1、12FTM10AGMA Technical PaperDevelopment of NovelCBN Grade forElectroplated FinishGrinding of HardenedSteel GearsBy U. Sridharan, S. Ji,S. Kompella and J. Fiecoat,Diamond InnovationsDevelopment of Novel CBN Grade for Electroplated FinishGrinding of Hardened Steel GearsUppili Sridharan, Shuang Ji, Srid
2、har Kompella and James Fiecoat, DiamondInnovationsThe statements and opinions contained herein are those of the author and should not be construed as anofficial action or opinion of the American Gear Manufacturers Association.AbstractThe unique requirements of an electroplatable superabrasive CBN gr
3、it used in profile grinding of hardenedsteel gears as well as the attributes and grinding behavior of a new CBN developed specifically for thisapplication are discussed. Profile gear grinding parameters were simulated in through-hardened AISI 4140steel(56HRC)andthegrindingperformanceofthenewCBNwasco
4、mparedagainstacompetitiveCBNgradewidely used in the application. Consistent with field criteria, grinding performance was characterized basedonoccurrenceofburnorformfailure. Theburnormetallurgicalphasetransformationfailurewasdetectedby Barkhausen Noise Analysis (BNA) and corroborated by microstructu
5、ral and microhardness evaluations.The form failure was simulated by tracking average radial wheel wear to a threshold value where form losswas expected to occur. Grinding tests indicate that the new CBN grit can grind 35% more parts compared tothe competitive CBN grade before burn failure. In additi
6、on, the new CBN displayed a lower wear rate. Thenew CBN grade also exhibited a uniqueability to grind with lower grinding power,resulting ina nearconstantBNAresponseonthegroundsurfacethroughoutthetest. Thisimpliedminimalmicrostructuralchangeonthegroundpartfromstarttoendofthetestcomparedtotheprogress
7、ivesofteningofgroundsurfacenoticedwiththe competitive CBN.Copyright 2012American Gear Manufacturers Association1001 N. Fairfax Street, Suite 500Alexandria, Virginia 22314October 2012ISBN: 978-1-61481-041-43 12FTM10Development of Novel CBN Grade for Electroplated Finish Grinding of HardenedSteel Gear
8、sUppili Sridharan, Shuang Ji, Sridhar Kompellaand James Fiecoat, Diamond InnovationsINTRODUCTIONGears are the most common power transmission devices used in a wide range of applications likeautomobiles,powergenerationandmachinetools. Dependingontheirusage,gearscanbequalitativelyclas-sified as commod
9、ity gears and precision gears. Commodity gears usually are small gears with minimal loadcyclesandgeometricaltolerancesrequirements. Precisiongearswhichareusedinhigh-endaerospaceandautomotive applications have ever increasing requirements imposed on them. These include increasedservicelife,lowerNVHle
10、vels,minimizedbacklashesandabilitytoaccuratelypredictservicelifeforpreventivemaintenance 1.Such stringent requirements have naturally had an influence on manufacturing and process control of preci-siongears. Thepasttwodecadeshasseenasteadyriseindemandforgearstobehardfinishedbymachiningor grinding to
11、 achieve the above mentioned performance metrics. Gear manufacture begins with formingprocesseslikeforgingorblankmachiningprocesseslikehobbing. Thisisfollowedbyheattreatingprocessessuch as case carburizing, through hardening, and nitriding. The heat treating process induces distortions inthe gear wh
12、ich necessitate hard finishing as the last step to ensure maximum load capacity and reducedtransmission noise 2.Hardfinishingiseitherdonebymachiningprocesswithdefinedcuttingedgeslikeskivehobbing,hardturning,etc or by abrasive finishing processes like grinding or gear honing 2. The classification of
13、abrasivefinishingprocesses is as shown in Figure 1 2.Thegrindingprocessesusedinfinishgrindingofhardenedgearscanbeclassifiedasgeneratinggrindingandprofilegrindingdependingonhowtheprofilesofgeartootharefinished. Thegrindingprocesscanbecontinu-ous where the gear profile is achieved progressively by gri
14、nding with a splined wheel or discontinuous profilegrinding where a specific profile is ground one gear tooth at a time.Figure 1. Breakdown of abrasive hard finishing processes 24 12FTM10Continuous generating grinding is typically adopted when grinding small batches with varying gear profilesandhenc
15、eusedressableconventionalaluminumoxidewheelsintowhichprofilescanbeeasilydressedusingadiamonddressingtool. Incertainspecialcaseswheremaintaininggearsurfaceintegrityisimportantvitrifiedbonded CBN wheels may be preferred over conventional vitrified wheels 3.Discontinuousprofilegrindingisgenerallyusedin
16、ahighvolumerepetitiveprocessandtypicallyusesanelec-troplatedsuperabrasiveCBN wheel. ElectroplatedCBN wheelscontain asingle layerof abrasiveembeddedin a nickel matrix and have the negative profile of the gear tooth ground with abrasive protrusion tolerancetypicallylessthan5mm. Thisisessentialtoachiev
17、eaccurateprofileoneachgeartoothduetothesinglepassnature of the process.This paper focuses on the unique requirements, physical attributes and grinding performance of a develop-mental grade of CBN designed specifically for electroplated profile grinding process.Profile Grinding CBN lDue to the nature
18、 of the grinding process itself and also the type of bond system used in the wheels, the re-quirements of a CBN grit used in an electroplated profile grinding wheel differs from a CBN grit typeused inavitrified bonded CBN wheel for other applicationsVitrified bonded wheels are characterized by high
19、natural porosity a multiple grit layers held together by theglass frit in the bond. Hence physical properties of a CBN such as shape and toughness only partially influ-encetheperformancemetricsofthewheelsuchasabilitytomaintainform,grindwithlowpowertoreduceriskofpartburnandgreaterintervalsbetweenwhee
20、lreconditioning. Incontrast electroplatedwheels exhibithighabrasive protrusion with 40-50% of the abrasive embedded in a non-porous nickel matrix. This is one oftheprimaryreasonswhyelectroplatedwheelswork bestin aprofile grindingset upas thehigh levelof gritprotru-sion enables high material removal
21、rates enabling fewer grinding strokes per tooth while achieving the re-quired gear form. Because of this, the physical properties of CBN grit have a greater influence on the wheelperformance in an electroplated wheel than a bonded wheel.CBNgritsusedinanelectroplatedwheel aretypically highlyblocky in
22、shape withaverage aspectratios oftheparticleslowerthan1.50topromoteuniformwheelwear. TheyarealsomuchtougherthantheCBNgritsusedin a bonded wheelto ensurelong wheellife. The rangeof materialremoval ratesused inprofile geargrindingapplicationresultsintheCBNcrystalwearbeingmoreattritiousthaninotherappli
23、cations. ThustheCBNgritstendtobecomeprogressivelydullwithwheelusage resultingin anincrease ingrinding powerand theassoci-ated risk of thermal damage.Ingeargrinding,wherethermaldamageadverselyaffectsthelifeandperformanceofthegear,itisimperativeto use a CBN which exhibits a tendency to fracture in sma
24、ll fractions and thus keeping the wheel sharp andfree cutting with a stable grinding power. This property is termed as “microfracturing” capability. It is alsohighlydesirabletohaveahighlyblockyshapetopromoteuniformwheelweartoachieveconsistentgeartoothform from the start to the end of grindingwheel l
25、ife. Achievinga uniformwheel wearin profilegear grindingisparticularly challenging as the depths of cut and corresponding grinding forces encountered by the grits varycontinuouslyfromthetoptobottomofageartoothflankascitedinpreviousattemptstomodelgrindingforcesin profile gear grinding 2.The requireme
26、nts of CBN crystals with uniform shape and lower toughness are truly unique to profile geargrinding. Generally, CBN crystal shapes and toughness are directly correlated i.e. more uniform crystalspossesshighertoughnessandviceversa. Thusdeveloping acrystal withlower toughnessand blockyshapepresents si
27、gnificant technical challenges. Development of such a novel CBN referred henceforth asDevelopmental CBN A is reported here along with grinding performance compared to a competitive CBNgrade that is commercially available and used in this application.CBN morphology and characterizationVisually, CBN A
28、 has much rougher, less well defined facets than the competitive CBN as seen from SEMmicrographsinFigure 2. Theroughsurface morphologyhelps promotemicrofracturing abilityin thecrystals.5 12FTM10Figure 2. Comparison of crystal morphologyCBN crystals are generally characterized as a function of their
29、friability using a metric known as ToughnessIndex (TI). It is a composite measure that takes into account the hardness, shape and fracture behavior of acrystal. The procedure entails placing a precisely measured amount of CBN in a capsule of pre-defined in-ternalgeometryalongwithatungstencarbideorst
30、eelball. Theclosedassemblyisvibratedatasetfrequencyfor a fixed amount of time. The contents of the capsule are then separated using a sieve stack and the CBNcollected in the sieve stack is accurately weighed. The ratio of the weight of CBNthat resistedfracture tothestarting weight is reported as the
31、 TI. Figure 3 shows that the normalized TI of CBN A relative to competitiveCBN A is similar.ExperimentalTypicalgeargrindingprocessrunsatwheelspeedsof25-30m/swithstraightoillubricationanddepthsofcutof 0.25 mm or lower. A laboratory-scale grinding test configuration, described below, was designed tofa
32、ithfully capture gear grinding conditions as closely as possible.Figure 3. Relative crystal toughness index6 12FTM10Gear grinding simulation test descriptionProfile gear grinding simulation tests were conducted using 1A1 electroplated wheels containing US mesh170/200(B91)gritofDevelopmentalCBNAandth
33、eCompetitiveCBN. Thewheelspecificationsareasbelowin Table 1. Twowheels ofeach abrasivetype werefabricated andtested toassess wheelto wheelvariability.Through hardened AISI 4140 steel which is a commonly used gear material was chosen as the workpiecematerial. The 4140 workpieces were chosen to be 3.2
34、 mm in thickness to ensure uniform throughhardenability. The specifications of the 4140 workpiece used for the tests are provided in Table 2.SurfacegrindingtestsweredoneusingaBlohmPrecimat306surfacegrinderwithdepthofcutof0.127mminupcutmodesimilartodepthsofcutsadoptedinprofilegeargrinding. Awheelspee
35、dof45m/swasusedalongwith a 5%-concentration water soluble oil coolant at a flow rate of 151 liters/min at the entry and exit of cut.This wheel speed was chosen to reduce wheel wear when using a water soluble coolant.Performance metricsThetwoprimarymetricsbywhichtheperformanceofageargrindingwheelisas
36、sessedaretheriskofthermaldamageandlossofforminthegroundgear. Theoccurrenceofeitheronthegroundgearwouldbeconsideredthe point of wheel failure. To capture these aspects, grinding performance in the simulation tests was meas-ured as a function of radial wheel wear, surface finish, and specific grinding
37、 energy. In addition, the thermalimpactofgrindingontheworkpiecewasmeasuredbyBarkhausenNoiseAnalysis(BNA). Awearthresholdof30 mm of wheel wear was chosen to correspond to “form failure”. The radial wheel wear and surface finishweremeasuredbyaHommelWaveline60stylus-basedprofilometer. Thewheelwearwasre
38、plicatedinasoftsteelcouponwiderthanthewidthofgrindingasshowninFigure 4. Thoughthe wheelwas 12.5mm wide,thewidth of cut was maintained at 3.2 mm. Thus only a portion of the wheel width was used for one test. Thisenabled use of the wheel for a second test if needed. The un-used portion of the wheel wa
39、s used as areference surface from which to monitor wheel wear. After a pre-determined volume of the material wasground, the entire width of the wheel was used to take a shallow cut in a soft steel workpiece. The resultantwheel trace on the soft steel block was recorded using a stylus-based profilome
40、ter. Such a trace provided ameasure of the wheel wear relative to the un-used (reference) wheel.Table 1. Wheel specificationsWheel type Electroplated 1A1Wheel diameter 152 mm (6.0 in)Wheel width 12.5 mm (0.50 in)Mesh size US Mesh 170/200 (B91)Table 2. Grinding test conditionsMachine Blohm Precimat 3
41、06, 15 hp CNC surface grinderGrind mode Transverse (upcut only)Test material AISI 4140 steel; through hardened to 54-56 HRCWorkpiece heat treating conditions Austenitized at 1575F for 45 minutesTempered at 325F for 2 hoursWheel speed, vs45 m/s (9,000 SFPM)Depth of cut, ae0.127 mm (0.005 in)Width of
42、cut, bd3.2 mm (0.126 in)Length of cut, lc305 mm (12 in)Specific material removal rate, Q/w 8.1 mm3/mm/s (0.75 in3/in/min)Table speed, vw3.8 m/min (150 ipm)Coolant Water soluble oil at 5% concentrationCoolant flow 151liters/minat6.9bar(40gpmat100psi;entryandexit nozzles)7 12FTM10Figure 4. Illustratio
43、n of radial wheel wear measurementItmustbenotedthatforUSmesh170/200,wheretheaverageCBNgritsizeis91mm,thenominalcoverageofnickel layer for electroplating is 40-50% of the mean grit size. Thus, in theory an electroplated wheel with170/200 mesh particles is expected to perform predictably to a wheel we
44、ar of at least 45 mm. Hence a wearthreshold of 30 mm is very rigorous for the purpose of gauging form failure in actual gear grinding process.The specific grinding energy was quantified by monitoring spindle power using a Hall-effect transducer TheBarkhausenresponseofthegroundworkpiecewasalso record
45、ed. TheBarkhausen NoiseAmplitude forthe4140steelworkpiecetypewasoptimizedtodetectgrindingburnasdiscussedinthesectionbelow. Theover-all effective wheel life was determined by the quantity of material ground at the point of occurrence of either“burn failure” as established by BNA or “form failure.”Bar
46、khausen Noise AnalysisBarkhausenNoiseAnalysisisanon-destructivemethodofevaluatingsurface integrityby detectingchangesin microstructure in ferromagnetic materials. Barkhausen noise is an inductively measured magneto-elasticsignal from ferromagnetic materials which consists of small magnetic regions c
47、alled “magnetic domains”which act like microcosmic magnets. These domains are separated by boundaries known as domain wallswhich rotate the magnetic vector of the domains 4.Whenan externalmagnetic fieldis applied,it initiatesan alternateorientation ofthe magneticdomains bythemovementofthedomainwalls
48、. Thischangeinorientationisstepwiseduetoirregularchanges ofmagnetiza-tion. The removal of the exciting magnetic field causes reversal of domain wall movements, but some do-mains are not able to take their original dimension and orientation due to a hysteresis phenomenon 4.This property of ferromagne
49、tic materials can be taken advantage of by applying an alternating current whichwould result in back and forth movement of the domains walls which can be measured and expressed asBarkhausen Noise Amplitude.BNA Optimization ProcedureSeveral workpiece properties such as the chemical composition, heat treating methods and conditions influ-ence a material s response to Barkhausen Noise Analysis and have been discussed in detail elsewhere4.Thus the excitation field conditions (voltage, frequency and amplitude filter) have to be optimized for
