1、09FTM04AGMA Technical PaperNew Developments inGear HobbingBy Dr. O. Winkel,Liebherr-Verzahntechnik GmbHNew Developments in Gear HobbingDr. Oliver Winkel, Liebherr-Verzahntechnik GmbHThe statements and opinions contained herein are those of the author and should not be construed as anofficial action
2、or opinion of the American Gear Manufacturers Association.AbstractSeveralinnovationshavebeenintroducedtothegearmanufacturingindustryinthelastyears. Inthecaseofgear hobbing, the dry cutting technology and the ability to do it with powder-metallurgical HSS-materialsmight be two of the most impressive
3、ones. But the technology is still moving forward.Theaimofthisarticleistopresentrecentdevelopmentsinthefieldofgearhobbing,asthereareinnovationsregarding tool materials, process technology and process integration.Copyright 2009American Gear Manufacturers Association500 Montgomery Street, Suite 350Alex
4、andria, Virginia, 22314September 2009ISBN: 978-1-55589-957-83New Developments in Gear HobbingDr. Oliver Winkel, Liebherr-Verzahntechnik GmbHIntroductionDuring the last years, a couple of newdevelopments have been introduced regardinggear hobbing. Theinnovations inthe substratema-terials andcoating s
5、ystems have ledto anincreaseof productivity by higher cutting speeds and longertoollife. But ofcourse, higherperformance leadstohigher prices for the tools, so the impact on the toolinvestment as well as the resulting costs per piecehave to be examined.Another perspective to improve productivity is
6、toshorten the process chain. Here, the processintegration via chamfering and deburring on thehobbing machine is discussed. Besidesconventional chamfering methods like theGratomat-principle or the rotary deburring, anewprocess using specially designed chamferingcutters will be presented.Finally, the
7、chance for cost savings by processsubstitution is discussed focusing on examples forfinish hobbing. To overcome shaving as thetraditionalsoft-finishing method,new toolconceptsare presented which aim to increase the processperformanceregardingtoollifeand workpiecequal-ity. Especiallythe chanceto elim
8、inateor controlthenaturaltwistoffinishhobbingmightopenupthewayfor new applications.Modern tool designAlthough also the machine tools have beenimproved a lot in the last decades,the majorimpacton the hobbing technology was related to the tooldevelopment. Ifwelookontheimprovementsinthepast there, the
9、focus was on the one hand on thesubstrate materials and on the other hand on thecoating systems (figure 1). Both developments to-gether lead to much higher cutting speeds and/orlonger tool life. Even processes like dry hobbingbecame reality.Figure 1. Tool improvements in the past4Coming from the con
10、ventional HSS substrates(e.g., EMo5Co5 or M35) with TiN-coating, the useof carbide hobs seemed to be necessary to realizedry hobbing applications. But after a phase ofenthusiasm, problems with the process reliabilityregarding the tool life of the reconditioned carbidehobs (e.g., due to cobalt leachi
11、ng during stripping)stopped the trend. Then the introduction of themore heat-resistant TiAlN-coatings incombinationwith higher alloyed and more homogeneous PM-HSS substrates brought the dry cutting back ontrack. Nowadays,dry-cuttingwithPM-HSSaswellas carbide is state-of-the-art. And since theAlCrN-b
12、ased coatings have been introduced suc-cessfully in the gear hobbing market a couple ofyears ago, speeds and feed could be increasedeven more in many applications.Besides their more homogeneous structure, themain advantage of the powder metallurgical HSSsubstrates is the ability to contain greater a
13、mountsof alloys. As shown in figure 2, the remainingcontent of iron was reduced from about 70% for astandard substrate (e.g., ASP2030, S590 orRex45) to a minimum of 55-60% for the so-called“bridge materials” (e.g., ASP2080, S290 or Rex121). Thoseextremelyalloyedsubstratesaremoresimilar to carbide ma
14、terial then PM-HSS, whichgivesontheonehandtheadvantageofhigherwearresistance but on the other hand the disadvantageof much worse resharpening. Therefore, mediumalloyed substrates like ASP2052, S390 or Rex 76are a good compromise for high performancehobbing applications.Regarding the different coatin
15、g systems, figure 3shows a comparison of the most importantcharacteristics.1 WhilethehigherhardnessoftheTiCN-coatingcomparedtotheTiN-coatingshowedpotential to improve the tool performance in wetcutting, the low red hardness (maximum servicetemperature) of both coatings was not reallysufficient for d
16、ry hobbing applications. Here theTiAlN- and especially the new AlCrN-coatingshave proven their performance ability. With maxi-mum service temperatures of 900 1100Cincombination with their thermal isolatingeffect tothesubstrate material, a new level of dry cutting couldbe reached.Figure 2. PM-HSS mat
17、erials5Figure 3. Properties of modern coating systemsAs usual, higher performance means higher pricing(figure 4). As a rule of thumb, on PM-HSS hobs aTiAlN-coating costs about 20% more than a TiN-Coating. AnAlCrN-coating willcost additional30%compared to a TiAlN-coating, so in total about 55%more th
18、an a TiN-coating. For carbide hobs, thecoating prices are typically about 20% higher thanfor PM-HSS-tools.Figure 4. Costs for different coatings6Due to the usually lower tool costs, PM-HSS isactually the preferred substrate material for hobsespecially in the smaller modules (e.g., automotiveand truc
19、k industry). Characteristic for PM-HSS isits reliable wear behavior in a widespread range ofapplications. Carbide offers advantages especiallyin the area of finishing and cutting of high-strengthworkpiece materials (Rm 900 N/mm) due to itshigh wear resistance. For low to medium strengthmaterial (Rm=
20、 500-700 N/mm2), typical cuttingdataaregiveninfigure5. PM-HSSnormallyallowshigher chip thickness (higher feed rate), carbideoffers higher cutting speeds.The final decision for the best choice of substrateshould be based on a detailed analysis of the costsper piece (figure 6). As assumed, the use of
21、PM-HSSleadstolowertoolcosts(here: 16%). Carbidetools offer lower machining times (here: 22%) andthuslowermachiningcosts. Inthepresentedexam-ple, both advantages are almost leveling out eachother concerning the costs per piece. This under-lines that the decision regarding the right substrateshould be
22、en made each time separately. If no sig-nificant advantages for carbide, PM-HSS is actual-ly favored in most applications due to the more reli-able performance and the lower investment costs.Figure 5. PM-HSS vs. carbideFigure 6. Cost calculation example7But the example also points out a couple of ot
23、heraspects. Since the process is running on an oldermachine, the automation is quite slow. Therefore,the idle times (e.g., loading and unloading of theworkpiece) is about 40%of thecycletime. Theruleis that, the higher the cutting data (speeds andfeeds)andthelowerthehobbingtime,thefastertheautomation
24、 should be. Another interesting point isthat although the tool costs of about 35% are quitehigh, the portion for the tool investment of about10% is very low. The conclusion is thatthe pricefora new tool should not be the primary criteria be-cause it has a very small impact on the total costs.The mes
25、sage is exactly the opposite. If the higherprice for substrate material or coating is connectedto a higher performance ability of the tool, this in-vestmentpays offin manycases. This isespeciallythe case, if the higher tool performance is used toincrease the cutting data which leads to minimizedmach
26、ining costs.Chamfering and deburring methodsBesides the generating of the gear teeth itself, alsosecondaryoperationshavetobecarriedout. Averyimportant one is the elimination of theburrs thatarecaused by the cutting process. Additionally, achamferingofthesharpedgesisrequestedinmanyapplications. Since
27、 the difference between debur-ring and chamfering is important and often mixed,the most important aspects are pointed out infigure 7.Deburring is necessary, to protect the workeragainstinjuries duringmanual handlingof thework-pieces. Insubsequentprocesses,burrsonthefacesidescanaffectthegearqualityif
28、thefacesareusedforlocatingor clamping. Finally,remaining burrsonthefinishedpartcancausehighernoiseemissionorwear in the gear box.Chamfering is often applied to avoid nicks duringworkpiece transportation. In addition, the sharpedges lead to over-carburization which causes anembrittlement and can lead
29、 to edge chipping. Thiswill lead to higher wear in the gear box. Otheraspects might be the support of the assembly pro-cessandtheimprovementoftoollifeduringthehardfinishing process (especially for gear honing).Figure 7. Why chamfering and deburring8There are two typical chamfering processes whichdif
30、fer from each other in flexibility and neededchamfering time. The first one is theGratomat-process (figure 8), where chamfersalongthetoothformarecreatedwithmillingcutters.Thetoolsarepressedontheworkpiecefacesunderpre-load and a specific setting angle. The appliedmilling cutters are made of carbide f
31、or a higher toollife. High-speed spindles are creating thenecessary cutting speed. The process is veryflexible regardingthe workpiecegeometry andrela-tivelyinsensitive towardsthe workpiecestrength. Ifan according chamfering unit is integrated in thehobbingmachineandthereis asufficient cycletimefor h
32、obbing, the chamfering can be done parallel tothe primary processing time. In this case, noadditional cycle time for chamfering is needed.Figure 9 shows one possibility for the integration ofsuchachamferingunit. Whileusinga4-stationringloader, the 90-position can be used for the cham-fering. The adv
33、antage of this idea is that noadditional floor space is needed. The foot print ofthe machine stays constant while an additionaloperation is integrated.Figure 8. GratomatFigure 9. Floor space of integrated unit9Thesameprinciplecanbeusedtoapply achamfer-ing unit for rotary deburring tools. Since theGr
34、atomat-principle is quite flexible regarding theworkpiece geometries to be machined due to theuseofstandardmillingcutters,the chamferingitselftakes some time. Here, the rotary deburring has itsbenefits in the extremely short chamfering timesdue to the applied special tools (figure 10).Withrotarydebu
35、rringthechamferingisdonebycoldforming. The chamfer is created by a specially de-signedtoolwhichrollsunderpressurewiththegear.The deformed material on the face side is shearedoff by deburring disks. Deformed material in thegear flanks can be flattened by burnishing wheelswhich are integrated in the r
36、otary deburring tools.Those tools can therefore be quite complex, sincethey consist of several gears. The typicalsubstratematerial is PM-HSS.As mentioned before, the process allows veryshortchamfering times which can be just a couple ofseconds. Thus, normally the chamfering is doneparallel to the pr
37、imary processing time even at veryshort cycle times. The economic limits are set bythe low flexibility and high strength workpiecematerials.An alternative to the conventional chamferingmethods which require additional chamfering unitsis the ChamferCut-technology (figure 11). 3, 4By adding additional
38、 chamfering cutters (the so-called ChamferCut-tools) to the hob, the chamfer-ing can be done on a standard hobbing machine inthe same setup directly after gear cutting. Due tothe specific tool design, the chamfering process isworking continuously. Its function and restrictionsare discussed in the fo
39、llowing.Figure 10. Rotary deburringFigure 11. ChamferCut10All tools for gear hobbing and chamfering aremounted onone arbor. Afterthe gearhas beencut,theChamferCutscomeintoplay. ThefirstChamfer-Cut creates a uniform chamfer at the top side of thegear. The second ChamferCut is then responsibleforthede
40、burringandchamferingofthebottomside.The result is a chamfered gear that needs noadditional machining.To get an impression of the chamfering quality,figure 12 shows an example. Due to the fact thateach ChamferCut is specifically designed for asingle workpiece geometry and the chamfering it-self is do
41、ne by cutting, it creates a very uniform andhomogeneous chamfer along the whole tooth gap.Even the chamfering of the tooth root area is not aproblem. Different to the hobbing process, thechamfer is not formed by several enveloping cuts.The whole chamfering contouris createdin asinglecut. Therefore,
42、it is not a generating process.Crucial for the feasibility of this technology forindustrialapplicationsisasuitablesoftwaresupportfortheoperator,whichmeansthequalityandusabil-ity ofthe accordingmachine software. Therefore,aspecially dedicated software package has beendeveloped, which uses the same da
43、ta andgraphicsas the setup sheets provided by the tool supplier tosimplify the programming and adjustments (figure13). Afterwards, the software calculates andrealizes the necessary axis movements.Figure 12. Chamfering quality and chipsFigure 13. User interface11After the tool is worn out, it could b
44、e easilyresharpenedontherakeface, identicalto hobshar-pening. The necessary adjustments of the setupdata after resharpening of the ChamferCuts aredone automatically by the software based on theactual outside diameter.The ChamferCut-technology is not only applicablefor small module gears, but also fo
45、r big modulegears, which is shown in figure 14. Here the pre-grind form milling is followed by a chamfering of thebottom side of the gear. Since rotary deburring ofthis module 16 gear is not possible and the Grato-mat-principle would require an additional machine,the ChamferCut offers the chance to
46、remove theheavy burr on the bottom side which is created bythe ICI-cutter in the same setup.But it has to be mentioned that there are also somepre-conditions to apply this chamfering process.Primarily, a sufficient amount of space on the hobarbor in combination with an according shiftinglength is re
47、quired. Furthermore, the clampingfixture has to be adapted because the ChamferCutis working at a lower center distance than theaccording hob. Finally, the ChamferCut-toolsshould not have interference with the workpiececontour. But the major drawback might be that thechamfering process always increas
48、es the cycletime. Big advantages are the reduced investmentcosts compared to the chamfering units and theshort setup times.Finish hobbingAlthough hard finishing (like honing or grinding) iscoming up strong to the gear market, the cost effi-ciency of soft finishing (like shaving) is stillunbeaten. Wh
49、ere applicable, finish hobbing offersthe shortest possible process chain (figure 15).Figure 14. Gear milling and chamferingFigure 15. Finish hobbing12Since shaving is still the most applied soft finishingprocess, finishing hobbing has made big steps for-ward by the improved accuracy of modern hobbingmachines in combination with high-quality tools(quality AAA or better). Therefore, the quality gapbetween finish hobbing and shaving closes moreand more.If both processes are compared directly (figure 16),shaving has p
