1、08FTM16AGMA Technical PaperHob Tool LifeTechnology UpdateBy T.J. Maiuri, The GleasonWorksHob Tool Life Technology UpdateT.J. Buzz Maiuri, The Gleason WorksThe statements and opinions contained herein are those of the author and should not be construed as anofficial action or opinion of the American
2、Gear Manufacturers Association.AbstractThemethodofcuttingteethonacylindricalgearbythehobbingprocesshasbeeninexistencesincethelate1800s. Advances have been made over the years in both the machines and the cutting tools used in theprocess. This paper will examine hob tool life and the many variables t
3、hat affect it. The paper will cover thestateoftheartcuttingtoolmaterialsandcoatings,hobtooldesigncharacteristics,processspeedsandfeeds,hob shifting strategies, wear characteristics, etc. The paper will also discuss the use of a commondenominatormethodforevaluatinghobtoollifeintermsofmetersorinchespe
4、rhobtoothasanalternativetotool life expressed in parts per sharpening.Copyright 2008American Gear Manufacturers Association500 Montgomery Street, Suite 350Alexandria, Virginia, 22314October, 2008ISBN: 978-1-55589-946-23Hob Tool Life Technology UpdateT.J. Buzz Maiuri, The Gleason WorksIntroductionUpu
5、ntilthe19thcenturyalmostallgearswerehandmade and the gears were cut with form cuttersshaped to correspond to the spaces between theteeth. The first known gear cutting by machinewasdeveloped by Juanelo Torrianno (1501-1575). Itwas recorded that he was able to produce up tothreegearsperday onhis hand-
6、poweredmachine,using cutting tools that were nothing more thanrotary files.1 Much information about the historyof gears can be found in the late Darle W. Dudleysbook on “The Evolution of The Gear Art”. The bookwas sponsored by the AGMA and published in1969.Figure 1 is taken from the English inventor
7、 JosephWhitworths patent of 1835 which clearly shows ahob cutting a gear. Whitworth claimed in the patent“ theconstructionandarrangementofmechanismby which I give a continuous rotary motion to thewheel or disc under operation, which motion is soproportionedtothespeedoftherotarycutterthatbyevery rota
8、tion of the cutter a segment of the wheelor disc shall be advanced equal to the distance ofone tooth and space”. The machine shown can be“ bolted on to a work bench,or placedin anyotherconvenient situation”.2Itisalsointerestingtonotethatinthe1871patentofPhiladelphias Henry Belfield the cutting tool
9、is re-ferred to as a “hub”, not as a hob.2 George B.Grant was issued a patent for a spur gear hobbingmachine in 1889.The first machine capable of cutting both spur andhelical gears was invented by Robert HermannPfauter of Germany in 1897. Hermann Pfautersfirst prototype machine Figure 2 included a h
10、ori-zontal workspindle on vertical ways, a hob swivel,ahobcarriagefedalonghorizontalwaysonthebedofthe machine, and an upright foroutboard supportofthe work arbor. The hob feed was accomplishedmanuallywithacrankon theend ofa feedscrew.2Figure 1. Whitworths 1835 machineFigure 2. Robert Hermann Pfauter
11、s prototypeToday,mosthobbingmachinesarefullsixaxisCNCcontrolled machines, capable of very high cutterand work table speeds. Many machines utilize di-rect drive hob and work spindles which present aninterestingscenarioinwhichwenowproducegearswith machines that do not have gears in them!4The hobbing p
12、rocessIn brief, to paraphrase what Joseph Whitworth saidinhispatentof1835,hobbingisacontinuousindex-ing process in which the cutting tool and the work-piece rotate in a constant relationship to each otherwhile the hob tool is fed into the work. For generat-ing helical gears, the rotation of the work
13、 is eitherslightly retarded or slightly advanced in relation tothe rotation of the hob. As the hob is fed across theface of the work once, all the teeth in the work arecompletely formed. The hob can be fed axially, ra-dially, diagonally, or tangentially, depending uponthe application and the machine
14、 options available.The hobbing process can be visualized as a wormandwormwheelrunningtogetherthehobisrepre-sented by the worm and the workpieceby thewormwheel. The hob has a worm thread that has beenfluted to provide cutting edges with each tooth re-lievedtoformclearancebehindthecuttingedges. Itmust
15、 be made from a material suitable for cuttingthe work piece material.Cutting tool materials - high speed steel(HSS)Early cutting tool materials from the 1900s to1940s consisted of high speed steels designatedas 18-4-1, which consisted of 18% tungsten, 4%chromium and 1% vanadium.3 Today we havemany m
16、aterials to choose from. Table 1 lists highspeed steel materials in use today, their chemicalcomposition and Rockwell C hardness.4In the movement to cut gears without the use ofcoolant, carbide was initially selected as the hobtool material. Because of the expense of the car-bide, the manufacturing
17、costs, and the special han-dling required, high speed steel “bridge” materialswere identified and have replaced the carbide inmanyapplications. Itisinterestingtonotethatisnotthecaseinbevelgear drycutting production,wherecarbide remains the choice materialfor stickbladesusedinbevelcuttersystems.4 See
18、belowformoreinformation on carbide material.Figure3isamaterialselectionupgradeguidebasedon what the desired output is: improving the redhardness(thepropertyforretaininghardnessatele-vated temperatures) of the material, increasing thehobspeed,and/orincreasingthewearresistanceofthe material. Rex86, Re
19、x121 and M35V aregrouped together on the chart, and the choice ofmaterial within the group should be based on a costper piece analysis. Because of the high alloy con-tentofREX121thehobsharpeningtechniqueiscrit-icaltoavoiddamagingthesubstratematerial. Avail-ability of a specific material can be a fac
20、tor in yourchoice.Table 1. High speed steel materialsC Cr W Mo V Co HRCCPM M2 1.0 4.2 6.4 5.0 2.0 - 64ASP 2023 1.3 4.2 6.4 5.0 3.1 - 64CPM M4 1.4 4.3 5.8 4.5 3.6 - 64CPM REX 54 1.45 4.3 5.8 4.5 3.6 5.3 65CPM REX 45 1.3 4.1 6.3 5.0 3.1 8.3 66ASP 2030 1.3 4.0 5.0 6.5 3.0 8.0 66CPM T15 1.6 4.0 12.3 - 5
21、.0 5.0 66CPM REX 76 1.5 3.8 10.0 5.3 3.1 9.0 67CPM REX 86 2.0 4.0 10.0 5.0 5.0 9.0 68ASP 2060 2.3 4.0 6.5 7.0 6.5 9.0 68CPM REX 121 3.3 3.8 10.0 5.3 9.0 9.0 70M35V Conventional 1.2 4.1 6.0 5.0 3.0 5.0 665Figure 3. Material upgrade selectionCarbide gradesEven though HSS “bridge” materials have replac
22、edmanyearlyapplications,carbideisusedforapplica-tions such as steering pinions and armature shaftpinions Figure 4 generally small diameter finepitch applications. Carbide is also used in hard fin-ishing applications where gears are finish hobbedafter heat treatment.Basically, there are two classific
23、ations of carbidegrades: “P” and “K”. It is important to understandthat the grades refer to the recommended workingconditionsandnot theexact compositionof thema-terial.Cementedcarbidesarearangeofcompositemate-rialsthatconsistofhardcarbideparticlesbondedto-gether by a metallic binder. The proportion
24、of car-bidephaseisgenerallybetween70-90%ofthetotalweight of the composite.ISO “K” grades of carbide are a simple two-phasecomposition consisting of tungsten carbide (WC)and cobalt (Co). A typical composition of a “K”grade carbide is 90% WC and 10% Co by weight.“K” grades have good edge stability and
25、 abrasionresistance with a grain size range of 0.5 mm0.9 mm.ISO “P” grades of carbide are three-phase alloyedcompositions consisting of tungsten carbide (WC),cobalt (Co) and cubic carbides. The cubic carbidebinders can betitanium carbide(TiC), tantalumcar-bide (TaC) and niobium carbide (NbC). A typi
26、calcomposition of a “P” grade carbide is 73.5% WC,8.5%TiC,8%TaCand10%Cobyweight. Thecubiccarbidesaresofterandhavelargergrainsize- 2mmto 4 mm is normal for these alloy materials.Most carbide applications in use today are ISO “K”grade. Note that traditional titanium base coatingscannot be stripped fro
27、m ISO “P” grade carbides.SeeTable2.Figure 4. Carbide shank hobTable 2. Coating re-conditioning guidelinesGuideline TiNiteTiNCarboNiteTiCNAlNiteTiAIN-STiAIN-X AlCroNiteAlCrNStrippable from HSS Yes Yes Yes Yes YesStrippable from K-gradecarbideYes Yes Yes Yes YesStrippable from P-gradecarbideNo No No N
28、o YesRecoatable over itself Yes NotrecommendedYes Yes NoNumber of recoatings 3-7 - - 2-4 2-4 - -6The following figures show some ofthe relativema-terial properties of carbide and steel.The density of carbides Figure 5 is nearly twicethatofsteel. Thismeansthatacarbidehobwiththesame geometric characte
29、ristics as a high-speedsteel HSS hob is much heavier.Figure 5. DensityCarbide is also much harder Figure 6 than steel,and is not as tough Figure 7. Think of toughnessas the ability to resist fracture. This means that ifyou drop an HSS hob you may just put a “ding” on acouple of teeth, but if you dro
30、p a carbide hob it mayshatter into pieces. Because of these properties,you must take certain precautions with the carbidehobs that you normally would not take with the con-ventional HSS hobs.Figure 6. HardnessThe linear expansion of carbide Figure 8 is lessthanhalfthatofsteel. Thiscanbeasignificantc
31、har-acteristic due to the fact that if you are using a shelltype carbide hob with a steel hob arbor, the hob ar-bor will expand at a greater rate than the carbidehob, and you must account for this thermal expan-sion difference in the clearance between the hobbore and the steelarbor, otherwiseyou may
32、shatterthe hob.5Figure 7. ToughnessFigure 8. Linear expansionCermetTest cutting with cermet hobs have also been con-ducted. The word cermet is derived from the termsceramic and metal. A cermet is a hard materialbased on titanium carbide or titanium carbonitridecemented with a metal binder.6 Cermet m
33、aterialsallow for higher cutting speeds over HSS tools andeven carbide tools. At this point cutting withcermettools has not proven to be cost effective.CoatingsToolcoatingscametothemarketinthe1980s. Themost popular at the time was Titanium Nitride TiN.This coating served well for high speed steel ap
34、-plications used with a coolant, and is still in usetoday.Titanium aluminum nitride TiAlN was developed inthe mid eighties and gained popularity in the 1990s7as a coating used in cutting hard materials and highheat applications, and became a very popular coat-ing for dry cutting applications.Aluminu
35、m Chromium Nitride AlCrN coating wasintroduced in 2006 and is today the coating ofchoice for best results in dry hobbing applications.The coatings used in gear production today are pri-marily AlNite (Balzers Balinit FUTURA NANO),TiAlN-X (Balzers Balinit X.TREME), andAlCroNite(BalzersBalinitALCRONA).
36、 Theper-formance of AlNite and TiAlN-X are about thesame, although some customers prefer one overthe other, and TiAlN-X is used primarily on carbidesubstrate material tools. AlCroNite (BalzersBalinit ALCRONA) has shown advantages for anumber of applications over the other coatings.Trials with Balini
37、tX.CEED a high deposition tem-perature, high aluminum single layer coating, Bali-nitHardlubeaduplexcoatingconsistingofawearresistant TiAlN base layer and a high lubricity (lowfriction coefficient) WC/C top layer did not showany significant advantage over other coatings.Nanocomposite coatings such as
38、 nACo by Platitare available. The nACo coating comprises ofAlTiN nano sized particles imbedded in an amor-phous (non crystalline) matrix of silicon nitride(Si3N4), yielding a high oxidation resistance.7The following is a brief description of coatings usedin production today:1) TiNite (Balzers Balini
39、t A) - TiNiteis a TiNTitanium Nitride coating and is a general pur-pose coating for all wet oil or water soluble ap-plications. It is not recommended for drycuttingapplications.2) CarboNite (Balzers Balinit B) - Carobo-Nite is a TiCN titanium carbonitride coatingrecommended for wet cutting only on m
40、aterialsthat are abrasive in nature, such as cast iron orotherhardtomachinematerialsthatrequirehighabrasion resistance.3) AlNite (Balzers Balinit FUTURA NANO) -AlNite is a single layer TiAlN coating with anominal 50:50 ratio of Titanium to Aluminum. Ithas high thermal stability and can be used forcu
41、tting all steels, cast iron, stainless steel andmay be used wet or dry.4) TiAlN-X (Balzers Balinit X.TREME) X.TREME is a single layer coating of TiAlN. It isspecialized for carbide mills for hardened steelworkpieces(50HRC). Itmay alsobe usedwetor dry, and is a very popular coating today forbevel gea
42、r carbide stick blade applications.5) AlCroNite(BalzersBalinitALCRONA)AL-CRONA is a high performance, titanium-freecoating (AlCrN) of the G6 generation.8 It hasexemplary wear resistance under both conven-tional conditions and severe mechanicalstresses.Table38listssome ofthe propertiesof coatingsinus
43、e today.Table 3. Coating propertiesTiNiteTiNCarboNiteTiCNAlNiteTiAINAlNite- XTiAIN-XAlCroNiteAlCrNBalzersBALINITABalzersBALINITBBalzersBALINITFutura NanoBalzersBALINITX.TREMEBalzersBALINITALCRONAHardness (HV 0.05) 2300 3000 3300 3500 3200Coefficient of friction 0.4 0.4 0.30 - 0.35 0.4 0.35Max. servi
44、ce temp.600C1112F400C752F900C1652F800C1472F1100C2012FCoating color Gold Blue-grey Violet-grey Violet-grey Blue-greyCoating structure Monolayer Multilayer Nano Monolayer Monolayer8The pie chart in figure 9 gives an indication of howthe trend in coatings has changed over the last de-cade. The chart re
45、presents coatings applied to allcutting tools, including bevel stick blades. You cansee the increase in use of the TiAlN coatings, andnow the trend to the AlCrN coating.Tool reconditioning guidelinesOnce the hob tool is used, it must be sharpened toremove the wear. The sharpening process will re-mov
46、e the coating on the face of the tool. To obtainbetter tool life it is good practice to re-coat the toolafter sharpening. However, consideration must begiventowhetherthecoatingmustbestrippedoffthetool before re-coating, or if it can be coated over it-self. Stripping the coating from a hob consists o
47、f achemical process where the coating is removedwith a peroxide base solution.Table 2 offers guidelines for stripping and re-coating of various coatings in use today.Hob edge preparationTool life improvements can be made by preparingthe edge of the hob tooth.9 The process for treat-ingtheedgeonHSSma
48、terialsconsistsofremovingtheburrinadryblastwithanabrasivematerial. Thisprocess is followed by a wet blast operation to re-move any residual dry abrasive to enhance thesurface for the coating application.Treatingthecuttingedgeofcarbidetoolsconsistsofa honing process with a diamond brush. Generallyspe
49、aking, an edge radius of about 0.0004” to0.0008” 10 to 20 mm is desired.Wear basicsFigure10identifiesthebasic typesof hobwear. Tipand flank wear are normal and eventually the wearwill break through the coating and abrade the sub-strate material of the hob. Cratering on the face ofthehobcanalsooccurandthetoolcanfailifthecra-ter becomes too large and extends to the cuttingedgeofthetool. Seetheappendixforactualphotosof
