A Novel and More Efficient Way to Grind Punching Tools

Transcription

1 A Novel and More Effiient Way to Grind Punhing Tools P. Krajnik 1,, R. Drazumeri, J. A. Badger 3, C. M. Niolesu 1, J. Kopa 1 KTH Royal Institute of Tehnology, Prodution Engineering, Stokholm, Seden University of Ljubljana, Faulty of Mehanial Engineering, Ljubljana, Slovenia 3 The Grinding Do Consulting, Austin, Texas, USA peter.krajnik@iip.kth.se ABSTRACT A simulation model of punh grinding has been developed hih alulates the instantaneous material-removal rate, ar length of ontat and temperature based on the kinemati relationships beteen heel and orkiee and determines the optimum mahine parameters to redue yle time and ahieve a onstant-temperature no-burn situation. To basi outputs of the simulation model inlude ar length of ontat and speifi material-removal rate. A thermal model is inluded in the simulation to alulate maximum grinding zone temperature rise. A novel method is developed to onstrain this temperature rise in the simulation. The thermal model inputs a onstant value of speifi grinding energy and the energy partition, hih represents the fration of the grinding energy onduted as heat to the orkpiee. The simulation-based optimization an lead to a drasti redution of grinding yle time. Moreover, the limitation of maximum grinding zone temperature rise belo the transitional temperature an help to avoid generation of orkpiee thermal damage, hih inludes thermal softening, residual tensile stress, and rehardening burn. The grindability of high speed steel (HSS) is also disussed in terms of poer onsumption, speifi grinding energy and undeformed hip thikness. Keyords: Grinding, Modeling, Simulation, Geometry, Thermal. 1. INTRODUCTION AISI M (EN HS6-5-) HSS for old ork appliations is idely used for prodution of punhing tools due to its high thermal stability ombined ith high hardness and ear resistane. These properties make HSS more diffiult to grind than other steels, espeially hen using onventional aluminium-oxide heels. The grindability is largely affeted by the hard tungsten-molybdenumand vanadium-arbides held firmly in the hardenedsteel matrix. High temperatures hih an our during grinding of HSS lead to various types of thermal damage, suh as surfae oxidation, softening, rehardening, residual stresses, and raks [1]. Tensile residual stress is partiularly ritial beause it redues servie life and reliability of an operating punhing tool under fatigue onditions. Lo tensile residual stress is therefore a key surfae integrity requirement. The fatigue properties of punhing tools are determined not only by the HSS material properties (e.g. arbide and inlusion ontents, phase transformations) [] but also by the thermophysial properties of the grinding proess (hip thikness, heel topography, energy partition, et.), hih affet the grinding temperatures. It as found that thermal stresses generated in the grinding proess are the primary soure of tensile residual stresses [3]. In this onsideration, the problem of ontrolling residual stress is redued into a problem of ontrolling grinding temperature. For thermal analysis of grinding, it is of major importane to understand the fators hih affet the grinding temperatures. An analytial temperature model is used in the simulation to avoid generating tensile residual stress. The model inputs the values of grinding geometry, kinematis and speifi grinding energy. Another major fator for predition of grinding temperatures is the energy partition, hih represents the fration of the grinding energy transported as heat to the orkpiee. Punh grinding is a speial type of outside-diameter plunge grinding, performed on a ylindrial grinder. In order to grind a non-round profile on a punhing tool, the radial infeed veloity (X-axis) must be synhronized ith the orkpiee veloity (C-axis) in a ontrolled manner. Improving the effiieny of punh grinding requires an optimization of grinding geometry and kinematis, hih is based on a proess simulation. The overall optimization objetive, hoever, is to minimize grinding yle time subjet to onstraint on maximum grinding zone temperature rise. Early, and to our knoledge the only researh on punh grinding inluded a development of the proess simulation [4]. The developed simulation as used to predit material removal rate, ar length of ontat, and the orkpiee veloity. The major onlusions of this early researh are summarized belo [5]: Punh grinding at onstant orkpiee veloity leads to lo material removal for most of the grinding yle.

2 Grinding yle time an be redued by inreasing the orkpiee veloity around the punh orner. The ar length of ontat is over three times that of an equivalent sized round part in ylindrial grinding, leading to an inreased threshold for the onset of thermal damage. The inreased material-removal rate is best ahieved by an inrease in the orkpiee veloity rather than by an inrease in the depth of ut.. GRINDABILITY It is ell knon that, ithin a family of HSS, different grades may have vastly different grindabilities, resulting in larger speifi grinding energies and higher heel ear (loer G-ratios) [6]. In HSS, tungstenmolybdenum- and, in partiular, vanadium-arbides ause rapid dulling of Al 3 grains. Figure 1 shos speifi grinding energy (measured via grinding poer) vs. the amount of material removed (measured as the volume of material removed per mm heel idth per mm heel irumferene) for grinding of different grades of HSS [7]. Experiments ere arried out on a surfae grinder using a 1%-hite-Al 3, 46-grit, H-grade, vitrifiedbonded grinding heel (diameter 35 mm, heel speed 5 m/s), ith 6% soluble oil grinding fluid and a luster-diamond dresser (depth of dressing 5 μm and dressing lead of. mm/rev). e [J/mm 3 ] V [mm 3 /mm ] WKE45 M9V ASP 6 M ASP 3 ASP 17 Fig. 1: Speifi grinding energy vs. amount of material removed for six different HSS grades. From the upper figure, e see that all grades begin at a speifi grinding energy of around 15 J/mm 3. Hoever, as some grades ontain more and larger hard arbides, the speifi grinding energies an inrease drastially as grinding proeeds, resulting in one grade generating values of grinding poer several times greater than grades ith high grindability. Sine temperature inrease is diretly proportional to grinding poer, this results in muh higher grinding temperatures. 3. SPECIFIC GRINDING ENERGY AND UNDEFORMED CHIP THICKNESS Speifi grinding energy e provides a valuable measure of the ability of a grinding heel to remove material. Speifi grinding energy depends on numerous fators, suh as sharpness of the grinding heel, the grindability of the orkpiee material, undeformed hip thikness, ooling and lubriation, et. There are several ays to alulate the undeformed hip thikness, h m, depending on the shape of the undeformed hip volume. For the alulation of undeformed hip thikness e adopted the ontinuity analysis [6]. For the need of a simulation, h m is expressed in terms of speifi material removal rate Q and ar length of ontat l : h m = 6 Q C r v l here C is the utting-point density, r is the shape fator, and v s is the heel veloity. Q and l are onstant in ylindrial grinding. In ontrast, both parameters vary onsiderably ithin a single orkpiee rotation hen punh grinding, as shon later in Figure 6 and Figure 7. Depending on the method, different values of shape fator an be used [8]. Here, a value of r = 1 as hosen. The utting-point density, C, depends on the grit diameter and the dressing onditions and an hange as the heel ears. One simple method to alulate this value is [6]: 1 n1 d g s (1) C C = () here d g is the grain diameter and C 1 and n 1 are onstants. Numerous values of C 1 and n 1 have been found [6] depending on the bond type, dressing and also on the method of measurement. Here, values of C1 = and n 1 = 1 ere hosen, hih orrespond to results found by Sha [9]. Although different methods an be used to alulate h m, e are most interested in relative values, hih means the onstants hosen above are adequate provided they are onsistent throughout. Therefore, e an pluk various values of speifi grinding energy from the already onduted experimental ork [1] and poer measurements made at various tool manufaturers grinding HSS ith vitrified- and resin-bonded Al 3 heels. In all experiments oil as used as a grinding fluid. The poer measurements ere taken before the amount of material removed V exeeded.1 mm 3 /m. Figure inludes measured values obtained in the folloing experimental sets: Set 1: resin-bond, M, medium dress Set : resin-bond, M, oarse dress Set 3: vitrified-bond, M, fine dress Set 4: resin-bond, ASP3 Set 5: vitrified-bond, M, oarse dress Set 6: resin-bond, ASP6

3 e [J/mm 3 ] Set 1 Set Set 3 Set 4 Set 5 Set 6 For the needs of modeling the punh-grinding geometry and kinematis, let us onsider instantaneous ontat onditions at the orkpiee rotation angle ϕ hen grinding one side of the orkpiee in an i-th grinding pass as shon in Figure 4. The oordinate system is held fixed on the orkpiee. This means that from the simulation point of vie the heel rotates around the orkpiee ith the relative rotational frequeny that equals the atual orkpiee rotational frequeny n h m [μm] 3 n δ s π v s Fig. : Speifi grinding energy vs. undeformed hip thikness for different experimental sets. From the same figure it an also be seen that speifi grinding energy, e, dereases asymptotially until reahing a minimal steady-state value e of around 5 J/mm 3. In all ases the heel as dressed relatively sharp. Therefore, e no have a relationship beteen h m in the ase of a sharp heel. If e fit a urve to the above data e get the folloing equation: C e = e + (3) n h m We have taken the value of speifi grinding energy e = e = 5 J/mm 3 to be fixed (sharp dress, aggressive ut). The parameters C and n depend on the dressing and grinding onditions (amount of material removed, heel ear, grindability, et.). In this stage of researh, the variation of speifi grinding energy has not been onsidered. 4. GEOMETRY AND KINEMATICS Punh grinding ith ontrolled infeed is aomplished by radially feeding a grinding heel into the rotating orkpiee to produe a punh to a desired geometry, as shon in Figure 3. n Retangular orkpiee (finish) Round orkpiee (start) Ar length of ontat l Grinding heel Fig. 3: Illustration of punh grinding operation. Fig. 4: Punh grinding geometry and kinematis. Grinding passes (utting paths) for eah side of the orkpiee are seleted as parts of irles, hose diameters inrease from the initial orkpiee diameter to infinite value that represents a straight line. The maximum utting depth, reahed in the middle of the side being ground, is the same for all grinding passes. Penetration area of the grinding heel into the orkpiee results an be haraterized by the ar length of the ontat l. Negleting motions and deformations of the heel and the orkpiee, l is alulated as: d l = s [ ψ 1 ψ ] (4) here d s is the heel diameter, ψ 1 is the ontat entry angle and ψ is the ontat exit angle. The instantaneous speifi material removal rate Q is alulated as: Q e = d a 1 + ae v (5) here a e is the depth of ut, d is the diameter of the utting path and v is the relative orkpiee veloity given by: v d 15 π = v d n s s v ( ) a e ϕ 1( ) δ s n (6)

4 here v s is the relative veloity of the heel entre: 3 δ s v s = n (7) π osψ and δ s is the distane beteen the orkpiee and the heel enters. To ahieve proper grinding kinematis, the radial infeed veloity should equal: 3 = δ s tanψ n (8) π 5. THERMAL MODEL In the simplified heat-transfer analysis, the maximum grinding temperature rise θ m is usually alulated by the adapted Jaeger model [11]: 6. SIMULATION Within the simulation model, geometrial mehanism of the orkpiee forming and proess kinematis are linked-up ith the thermal model. The simulation model inputs the folloing parameters: Wheel diameter: 38 mm Initial orkpiee diameter: 5 mm Width and height of the punh: x 15 mm Speifi grinding energy: 5 J/mm 3 Energy partition: 6% Thermal ondutivity of the orkpiee: 4 W/mK Workpiee density: 8.1 g/m 3 Speifi heat apaity of the orkpiee:.46 J/gK The features of the punhing tool, used as a ase-study for the simulation, are shon in Figure q l θ m = (9) k ρ v p here q is the heat flux entering the orkpiee, l is the ar length of ontat, k is the thermal ondutivity of the orkpiee, ρ is the orkpiee density, and p is the speifi heat apaity of the orkpiee. The generation of tensile residual stress is strongly dependant on orkpiee properties, in partiular material yield stress. The value of the transitional temperature, i.e. the grinding temperature for the onset of tensile residual stress varies from 4 to 6ºC for AISI M, depending on the heat treatment [3]. For our optimization e are onstraining the θ m to 6ºC, presuming that the likelihood of tensile residual stress generation on the orkpiee surfae ill be minimal. For alulating θ m, the parameter hih remains to be determined is the heat flux entering the orkpiee q. Of the total grinding energy, only the fration, ε knon as the energy partition, is onduted as heat to the orkpiee at the grinding zone. For a speifi grinding energy e or poer P (measured using a poer meter), the heat flux to the orkpiee an be ritten as: q ε P ε e Q' = = (1) l b l here b is the grinding idth. In the upper equation, the numerator represents the portion of the grinding poer entering the orkpiee as heat and the denominator is the area of the grinding zone. Both Q and l an be predited by the developed simulation model. In order to alulate q, it is neessary to speify the energy partition to the orkpiee ε. The energy partition an be obtained experimentally using temperature mathing and inverse heat transfer methods [1]. For shallo ut grinding of steels ith aluminum oxide heels, the energy partition typially varies from about 6 to 85% [13]. Fig. 5: Punhing tool. The variations in ar length of ontat l and speifi material removal rate Q are shon in Figure 6 and Figure 7 respetively. Both diagrams are shon for the optimal grinding senario, onsisting of 31 grinding passes (total number of orkpiee revolutions). Variations in l and Q for the first grinding pass are shon in blue (i = 1). l [mm] i=1 i=15 i= ϕ[ ] Fig. 6: Ar length of ontat vs. angle of orkpiee rotation.

5 i=1 i = 15 i = i=1 i = 15 i = 31 Q [mm 3 /mm s] n [rpm] ϕ [ ] Fig. 7: Speifi material removal rate vs. angle of orkpiee rotation. 7. OPTIMIZATION In punh grinding, drasti hanges in the grinding onditions our if the orkpiee rotates at onstant veloity. The instantaneous speifi material removal rate Q an ause loalized thermal damage on the punh leading or trailing edge. An approah to overoming this problem is to vary the orkpiee and infeed veloities as to maintain a more onstant grinding temperature. In theory, the punh-grinding yle an onsist of a single or multiple passes (number of total orkpiee rotations). In the single pass (reepfeed) operation, all material is removed during one rotation of the orkpiee. Similarly, punh grinding ith multiple passes requires higher orkpiee and infeed veloities that are onstrained by the orkhead speed range and maximal speed as ell as aeleration of the ross slide (X-axis). The ase study onsiders grinding on a Studer S33 ylindrial grinding mahine, hose major limitation is the maximal infeed (X-axis) of 1 m/min. For this speed, the orkpiee rotational frequeny is limited to 1 rpm, even though the maximal speed of the orkhead is 15 rpm. The optimization problem an hene be ritten as: minimize subjet to: T θ m t n 1 rpm v fr 1 m/min The optimal number of grinding passes that yield minimal grinding yle time is not knon. The optimization problem is solved by the simulation model that yields optimal number of grinding passes and grinding parameters satisfying the optimization objetive and onstraints. Considering the heel speed is fixed, only the orkpiee and infeed veloities are to be optimized. The major output of the optimization is n, shon in Figure ϕ[ ] Fig. 8: Variation of orkpiee rotational frequeny. We an see from the upper figure that the orkpiee veloities, or n, are inreasing during suessive grinding passes. The upper limit of orpiee veloity is ahieved in the last grinding pass. In the optimal senario, onsisting of 31 grinding passes, the minimal grinding yle time of t = 6.4 minutes is ahieved. Grinding yle time depends on the number of required grinding passes N, i.e. the total number of orkpiee revolutions. As e see in Figure 9, the minimal grinding yle time is ahieved hen N=31. Grinding in one pass (reep-feed) senario ould be far from optimal, resulting in the yle time of t = 85.3 minutes. t [min] N Fig. 9: Grinding yle time vs. number of grinding passes. 8. CONCLUSIONS The idea of this paper is to present an overvie and partiularities of punh grinding operation and to disuss analytial approahes to simulate thermal behavior, geometry and kinematis of the proess. Simulation model, inorporating all three aspets, is used to optimize the proess so that the grinding yle time is minimized.

6 Tensile residual stress is one of the main limitations of a punhing tool, so it is espeially important to avoid their generation. This an be ahieved by onstraining maximum grinding zone temperature rise. A ritial fator needed for alulating this temperature and ontrolling the onset of tensile residual stress is the speifi grinding energy and the energy partition. More experimental ork is required to adequately estimate the energy partition and to aount for speifi energy variation ith dressing and grinding onditions. Experiments are urrently being onduted, ith measurements of speifi grinding energy, depth of thermal damage (i.e. rehardening burn and thermal softening). These measurements ill enable the alibration of the simulation model. While this thermal analysis is urrently being developed further, the simplified model presented here is suffiient to give relative temperature values, enabling the model to determine parameters that give minimal yle time. It is lear, hoever, that the speifi energy and its partition to the orkpiee should be kept lo. One ay to ahieve this is to have suffiient self-sharpening of the heel that redues the speifi grinding energy. Another ay to ahieve ooler grinding ould be to use CBN heels, hih have higher thermal ondutivity (arrying more heat aay from the grinding zone than aluminium-oxide heels). Last but not least, ooling effiieny must be separately optimized to take maximal amount of heat by the grinding fluid and to further loer the energy partition. 9. ACKNOWLEDGEMENTS This researh as onduted ithin the E!4957- PUNCH-GRIND projet, hih is o-funded by the EUREKA Initiative. The authors thank Kern Tool Tehnology for enabling the required experimental ork. 1. REFERENCES [1] Snoeys, R., Maris, M. and Peters, J. (1978), Thermally indued damage in grinding, CIRP Annals - Manufaturing Tehnology, 7/, [] Meurling, F., Melander, A., Tidesten M. and Westin, L. (1), Influene of arbide and inlusion ontents on the fatigue properties of high speed steels and tool steels, International Journal of Fatigue, 3, [3] Chen, X., Roe, W. B. and MCormak, D. F. (), Analysis of the transitional temperature for tensile residual stress in grinding, Journal of Materials Proessing Tehnology, 17, [4] Baliga, B., Doyle, E. D. and Thompson, W. (1998), Computer simulation for punh grinding, Transations of Mehanial Engineering, 3/1, [5] Baliga, B., Hodgson, P. and Doyle, D. (1999), Effet of ontat length in off-round grinding, In: Abrasive Tehnology: Current Development and Appliations I (Jun Wang. (Ed.)), World Sientifi Publishing, Singapore. [6] Malkin, S. and Guo, C. (8). Grinding tehnology: Theory and appliations of mahining ith abrasives. Industrial Press, Ne York. [7] Badger, J. A. (7). Grindability of onventionally produed and poder-metallurgy high-speed steel, CIRP Annals - Manufaturing Tehnology, 56/1, [8] Roe, W. B. (9). Priniples of modern grinding tehnology. William Andre Publishing, Norih. [9] Sha, M. C. (1996). Priniples of abrasive proessing, Oxford University Press, Oxford. [1] Badger, J. A. (1999), A omputer model to predit grinding fores from abrasive heel topography, Ph.D. Thesis, Trinity College, Dublin. [11] Jaeger, J. C. (194), Moving soures of heat and the temperature at sliding ontats, Proeedings of the Royal Soiety of Ne South Wales, 76, 3-4. [1] Malkin, S., Guo, C. (7), Thermal analysis of grinding, CIRP Annals - Manufaturing Tehnology, 56/, [13] Kohli, S., Guo, C., Malkin, S. (1993), Energy partition to the orkpiee for grinding ith aluminum oxide and CBN abrasive heels, ASME Journal of Engineering for Industry, 117,