low surface temperatures during speed stroke grinding, residual compressive stresses occur in the surface layer of the ground J-titanium aluminides (Figure 5). Accordingly, HPG processes of difficult-tomachine materials are also achievable if the chipping mechanisms are known and the process design is adapted to the ground material [12].

Furthermore, the identification of the process boundaries of machining e.g. steel materials is an important factor for High Performance Grinding processes. If the process boundaries are known, the required component properties can be achieved during a superior level of productivity and process safety. Especially thermal and mechanical loads during the grinding process determine the properties such as the residual stresses of a workpiece surface. Because of thermomechanical overload, phase transformations can occur within the surface layer, which can lead to premature failure of the component. Duscha conducted experimental investigations to identify the thermo-mechanical load during pendulum and speed stroke grinding of 100Cr6 steel. Therefore, he did dilatometer measurements in order to identify the material behavior depending on the predominant temperature and strain. Among other things, the results of his investigations expand the fundamental scientific knowledge of phase transformation by the mathematical description of the influence of strain and strain rate on the resulting austenite start Ac1b and finish temperatures Ac1e (Figure 6). The obtained results can be transferred to grinding. During grinding, thermo-mechanical loads are applied to the workpiece surface layer by the grinding wheel interaction and lead to deformations. This results in strains. During grinding, the Ac1b temperatures are lowered according to acting mechanical loads. The phase transition is supported due to the pressure. The distance between the austenite start temperature Ac1b and the austenite finish temperature Ac1e increases with higher strains. The austenite transformation starts earlier, but more time and/ or more thermal energy for the complete transformation is required. These new insights allow the prediction of the phase transformation during grinding, which enables the manufacturer to adapt the conventional grinding process towards a HPG process while ensuring high product quality [13].

270


Temperature T [°C]

800 780 Ac1e-Temperature

760

Ac1b-Temperature

740 720 700

0 0

0.1 0,1

0.2 0,2

0.3 0.4 0,3 0,4 Strain H [-]

0.5 0,5

0.6 0,6

conventional grinding, the productivity of the combination grinding and laser machining can be significantly increased. Thus, in the present case the machining time was reduced by 85%, the power consumption was reduced to only 11% of the power consumption of the conventional grinding process and the processing cost were reduced by almost 20% [18]. Grinding processes providing an extremely high surface finish quality can be regarded as High Performance Grinding processes, too. Extraordinary finishing results can be achieved, using precise process control and a special tool design.

Material: 100Cr6 Parameters: . = 100 °C/min T. up Tdown = 26 °C/min

Laser process

Grinding process

Microscope image of the cutting edges

Tend ε

= 800 °C = 2 s-1

(a)

Rake face Burr

Figure 6: Austenite temperatures in dependency of the plastic strain during grinding 100Cr6 steel

200200 µm µm

4.5. Combination of grinding processes with other manufacturing processes More sophisticated processes and a combination of grinding processes with other manufacturing processes also open up potential for high performance processes in the field of grinding technology. Machining of polycrystalline diamond (PCD) tools is a great challenge for manufacturers because of its physical properties, especially its high hardness and its high wear resistance. The conventional machining process is grinding, implicating low material removal rates and high wear rates of the grinding tools. During the last decade, PCD machining using a laser process became more efficient [14, 15]. However, for many applications, the use of ground and laser machined PCD as a cutting material failed due to implementing an economic production process and the technological understanding of the continued process [16, 17]. For many years the manufacturing industry limited itself to either grinding or laser machining. A new approach by Schindler, who identified and classified the removal mechanisms in grinding of PCD indexable inserts showed that the combination of both manufacturing methods is much more efficient than just using one single method. By combining the advantages of both methods, manufacturing of PCD can be performed very efficiently in a high performance process. Hereby, the laser ablation is used to reach the desired geometry of the PCD with high productivity. Because of the high energy density of the laser, the PCD surface layer usually is thermally damaged, so that the use of the indexable inserts is not possible without additional grinding (Figure 7, (a)). Consequently, finishing of the inserts is performed by a grinding process, generating a high surface quality and low cutting edge roughness (Figure 7, (b)). During this grinding process only a small total depth of cut ae, tot is ground so that the surface layer, which was damaged by the laser process, is removed. The small total depth of cut ae, tot ensures low wheel wear and short grinding time. Respectfully, the tool and processing expenses can be reduced. In comparison to

(b)

Rake face

Flank face 200 µm

Focused ion beam-preparation + transmission electron microscopy-analysis Grinding direction Therm. influenced surface layer Cobalt Diamond Laser system: Pro PKD Laser parameters: = 200 ns tpulse Epulse = 1 mJ = 10 Hz fpulse = 35 µm df Normal atmospheric conditions

Diamond 1 µm

Cobalt

Machine: ISOG Technology S22P Turbo Grinding parameters: ae,tot = 500 µm = 5 µm ae = 15 m/s vs = 150 mm/min vf Grinding oil Grinding wheel: D15A PCX V

Figure 7: PCD surface layer after laser machining and after grinding

4.6. HPG of unique materials The rotational grinding of silicon wafers for the semiconductor industry represents a particular challenge in High Precision Grinding. In order to guarantee the highest surface quality, the process must be understood in detail. Therefore firstly, the distribution and structure of the subsurface damage in rotational ground wafers must be known. Secondly, the occurring process forces within the contact zone must be figured out and thirdly, the respective contact zone temperatures must be investigated. Pähler explored these three scientific issues by grinding (100) oriented silicon wafers. He found out that the surface roughness of the wafer does not depend on the mechanical anisotropy of the crystal, but increases with grain size, feed rate and also with increasing radius of the wafer (Figure 8). On the other hand, the subsurface-damage (SSD) strongly depends on the orientation of


Rotational grinding

the crystal, in detail on the relative position between the cutting direction and the main axis of the crystal. Based on the results a three-zone model of the defected structure of rotational ground wafers was derived. In addition, a direction dependence of the process forces was determined. Concurrent with the SSD investigations, areas with high and low process forces coincide well with the areas of high and low SSD depth, respectively. By means of an innovative temperature measuring concept, which enables the determination of the temperatures in the contact zone during rotational grinding, Pähler found out that within the contact zone slight temperature differences can be resolved [19]. Such improved process insights open up new possibilities of improving the grinding procedure. They offer opportunities to achieve specific sub-surface properties of the ground wafers in combination with improved grinding wheel lifetime. Grinding tool

vs

Contact zone

Wafer: (100) oriented m-Si Diameter = 200 mm Edge exclusion: 3 mm Tool: D3 C120

vw

rw

vc

rw

Fn Ft

r t

Process parameters: vs = 50 m/s = 20 µm/min ae Ra measurement: System: WYKO NT 2000 Ra values: average of 5 measurements

Figure 8: Surface roughness distribution on a 200 mm fine ground (100) oriented Si wafer

5. Conclusion and outlook In order to meet today's challenges resulting from globalization, cost pressure and increasing product quality demands, powerful grinding processes for manufacturing companies are becoming increasingly important. To optimize conventional grinding processes and transform them into High Performance Grinding processes, however, many “resistors”

need to be resolved. To overcome these “resistors” it is of great importance to know the mechanisms of the respective grinding process and to take the interactions between all components into account. The presented examples and methods in this paper show that further research and optimization potential still exists in many areas of grinding technology. If an increase in knowledge about the relationships in the grinding process enables a significant improvement of at least one of the four performance characteristics time, cost, quality or feasibility, there is the opportunity to transform a conventional grinding process into a High Performance Grinding process. References [1] Heisel U, Klocke F, Uhlmann E, Spur G. Handbuch Spanen. Carl Hanser Verlag, Print ISBN: 978-3-446-42826-3, 2014. pp. 14-17 [2] Jackson MJ, Davis CJ, Hitchiner MP, Mills B. High-speed grinding with CBN grinding wheels—applications and future technology, In: Process. Technol., Vol: 110, 2001. pp. 78–88. [3] König W, Klocke F. Manufacturing Processes 2: Grinding, Honing, Lapping. Springer Verlag Berlin Heidelberg, ISBN: 978-3-540-92258, 2005. pp. 7-10. [4] Denkena B, Köhler J, Kästner J. Chip formation in grinding: an experimental study. In: Production Engineering Research and Development, Vol. 6, 2012. pp. 107-115. [5] Kopac J, Krajnik P. High Performance Grinding. In: Journal of Materials Processing Technology, Volume 175, 2006. pp. 278-284. [6] Gühring K. Hochleistungsschleifen Eine Methode zur der Schleifverfahren durch hohe Leistungssteigerung Schnittgeschwindigkeiten. Dissertation RWTH Aachen, 1967. [7] Kumar S, Paul S. Numerical modelling of ground surface topography: effect of traverse and helical superabrasive grinding with touch dressing. In: Production Engineering Research and Development, Vol. 6, 2012. pp. 199-204. [8] Kassen G, Werner G. Kinematische Kenngrößen des Schleifvorganges. In: Industrieanzeiger, Vol. 91, No. 87, 1969. pp. 2087-2090. [9] Weiß M, Klocke F, Barth S, Rasim M, Mattfeld P. Detailed Analysis and Description of Grinding Wheel Topographies. In: ASME 2015 International Manufacturing Science and Engineering Conference, Vol. 1, ISBN: 978-0-7918-5682-6,2015. [10] Inasaki I. Speed-Stroke Grinding of Advanced Ceramics. In: CIRP Annals – Manufacturing Technology, Vol. 37, 1988. pp. 299-302. [11] Takao N et. al. Surface Grinding Characteristics of Si3N4 Ceramics under High-Speed and Speed-Stroke Grinding Conditions. In: Journal of the Ceramic Society of Japan, Vol. 103, 1995. pp. 1238-1242. [12] Zeppenfeld C. Schnellhubschleifen von γ-Titanaluminden. Dissertation RWTH Aachen, 2005. [13] Duscha M. Beschreibung des Eigenspannungszustandes beim Pendelund Schnellhubschleifen. Dissertation RWTH Aachen, 2014. [14] Wegener K, Dold C, Henerichs M, Walter C. Laser prepared cutting tools. In: Physics Procedia, Vol. 39, 2012. pp. 240-248. [15] Eberle G, Wegener K. Ablation study of WC and PCD composites using 10 picosecond and 1 nanosecond pulse durations at green and infrared wavelength. In: Physics Procedia, Vol. 56, 2014. pp. 951-962. [16] Kenter M. Schleifen von polykristallinem Diamant. Dissertation Universität Bremen, 1990. [17] Michels C. Nur mit optimalem System Schleifen von CBN-/PKDWerkzeugen. In: VDI-Z Special Werkzeuge, 2003. pp. 59-61 [18] Schindler F. Zerspanungsmechanismen beim Schleifen von polykrisallinem Diamant. Disseration RWTH Aachen, 2015. [19] Pähler D. Rotational Grinding of Silicon Wafers. Dissertation RWTH Aachen, 2009.

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