1、06FTM14The Optimal High Speed Cutting of BevelGears - New Tools and New Cutting Parametersby: Dr. H.J. Stadtfeld, Gleason CorporationTECHNICAL PAPERAmerican Gear Manufacturers AssociationThe Optimal High Speed Cutting of Bevel Gears -New Tools and New Cutting ParametersDr. Hermann J. Stadtfeld, Glea
2、son CorporationThe 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.AbstractHighspeedcarbidedrycuttingwentthroughanevolutionwithrespecttospeedsandfeedsandthekinematicrelationshi
3、p between cutting blade and work, which eventually resulted in many improvements, availabletoday. The dependency of many important parameters upon the particular situation of a certain job makes itoften difficult for a manufacturing engineer to establish an optimal cutting scenario.Formanyyears,theq
4、uestionofthreefacesharpenedversustwofacesharpenedbladeshasbeendiscussed,butitneverreallywasaquestionofhowmanyfacesweresharpened,butratherhowflexiblethegeometryofthemajorsurfaces,thatformthecuttingedgecouldbeadjustedtotheparticularsituationofacertainjob.Theexpression “certain job” includes next to th
5、e cutting method: face hobbing or face milling, generating ornon-generating, also the basic parameter of the gear set design as well as the blank material and its crystalstructure.Thesecondsignificantaspectofthethreefacesharpeningistheallaroundcoating,thatisrequiredif the protecting layer of coating
6、 was removed due to the grinding of front face and side relief surfaces.An analysis of the different parameters and their influence to the performance of the cutting process, allowsestablish five, nearly independent areas of attention:- Blade geometry and placement in the cutter head- Cutting edge m
7、icro geometry- Surface condition of front face and side relief surfaces- Speeds and feeds in the cutting process- Kinematic relationship between tool and work (climb or conventional cutting, vector feet)Thispaperpresentsexplanationsandguidelinesforoptimalhighspeedcutting,dependingoncuttingmethod,par
8、t geometry, blank material and manufacturing environment, which also help to choose the right bladesystem to give the manufacturing engineer detailed information to support the effort of optimizing cutterperformance, tool life and part quality.Copyright 2006American Gear Manufacturers Association500
9、 Montgomery Street, Suite 350Alexandria, Virginia, 22314October, 2006ISBN: 1-55589-896-31The Optimal High Speed Cutting of Bevel Gears - New Toolsand New Cutting ParametersDr. Hermann J. Stadtfeld, Gleason CorporationIntroductionHigh speed carbide dry cutting went through anevolution with respect to
10、 speeds and feeds and thekinematic relationship between cutting blade andwork, which eventually resulted in many improve-ments, available today. The dependency of manyimportant parameters upon the particular situationofacertainjobmakesitoftendifficultforamanufac-turing engineer to establish an optim
11、al cuttingscenario.For many years, the question of three face sharp-ened blades versustwo facesharpened bladeshasbeendiscussed,howeveritneverreallywasaques-tion of how many faces were sharpened, but ratherhowflexiblethegeometryofthemajorsurfacesthatform the cutting edge could be adjusted to the par-
12、ticular situation of a certain job. The expression“certain job” refers to the cutting method: face hob-bing or face milling, generating or non-generating,thebasicparameterofthegearsetdesignaswellastheblankmaterialanditscrystalstructure.Thesec-ond significant aspect of the three face sharpeningis the
13、 all around coating that is required if the pro-tecting layer of coating was removed due to thegrinding of front face and side relief surfaces.An analysis of the different parameters and their in-fluence to the performance of the cutting process,allowstoestablishfive,nearlyindependentareasofattentio
14、n:- Blade geometry and placement in the cutterhead- Cutting edge micro geometry- Surface condition of front face and side reliefsurfaces- Speeds and feeds in the cutting process- Kinematic relationship between tool and work(climb or conventional cutting, vector feed)The proposed paper presents expla
15、nations andguidelinesforoptimal highspeed cuttingdependingon cutting method, part geometry and manufactur-ingenvironment,whichalsohelptochoose therightblade system to give the manufacturing engineerdetailed information to support the effort of optimiz-ing cutter performance, tool life and part quali
16、ty.Cutting Tool MaterialThepreferredtoolmaterialfordrycuttingprocessesis extra fine grain carbide. Carbides are cemented,sintered composite materials of tungsten carbide(WC) and one or more metallic binding elements.The best experience in dry cutting of bevel gearshave been gained in applying K-grad
17、e (ISO desig-nation) carbides which only contain the single bind-ing element cobalt (Co). Depending on the cuttingdepthithasbeenfoundthatacobaltcontentof12%shows the best cutting results for larger size gearsand a cobalt content of 6% is advantages for smallsize jobs.The lower contents of cobalt inc
18、reases the hard-ness and the wear resistance of the cutting tool butmakesitmorebrittlewhichincreasestherisk ofcut-ting edge chipping. The higher contents of cobaltenhances the toughness of the tool, which reducesthe wear resistance. Figure1shows two diagramswhich indicate fracture toughness and wear
19、 resist-anceasfunctionofthecobaltcontents1. Thebestcompromise for different work piece materials andjobs from automotive to truck size is evident by acomposite from 10% cobalt and 90% tungstencarbide.The diagrams in Figure 1 show with their differentcolor graphs the influence of the grain size with
20、re-spect to toughness and wear. The extra fine grainsintered structure delivers the best overall perfor-mance compared to finer as well as courser struc-tures materials. The excellent cutting experiencewith extra fine grain materials is also a result of thehighsinteringqualitywithlowvariationincompo
21、sitedensity and a highly homogeneous structure as aresult of the sintering process of extra fine graincarbide particles.2Figure 1: Fracture toughness and wearresistance of carbideCutting Tool Coating and Cutting EdgeRoundingThe purpose of cutting tool coating in the area ofchipformingactionis toprov
22、ide asurface layerwithhighhardnessandlowsurfaceroughness.Thispro-tects the carbide cutting edge and surroundingsfrom chemical reactions and isolates the basic sub-strate from the chip flow with respect to wear andtemperature.Itseemstobeabasicallydifferenttasktoprotectthecutting edge of a two face sh
23、arpened blade vs. thecutting edge of a three face sharpened blade. Thethreefacesharpeningrequiresanallaroundcoatingwhich delivers a cutting edge that is formed by acoated front face as well as a coated side relief sur-face including top relief area and also covering theclearance side surface.The bla
24、de that is only coated on the front face (twoface sharpened blade as shown in Figure 2) has aprotectiononthefrontfacewherethesurfacestressfromchipformingisthehighestandwherethechipsare being molded to their final form with a high rela-tive surface speed. The unprotected side relief sur-face has a co
25、ntinuous sliding contact with the flanksurface of the work. The individual parameters of aparticular cutting process will make the differencebetween high contact forces that lead to rapid wearand cutting edge chipping or a result in a negligibleside relief wear with an un-chipped appearance ofthe cu
26、tting edge 2.Figure 2: Chip removal mechanicsThe edge rounding radius dimension and the microblendbetweenedgeroundingandfrontfacecoatingare the key parameters to achieve low cuttingforces, optimal plasticization and chip shearing ac-tion. The optimal edge rounding radius for todayshigh speed dry cut
27、ting for only front face coatedblades lies between 5mm and 10mm. Figure 3shows on the left side a device for edge rounding ofcarbide blades with permanent front face coating.The nylon bristles of a rotating brush contain alumi-num oxide. The brush traverses sidewayswhile itisrotating with a circumfe
28、rential velocity vector that islined up in the 3mm to 7mm interference zonebetweencuttingedgeandbrushwiththedirectionofthe cutting edge. The photographs on the right sideof Figure 3 show the cutting edge before edge3rounding and to the right after rounding. The uppersection in the photographs with t
29、he darker color isthe coated front face, the light colored stripe is thecutting edge and the area below is the side reliefsurface. The influence of edge rounding is evidentbythereducedmicrocratersandthesmoothertran-sition between the cutting edge and the coating.At the beginning of carbide dry cutti
30、ng operationsthe best cutting results were obtained using a TiAlNcoating with a composition of 50% Titanium and50%Aluminum-Nitride.Both, singleand multilayercoatings were tested in many cutting trials. Today,the commonly used and highest performing coat-ings are derivatives of a single 10mm thick la
31、yer ofTiAlN with the trade names AlNite and TiAln-X.The coating table in Figure 4 gives an overview ofsome physical properties of coatings in use todayfor chip removing operations 1.Work Piece Material and StructuralTreatmentGearsinindustrialgearboxesandautomotive/trucktransmissions are generally ma
32、nufactured fromcarburizing steels which provide a hard and highlydurable surface and a core with high toughness af-ter case hardening. Commonly used materials forthe manufacturing of bevel gears are:A) AISI 5117 or ISO 16MnCr5 tensile strength 600N/mm2B) AISI 3215 or ISO 18CrNiMo6 tensile strength80
33、0 N/mm2C) AISI 4119 or ISO 20MoCr4 tensile strength 700N/mm2Figure 3: Edge rounding device and cutting edge before and after roundingFigure 4: Table with commonly used Physical Vapor Deposition (PVD) coating types4Aprovidesbestmachinabilityandbehaveswelldur-ing heat treat and quenching with respect
34、to distor-tion, internal stresses and the mechanicalproperties of surface and core.B delivers high core hardness after heat treatment(above40HRc),issensitivetoheattreatdistortionsand is critical with respect to tool life.Ccannotprovidethesamehighmechanicalproper-tiesthanBbutshowsbettermachinability.
35、It isoftenused as a compromise between A and B.Manufacturing of small batch production is mostlydone from bar stock material. In a mass productionenvironment the blank material is forged to a shapethat approximates the turned blank contour closelywith a decent stock allowance. In both cases, thestee
36、lstructureisnotoptimalwithrespecttomachin-ability and should be annealed or normalized.ThephotographsinFigure5showthestructureofaforgedblankafteretchingwithnitricacidwithamag-nification of 200. The upper photograph in Figure 5shows a coarse structure from ferrite with little butconcentrated grain bo
37、rders of perlite and graphite.Although the structure will change during the casehardeningandquenchingprocesstoprovidethede-siredpropertiesofthefinishedgear,turningandcut-ting ofthe softmaterial accordingto theleft photoinFigure 5 will deliver medium to low tool life.Foroptimalcuttingperformancewithr
38、especttosur-facefinishandtoollifeforboth,highspeedsteelwetcutting or carbide dry cutting it is recommended, tochange the structure to a more homogeneous ap-pearancewithasmallerferritegrainsize.Theresultof normalizing is shown in the center of Figure 5.The bottom photograph shows thefine structureaf-
39、ter annealing the forged material. Forming a chipmeans to generate a crack through the structuresshown in the three photos. Such a crack will followthesoftergraphite,whichisbetweenthehardferriteparticles. It is obvious, that in case of a very coursestructure the cutting edges frequently hits large f
40、er-rite particles which in some cases splits the ferritebut more likely will move the large ferrite particlesout of its way, which causes extensive wear of thecuttingedgeandresultsinaroughsurfacefinish.Anextremely fine structure will cause a build up edgeand a tearing with a scuffed appearing surfac
41、efinish.Figure 5: Steel structure of a forged blank(top), after normalizing (center) and afterannealing (bottom)Optimalfordryhighspeedmachiningwithcarbideisa structure between the center and right hand sidephoto (Figure 5).Feeds and SpeedsThe original surface speed recommendation of330m/min for high
42、 speed carbide bevel gearcuttingcame from thecylindrical gearhobbing withcarbidehobs.Atthebeginning ofhighspeedbevelgearcut-tingitbecameevident,thatthechipthicknesshastoexceed a certain minimum value to reduce cutting5edge chipping and abrasive wear on the side reliefsurface. In other words, it was
43、not only possible touse largechip thicknessesin highspeed bevelgearcutting but it proved to be required in order toachieve good and consistent tool life. Figure 6shows the dependency between chip thickness,surface speed and tool wear as a qualitative dia-gram. A medium chip thickness which is equiva
44、lentto an average end chip of 0.08mm (automotive sizeparts) delivers the best results from medium tohighsurfacespeeds.Lowsurfacespeedscausebuildupedge and high surface speeds result in excessiveside relief wear. An optimal surface speed is foundastheoptimum betweenlowest wearand shortcut-ting times
45、between 250 and 300 m/min.Figure 6: Tool wear as function of chipthickness and surface speedThere are certaincutting conditionsto considerthatbasicallydifferfromeachother.Atthebeginningofanon-generated plunge cut only the blade tips areexposed to a cutting action. As the plunging pro-ceeds the cutti
46、ng edges perform more and morechipremovaluntilthe endof theplungecycle,whenthe chip load is the highest. The tips of the bladesare cutting material from the first contact betweencutter and part until the end of the cycle. Cuttingedge sections away from the tip participating onlyfor a short time in t
47、he chip forming process. The re-quirementfromacuttingcycleistoprotectthebladetips from chipping and wear and minimize a bladepressure angle change that is caused by the differ-ence in time the different sections of the cuttingedges have in contact with the slot they cut. Thesingle ramp, shown in Fig
48、ure 7 (orange graph) istheresultofmanycutting trialsand parameterstud-ies that have been conducted to optimize cuttingperformance. The proportionally reduced plungefeed rate while the full depth position is being ap-proached(abscissa)leadsasresulttoa constantorslightlyincreasingchipload.Thethirdcutt
49、ingcondi-tion is the “dwell” on the end of the cycle.Figure 7: Single ramp for optimal chip loadIn face milling the cutter has to make one additionalrevolution at the end of the plunge. This assuresthat the widest or tallest blade has the possibility totake a last small chip to achieve consistent flankform,fromslottoslotandalsotoreducethespacingerror. The dwell time is calculated to be just 5% lon-ger than required for one revolution. The fact thatsome blades will take a small chip during the dwellrotationandothersmayonlyrubalongtheflanksur-face is very negativ
