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    Influence of grinding parameters on the quality of high content PCBN cutting inserts.pdf

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    Influence of grinding parameters on the quality of high content PCBN cutting inserts.pdf

    Journal of Materials Processing Technology 214 2014 276– 284Contents lists available at ScienceDirectJournal of Materials Processing Technologyjou rn al h om epage www.elsevier.com/locate/jmatprotecInfluence of grinding parameters on the qualitycontent PCBN cutting insertsB.Institute der aArticleReceivedReceivedAcceptedAvailable online xxxKeywordsToolPCBNGrindingin grainroughnessof theof1.geouscuttingresistance,propertiesKlocke and Knig, 2008. Since diamond tools have a low chemi-cal stability during cutting ferrous materials, polycrystalline boronnitride PCBN was created to replace its use McKie et al., 2011.Currently, PCBN is mainly applied in high speed and hard machiningCook and Bossom, 2000.geometriesandformationfragments.rial,frompermanentKnig,materials.criticalcrackkoehlerifw.uni-hannover.decarlos.ehvgmail.comcutting speed, lead to a decrease of the process forces, surfaceroughness, as well as dimensional and form errors Kopac andKrajnik, 2006. Furthermore, higher cutting speeds enable a pro-ductivity increase, since an increase of the feed velocity withoutroughness deterioration becomes possible Klocke et al., 1997.0924-0136/http//dx.doi.org/10.1016/j.jmatprotec.2013.09.013Due to its high hardness and abrasiveness, targeted surfaces andin PCBN inserts are manufactured by grinding MalkinHwang, 1996. Grinding of brittle materials is dominated by theof micro-cracks and the resultant break out of workpieceWith the penetration of grain cutting edges in the mate-radial and lateral cracks are formed. The chip removal resultsthe lateral cracks in particular, while axial cracks lead to adamage of the workpiece peripheral zone Klocke and2005.However, a ductile machining can be carried out even on brittleFor this, single grain chip thicknesses under a determinedvalue are necessary. Under this value the required energy forpropagation is higher than for plastic yielding Bifano et al.,∗Corresponding author. Tel. 49 5117625207; fax 49 5117625115.E-mail addresses denkenaifw.uni-hannover.de B. Denkena,J. Khler, venturaifw.uni-hannover.de,C.E.H. Ventura.Otherwise Tso and Liu 2002 note during grinding of polycrys-talline diamond PCD that lower cutting speeds reduce shocksbetween wheel diamond grains and PCD grains. In addition, thecarbonation of PCD grains due to high temperatures can be avoided.Friemuth 2002 establishes that during the grinding of cuttinginserts the use of grinding wheels with smaller abrasive grain sizesalso brings advantages. A reduced grain size leads to a significantincrease of the active cutting edges, contributing to the decrease ofthe single grain chip thickness. Consequently, the load on each grainis reduced and an improvement of the surface and edge quality isobtained.As already noted, the single grain chip thickness is an importantfactor, which describes the contact relations between workpieceand grinding wheel. Thus, it enables the characterization of theinfluence of different process input parameters on the workpiecequality. Main factors affecting this characteristic value are feed andcutting speed, abrasive grain size and grain concentration. Feedand cutting speeds are related to the kinematic of the process,while grain size and concentration contribute by describing the– see front matter 2013 Elsevier B.V. All rights reserved.Denkena, J. Khler, C.E.H. Ventura∗of Production Engineering and Machine Tools, Leibniz Universitt Hannover, Anr t i c l e i n f ohistory11 June 2013in revised form 28 August 20139 September 2013grindingcutting insertwheel weara b s t r a c tPlunge-face grinding is commonlyprocess design, an investigationPCBN inserts is carried outincluding a variation of edge chipping and flank facesize as well as an increasequality. Moreover, a variationquality.IntroductionIn order to attend to the demands of an economically advanta-process and to lead to a proper workpiece surface quality,tool materials must have a high hardness and abrasivea sufficient toughness, good mechanical and thermalat high temperatures as well as a high chemical stabilityof highUniversitt 2, 30823 Garbsen, Germanyused to finish PCBN cutting inserts. In order to reach an adequateof the influence of the grinding parameters on the quality of high contentthis work. For this, the inserts are ground with different grinding wheelssize and bonding, dressing feed rates, feed and cutting speeds and theare measured. It was found that a reduction of the abrasive graindressing feed rate lead to an improvement of the insert edge and surfacethe cutting and feed speeds has only a small influence on the PCBN insert 2013 Elsevier B.V. All rights reserved.1991. In this way, the removal mechanism with small single grainchip thicknesses and high specific energy is caused predominantlyby ductile flow. In contrast, low specific energy and larger chipthicknesses indicate a brittle fracture Chen et al., 2010.The right choice of the feed and cutting speed, as well as thegrinding wheel specification, is relevant to a ductile grinding Yasui,2007. Smaller chip thicknesses, resulting from an increase of theB. Denkena et al. / Journal of Materials Processing Technology 214 2014 276– 284 277supergrindingplunge-facetoincreasesnotationgrainincreaseschipbemicroscopicicalcausesflatteningthiscausebetweenofwheelspiece1980enedtwobondHowever,grindingetersprotrusionmeanthatity.sharperLargeronforces.arisedressingdecreasingFig. 1. Dynamics of diamond wheel during grinding ofwheel layer. Friemuth 1999 demonstrates that ingrinding the number of active edges is inversely relatedthe square of the grain size and the single grain chip thicknesslinearly with the grain size. However, this approach doestake into account the wheel wear and, consequently, the vari-of the grinding wheel topography during grinding. Although,flattening, for example, leads to a reduction of the grain size,the number of active grains and decreases the single grainthickness. Other factors such as feed and cutting speed mustalso considered with regard to their effects on wheel wear.Grinding wheel wear can be characterized as macroscopic orwear and results from mechanical, thermal and chem-loads. Mechanical loads affect the wheel macro geometry anddimension and form deviations.Microscopic wear describes grain wear like grain break out,or pullout of grains Hitchiner and Wilks, 1983. Incontext, thermal loads play an important role, since they cangraphitization of diamond grains and damage the interfacegrains and bonding Wimmer, 1995. During grindingsuper hard polycrystalline materials with diamond grindingthe wear results mainly from the flattening of grains Fig. 1Mamalis et al., 2000.In order to reduce the effects of the wheel wear on the work-an adequate dressing process is needed Komanduri and Reed,. While conventional abrasive wheels are trued and sharp-in a single operation, super abrasive wheels require frequentlydistinct operations, since a sufficient grain protrusion from thelevel is normally not achieved Tnshoff and Geisweid, 1979.simultaneous truing and sharpening of fine-grit diamondwheels with a SiC-roll is possible, when the right param-are chosen. Thereby, it is possible to reach an adequate grainand wheel form in a single step Friemuth, 1999.Huang 2001 defines the dressing intensity as the resultantforce acting on each abrasive grain during dressing and notesthe reduction of this intensity leads to a better surface qual-He explains that a higher dressing intensity produces largeredges and a lower intensity produces more micro edges.edges cause lower forces, but deeper grooves. Micro edgesthe other hand produce lower roughness and higher grindingParticularly in grinding of PCBN inserts high wear and forcesand geometry deviations can occur. In this case, an adequateprocess cares for a proper flatness of the abrasive layer,the risk of deviations Michels, 2005.hard polycrystalline materials Mamalis et al., 2000.Considering the difficulties in grinding PCBN tool material andbecause of the high costs involved to investigate this process, alimited number of works related to this topic is published. So thereis a lack of knowledge, which has to be resolved due to the grow-ing importance of PCBN cutting tools in machining, the so-calleddifficult to cut materials.With the goal to reach an adequate process design, investiga-tions on the influence of the grinding parameters on the qualityof a PCBN insert were performed. For this, a systematic variationof the feed and cutting speed as well as the dressing feed and thegrinding wheel specification i.e., grain size and bonding materialwas carried out. Grinding wheel wear and grinding forces were alsoexamined in order to support the obtained results.2. Materials and methodsThe experiments were carried out on a five-axis grindingmachine type Wendt WAC 715 Centro with a maximum rotation1625 rpm. The dressing system of the applied machine consists ofa cup wheel and its axial feed movement enables sharpening andtruing of the grinding wheel with a single dressing roll in only oneoperation.In these investigations cutting speed vcwas varied from 10 to30 m/s, feed speed vfafrom 1 to 10 mm/min and dressing feed ratevfadfrom 0.5 to 2.0 H9262m/s. Different diamond grain sizes were testedD10, D15, D36 and D46. Moreover, for the grain size D36 a vitrifiedand a resinoid bonded wheel were applied. All grinding wheels haveconcentration C120 5.5 Kt/cm3.The insert has specification SNMA120408 and has eight PCBNcorners brazed onto a cemented carbide substrate of grade K10-20. The PCBN part is composed of 90 PCBN and TiCN and Co asbond materials. PCBN grain size is 4 H9262m and its Vickers hardness is36.4 0.48 GPa.In comparison with the grain and bond wear caused by PCBN,the cemented carbide part plays a minor role and can be neglected,but clogging of the wheel during grinding of the cemented car-bide part can occur. Aiming to avoid this situation and to eliminatewheel form deviations a continuous dressing is performed in alltests. The total plunge depth depends on the dressing feed rateand on the grinding feed speed, which defines the process time.The rotational speed of the dressing roll cannot be adjusted and isfixed in 375 rpm. Grinding wheels with different grain sizes mustbe dressed with dressing rolls of different grain sizes in order toprovide wheel flatness, sharpness and to avoid high wear rates of278 B. Denkena et al. / Journal of Materials Processing Technology 214 2014 276– 284Fig. 2. Measurement setup for measurement of wheel topography.the wheel and roll. In this way, the variation of the wheel grain sizeimplies at the same time a variation of the dressing roll grain size.All dressing rolls are of Al2O3. The combinations “grinding wheelgrain size–dressing roll grain size” follow the recommendation ofthe grinding wheel manufacturer and are D10–320 Mesh, D15–320Mesh,measuringchippinginsert,methodferentH9262ingvaluelated.process has been quantified by the optical 3D measurement deviceMikrocad GFM. Twenty measurements per insert were performedin different positions of different edges. In each grinding test, foursides of the insert flank faces were ground and each test wasD36–220 Mesh, D46–180 Mesh.The quality of the inserts after grinding was evaluated bythe average surface roughness Ra and maximum edgeRk. Due to the small PCBN area in the flank face of thethe measurement of the surface roughness by a tactilewas not possible. In this way, two areas of 1 mm2in dif-sides of the insert were acquired by the laser point sensorScan, Nanofocus, and five roughness profiles, transverse to grind-direction, were measured in each area. After that the averageand standard deviation of ten measurements were calcu-The measurement of the edge chipping after the grindingFig. 3. Edge chipping Rk and flank face average roughnessrepeated once. The normal and tangential force values correspondto an average of the obtained forces in each side of the insert.The forces were measured with two Kistler sensors type 9213. Theacquisition was carried out with a charge amplifier type 5015 ofthe same company. An acquisition rate of 2500 Hz and a low passfilter with cutting frequency of 1000 Hz were applied. Details of theforce measurement system can be seen in Denkena et al. 2013.It is known that the total force FT during grinding of an insertside is made up of the force acting on the PCBN part FCBN and ofthe force acting on the cemented carbide part Fcarbide. The magni-tude of these components is not only due to the properties of eachmaterial, but also proportional to the respective ground area ACBNRa under variation of grinding wheel grain size.B. Denkena et al. / Journal of Materials Processing Technology 214 2014 276– 284 279Fig. 4. Grain protrusion Rpk and area-related grinding forces under variation of grinding wheel grain size.and Acarbide. Aiming to just evaluate the forces acting on the PCBNpart of the insert, the following expression Eq. 1 was usedFprimeprimeCBN,R[FT,R− Fprimeprimecarbide,R Acarbide]ACBN1wherearea-relatedtangential.andmentcarbidenottreatedprocessterizationtopographyAfter each test two negative samples of the topography in dif-ferent regions of the grinding wheel were produced with a lighthardening mass and scanned by means of the laser point sensorH9262Scan, Nanofocus. The samples were then inverted and measured.This method enables the acquisition of the grinding wheel topog-raphy without the need to remove the tool from the machine. Fig. 2FprimeprimeCBNis the area-related force for PCBN part, Fprimeprimecarbideis theforce for cemented carbide and R means normal orIn order to apply the equation, Fprimeprimecarbideand FTin normaltangential directions were measured separately. The measure-of Fprimeprimecarbideis obtained from the grinding of the cementedpart alone, since its area is large enough and continuousinterrupted by the PCBN part.A quantification of the wheel wear by means of the G ratio is notin this study. However, the evaluation of the influence ofparameters on the workpiece quality demands the charac-of the abrasive layer, since changes of the grinding wheelcause force and workpiece surface variations.Fig. 5. Edge chipping Rk, flank face average roughness Ra, grinding forces andshows the measurement setup.Werner 1994 affirms that roughness parameters are foundunsuitable for the assessment of vitrified grinding wheels. Local,deep pores affect the profile, so that these surface parameters donot provide statistically significant statement about the wheel layercharacteristic. He still says that the Abbott curve provides a moredetailed statement about the topography. In this way, the reducedpeak height Rpk can be used to characterize the grain protrusion,regarding their mutual dependence Sroka, 2005. From each sam-ple an area of 4 mm2was acquired and five profiles were measured.Therefore the average value and standard de

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