Diamond technology Abrasive machining of ductile iron with CBN

n 2002 ductile iron accounted for 31% In this paper by K. Tuffy and M. O’ Sullivan, The performance of a CBN abrasive Iof casting shipments from 13.14 billion in an electroplated tool is quantified when high speed grinding ductile iron in tons in the US. Although pressure pipes are a large portion of this market terms of specific grinding energy, cutting forces and spindle power requirement. components for the car, truck and power The range of operation of the product was defined in the range of specific transmission sectors account for 27% material removal rate up to 220 mm3/mm/s, therefore operating under high of these ductile iron shipments [1]. Ductile efficiency deep grinding conditions. An initial ranking is developed by comparing iron or spheroidal graphite iron (SGI) has many properties such as good the performance of this CBN with another commercially available abrasive type, strength to weight ratio, excellent with a different impact strength and thermal stability. The mode of tool failure damping characteristics, good wear is different and is identified in this paper. The tool life was defined as the amount resistance and fatigue strength including good machinability [2]. Austempered of material removed before a specific normal force level of 90 N/mm was reached. ductile iron (ADI) is thought to offer all the It was found there was a direct correlation between force and power and either benefits of ductile iron as well as superior could indicate the end of tool life once a target value was specified. The tool mechanical properties common to many life of the stronger CBN product was twice that of the other weaker abrasive steel forgings and castings with lower manufacturing costs and offers potential type, which required higher specific grinding energies and grinding forces over for weight savings and associated the range of material removal rates studied. reduced fuel consumptions costs for example in the automotive industry [3]. ADI is thought to machine similar to Fig 1a HP cleaning coolant high-strength ductile irons [4]. Typical Schematic of grinding 76 L/min @ 50 bar application areas for ductile iron include arrangement on Blohm Profimat MT 408 crankshafts, camshafts, piston liners, gears, piston rings, wheel hubs, x manifolds, shafts, railway wheels and other applications such as machine frames and suspension brackets. vs An excellent review was given by Inasaki et. al [5] in 1993 which indicated the LP supply coolant 130 L/min @ 9.2 bar direction of fixed abrasive processing or grinding preferring the term ‘abrasive vw machining.’ The continued growth and development of vitrified bonds has extended the productivity barriers of Fig 1b many high volume automotive and Close up of surface aerospace component producers [6]. of electroplated CBN wheel used Central to the success of the rapidly in this project developing technology is the superabrasive products and product offerings, in particular with regard to CBN [7]. The emphasis on processes such as high efficiency deep grinding (HeDG) has firmly focused the spotlight on the unexplored potential of monolayer CBN wheels and the performance benefits [8]. The HeDG grinding process has been the focus of intensive research from a

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Test Conditions - Various CBN B181 16

) 14 Wheel speed, vs (m/s) 100 3 12 Workpiece speed, v (mm/min) 100 to 13,200 w (J/mm 10 net Depth of cut, ae (mm) 1 8 Coolant - Emulsion (%) 4 6 4 Workpiece EN GJS 700-2 (FCD 700-2 or D7003) P = 0.0132Q+0.9032 Grinding power P 2 Delivery LP cooling 9.2bar, HP cleaning 50bar 0 0 200 400 600 800 1000 3 Wheel type 1A1 - 250(20(127 (bs = 10) Material removal rate Qw' (mm /s)

Table 1 Standard grinding test condition Fig 2 Variation in Pnet versus Qw for product A process point of view, with an intensive ignored. It will be assumed that the only 16 effort on thermal modelling and workpiece difference between the electroplated 14 Pnet A subsurface integrity [9]. This paper will tools will be the abrasive type. P = 0.0138Qw+1.6422 12 Pnet B (kW)

concentrate on the abrasive product; The power was recorded directly from net 10 with two CBN grade types offered by the Sinumerik controller and the forces 8 Element Six reviewed in a high material were monitored using a Kistler three P = 0.0132Qw+0.9032 6 removal rate application on a pearlitic SGI component force dynamometer and 4

variant with electroplated CBN. In a follow attendant data acquisition system. In this Grinding power P 2 up paper the grindability of this pearlitic test the limit for the specific normal force 0 SGI material will be compared with a was 90 N/mm and this was called the end 0 200400 600 800 1000 Material removal rate Q ' (mm3/s) high strength ADI product. of tool life. The surface integrity was w checked at this specific normal force level Fig 3 Comparison between product A Test setup and no obvious phase transformations and product B in terms of Pnet were evident indicating that the workpiece The following tests were carried out on has not suffered from thermal damage. Power and specific grinding energy a Blohm Profimat MT 408 with a 45 kW The dominant pearlitic structure was still Fig 2 details the variation in the net spindle motor with a maximum speed of evident. Microhardness measurements power consumption with material

8300 rpm. Coolant was supplied as revealed no thermal softening and/or removal rate (Qw) for product A. The Pnet shown in Fig 1 at 9 bar using a Brinkmann rehardening had taken place. Given the being defined as the total power less the impeller pump and Brinkmann screw efficient removal of heat from the power consumed in driving the spindle pump was used to deliver coolant at 50 workpiece in the form of a stream of plus the coolant supply when grinding bar to a cleaning nozzle. The coolant sparks it was thought that the workpiece only air. Any reference to power from used was a 4% emulsion (Hocut 3380). would have tolerated a much higher level here will refer to Pnet. It is clear a

The distance, x between the cleaning head of Fn’, however this is outside the scope relationship exists with: and the wheel was kept constant. The up- of this investigation. grinding mode is depicted in Fig 1 was Pnet = Po + xQ (1) chosen for this test. In general for creep Discussion – from a superabrasive feed grinding, down-grinding requires machining point of view Where Po is the intercept. The slope of the lower specific energies and cutting forces line, x has units of u since the resultant due to the chip formation process but is In order to evaluate any grinding product must be equivalent to W (or J/s). The less efficient in terms of cooling. As Qw’ in a specific process it is reasonable to limit of the slope is thought to be the increases the efficiency of chip formation evaluate the performance range of the melting energy of the cast iron, which is increases also and the cooling and product and to attempt to assess what essentially similar to that of unalloyed lubrication aspects of the grinding process limits the product performance. In iron. The intercept can be taken as a become more dominant. Down-grinding superabrasive machining the change in measure of the efficiency of the cutting is more efficient in terms of cooling, specific grinding energy (u) and the net action of the abrasive since a lower value lubrication and swarf removal and was power consumption (Pnet) over a wide range for this constant will infer a lower Pnet. therefore chosen for this investigation. of specific material removal rate values Likewise it is believed replacing the

A step grinding method was employed (Qw’) are extremely informative. The test emulsion with a neat oil for example will in this process using half the wheel width, conditions used can be viewed in Table 1. also reduce the value of Po. allowing the other half of the wheel to be Product A is a brownish CBN product, The comparative performance of the used as a reference. The standard set of with a predominant tetrahedral morphology two abrasive types A and B is shown grinding conditions chosen can be seen and with an impact strength typically in Fig 3 in terms of Pnet. Here product B in Table 1. The purpose of this research 30% greater than product B. Product B has a larger intercept and slightly larger was to investigate the process performance is an amber product with a more value for the slope of the line over the with different abrasive types, therefore predominant cubic/octahedral morphology range of values studied therefore issues such as platability will be largely and is not as thermally stable. demands a higher level of power, and this

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difference increases as Q is increased. is critical in determining the magnitude w 180 )

It is important to note that the wheel of this constant and exponent, as well as 3 160 condition is assumed to be constant over the ability of the abrasive to maintain 140 the full range of Q evaluated for both this sharpness. It is appreciated that most w 120 products A and B. characterisation is done at the macro- 100 Fig 4 details the variation in the u with Q ’ level with respect to the abrasive and not w 80 for product A. The u is seen to decrease at the micro level where the active cutting 60 with increasing material removal rate. In point or plane is transient. In addition the 40 u = 81.035Q'-0.3654 the present representation no reference shape of the abrasive does not necessarily 20 Specific grinding energy (J/mm is made to the variation in wheel wear with mean it will be presented to the 0 050100 150 200 increasing material removal rate. However workpiece in this manner – this will be 3 Specific material removal rate Qw' (mm /mm/s) at increased values of Qw’ a more efficient determined by the lay of the particle in Fig 4 Variation in the u with Qw’ for product A grinding operation is clearly taking place. the tool making process. At higher Qw’ Relatively less ploughing and sliding are values the relative larger chip height may taking place at higher Q ’ and in the limit mean the size effect is less dominant but w 180 where the u approaches the minimum at low Q ’ the size effect becomes w 160 u A specific energy for chip formation, the dominant and therefore the sharpness of 140 u B ploughing and sliding energies would the abrasive becomes critical. Of course 120 then be zero. The specific grinding at higher Qw’ the number of active particles 100 energy is insensitive to alloying and heat changes also. 80 60 treatment since the melting energy of u = 156.87Q'-0.4772

Specific force (N/mm) 40 the material is independent of this [10] – Variation in the grinding u = 81.035Q'-0.3654 20 it is very important to consider this for forces with increasing Q ’ w 0 high material removal rate applications Fig 6 details the change in the specific 050100 150 200 3 Specific material removal rate Qw' (mm /mm/s) and helps explain when HeDG can normal (Fn’) and the specific tangential operate for materials with a relatively forces (Ft’) over the corresponding range Fig 5 Comparison between product A and product B in terms of u low grindability. of Qw’. At low Qw’ there is little difference A power law expression of the form u in the specific normal and tangential = AQ ’-b is expressed where A is a components of the force measured, w 100 constant and b is the exponent describing however at higher Qw’ the stronger, more Fn A the curve. The constant A is thought to tetrahedral product A has a distinctly 80 Ft A be dependent on the workpiece material lower specific normal and tangential Fn B and its resistance to shear whereas the component in comparison to product B. 60 Ft B exponent is thought to be more related In fact for a given force level for product 40 to the chip formation process [11]. It is B, say, Fn’ = 80 N/mm, almost a 10%

Specific force (N/mm) 20 worth stating that such curve exponents greater Qw’ is achievable producing the can only compare two different products same F ’ with product A. n 0 under the same process conditions if 050100 150200 250 Specific material removal rate Q ' (mm3/mm/s) the range of values being considered Variation in the surface w are the same. Fig 5 shows the difference finish over a range of Qw’ Fig 6 Specific force comparison in the grinding efficiency between the two The change in surface finish of the ductile between product A and product B abrasives – product A and B. Such a iron workpiece with increasing Qw’ in curve is less informative if the Q ’ is terms of R and R can be seen in Fig 7. w a z 18 increased by both the a and v in a What is surprising initially is that the e w 16 m) surface grinding operation since these magnitude of the Ra and Rz is as great at µ 14 curves will be largely independent. low Qw’ as it is at high Qw’, particularly in 12 Ra A Increasing Qw’ resulted in a more the case of product A. The chip height 10 Rz A efficient grinding process. Interestingly equation tends to predict that at 8 Ra B at low Q ’ the product B required a much increasing chip height (as Q ’ increases) 6 w w Rz B higher u in comparison to product A. an increase in the roughness parameters 4 Abrasive A with its dominant tetrahedral would be expected. However the surface Roughness parameter ( 2 0 morphology has a distinctive angular shape topography of the grinding wheel is 050100 150 200 Specific material removal rate Q ' (mm3/mm/s) with a relatively larger aspect ratio. Bailey changing continuously and despite this w and Juchem [7] state that CBN products occurring only on the micro level it will Fig 7 Variation in surface finish with with a more tetrahedral morphology have a significant bearing on the resultant increasing Qw’ for product A and B require lower grinding powers and grinding action and hence the surface produce lower cutting forces. It is clear finish of the workpiece. This partly that product A with its sharper cutting explains why high surface finish values action has a power law relationship with were obtained at low Qw’ since these a lower constant value and exponent. It were ground first using a relatively newer is thought the sharpness of the abrasive and hence coarser grinding wheel.

INDUSTRIAL DIAMOND REVIEW 1/06 RHP Diamond technology

Also a relatively inefficient grinding would have continued to grind for a 100 operation is taking place at low Qw’ and much longer time, if the tool life was the size distribution (B181) of the abrasive dictated by the wear on the particles or 80 product would indicate that it is more the radial wheel wear. 60 suited to a roughing type operation. The A more appropriate way in terms of a Ft' Fn' surface finish produced by product A is production environment of monitoring 40 superior to product B. This will be further tool life in electroplated tools is by 20 Specific force (N/mm) discussed later in this article. monitoring the spindle power since this 0 can be more easily achieved without the 0 Continuous monitoring need for installation of secondary 100000 200000 300000 400000 500000 600000 700000 Specific volume removed V ' (mm3/mm) 3 w at a Qw’ = 100 mm /mm/s transducers. Fig 10 shows that the Pnet The change in specific forces produced can be directly related to the forces in Fig 8 Variation in the specific forces for when machining ductile iron with product grinding and is an excellent indicator of product A with increasing volume removed 3 A at a Qw’ of 100 mm /mm/s with the end of tool life. The machine output cumulative volume removed is shown in or gross spindle power is simply a constant 100 Fig 8. A freshly plated wheel was used for plus Pnet so interpreting the machine this test. It is clear over the initial 500 cm3 output is also very straightforward once 80 a transient behaviour is observed. This the value of the power is determined Ft' A Fn' A 60 F ' B F ' B is characteristic of an electroplated tool where the end of tool life is reached. t n showing a ‘settling in’ period where the Depending on the given operation or 40 topography of the grinding wheel is application, this may be the power level 20 Specific force (N/mm) altering to achieve a more collective at which the surface integrity of the 0 uniform grit height of protrusion. This will workpiece becomes unacceptable or 0 be further referred to later in this article. simply the power level at which the tool 100000 200000 300000 400000 500000 600000 700000 Specific volume removed V ' (mm3/mm) It is worth noting that the specific normal becomes unusable due to the excessive w force value in the steady state is almost wear on the grinding wheel. Fig 9 Specific force progression double the value it started out when the for product A and product B new wheel was used. An almost constant Variation in workpiece surface force level is then reached and this is finish with cumulative volume 16 preserved for a sustained period of time of workpeice machined 14 emphasising the high strength of the The products are specified by a particle 12 product. For the purposes of this test size distribution but also have a shape 10 program a specific force level of 90 N/mm distribution. The combination of these two 8 Pnet A was specified as the end of tool life. This factors means following the electroplating 6 Pnet B value can therefore be thought as arbitrary process there will be a spread of particle 4 2

and or as an accelerated tool life test. heights of protrusion of the CBN particles Net power consumption (kW) 0 The difference in the performance on the wheel hub. Of course the shape 0 between product A and B in terms of of the particle will also affect the lay of 100000 200000 300000 400000 500000 600000 700000 Specific volume removed V ' (mm3/mm) the transient behaviour and duration of particle on the wheel hub but many w the relatively steady state period can be manufacturers can now control this. A Fig 10 Change in net spindle power with seen in Fig 9. Both products have a large spread of particle height of protrusion cumulative volume of workpiece removed similar specific normal and tangential will result in a sharply acting grinding force value at the start of the test wheel, consuming low grinding power 12 indicating that this value is indeed and producing a relatively rough surface m) Ra µ 10 dictated by the kinematic roughness on topography on the workpiece. Fig 11 details Rz the grinding wheel and not the particles the variation in the surface topography 8 6 themselves. The performance of product with reference only to Ra and Rz with

B is almost devoid of any steady state increasing Vw’. Again a constant Qw’ of 4 3 period with an approximate linear 100 mm /mm/s with product A was used 2 increase in the specific forces after a for this assessment. After an initial Roughness parameter ( 0 3 0 specific volume (Vw’) of 100,000 mm /mm grinding period the Ra and the Rz decrease of GGG 70 is machined. This is in substantially. This suggests a finer grinding 50000 100000 150000 200000 250000 300000 350000 400000 Specific volume removed V ' (mm3/mm) complete contrast to the product A, with wheel surface with a more controlled w on average a 30% stronger impact height of protrusion. The production of Fig 11 Change in surface roughness with strength as measured by the Friatest. a finer surface finish on the workpiece cumulative volume ground for product A The tool life of product A is almost double correlates with an increase in the specific that of product B according to the grinding forces (and of course the spindle specification of the tool life. From analysis power). It can therefore be concluded that of the surface topography of the grinding the grinding wheel surface does actually wheels the resultant wear levels were still change quite significantly over the duration under 13 microns, meaning that the tools of the test.

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Fig 12 Image of grinding wheel produced with product A prior to final test

It is worth stating that no wheel loading was observed at any point. The image in Fig 12 shows a section of the grinding Fig 13 Electroplated CBN wheels for this project were supplied by Cranden Diamond Products wheel surface produced with product A 50,000 mm3/mm prior to the end of the References tool life test. No wheel loading is visible Nomenclature [1] Kirgin K.H., (2003), Castings in the that would have contributed to an Q Material removal rate (mm3/s) Global Market, AFS Metalcasting increase in power consumption. With a w Forecast and Trends 2002. few exceptions the abrasive particles are [2] Cast Irons, (1996), Ohio: ASM International. Qw’ Specific material removal rate relatively unworn, with no wear flats and (mm3/mm/s) [3] Warrick R.J., Althoff P., Druschitz A.P., Lemke J., Zimmerman K. Mani P.H. and little abrasive loss from the bond matrix. u Specific grinding energy Rackers M.L., (2000), Austempered Ductile Iron Castings for Chassis Applications, (J/mm3) Conclusion Proceedings SAE 2000-01-1290. [4] www.kennametal.com P Nett power in grinding (kW) [5]Inasaki I., Tönshoff H.K. and Howes T.D., Selecting the correct product for a specified (1993), Abrasive Machining in the Future, process and application is critical to Ft’ Specific tangential grinding Annals of CIRP, Vol 42-2, pp. 723-732. achieving the most economic processing force (N/mm) [6] Wolter H., A New Generation of Highly route. It is clearly evident that product A Porous Vitrified Bond CBN Grinding F ’ Specific normal grinding force Wheels, Industrial Diamond Review, in this analysis contributed to significantly n (N/mm) Issue 3 1996, pp 68-70. increasing tool life. It can be expected that [7] Bailey M. and Juchem H.O., the reduced grinding forces would produce V ’ Specific volume of workpiece The advantages of CBN grinding: w low cutting forces and improved components with a superior surface removed (mm3/mm) workpiece integrity, Industrial Diamond integrity. The superior performance of Review, Issue 3 1998, pp. 83-89. product A is due to a greater impact vs Wheel speed (m/s) [8] Stephenson D.J., Corbett J., Laine E., resistance as well as a tetrahedral Johnstone I. and Baldwin A., (2001), Burn Threshold Studies for Superabrasive vw Workpiece speed (mm/min) morphology. It is recognised that this Grinding using Electroplated CBN research was completed independent Wheels, Proceedings of the Society of ae Depth of cut (mm) of the production complexities in the Manufacturing Engineers, MR01-219. b Width of grinding wheel (mm) [9] Jin T. and Stephenson D.J., (2004), electroplating process. The spindle power s Three Dimensional Finite Element is an excellent process control tool, R Average surface roughness Simulation of Transient Heat Transfer in capable of indicating the end of tool life a High Efficiency Deep Grinding, Annals (µm) of CIRP, Vol. 53-1, pp. 259-262. with excellent correlation with the grinding [10] Malkin S., (1989), Grinding Technology, forces in an electroplated application. The Rz Mean roughness depth (µm) Theory and Applications of Machining surface finish of the workpiece changes with Abrasives, Wiley, New York. significantly during the lifetime of an [11] Marinescu I.D., Rowe W. B., Dimitrov B and Inasaki I., (2004), Tribology of electroplated grinding wheel, but in Acknowledgements Abrasive Machining Processes, particular during the early transient or settling William Andrew Publishing. The authors of this paper would like to in period. The duration of this period will thank Cranden Diamond Products Ltd. depend on the process conditions, workpiece for the production of electroplated CBN Appendix grindability and the abrasive type. ◆ wheels used in this project (Fig 13). In In this paper, Product A refers to addition the private communication with Element Six standard product ABN 800 Dipl. -Ing H.O. Juchem and Professor Brian and Product B to ABN 300. Rowe and was much appreciated. This article is based on a paper presented at the 1st International Industrial Diamond Authors Conference held in Barcelona, Spain in K. Tuffy and M. O’Sullivan work for Element October 2005 and is printed with kind Six Ltd, Shannon Airport, Co Clare, Ireland. permission of Diamond At Work Ltd. www.e6.com

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