1、02FTM6Contemporary Gear Hobbing - Toolsand Process Strategiesby: C. Kobialka, Liebherr Verzahntechnik GmbHTECHNICAL PAPERAmerican Gear Manufacturers AssociationContemporary Gear Hobbing - Tools and ProcessStrategiesC. Kobialka, Liebherr Verzahntechnik GmbThestatementsandopinionscontainedhereinaretho
2、seoftheauthorandshouldnotbeconstruedasanofficialactionoropinion of the American Gear Manufacturers Association.AbstractGear manufacturing without coolant lubrication is getting more and more important. Modern hobbing machines aredesigned to cope with dry hobbing. In the last years carbide hobs were
3、prevailing in high-speed hobbing due to theirexcellent thermal stability. Today thishigh performance rate isconfronted with rather high tool costs and a critical toolhandling.PowdermetallurgicalHSScombinedwithextremelywearresistantcoatingsonthebaseof(Ti,Al)Noffernowinteresting alternatives for dry h
4、obbing. It is evident that existing conventional hob geometries can be optimizedrespectinglimitingfactorslikemaximumchipthicknessandmaximumdepthofscallops.Theexistingmachineconceptswithdirectdriventoolandworkpieceaxishavesufficientreservesforincreasingfeedsandspeedsinapplicationsofthefuture. Ananaly
5、sisofcontemporaryusedhobgeometriesshowsthatmodificationsinno.ofgashesandno.ofstartscanincreasetheefficiencyofthehobbingprocess.Thatmeansthatalargernumberofgashesincreasesthenumberofteethintheshiftingzone.Duetothisthehobcanbeusedforalongertime. Regardingtothequestionofincreasingcuttingspeedsthe textu
6、re analysis of a gear that was hobbed at a cutting speed of 1000 m/min shows that there is no influence of thecutting speed on the structure.Copyright 2002American Gear Manufacturers Association1500 King Street, Suite 201Alexandria, Virginia, 22314October, 2002ISBN: 1-55589-806-81Contemporary Gear H
7、obbing Tools and Process StrategiesClaus Kobialka, Dr.Ing.LIEBHERR VERZAHNTECHNIK GmbH, Kempten, GermanyDry cutting versus wet cuttingIf in the hobbing process coolant is used or not, isprimarily a question of the requiered workpiecequality and in a second step a question of themanufacturing environ
8、ment. When an othermanufacturing operation before heat treatment isperformed by using coolant, a washing operation isnecessary.Today, more than eight years after the firstpresentation of a hobbing machine for up to date drycutting, the process is verified by variousapplications. There are advantages
9、 anddisadvantages regarding to an ecological processwithout coolant (figure 1).Figure 1: Wet or dry?Due to higher temperatures during the chip flowcutting forces are reduced. This indicates a higherconstant workpiece quality. But this may causesurface defects due to critical chip flow effects.The to
10、ol life by using coolant depends on theapplied technology and the resulting chipthicknesses. At low cutting speeds smallest chipthicknesses can be cut. With increased cuttingspeed also the cutting temperature is increased.Based on this fact the tensile strength of theworkpiece material is decreasing
11、. At lowest tensilestrength rates the forging grade is decreasing sothat the workpiece material may flow betweencutting edge and the pre-cut workpiece surface. Theresults are optical damages on the workpiecesurface.Tool LifePerfor-manceQualityWet Hobbing Dry Hobbing+ low tool wear at moderate + high
12、er cutting speeds withcutting speeds due to “modern” tool systemslubrication effects ( HSS-PM, carbide, coatings )+ no minimal chip thickness - Pressure angle 10+ reduced cycle time by + prepared for innovations in part stacking substrates and coatings+ low part temperatures, o checking of warm part
13、s possibleeasy to check ( staggered tolerance field )+ no surface defects due to + better SPC-Quality due to bettercritcal chip flow effects energy balance ( more energyconveyed by chips and parts ) 2The cutting temperature of the dry hobbing processcan be influenced by cutting speed, feedrate andto
14、ol coating. This is the reason for different weardevelopments in identical cutting operations,figure 2. The differences in tool life between TiN-coated and (Ti,Al)N-coated tools in the wet processdepends only on coating hardness, without anyfurther effects.Cutting Speed m/min4030201050 100 150 200(T
15、i,Al)N, dryTiN, dryTool Life m/tooth0module = 2 mmtool dia. = 93 mmhcu= 0,18 mm16MnCr5SAE 86202= -20z2= 92(Ti,Al)N, wetTiN, wetFigure 2: Fly cutter test - Tool life of coated highspeed steel /1/In the dry hobbing process the cutting temperatureincreases because of the missing surroundingliquids. Thi
16、s is the main reason for a different wearmechanism with (Ti,Al)N-coated hobs. At the end ofthe cutting action the heated cutting edge comesinto contact with air. At this moment the aluminium-content of the coating reacts with the oxygen of theair. This oxydized aluminium (Al2O3) remains on thesurfac
17、e of the tool and gives a thin protective layerthat indicates a higher tool life.Requirements on the machine toolDepending on the workpiece tensile strength,material composition and the recommondedworkpiece quality, different substrates and coatingsare used parallel for gear hobbing. Up today there
18、isno solution existing, that combines the red hardnessof carbides with the ductility of high speed steel inone and the same substrate. The substrates of thepowdermetallurgical high speed steels have closedthe gap between carbides und conventional HSS,figure 3.The powdermetallurgical HSS can be descr
19、ibed asa combination of restricted red hardness togetherwith a sufficient ductility for the interrupted cut. PVD-coatings guaranty a red hardness on the hobsurface that copes with the enhanced requirementsof PM-HSS tools for dry cutting. Cuttingtemperatures in dry cutting are calculated for acutting
20、 speed of 200 m/min with approximately500C /2/. Compared with HSS-substrates allcarbide substrates render a higher hardness, butwith a significant lower toughness. Therefore, forinterrupted cuts like the hobbing process carbidesubstrates require a high stiffness of the cuttingsystem and a careful to
21、ol handling.The advantage of carbide tools are a low cuttingedge roundness for creating tiniest chips and thered hardness for high cutting speeds. In certainapplications it could be realized, that a carbide hobwas used with six times faster cutting speeds versusHSS.Figure 4 shows the influence of th
22、e tool substrateon the cutting forces and torques for a hobbingprocess for a constant machining time. With PM-HSS hobs the process is characterised by a higherfeed rate at a lower cutting speed. The result is ahigher chip thickness and indicated by this anincrease of the cutting force by approximate
23、ly 40 %and consequently the spindle torque is 60 % higher(with bigger HSS-hob diameter).Based on this fact today two different machinesystems are existing. If high spindle torques areneeded the stepped gear drive is the most flexiblesolution. The gear box between motor and hobspindle reduces the rev
24、olutions per minute andincreases the available spindle torque. Thismachine concept is for PM-HSS applications thestate of the art and widely used for prehobbingoperations before shaving, grinding, honing etc3Figure 3: Properties of cutting tools020406080100120140160180200Machining timeCutting speed
25、FeedrateChip thicknessCutting forceSpindle torquePercentPM-HSSCarbide- PM-HSS involves higher cutting forcesand spindle torquesFigure 4: Influence fo the substrate on the cuttingprocessFor applications where smallest OD-hobs arenecessary the high performance of stepped geardrive hob heads is not nee
26、ded. The required optimalcutting speed is realized by high rpm and due to thesmall outside diameter of the hob the necessaryspindle torque is lower. Here, as well as for highestcutting speeds needed for carbide hobs, theconcept of direct driven machine systems is the bestsolution. For example the fi
27、nish hobbing of gearsrequire highest cuttings speeds (up to 800 m/minand higher) for best surface qualities.Regarding the drive concept of the worktable theavailable direct driven motors have enough potentialin torque and rpm for the requiremnts of theworkpiece motion, figure 5.Features : Direct dri
28、ves for table-spindleand tool - spindle(optional reduction gear drive) Hob spindle speed max.: 9000 min-1 Table speed max.: 800 min-1 Cutting power : 10 - 22 kWLC 80 - pursuing a successful conceptFigure 5: Concept of the hobbing machine LC 80Tool design and process conditionsIn most of all manufact
29、uring applications themachining time is dominating the cycle time. Thenumber of starts has a major influence on themachining time. Figure 6 shows the machining timefor different numbers of teeth for single- and multi-HSSRed Hardness, AbrasionResistance(= Cutting Speed)Tensile Strength, Toughness(= F
30、eed)coatedCarbideIdeal Tool MaterialPCDCBNCeramicCermetCarbide coatedPM-HSSCoated Cermetcoated HSSHSS4start hobs. The maximum chip thickness (hcu max)and the biggest depth of feedmark scallops (x) islimited to respresent a realistic hobbing application.By using a single start hob, the manufacturing
31、timeis shortest with 35 teeth maximum.tool:mn= 2 mmda0= 80 mmni= 17gear data:b2= 302= 26,75technology:vc= 300 m/minhcu max 0,15 mmx 15 mno. of teeth z20,70,60,50,40,30,20,10machiningtimethmin0 10 20 30 40 50 60 70 80z0 = 3z0 = 2z0 = 1carbideFigure 6: Machining time for different no. of teeth fora ca
32、rbide hobIn the range up to 22 teeth the limitating factor is themaximum chip thickness. For the single start hoband a number of teeth more than 22 the feedratecan not be increased because the max. depth ofscallops is reached. The same fact occurs for thetwo start hob at a number of teeth more than
33、65. Fora three start hob this effect happens only beyond 80teeth.The hob outside diameter has an influence on theusable tooth length of the hob and on the scallopdepth on the wortkpiece. At the same time theapproach length is influenced by the hob outsidediameter. The approach lenth is significantly
34、influenced by the workpiece helix angle, the hoboutside diameter, the workpiece outside diameterand the cutting depth. The influence of all theseparameters on an optimal hob geometry issuperimposed by the maximum chip thickness andthe maximum scallop depth. Only if both items arerealized, the maximu
35、m chip thickness and themaximum scallop depth, the tool geometry isoptimized for the given hobbing application.Figure 7 shows for a constant usable tooth length of4 mm and a constant cutting depth the dependencyof number of teeth on manufacturing time fordifferent helix angles. If the maximum chip t
36、hicknessis the limiting factor a larger hob outside diameterfeatures a reduction of the machining time. If themaximum scallop depth is the limiting factor, adecrease of the hob outside diameter renders alower machining time. Significant for all examples isthat die highest helix angle indicates the h
37、ighestmanufacturing times.The efficiency of the hobbing process is influencedby the tool gemetry as well as in every othermanufacturing process. For a defined hob outsidediameter and toothed length, the productivity isinfluenced by the number of gashes. However, thepossible number of hob resharpenin
38、gs depends onnumber of gashes and hob diameter.For the following analysis is based on theparameters:single regrinding rate mm 0,25toothed length of hob mm 170max. usable tooth length mm 6min. usable tooth length mm 0,5max. no. of resharpenings 24machine costs per hour $/h 50Based on a carbide hobbin
39、g application theinfluence of the number of gashes on themanufacturing costs is shown in figure 8.With more gashes the machine costs per workpieceare decreasing because the z-axis-speed isincreasing. This means a decrease of totalmanufacturing costs up to 24 gashes on the tool.For tools with more ga
40、shes than 24 the tool costsper workpiece override the savings of shortermachining times.5Figure 7: Machining time depending on tool dia. andhelix angleFigure 8: Manufacturing costs depending on no. ofgashestool:n= 20mn= 2 mmusable tooth length = constgear data:z2= 30b2= 20 mmtechnology:vc= 300 m/min
41、T = 4,5 mmsense of lead60 65 70 75 80 85 900,40,30,20,10hcu max= 0,15 mm60 65 70 75 80 85 900,40,30,20,10x= 15 mmachining timeth mintool diameter da02= 012342= 102= 202= 3043124321machining timeth mintool diameter da0tool:n= 20mn= 2 mmda0= 70 mmhob costs: 2500 EURgear data:z2= 30b2= 20 mm2= 30techno
42、logy:vc= 300 m/minT = 4,5 mmhcu max= 0,15 mmsense of leadresharp. costs: 200 EUR/regrindingsingle-regrinding rate: 0,25 mm/regrinding0,70,60,50,40,30,20,10manufacturing costs Kf EUR/workpiece15 16 17 18 19 20 21 22 23 24 25 26 27no. of gashes nihob costsmachine costs6Work examplesAs described in fig
43、ure 2 the (Ti,Al)N-coating has thebest performance for dry hobbing with PM-HSSsubstrates. Figure 9 shows one example for a dryhobbing application, realized on a hobbing machinewith stepped gear drive hob head to achieve highesttorque performance on the tool spindle. The tool isused at a cutting spee
44、d of 170 m/min with afeedrate of 2,2 mm/tr. With this parameters a toollife of more than 4 m/tooth is reached. The hobbingoperation is before grinding, due to a multi-used toollayout the tool geometry has been optimized,regarding to the depth of scallops, for another, mostcritical part.Figure 9: Wor
45、k example for hobbing with PM-HSS,dry cutDue to a high workpiece quality the hobbingoperation is realized by one part in stack. Thequality results in lead waviness of approximately 15m is typical for a shaving operation after hobbing,figure 10. All deviations in profile are less than 10m and the run
46、out deviation of the hobbed gear is13,5 m.A further contemporary application for gear hobbingis the finish hobbing with carbide hobs. Figure 11shows a workpiece with two different gears. Bothgears (V1 and V2) are finished by milling andhobbing operations.For milling gear V2 a cermet profile cutter i
47、s used ata cutting speed of 1000 m/min. The operation isperformed on a direct driven machine concept. Thetool outside diameter of 50 mm requires highest rpmat a sufficient torque for this operation.Both tools are (Ti,Al)N-coated and used withoutcoolant. The gear V1 is hobbed with a carbide hobin two
48、 passes. In the first pass the tool is used at anoptimal chip thickness at a feedrate of 3.5 mm/tr.For a fine surface structure a second pass isnesessary and indicates a quality as shown infigure 12, by using a feedrate of 1 mm/tr at a cuttingspeed of 600 m/min in the conventional cut.The lead wavin
49、ess of gear V1 is 3.7m in the maximum and sufficeslike all other deviations in lead agear of quality class DIN 6. Theexample shows the limits of finishoperation in dry cutting. The profilevariation (left flank no. 6) is causedby chip welding on the flank andrequires for a safe process understatistical restrictions (SPC) ahobbing operation using coolant.In figure 13 lead and spacingdeviations are shown of gear V2.Lead waviness of 4.8 m in themaximum and run out error of4.4 m are sufficient for a finishing process. In theri
