1、07FTM08Manufacturing Net ShapedCold Formed Gearsby: Dr. D.M. Engelmann, Milwaukee Wire ProductsTECHNICAL PAPERAmerican Gear Manufacturers AssociationManufacturing Net Shaped Cold Formed GearsDr. Dennis M. Engelmann, Milwaukee Wire ProductsThe statements and opinions contained herein are those of the
2、 author and should not be construed as anofficial action or opinion of the American Gear Manufacturers Association.AbstractAn innovate metal forming process has been developed for manufacturing quality, durable and costefficientgears for high volume production. In this paper, the development of Cold
3、 Formed Gears (CFG) will bepresented along with their suitable applications. The manufacturing technique and equipment will beintroduced as well as the advantages and limitations. Applicable materials and heat treatment practices willalso be discussed. Gear tooth inspection charts will be presented
4、and compared to conventionalmanufacturing methodologies.Copyright 2007American Gear Manufacturers Association500 Montgomery Street, Suite 350Alexandria, Virginia, 22314October, 2007ISBN: 978-1-55589-912-71Manufacturing Net Shaped Cold Formed GearsDr. Dennis M. Engelmann, Milwaukee Wire ProductsIntro
5、ductionA net shaped metal forming process has been de-veloped for manufacturing quality, durable, highyield and cost efficient gears for high volume pro-duction. In this paper, the development of NetShaped Cold Formed Gears(CFG) byusing theDi-videdFlowMethodwillbepresentedalongwiththeirsuitableappli
6、cations. Themanufacturingtechniqueand equipment will be introduced as well as the ad-vantages and limitations. Applicable materials andheat treatment practices will also be discussed.Gear tooth inspection charts will be presented andcompared to conventional manufacturing method-ologies.Background In
7、formationCold forming/cold forging is a bulk metal formingprocesswhereametallicworkpiece(blank)isplasti-cally deformed (pressed)into atool (diecavity) byapress. The workpiece and tooling are at room tem-perature,i.e.,theyarenotpre-heatedasinthecaseof hot forging.The advantages of using a cold formin
8、g process tomanufacture parts are: 1) there is less materialwaste since material is being displaced and not re-moved, 2) the parts can be made at very high pro-ductionrates,3)thepartsarestrongduetotheworkhardeningofthematerial,4)the partshave tighttol-erances, and 5) the parts have excellent surface
9、quality.Cold forming by the Divided Flow Method is a pro-cess where a workpiece is simultaneously pressedfromanupperpunchandalowerpunchallowingthematerial to flow radially outward into the die cavity(Figure 1). This process is typically used forforming rotational symmetric components.A Net Shaped or
10、 Near NetShaped formingprocessis a process where no or minimal secondary and/orfinishing operations are necessary. A Net Shapedgear is a gear where the teeth do not need to bemachined or finished.Figure 1. Cold forming by the divided flowmethod.DevelopmentTo begin the development of cold forming gea
11、rs,Finite Element Analysis (FEA) was used to deter-mine how much forming load is required, how thematerialwillflow,andhowmuchthetoolswilldeflect(Figure 2).Figure 2. Gear die stress distribution.2Since metal forming involves movement of largeamountsofmaterial,aplastic deformation(non-lin-ear) Finite
12、Element software program is required.Today there are over a dozen commercially avail-able FEA software packages capable of simulatingmetal forming processes, cold or hot.An advantage of using FEA simulation during thedevelopment phase is to be able to determine theoverallaffectsofparametersontheresu
13、lts,e.g.,theaffects of the punch geometry on the material flow,the affects of the material alloying and processingon the forming load, etc., (Figure 3). As a develop-mental tool, FEA simulation allows the user to testseveral “what if” scenarios without having tomanufacture expensive “hard” tools. FE
14、A simula-tion can also be used to help determine what typeand size of manufacturing equipment is required tocold form a family of gears.Figure 3. CFG strain distribution.TheFEAsimulationresultswereusedtoconductaninitial Feasibility Analysis to determine the processlimitations, the manufacturing equi
15、pment require-ments and to estimate if the process yields a com-petitive product.Processes and equipmentNetShapedColdFormingofgearsdoesnotrequireany additional manufacturing processes over thetraditionalgearmanufacturingprocess. Simplyput,the gear cutting process is replaced by a metalforming proces
16、s. Furthermore, conventional pre-and post-processing operations are applicable.Figure 4 illustrates atypical ColdFormed Gearpro-cess flow.Figure 4. CFG process flow chart.Gear materialsThe cold formability of steel is dependent primarilyon the amount of carbon in the steel. The lower theamount of ca
17、rbon content, e.g., 8620 vs. 8640, thesmaller the forming load and the less the tools willbestressed. Thealloycontent andthe rawmaterialprocessing also play a role in how the material willflow, e.g., 1040 vs. 4140. For most gear configura-tions, the common gear materials ranging from lowcarbon steel
18、s (1020) to medium carbon alloyedsteels (4340) can be formed. If the carbon contentisabove40 points,itisbettertoconsiderhotforgingwhich will improve the metal flow and reduce theforming loads.Gear blank manufacturingAs with many manufacturing processes, it is impor-tanttostartwithaninexpensiveblankw
19、hetheritisablank turned from bar stock or a blank cold formedfrom coiled wire. The optimal blank manufacturingprocessdependsonthesizeandshapeoftheblankrequired.If a gear blank is under 50 mm, then the blankscould be cold formed using coiled wire on a cold3header at speeds ranging from 60 to 180 part
20、s perminute. If a gear blank is larger than 50 mm, then itcouldbeturnedfrombar stockor insome caseshotformed.In order to cold form gears, it is generally better toanneal the gear blanks to remove the residualstresses and any “work hardening” affects fromprior processing of the material. Annealing so
21、ftensthe gear blanks and makes them more formablewhich reduces the forming load required to deformthe material.Since the gear blanks will be cold formed in a toolcavity, lubricants and coatings must be used in or-der to avoid galling. Forming oils and/or ZincPhos-phatecoatingsaretypicallyusedtocoat
22、theblanks.PVD (Physical Vapor Deposition) coatings such asTiCN (Titanium Carbo-Nitride) are used to coat thetools.Forming pressIn order to plastically deform the workpiece into thediecavity,aformingpressisrequired. FortheDivid-edFlowMethod,thepresscanbeeitheramechani-cal or hydraulic forming press d
23、epending on thecomplexity of the part and the accuracy required.The main advantage of a single action mechanicalpress is the high productivity rates. However, a hy-draulic press can have more than one action anddeliverdesiredformingloadsandspeedsatspecificpointsthroughoutthestrokeofeachactionwithpre
24、-cision.For the development and production of cold formedgears, a 1200 ton triple action CNC hydraulic pressis used (Figure 5).Gear toolingSince cold forming processes occur at room tem-perature, the forming loads required to plasticallydeform steels are very high. Hence, the stressesinthe tooling a
25、re also very high which can lead to pre-mature tool failure. Advancements in tool steelsand the development of more formable steels hasenabled cold forming processes to be applied tomore complex precise components that were notpossible earlier.Coldformgeartoolingconsistsofadieset,geardie,punches and
26、 die clamping rings (Figure 6). Sincethesetoolstouchtheworkpiece,theyare uniqueforeachgear. Thesetoolshavealimitedtoollifeduetothe high stresses (fatigue) and frictional contact(wear), and as such are considered to be“perishable” tooling.Figure 5. Triple action hydraulic press.The die set which hold
27、s the perishable tools is con-sidered to be durable tooling. Thedurable toolsarenot unique for each gear and have a much longertool life because they typically are not as highlystressed and do not experience as much wear.Standard tool steels such as Shock Resistant (S),Molybdenum (M) and Tungsten (T
28、) High Speed,Water (W), Oil (O) and Air (A) Hardening andHigh Carbon High Chromium (D) Tools Steels canbe used for cold forming gears.4Figure 6. Gear forming tools.Due to the high forming loads during cold forming,the gear tooling elastically deflects. When the tooldeflection is consistent from part
29、 to part, it is pos-sible to compensate for theelastic deflectionduringthemanufacturingofthe perishabletools. General-ly, a gear die should be manufactured a few qualitylevels better than the final product.Machining operationsAfter the gear teeth are cold formed, the gears aresaid to be in the “Gree
30、n” state because they havenotyetbeenhardened. Inthisstate,featuresonthegear that could not be cold formed, such as cham-fers, grooves,clutch facefeatures, etc.,can bema-chined using standard CNC equipment.Heat treatmentThe heat treatment of cold formed gears, as withconventional gear manufacturing p
31、rocesses, de-pends on the base material. Cold formed gears oflower carbon content can be Case Hardened, whilemedium carbon content gears can be ThroughHardened. Theuseof heattreat basketsaids inthehandlingofthegears andthe uniformityof thehard-ening results (Figure 7). Cold formed gears canalso be I
32、nduction Hardened inline.Figure 7. Gear heat treat baskets.Finishing operationsAfter hardening of the gears, the centerhole canbehoned, the faces or any bearing surfaces can begroundandforhighqualitygears,thegearteethcanbe ground, all using standard production equip-ment.Gear qualityThe gear quality
33、, like in all conventional gearmanufacturing methods, is dependent on each ofthe manufacturing processes tolerances and con-sistencies. Since a gear blankis formedin acloseddie, the volume (weight) of the gear blank is impor-tant. Ifthereisnotenoughmaterial,therewillbeun-derfillinareasofthegear. Ift
34、hereistoomuchmate-rial, the die cavity will be overfilled causingexcessivetoolstressesandeventuallybreakingthedie.Theformingpressismorethanjustabighammertosmash the gear blank. In fact, the more accuratelythepresscanbecontrolled,themoreaccuratethegears will be. The more consistently the press canapp
35、ly the forming load, the more consistent thegears will be from part-to-part.The gear die has the largest influence on the gearquality. Asstatedearlier,thegeardietoolingwillbemade a few quality classes better than the desiredgear quality. In addition, the elastic deformation ofthe tooling must be com
36、pensated for and incorpo-ratedinthetooldesign. Theselectionoftoolmateri-als, coatings, and manufacturing methodologiesarevery importantin makinga highquality geardie.5Machining, finishing and heat treatment affect acold formed gears quality as much as and in thesame ways as it does for a cut gear.De
37、pendingonthegear configuration,a NetShapedCold Formed Gear will typically yield an AGMAQ8-Q9 finished quality gear.Advantages of cold formed gearsThe advantages of manufacturing gears by coldforming are: 1) high production rates, 2) effectivematerial usage, 3) small tooth-to-tooth variation,and 4) h
38、igh strength and high durability.High production rateSincealloftheteethareformedsimultaneously,thetime it takes to fill the die cavity is only a few sec-onds. Cycle time varies based on the size andcon-figuration of the gear, but most gears can be coldformed at rates of 2-10 parts per minute.Effecti
39、ve material usageAn advantage to closed die forging process is thatmaterial is not being removed, but displaced. Thistypically results in a 5%-10% material savings justfrom forming the gear teeth as apposed to cuttingthe gear teeth.An additional material savings of 10%-30% is pos-sible by designing
40、a thinner web (Figure 8). Theforming punches can be designed so that a thinnersymmetrical web can be formed without “puttingchips on the ground”. Hence, a lighter blank can beused with shaped forming punches.Figure 8. Material savings.Small Tooth-to-Tooth VariationAs can be seen in Figure 9, a cold
41、formed gear hasless tooth-to-tooth variation than hobbing since allofthegearteethareformedsimultaneouslyinapre-cisegeardie. Thiscouldresultinasmoother,quiet-er and longer running gear.Figure 9. Comparison of tooth-to-tooth variation for a cold formed gear and a hobbed gear.6High Strength and High Du
42、rabilityColdformingsteelinduceslocalworkhardeningintothe material which will yield a stronger product. Ascan be seen in Figure 10, most of the work harden-ing occurs in the root. Durability tests have shownthatcoldformedgearscanalsowithstandhigherim-pact loads than Powder Metal gears.Figure 10. Stra
43、in hardening.Applications and limitationsTo be competitive with conventional gear manufac-turing processes, a cold formed gear must bemanufacturedsothatitcaneconomicallyreplaceanexisting gear. Some potential applications of coldformedgearsareintheautomotive,recreationalve-hicle, power hand tool, law
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45、apanese Patent No. 11010274, 1999.Keppler-Ott, T.; “Optimization of Lateral Extrusionof Helical Gears”, German Doctoral Dissertationfrom the Institut fr Umformtechnik, UniversittStuttgart, Germany, DGM Verlag, 2002.Sweeney,K.; “ColdForming ofHelical Gears”,Ger-man Doctoral Dissertation from RWTH Aac
46、hen,Germany, Shaker Verlag, 2000.Tekkaya, A.E.; “Report Subgroup Process Simu-lationInternationalColdForgingGroup“,ICFGDoc-ument No. 15/02, 2001 Workshop in Ankara,Turkey.Meidert, M.; Hnsel, M.: “Net shape cold forging toclose tolerances under QS 9000 aspects”, Journalof Materials Processing Technology, Volume 98,Is-sue 2, 29 January 2000, Pages 150-154.Kondo, K.; “Profitable Net Shape Forging of Auto-motive Components” (in German), Inter-nationalConference on New Developments in ForgingTechnology, Fellbach, Germany, 2005.
