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Yogi Sharma, Philadelphia Corporation King of Prussia, PA

Introduction cutting in bevel . Initially, the pro- gear grinding, with almost all the opera- Some years back, most spiral bevel cess was limited to special requirements tions remaining the same, with the ex- gear sets were produced as cut, case because oflowcarbide tool life, the need ception of tooth grinding, which is re- hardened, and lapped. The case harden- for frequent sharpening, and limited ex- placed by hard cutting. ing process most frequently used was perience. But the picture changed dra- A simplified manufacturing process and is case carburizing, Many large matically after some time. The ex- sheet for a hard cut bevel gear will con- gears were flame hardened, nitrided, or perience gained and the development of tain the following: through hardened (hardness around 300 the CBN (Cubic Boron Nitride) tool • Complete of gear blanks BHN) using medium carbon alloy made bevel gear hard cutting very efiec- for teeth cutting - including various steels, such as 4140, to avoid higher tive from cost and quality viewpoints. operations, such as turning, , distortions related to the carhurizing As with any PIlOCe5S, hard cutting has drilling and tapping, etc. and hardening process. its limitations and problems. A properly • Bevel teeth cutting - consisting of The use of a quench press can con- controlled process, starting from the teeth cutting, testing, and any tooth trol, but not eliminate distortions .. A design concept, good bevel generators, contact development with master or operation cannot remove run- hard cutting tools, including sharpening mate, teeth deburring, etc. out, pitch error, profile error, and other fixtures, special machines, and trained • Heat treatment- mostly case car- errors caused by heat treatment dis- work force, are a must for successful burizing and hardening (nitriding, tortions. It can only improve active bevel hard cutting. flame hardening, and induction tooth profile finish and tooth contact This article describes the process and hardening are rarely used for hard cut location, provided that gears did not steps required for spiral bevel hard cut- bevel gears). have excessive errors during soft cutting ting on small batches or in a jobbing • Finish machining - all machining or high distortions in heat treatment. atmosphere. operations required before teeth hard As a matter of fact, overlapping some- cutting, such as turning O.D.ll.o.. times does more harm than good on a grinding, special machining, etc. bevel set, Process Description • Hard cutting - bevel gear arrives at Bevel tooth grinding was very limited Bevel hard cutting can be defined as hard cutting with most or all opera- due to cost and size. an operation in which gear teeth flanks tions done. It is important that the Advancement in bevel generators, are finished by removing the stock mating part or master is available for carbide technology, and many other allowance left during the soft or rough testing purposes. Normally, the gear factors allowed introduction of hard teeth cutting. The process is similar to (member with higher number of 36 Gear Tecnnology teeth) is finished first and pinion teeth practically unknown problems in hard delivery times and cost very effectively are modified to get correct tooth con- cutting . in spiral bevel gear boxes, tact along profile and length of tooth. •' Hard cutting is performed on the The modification for length of contact same 'type of machines as soft-cutting, is nonnally made by change of radius making the process much more Preparation of curvature of the cutting blades. The economical ..Of course, special tooling Following are some items which location of tooth contact along the is required. should be considered in detail for suc- length is usually controlled by • The same person or group ofper- cessful and economical bevel hard machine settings. The profile correc- sons are involved in soft and hard cut- cutting. tion or modification can be made by t:ing. It has been found that control in Practical Tooth Design. A balanced different means depending on the roughing operation (soft cutting inbevel tooth geometry is a must fora good type ·ofmachine or system used to cut gears) is quite important fora successful hard cut set. All new tooth geometry bevel gears, size of gears, and pitch. finishing operation. Proper stock allow- must be reviewed carefully from a On fine pitch gears, high pitch line ance, tooth depth, tooth contactretc. manufacturing point of view. In a job- hearing can be obtained by using cut- are necessary for bevel hard cutting. bing or low batch environment, the new ter blades wi th modified profiles ..On Too much stock. allowance can cause tooth geometry should try to use ex- coarse pitch gears, profile modifiea- loss of case depth and longer cutting isting tools, as new tool requirements tion 'can be made using taper shims. times, while too Iittle stock allowance can cause cost and delivery problems. Once the tooth contact requirements also can cause a variety of problems. Even in high batch production where are met, the gear teeth are finally Hard cutting time cycles can be reduced tools can be designed around gears, checked for spacing and mounted on by making some adiustrnents at soft poor tooth geometry can cause multiple a gear checker. cutting for heat treatment distortions, problems, such as low too] life, smaller • Final inspection - includes dimen- and it can be done very simply, as both fillet radius, etc. sional checks, magnaflux, and a-"y operations are performed by the same Gear Blank Design. Fig. 1 to Fig. 4 other specialrequirements. person or group of persons. show a bored pinion, a stem spiral bevel • Consistency in bevel hard cutting pinion, iii solid gear, and a ring type Ad.v.antages ,ofHard. Cutting can eliminate the need for matched gear. As shown in F.ig. 2, the bevel pin- • Higher power transmission by bevel sets by careful. planning. Some of ion can be indicated in both planes by spiral bevel gears, as both members are the requirements for elimination of means of an extra extension in front of case carburized and hardened and matched sets are as follows: the teeth. Both gears show prool bands finished by hard cutting. - Manufacturing and storing of case for indicating the gear blank at hard cut- • Higher and predictable quality hardened and hard cut master gearand ting. Special attention must be paid in levels in gear teeth. pinion for checking the gears and pi- blank design so that revalidated proof • A surface finish of 16 Rl\1S or nions in all future setups. surfaces at final machining can be used better. - Optimization of design and cutting for indication purposes at hard cutting. '. Lower noise level lower internal data so that it does not have to change In high batch production, blanks, tru- dynamic forces due to higher geometric for the period unmatched sets are ing, and use of proof surfaces are not re- accuracy, and better load distribution. required. quiredbecause of special customized A hard cut bevel set signifkantly - Tighter control of critical dimen- fixtures. Still, it is good practice to create reduces the gear box vibration problem sions on gear Ipinion blanks. Hard cut proof bands at final machi,ning for in- caused by higher intemal dynamic unmatched sets are not only useful. in spection or assembly purposes. In the forces due to poor quality gear teeth, assemblies, but they can eliminate which can cause premature gear box many production problems, as each AUfHOR: failure, member can be processed independent On the other hand the gear grinding of others. The unmatched set approach YOCI SHARMA is employed in gear operation is always very sensitive to must be used very selectively, as it needs manufacturing .and design at Philadelphia many factors, such as rate of material. careful planning, customized fixtures, Getlr Corp. He holds an M.S. degr:ee ,in removal, grinding wheel, coolant, 'etc. optimized design, cutting summaries, mechanical engineering from ViIlanova University and one in jndustrial.engineering Any compromise or loose control can and long term commitment. The un- from Perm State. Mr. 5hatmais a .licensed cause surface t.emperingor cracks or matched set approach combined with mechanical engineer .in tne state .of Penn- both ..Surface temperingor cracks are standardized bevel sets can cut down syivaniJ:! and' a-senior member of SME. November/December 1990 37 -

Fig. 2

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311 511

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:F!g.4 November/December 1,990 39' case of a bored pinion/gear, the Soft and Hard Cutting Summaries. A mounting distance problems. Ring subassembly with shaft should be done tooth cutting summary for a beveJ gears must be die quenched' utilizing wherever possible to avoid concentric- generator provides machine set up data, proper quenching dies. Fig. 7 shows a. ity and squareness problems (Fig. 5). cutter data, tooth measurements, and quench press. A pre-quench and temper Fig. 6 shows the subassembly of a ring other tooth geometry information operation of rough-turned blanks for gear and spider. In pinions, the one- related to a specific spiral bevel set. large gears and pinions can assist in piece design does offer some manufac- Bevel hard cutting blades are very sen- stabilizing the pieces during heat treat- turing advantages over the two-piece sitive to proper feed and speed. Selec- ment. In the jobbing atmosphere, gears design. Also, small to medium size gears tion of correct amount of crowning and pinions should be checked. for should be investigated for one-piece stock allowance affects the overall distortion belore releasing for final design, as there are some manufacturing qual.ity and cutting times. Normally, in machining. advantages in the one-piece high batch production, a sample set or Finish Machining Operation. The configuration. dummy set is always useful to fine tune finish machining of gears and pinions Previously, gears were mostly de- the summary ..In a jobbing or low batch for hard cut bevel. gears is very im.por- signed as bored rings so that they could atmosphere, a dummy set or experi- tant. The runout and squal'eness and ali be die quenched, but hard cutting has mental set mayor may not be possible other geometric tolerances of beari:ng changed the picture somewhat. Distor- for various reasons; therefore, a jobbing diameters, proof diameters, etc. must be tions in free quenching of small to atmosphere requires initial selection of kept to less than 40 % of the values medium size solid gears can.be kept low all variables affecting tooth contact. allowed on the gear teeth. Any loose with proper control. Hard cutting will The length and location of tooth contact control in finish machining operations remove the distortion from teeth. Cau- should be based on factors, such as can add to distortion, leading 'to longer tion must be used for any switch ove.r assembly conditions (overhung or cutting cycles and various problems. It from two-piece design to one-piece. straddle mounted members), tooth stif- is vita] that proof surfaces in both p1anes There are many factors which affect feners, and loading, application,and are indicated within specified values distortions in heat treatment, such as past experience. Wherever possible, fine before proceedingwi.th final machining. material blank configuration, rixturing tuning of tooth contact can be achieved The big difference in the fina1 machin- in heat treatment, etc. Vl/herever possi- through load testing of bevel gears. ing of a lapped and a hard cut gear is ble, some experimental pieces should be Heat Treatment Process ControL Ex- that, in the event of a lapped set, the manufactured to evaluate the situation cessive heattreatment distortions are back face or mounting face is usually before making the final decision, as ex- highly undesirable for hard cutting of left as it is, while in hard cutting, it is cessive distortion is highly undesirable bevel gears for a variety of reasons, such ground or turned square to bore. In ad- and can cause various problems at hard as loss of case thickness, longer cutting dition, it is critical 'that som extra cutting. times, tooth thickness problems, and material is left in the back of a hard cut 40 Gear lechnoloov .fbI' Y01ur tougllestg ~.._~rcutt.--ng Jobs,

,tile lIardest ,ste,el JS ' ,the easl ~'.-t clJQ -'ce • I•• CPMREX 76

For hobs, cutters and other gear cutting CPM REX 76 is just one of 10 high speer! steels tools. that are more than a cut above the rest, specify produced by the Crucible Particle Metallurgy pro- Grucible CPM REX~ 76. With 33% total alloy con- cess. With the industry's widest material selection, tent and an attainable hardness ot HiRC 68-70, this Crucible can meet your specific needs at any pro- hliglh speed steel provides. the highest available ductivity level. You can selectively upgrad:e to the co-mbination of red hardiness, wear resistance and best CPM material for the right eppllcaton. toughness, either coated or uncoated, On your next order, specify a high speed steel A 9ig- Three US. auto maker mcenUy realized a that's hard to beat ... CPM REX. 76 or another rnem- 300% -improvement in gear cutting tooll life by ber of the CPM REX family. To learn mare, contact switchingl from 1M3 HSS to CPM REX 716. With your nearest Grucible service center or call to'll free: gr,ealer too~lllifeand excellent grindability, CPM REX 76 means less downtime because resharpening is 11..800~PAR;"XC;EL easier and less frequent - (1~800-7.27-9235)

Ill/iii?!_____ 11 A Division of Crucible Matenals Corporation

CiReLli A-19 ON REAIDERIREPlY CARD gear at blank turning, which will then '[xPAIIDIr«; DIE be removed at final machining .. This Z tLAHP ING :DI[ will reduce the variations in mounting

L_ CONE distances .. In multiple pieces, consis- tency of the crown to back dimension is important for hard cutting cycles, Variation will cause adjustments for every piece on 'the machine, resulting in longer cutting cycles. Proper proof bands and surfaces must be created at final machining, with concern given to their location as well, so that they can be easitly used for indication purposes at hard cutting .. Soft Cutting Blades. Fig. 8 depicts a blade with protuberance.Where ever fig. 7 possible, protuberance blades should be used ..They will cut down hard cutting

__ -- StTTCUT time, extend too] life, and reduce the possibility of steps in fillet. The use of IlARDCUT BASIC I'IiESSUR£ ,ANGLE customized protuberance blades is no problem in high batch production, but TOOTH TIP injobbing or low batch production the picture is quite opposite. Amount and ,..__ ~C!i_ height of protuberance selection be- I DIAMETER ,I comes difficult as blades are stretched to I J cover maximum range. Blades can still I J be obtained with protuberance selected ,I I very carefully, which 'Will help hard cut- J , ting. Setting middle blades slightly deeper than side blades also helps in hard cutting and is a common practice

R[]lJT rlLLH in bevel cutting. Fig. 9 shows a fillet created witha deeper middle blade. Fig. Bg.B 10 shows a blade for soft cutting. Hard Cutting Blades. Bevel hard cut- ting began with the use of carbide inserts. Carbide inserts are damped to steel bodies for coarse pitches and brazed for medium to fine pitch gears. In the beginning, low tool life with car- bide inserts, frequent tool sharpening, insufficient experience, high cost. and many other Factorskept hard cutting's use quite low. As hard cut bevel gears reached into the field, their performance in all aspects was the single most reason to increase the use ofbevell hard cutting. ,", In the meantime, CBN inserts were rl NI SHED PROf ILE BY HAROCUTT I NGBLAOC introduced for hard cutting, The BZN (BZN is a trade mark of General Electric DEEPER RDOT PRODUCE D .BY HIOlli.t BL~DE Company) compact blank isa combin- Fig. 9 ation of a layer of Borozon CBN (Cubic 42 Gearlechnology 'Gear Tooll S;peela:lIsls 3601 WEST T'OUHY AVENUE UNC'OLNWOOD" 11'lILlNOIS 60645 1'08-6,75-2100 1-·800k628-2220 In IlL '·8'00'-628·2221 GEAR TRAINING PROGRAM

1. BASIC FUNDAMENTALS d) Schematic - Principles B. Rolling a) Speeds - Feeds 1. a..r HImIry 2. a..r F1rHehlngaft ... HMt: TfNl II Machine Cutting Conditions A. Honing A. Cyefotdal Teeth B. B. Involute Teeth B. Lapping a! The Mobbing Machina C. Grinding C. Gear Cutting Machines b Types and Manufacturers D. Gear Cutting Tools a) Methods - Formed Wheel· c Schematic - Differential and Generating - Threaded Wheel 2. a..rTypee Non-Differential b) Machine Typas A. Par811e1Axis dl Sl*KIs - Feeds B. Intersecting Axis a Climb CuI - Conventional CUi 5. GEAR INSPECTION C. Skew Axis Shifting - Types II 1. Functional 3. OHr Aattoa 3. The Hob ... Cutting Tool A. Gear Rollers 4. Involut. Gear o.om.try A. How It Cuts B. Gear Charters A. Nomenclatura B. Tolerances and Classes a) Reading the Chart B. Involutomelry - Contract Ratio, etc, C. Multiple Threads b) Tooth-to-Tooth Composite Error C. Helical Gearw - Lead - Helical Overlap D. Hob Sharpening and Control c) Total Composite Error E. The Effect of Hob and Mounting Errors Gear C. Master Gears 5. Tooth Bysteme on lhe Gear A. Full Depth B. Full Allet 4. The Shaper CUtt..... a Cutting Tool ~!=~~i" C. Stub Depth A. Know Your Shape! Cutters c Special Types I. a.n.r.I Formu_ B. Design Llmltallons 2.~1 C. Sharpening A. Size - Tooth Thlcknen 7. M8them8tIca - (I.T.W. Trig Book) D. The Effect of Cutter Mounting and B. Runoul Errors on the Gear C. SpacIng 2. HI SPEED STEELS E. Manufacturing Methods D. Lead A. Common Types 5. Tool Tolet'ance Va. GHr Toter.nc:e E. Involute B. Special Types A. Machining Tolerances 3. Automatic: and SamI-AutCMMtk: C. Heat Treatment - M8IaIlurgy B. Gear Blank Accuracy and Design A. How They Work D. Controls Umitations B. What Can Be Checked E. Sur1lK:e Treatments C. How Fast F. Special C_ ... FINISHING THE GEAR 4. ChIIr1I~ - An8IytIcIII and Functional 3. CUTTING THE GEAR 1, Gear F1nl8tMng - BatON Hardening A. Reading the Charta A. ShaVIng B. Which Errors Affect Other Elements 1. FonnIng a) The Shaving Cutter C. How to COI'rectthe Error the Chart Shows A. Milling b) Types of Shaving - Conventional. B. Underpass. Diagonal 6. INDIVIDUAL INSTRUCTION C. Shear Cutting c! Crown ShaVing AND SPECIFIC PROBLEMS 2. o....,.ung d Shaving Cutler Modifications A.She e Co-ordinatlng Tool Design- OR PLANT TOUR The Shaver and Pre-Shave Tool This Program is IleKlble and chenged 10 meet !he ~lCircu'!'~ype II Re-Sharpening needs of each group. c) Machine Types and Manufacturers g) Machines

ITW ILLINOIS TOOLS The 4-day program consIIts of a coordinated aeries of lectures given by the Engineering, A Division of IIIInoI8 Toot8 Wofka Inc. Production and lnapection ata1'l8 of Illinois Tool Works Inc. They represent ovar sixty years of 1991 GEAR TRAINING PROGRAM experience In gear and tOOlspecialization. Four Day Seminara The .... ion. are conducted with a technique and approach that leads to Interning January ''''7 May 13-1e ,e-li discussion and participation. Staning with the basic fundamentale, the program I. directed Februaly "." June 10-13 '417 into those areas of g.... test interest and value to the people attending the particular session. March . 11·" July 15-18 "·:4 Aprj "" 15-1e Augua1 12·15 The groups are kept .maIllO that this object is readily accomplished. 2·5 All mentioned, the planned program IU18 tour days. One-half of the fourth day I. for In- TUITION FEE: II1II8.00 dividual dlscunion of apeclflc problems In a detailed manner with members of the illinois NOTEI Additional siudenla, aame CCIInPMy, __ ct.. teeo.OO TOOlWorks' ataff. More than 4,000 Individual. from hundreds of companies repreeenting manufacturing, Includes ,he ,r8l18pO!1abon from !he hotei to ITW and bKk. one group dIM«. engineering, Inspection and management, have come to Chicago for these programs. They hoepI!aI1\y meeting. conllnantai br~. and aIIl~ haw been conducted on a monthly basis .Inea 1958. CI.... have also been conducted In Ths 5ocI8Iy 01 Manufaclurlflg Englneera ISM E.) has IIPP"\MId m,. School lor Europe. We are c.rtaln that this well rounded program has helped all of them to a better Job Pr01888iona1 Cred<18 10Ward !he S.M E. Recer\IIIca1oo Program Creda are ...... m.d and a110 given them a better underltandlng of engineering, manufacturing and Inspection. on !he baN 01 ont creel' per c:ontenl hOur For addII#oMI ~ aonIICt; All thole attending are aaaIgned to the same hotel. This promote. friendly contact and ROBERT H. MOOiAOW diacuaalon of mutual problems and Interesta. Tuition for the course Includes transportation Manager. TI'1Ii'*'II from the hot.' to ITW and back, one group dlnn.r, ali continental breakfasts and all lunche •. 708-675-2100 We hope we may Include your company in one of our Training Program •. l.aoo.e21-2Z!O CIRCLE A-20 ON READER REPlY CARD In IL CIlI: l.aoo.e21-2221 blades are manufactnred with a certain fillet radius, which should closely match with the roughing blade radius. During hard cuttiflg, the fillet comers can be slighHy skimmed with hard cutting blades, but any excessive fillet material removal compromises the tool life.

Summary The hard cut bevel gears have per- formed very well in all applications. Hard cutting has improved load carry- ing capacity and has made quality bevel. gears available Ito the industry at justifiable costs. The improvement in large gears is even more noticeable as tooth grinding

Fig. 10 - Courtesy of Klingelnberg Corp. was not possible because of size. The in- troduction of CBN tooling has brought Boron Nitride) and a cemented tungsten ter machine for hard cutting all the time. the toolingcest and cutting times to carbide substrate produced as anin- • Single Indexing Versus Continuous reasonable and economical values. Fur- tegral blank using an advanced high Indexing System ..Bevelgenerators with thermore, consistency of hard cutting pressure, high temperature process. The single indexing directly affect tooth has made unmatched sets a reality use of CBN inserts increased the tool life spacing; whereas, continuous indexing under a controlled atmosphere and for many times. Cutting times decrease aJF offers significant accuracy:in tooth spac- certain applications. Hard cutting also preciably as less sharpening isrequ.ired. ingdue to the natural hunting action provides. gear teeth with the quality and From a performance standpoint, 'the between gear teeth and cutter blades. surface finish of grinding without any CBN mserts excelled considerably over • Multi-Start Cutter System. In rut- possibility of any metallurgical damage . theecarbide inserts in every asped , even ting systems which utilize multi-start FinaJ]y, the utilization of CBN tools for though the price of the CBN insert is cutter heads, attention must be paid, so hard cutting is being applied more fre- .mUUI ':1;.. rugnerk;~k .t...hh