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Home , Gear, Gear cutting, Gear shaper, Grinding machine, Hobbing, Shaper

Cutting and Grinding and Precision Cutting Developed for for Automobile Transmissions

MASAKAZU NABEKURA*1 MICHIAKI HASHITANI*1 YUKIHISA NISHIMURA*1 MASAKATSU FUJITA*1 YOSHIKOTO YANASE*1 MASANOBU MISAKI*1

It is a never-ending theme for motorcycle and automobile manufacturers, for whom the Division of Mitsubishi Heavy Industries, Ltd. (MHI) manufactures and delivers machines, gear grinding machines and precision cutting tools, to strive for high precision, low cost . This paper reports the recent trends in the automobile industry while describing how MHI has been dealing with their needs as a manufacturer of the machines and cutting tools for gear production.

process before heat treatment. A gear shaping machine, 1. Gear production process however, processes workpieces such as stepped gears and Figure 1 shows a cut-away example of an automobile internal gears that a gear machine is unable to transmission. Figure 2 is a schematic of the conven- process. Since they employ a generating process by a tional, general production processes for transmission specific number of cutting edges, several tens of microns gears. The diagram does not show processes such as of tool marks remain on the gear flanks, which in turn keyways and oil holes and press-fitting bushes causes and noise. To cope with this issue, a that are not directly relevant to gear processing. Nor- gear shaving process improves the gear flank roughness mally, a gear hobbing machine is responsible for the and finishes the gear tooth profile to a precision of mi- crons while anticipating how the heat treatment will strain the tooth profile and tooth trace. After heat treat- ment, it was usual only to finish the portion of the gear that accommodates a bearing.

2. Recent trends in gear production1 As described above, gear tooth finishing based on the predicted distortion due to heat treatment takes place conventionally before the heat treatment process. The recent stringent requirements for higher precision, how- ever, prompts processes such as gear grinding and honing to finish the gears to take place more frequently after heat treatment. Since sophisticated processing tech- niques such as gear shaving have not readily taken root Fig. 1 An automobile transmission overseas, gear grinding is more often the choice.

Gear Hard Material cutting turning Complete Heat treatment Gear Hard Material Turning Shaving Complete cutting turning

Fig. 2 Conventional production processes for transmission gears. Normally up to the gear hobbing process. Gears requiring high-precision call for the shaving process.

*1 Division Mitsubishi Heavy Industries, Ltd. Technical Review Vol. 43 No. 3 (Sep. 2006) 1 Gear Hard Gear Material Turning Complete cutting turning grinding

Gear Heat Hard Gear Gear Material Turning cutting treatment turning grinding honing Complete

Gear Gear Hard Gear Complete Material Turning cutting shaving turning honing

Fig. 3 Recent production processes for transmission gears Gears requiring high-precision call for finishing processes after heat treatment. Gears of conventional accuracy end at the shaving process.

Figure 3 shows recent gear production processes. The environmentally friendly but also reduces costs. To honing process improves gear flank roughness and cre- this end, MHI has developed the world's first dry gear ates a three-dimensional tooth profile. However, since cutting system.2 More specifically, the development finishing after heat treatment pushes up production costs, of cutters protected by Super Dry Coating as well as attempts to reduce the total cost are in progress. Since gear hobbing machines suitable for dry cutting has each gear in a transmission is separately responsible for realized dry gear hobbing. MHI's gear also vibration and noise, each calls for its own optimal finish- employs the dry cutting method. ing method regarding precision and cost. For example, a During hob cutting, the tip of the hob cutter reaches gear undergoing the grinding process calls for improved a high temperature which makes it wear quickly. Hob hobbing precision instead of eliminating the shaving pro- cutters protected with Super Dry Coating have im- cess. Another example is that the honing process calls proved thermal strength. Since this enables the cutting for shaving before heat treatment to save tool costs while speed to be increased from the conventional 100 to 120 stabilizing the pre-process precision and reducing the m-min-1 to as high as 200 m-min-1 while suppressing machining allowance. processes such as forg- tool tip wear to double the tool life, productivity has ing are also recent choices that increasingly employ a improved considerably. monoblock method to produce clutch teeth among Figure 4 compares the changes in the coat composi- others. The following summarizes the recent trends in tions between Super Dry Coating and the conventional transmission gear production. TiN coating. When exposed to high temperatures, the (1) Employing dry cutting TiN coating has its Ti oxidized and is transformed into .To improve productivity of gear cutting by high a brittle structure. Super Dry Coating, on the other speed cutting. hand, is considered to have its aluminum selectively .To reduce tool costs by making tool life longer. oxidized to form a strong, hard layer. .To reduce cutting oil costs by employing dry cutting. Recently, MHI has developed the Super Dry II Coat- (2) Skipping shaving process ing with its high temperature oxidization .To skip the shaving process if tooth grinding comes characteristics enhanced even further to realize a after heat treatment. cutting speed as high as 250 m-min-1. The Super Dry (3) Employing a monoblock forging process for clutch II Coating also demonstrates high anti-oxidization teeth characteristics at the high temperature of 1,200oC. .To use plastic processes (eliminating machine cut- ting processes). .To make gears compact. (4) Developing gear production overseas and operators 100 100 Al oxide being less skilled Ti oxide .To call for machine tools with ease of operation and O maintenance. 50 50 N Ti .To enhance and improve operator support software AI Ti

and pre-services. Composition (%) N Composition (%) O (5) Employing optimal processes for workpieces 0123456 0123456 .To provide various machine tools to satisfy each Depth from surface Depth from surface method of processing. TiN coat (thickness TiAlN coat (thickness 3. Precision cutting tools

(1) Coating for dry cutting Fig. 4 Comparison of coat compositions o Getting rid of coolant from workshops is not only 800 C, in atmosphere, maintained for 5 hours

Mitsubishi Heavy Industries, Ltd. Technical Review Vol. 43 No. 3 (Sep. 2006) 2 (2) Carbide hob cutters for finishing after heat treatment Finishing by a carbide hob cutters is gaining ac- 6 000 5 080 4 280 ceptance for small diameter gears such as steering 5 000 4 000 because they are difficult to process on a gear 2 440 3 000 after heat treatment due to the in- 2 000

terference of the grinding . MHI offers carbide re-sharpening 1 000 hob cutters for finishing that are a combination of

Quantity machined/cutter 0 ultra-fine particles, carbide alloy and a coating. Other make High-rigidity High-rigidity design of cutter design cutter cutter + super coat (3) Surface-treated shaving cutters The shaving process accounts for the most of the Fig. 5 Improving life of shaving cutters gear finishing process. Surface treatment raises the sures against noise. surface hardness of the shaving cutter to extend the (3) Improving ease of operation and maintenance tool life which in turn reduces the tool cost. Com- Improvement in operator support systems and mea- bined with the high rigidity design that takes the sures against operators being less skilled. elastic deformation of teeth into account, the tool life (4) Unification of line concept actually improves 1.2 to 2 times (Fig. 55). Establishing a unified, total concept from hobbing machines, gear , shaving machines, gear 4. E series gear cutting and grinding machines grinding machines to cylindrical grinding machines. To answer the needs of the automobile industry as (5) Compatible with flexible production described in Section 2 of this paper, MHI has integrated Organizing major units into packages and modules the gear cutting and grinding machines such as gear to make line configuration and re-configuration easy. hobbing, gear shaping and gear shaving machines into These developments have involved striving for cus- the E series along with the development of cutting tools. tomer satisfaction and implementation of CFT activities The newly developed ZE series of gear grinding machines comprising the marketing, design, assembly and service for the generating process are also available. The letter functions. The standard specifications incorporate past "E" of the E series stands for being ecological, economic complaints and defects, customers' requirements and and excellent. The following concepts are the basis of specifications in an organized manner. the developments. The development design thoroughly mobilized the 3D- (1) Global standard CAD system while a full design review for ease of Clearly defining the standard specifications com- operation and better workability took place in addition patible with development of production overseas. to FEM analysis. MHI also strives to develop servo tech- (2) Thorough measures for energy saving and the envi- nology that has resulted in the NC guide ST ronment series, synchronous shaving machines and synchronous Standardization of energy-saving circuits and mea- honing machines. Figure 6 lists MHI's lineup of gear

Gear production process

Gear shaping Chamfering Shaving Cylindrical Gear Gear Broach Hobbing Heat machine machine machine machine treatment grinding machine grinding machine honing machine

Full dry cut Production cylindrical Generating gear Synchronous gear grinding machine grinding machine honing machine GE15A SE25A ME20 FE30A PD32 ZE15A ZS25A GE20A GT06R (Hobbing + Synchronous gear PD23 ZE24A GE25A Chamfering + ) shaving machine General-purpose Helical guide-less GD10 FS30A cylindrical grinding Form gear SD15 gear shaping machine grinding machine machine Machine GD20 tools SD25 ST25 RD23 ZG400 GD30 SC40 ST40 RD32 ZG1000 GD50

GB63 SC63 RC45 Shaving cutter grinding machine GB100 SC100 ZA30A GBH18

Tools Shaving broach cutter

Fig. 6 Lineup of MHI's gear processing machines and gear cutters

Mitsubishi Heavy Industries, Ltd. Technical Review Vol. 43 No. 3 (Sep. 2006) 3 processing machines. By manufacturing and marketing integration type shown in Fig. 7 that can later hook gear processing machines such as gear hobbing, shaping up with the modular-designed gantry loader with a and shaving machines, various gear grinding machines, narrowed width. cylindrical grinding machines and cutting tools and even This machine is capable of machining a workpiece commercially producing actual gears in quantity, MHI as shown in Fig. 8 at a cutting speed of 250 m-min-1 is a total gear processing system manufacturer unparal- for a cycle time of 14.3 seconds, reducing the cost by leled in the world. approximately 45%. Figure 9 shows the benefits of 4.1 GE series gear hobbing machines compatible with introducing this machine. If equipped with a coolant dry cutting unit, the machine becomes capable of conventional wet To be compatible with dry cutting, the E series gear cutting. hobbing machines incorporate the following features. 4.2 Dry cutting gear shaper SE25A (1) Discharging chips: A cover provided in the machine Figure 10 shows the newly developed E series gear prevents chips from scattering. A steeply sloped cover shaper model SE25A. With the structure around the made of smoothly discharges chips out redesigned for high-speed dry cutting, the ma- of the machine. chine improves the maximum spindle stroke speed from (2) High-speed operation: Adopting a spindle and table 1,500 min-1 to 1,800 min-1 and the cutting speed from 90 spindle structure compatible with a cutting speed of m-min-1 to 130 m-min-1. To suppress vibration during 250 m-min-1. high-speed cutting, which is an issue with the gear (3) Heavy cutting: Improving the machine rigidity and shaper because of the reciprocating motion of the cutter, employing large-capacity spindle motors. the SE25A has a new model balancer shaft. The arrange- The GE15A is available for choice in either the ment successfully suppresses the vibration during standard type with a conventional layout or the line- high-speed cutting to 1/6 or less of conventional models.

Workpiece specifications Module 1.4 Number of teeth 34 Gantry loader Face width 25 mm Pressure angle 17.5o Control box o 22.5 RH installed on Tool specifications left side of Type Solid hob machine lessens Outside diameter 58 mm machine width. Starts 4 Flute 16 Coating Machining parameters Machining method Climb cut Hob 1 370 min-1 Gantry loader Cutting speed 250 m/min Control box can be installed later. Axial feed rate 2.0 mm/rev Cycle time 14.3 s Before machining After machining Fig. 7 General view of GE15A Fig. 8 Actual processing data (compatible for line integration)

Electricity 45% reduction cost 1 1 0.06 cost: 0.9 0.09 40 % reduction Coolant cost: 0.8 Coolant cost 100 % reduction Electricity cost: 0.7 33 % reduction 0.6 0.55 0.5 Electricity 0.4 Cutting cost 0.04 tool cost Coolant 0.3 0.85 Cutting cost 0 tool cost 0.2 0.51

0.1

Index ration compared with conventional machine 0 Fig. 10 General view of gear shaper Conventional machine GE hobbing machine model SE25

Fig. 9 Case example: Benefits of introducing dry cutting

Mitsubishi Heavy Industries, Ltd. Technical Review Vol. 43 No. 3 (Sep. 2006) 4 Stroke width 21 mm, module 2.5, diameter 75 mm, Stroke speed 760/1 200 min-1, cutting speed 50/80 m-min-1 Cycle time 49 seconds (conventional machine 73 seconds, production efficiency improvement 1.5 times)

Tooth profile error Tooth lead error JIS class N6 JIS class N4 500X 4.0X 500X 4.0X ROOT TIP ROOT TIP BOTTOM TIP BOTTOM TIP FF FFA FF FFA FQ FH FQ FH FA FA FQA FQA R1 4 L1 3 4 0 R1 4 4 L1 2 -3 0 7 1 2 3 R2 3 L2 0 R2 4 L2 2 4 2 6 4 2 3 1 R3 4 L3 3 3 10 9 R3 L3 3 1 4 0 4 2 2 R4 4 L4 3 4 4 10 8 R4 L4 2 -1 4 4 2 2 MEAN 2 MEAN 3 3 0 7 MEAN 4 MEAN 0 1 4 2 1 1 BIAS BIAS 5 4 BIAS 11BIAS 2 4 FF: F total profile FA: FH profile FFA: FF profile form FQ: F total helix FH: FH helix slope FQA: FF helix form deviation slope deviation deviation deviation deviation deviation

Fig. 11 Example of machining by SE25A

OBD diviation in continuously Workpiece specifications Machining parameters dry machining 40 Stroke speed 2.5 600/1 000 30 Module (min-1) Cp=3.13 20 Cutting speed Number of teeth 2.8 40/66 (m-min-1) 10 0 Diameter (mm) 75 Circumferential feed 2.618/1.1122 (mm/str) -10 -20 Radial feed Face width (mm) 15 0.009/0.01 (mm/str) -30 -40 Helix angle Spur gear Cycle time 90s 0 5 10 15 20 25 30 35 40 Quantity

Fig. 12 OBD variation at continuously cutting Figures 11 and 12 show case examples of machining by SE25A. Restricting vibration results in stable, favor- able machining accuracy. The thermal deformation that could be a problem with dry cutting remains low while the over ball diameter during continuous machining is also stable. As with the GE15A, the SE25A is also ca- pable of both dry and wet cutting. 4.3 Gear shaving machine FE30A Figure 13 shows the newly developed model FE30A. Reviewing the column and saddle structures using FEM analysis improved their rigidity. The operation panel has a large color graphics display to ensure easier op- eration and maintenance. The feature for monitoring Fig. 13 General view of FE30A the spindle load currents helps to establish the machin- (1) Use of threaded, general grinding and the shift ing parameters. In addition, data entry on the NC grinding method realizes stable, high precision grind- screens alone allows tapers and the crowning amount to ing and reduces the tool cost. be corrected. Figures 14 and 15 illustrate the principle (2) Use of built-in motors on the grinding and table and actual data samples, respectively. spindles realizes high-speed, high-precision grinding. 4.4 Gear grinding machines (3) With the dresser mounted on the ring loader, dress- Grinding gears after heat treatment to reduce vibration ing while being clamped by the Hirth realizes and noise from transmission gears and improve final accu- high precision dressing on the machine. racy is drawing attention. As the E series gear grinding (4) Use of the standard 8-axis NC system and various machines, MHI has developed models ZE15A/24A3 to meet machining support software offers ease of operation these needs. These machines have the following features. and maintenance.

Mitsubishi Heavy Industries, Ltd. Technical Review Vol. 43 No. 3 (Sep. 2006) 5 Face width Face width b K K c Cutter K Cutter face d a d' b'

Workpiece d c b Workpiece Distance width d' a between b' centers Face width Taper Z c' Face width a' Conventional crowning method

Center of oscillation Guide rail Allowable table feed length With table at extreme left end With table at center With table at extreme right end

Fig. 14 Taper/crowning principle

Machining example A simple data entry on the operation display can revise crowning and taper/crowning. Zero setting Tooth trace correction Crowning Taper/crowning Data Amount of crowning 0.0 0.0 0.03 0.03 entry Axis A compensation 0.0 0.02 0.0 0.0 (mm) Amount of taper 0.0 0.0 0.0 0.02 Tooth trace accuracy

Fig. 15 Revised of taper and crowning

4.5 High-speed, high-precision synchronization Placing emphasis on developing a high-speed, high- precision synchronous system, MHI has been the world leader in developing the NC guide gear shapers and syn- chronous shaving machines. (1) NC guide gear shaper When machining a helical gear, a gear shaper uses a device called a guide to make the cutter have a spi- ral motion along the tooth trace. Since a dedicated guide that matches the lead of each cutter is neces- sary, a skilled operator has to change the guides when using cutters with different leads. MHI's ST series gear shapers numerically control the spiral motion. Fig. 16 Gear grinding machine model ZE15A Figure 19 illustrates the guide structure. While an operator conventionally has to fine tune the These machines are capable of machining a two- dresser to correct the pressure angle of a gear tooth, these step gear with different helix angles by a single machines, as shown in Fig. 1818, only require data entry chucking as shown in Fig. 2020. on the NC screens to easily correct the gear tooth pres- (2) Synchronous gear shaving machine sure angle, which in turn contributes to a considerable In general, a gear shaving machine drives the cut- improvement in workability and stable gear quality. ter while the workpiece trails it. With no control Figures 16 and 17 show the general machine view and exercised over the workpiece in its direction of rota- an example of grinding parameters, respectively. tion, conventional shaving machines are unable to

Mitsubishi Heavy Industries, Ltd. Technical Review Vol. 43 No. 3 (Sep. 2006) 6 specifications Gear specifications Outside diameter 300 mm Module 3, number of teeth 31, pressure angle 20o, Number of threads 3 helix angle 20o (RH), outside diameter 105 mm, face witdth 40 mm Overall length 125 mm

Dressing parameters Grinding parameters Grinding wheel revolution speed 70 min-1 Grinding wheel revolution speed 2 580/3 800 min-1, Cycle time: Dresser revolution speed 3 260 min-1 axial feed 0.8/0.4 mm/rev, 78 seconds Dressing time: 3 minutes 43 seconds radial depth of cut 0.25/0.8 mm, climb/conventional

Tooth profile error JIS class N1Tooth lead error JIS class N1 500X 4.0X 500X 1.0X ROOTTIP ROOT TIP BOTTOM TOP BOTTOM TOP MAX MIN MAX MIN MAX MIN MAX MIN FQ FH FQ FH FQ FH FQ FH FQA FQA FQA FQA 5 5 2 0 0 1 0 -2 0 1 R1 2 L1 -1 R1 L1 5 2 -2 1 1 5 5 6 5 0 0 0 -1 5 0 -2 0 1 R2 1 1 R2 L2 2 -1 L2 2 1 5 5 6 5 1 0 5 0 -1 5 0 2 R3 2 0 -1 1 R3 L3 2 -1 L3 1 1 5 5 6 5 2 0 4 0 0 5 0 2 R4 2 1 -1 1 R4 L4 2 -1 L4 1 1 5 5 6 5 1 0 5 0 2 1 0 1 5 0 -1 MEAN 1 MEAN 2 -1 MEAN 1 1 MEAN 5 5 6 5 BIAS 0 0 0 2 BIAS 2 BIAS 1 BIAS

Fig. 17 Example of gear grinding by ZE15A

Correction right + 3/left +1 Standard (right -2.7/left -0.9 m) Correction right -10/left -10

Right 0.5 Right -14 0.5 -2.7 -14.0 Aiming at Aiming at -2.7+3=0.3 -2.7-10=-12.7

0.5 Left 0.5-0.9 Left -12.3 -12.3

Aiming at Aiming at -0.9+1=0.1 -0.9-10=-10.9

Fig. 18 Pressure angle correction

Conventional ST25CNC machine Spindle motor Spindle motor Crank

Helical guide (male)

Helical guide (female)

Spindle Servomotor for helix angle control cutter

Fig. 19 Illustrated explanation of guide structure

Mitsubishi Heavy Industries, Ltd. Technical Review Vol. 43 No. 3 (Sep. 2006) 7 (1) Accuracy distribution after hobbing 10 : L cumulative pitch 8 : R cumulative pitch 6 : Tooth run-out 4

2 0 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

(2) Accuracy after conventional gear shaving with workpiece trailing 12 10 8 6 Fig.20 Machining two-step gears 4 2 0 improve the pitch accuracy. This has moved MHI to 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 develop the synchronous gear shaving machine model Almost no pitch accuracy improvement. FS30A.4 Figure 21 shows how much this machine improves the pitch accuracy. While the synchronous (3) Accuracy after synchronous gear shaving shaving finishes the teeth with the workpiece rotat- 14 ing in one direction only, the conventional machines 12 have to change the cutter between the for- 10 ward and reverse directions. Eliminating the 8 switching between the two directions contributes to 6 reducing the cycle time. In addition, since the syn- 4 2 chronous control maintains the correct positional 0 relationships between the cutter and the workpiece, 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 the cutter can copy its tooth profile correctly on the Pitch accuracy improves. workpiece. It has been reported that this makes the tool life control easy and has a favorable effect on ex- Fig. 21 Improvement in pitch accuracy by synchronous gear tending the tool life. shaving machine

5. Conclusions As a comprehensive manufacturer of gear processing systems producing both machine tools and cutting tools, MHI has developed the dry cutting system, E series machine gear grinding machines, among others, while anticipating the future trends of the automobile indus- try. While continuing to develop product systems that anticipate the need of customers, MHI is also willing to Masakazu Nabekura Michiaki Hashitani Yukihisa Nishimura offer the comprehensive services including remote moni- toring of production lines, implementation of pre-services and enhancing production support systems.

References (1) Hashitani, Dry Cutting System and Gear Finishing Process, Japan Society for Special Symposium Masakatsu Fujita Yoshikoto Yanase Masanobu Misaki 2004.1.19 (2) Kage et al., Dry Hobbing of Gear, Japan Society of Mechani- cal Engineers Mechanical Engineering Congress Review 2003 VIII p.177 (3) Hashitani et al., ZE15A Gear Grinding Machine for economi- cal hard gear finishing of quiet, small gears, Mitsubishi Heavy Industries Technical Review Vol.42 No.2 (2005) (4) Nabekura et al., The Synchronous Gear Shaving Machine Aimed at Low-Noise Gear, Mitsubishi Juko Giho Vol. 39 No.4 (2002)

Mitsubishi Heavy Industries, Ltd. Technical Review Vol. 43 No. 3 (Sep. 2006) 8