AN OVERVIEW OF GEAR MANUFACTURING PROCESSES ( ENGLISH VER.)

CHAPTER 1

AN OVERVIEW OF GEAR MANUFACTURING PROCESSES

Gear manufacturing has been one of the most complicated of the metal cutting processes. From the beginning of the century, the demand for better productivity of gear manufacturing equipment was posed by “The Machines that changed the World” i.e. AUTOMOBILES.

GEARS IN AUTOMOBILE TRANSMISSIONS

A gear box transmits the engine power to the driving wheels with the help of different gearing systems. Different gear combinations are used to give the smooth running, the lower fuel consumption, and the optimum driving comfort. Generally, passenger car transmissions are provided with 4-5 forward speeds and one reverse speed. In front wheel models, hypoid gears have been replaced by helical gears. Fig. 4.1 shows a typical transaxle of front drive model. Involute splines, both external and internal, are also widely used on various shafts and hubs for slide meshings in transmission system. Bevel gear and pinion are still used in differential of automobiles. However, parallel axes spur and helical gears are the main gears in automotive transmission. Manufacturing of gears presents a demanding challenge for metallurgists in heat treatment, for supervisors in machining and gear cutting, and for quality engineers in keeping the quality to the required standards.

Fig. 4.1 A Typical Transaxle of a Modern Passenger Car

Gear manufacturing process dynamics are undergoing a major breakthrough in last two decades. Solutions being sought are not corrective but preventative. Normally, either soft gear process dynamic or hard gear process dynamic is being aimed. Objective is to cut the number of operations or machines through which a work gear needs to pass to attain the final specifications of dimensions and tooth form quality.

In soft gear process dynamic, the gear teeth are generated by gear hobbing or shaping depending on the component design constraints. Soft finishing of gear teeth is carried out by gear shaving, rolling or grinding to attain the gear quality grade. Even after the heat treatment deterioration, the quality specification remains well within the desired final specification to meet product final performance requirements such as noise, etc.

In hard gear process dynamic, hobbed and/or shaped, or warm forged/rolled gears after heat treatment undergo final finishing operation, such as hard finishing, honing, or grinding. Overall economy becomes the deciding factor for selection of the process dynamic.

GEAR QUALITY AND MANUFACTURING PROCESSES

The functional necessity of a gear pair defines the limits of the deviations of all gear specifications. Gear quality refers to these permissible limits of deviations. Gear quality grades are standardised for different normal module/DP ranges and different ranges of reference diameters in AGMA, DIN, JIS and other standards. AGMA provides 8 grades from 15 to 8, where the higher grade number indicates the better gear accuracy. In DIN and JIS, a lower grade number means better gear accuracy.

Manufacturing processes used to produce finished gear specifications have certain capability limitations. Machine, work fixture, cutter, arbor, machined blanks, and also the cutting parameters add some amount of errors to different gear elements. Stages of manufacturing processes are to be accordingly decided. Fig. 4.2 gives a guideline for the capability of different manufacturing processes in terms of achievable quality grade requirements.

Fig. 4.2 Process Capability of Different Gear Manufacturing Processes

RAW MATERIALS FOR TRANSMISSION GEARS

Gears are generally designed for a finite life. Alloy steels are most favoured gear material. Case hardening steels provide the ideal features required for gear material. For gear teeth,

the surface is to be hard with soft and tough core to provide wear and fatigue resistance. Case hardening steels do have varying chemical composition, and are named accordingly, e.g. Chrome Steel, Low molybdenum steel, Chrome molybdenum steel, Nickel-chrome-molybdenum steel.

Basic requirements of good gear materials may be summarised as follows:

• Well controlled hardenability, that helps in getting consistent and predictable result after heat treatment. Hardenability is the property of a steel that determines the depth and distribution of the hardness induced by quenching.
• Least non-metallic inclusions especially oxides that generally present machining difficulties.
• Good formability for better forge die life and consistency of forge quality.
• High and consistent machinability.
• Low and stabilised quenching distortion.
• No grain growth during present practice of high temperature carburising, which can cause higher quenching distortion and lower toughness.

During recent years significant progress has been made in production of steels ideally required for gear. Gear steels are being developed to have totally controlled hardenability reducing distortion or making it accurately predictable and repeatable. With improved steel making processes, chemical compositions are being established to reduce inter-granular oxidation. Toughness and fatigue strength are getting improved dramatically. All these are through the improved steel manufacturing technology - especially the development of secondary refining (vacuum degassing and ladle refining applying arc heating) and related techniques.

BASIC FORMING PROCESSES

Hot forging is most commonly used for gears. Maximum and highly uniform density is ensured by complete filling of forging die. During forging or upsetting, material grain is made to flow at right angle to the direction of the stress on gear teeth in actual dynamic loading. Uniform grain flow also reduces distortion during heat treatment. Generally shaft gears are upset. Even roll forging is used for cluster gears for high productivity. Cold/warm formings are high production though capital intensive methods used presently to produce gear blanks with much better dimensional control and about 20% material saving. Parts are formed without flash or mismatch. Draft angles are held to 1/2 degree on long parts and concentricity upto about 1 mm.

A good forging is a necessity. With faulty forging, no amount of excellence of design and care in manufacturing of gears from the best available material can ensure production of good quality gears. Machinability, ultimate strength, final quenching distortion, and surface finish will all be affected by the forging practices.

New cold forging methods produce a neat finished gear profile combining forming with rolling. Differential gears of automotive transmission are being commercially produced with neat tooth forms. Even, the gear teeth of spiral bevel gears are reported to be formed by plastic deformation of induction heated bevel gear blank using tooth rolling tool. The process produces a very high tooth finish, and results in a lot of material saving. On a larger gear, depending on application, a finishing operation of hobbing or grinding may be necessary with a material stock removal of 0.4 mm-0.8 mm on tooth flanks. Cold rolling is already practiced for high speed production of splines and serrations with many built-in advantages.

GEAR BLANK MACHINING

Quality of gear manufacturing starts with blank machining. Accuracy in blank machining is a necessity for attaining the desired quality standard of finished gears. According to shape, the gears are called round gears and shaft gears.

For round gears, the dimensional and/or inter-related tolerances that must be closely controlled are as follows :

1. Size of the bore (inside diameter).
2. Out of roundness or straightness of bore.
3. Squareness of the bore axis with respect to face.
4. Parallelism of the two faces.
5. Outside diameter and runout with respect to bore.

Different defects in blank machining and their effects in subsequent gear manufacturing are:

1. Oversize bore results in poor clamping efficiency of the gear. Even a slight tendency to slip on the work holding arbour may cause lead error.

Geometrical error of the bore also results in poor work holding efficiency.

2. Error in perpendicularity of the bore axis with respect to the locating face, results in lead error and variation in lead.

3. Excessive parallelity error of work clamping face with respect to work locating surfaces, results in non uniform clamping and may twist the blank. In stack hobbing (when numbers of blanks are placed one over the other and are cut simultaneously), it causes lead error.

4. Excessive eccentricity of the outside diameter with respect to bore results in uneven cutting load and causes varying tooth depth around the periphery.

Round gear blanks are machined generally in two setups on many types of chucking lathes. Three-operation blank finishing ensures clean outside diameter. Two-operation finishing leaves a step on outside diameter. However, with accuracy of present work holding chucks, the amount is well within a limit that does not cause any trouble for ultimate performance.

For shaft gears, the axis of rotation is created by a face milling and centring operations on both the ends. The accuracy of the operation is important to maintain accuracy in the subsequent operations. Generally a protected type centre drill is used to avoid damage to the actual locating surface of the centre during handling. Shaft gears blank machining requires careful planning to achieve the concentricity between different locating surfaces and gear diameters. The tailstock pressure and the cutting forces may bend the shaft depending upon the length/diameter ratio, that may necessitate a judicious application of well-designed steady rest.

GEAR MANUFACTURING PROCESSES - IN GENERAL

Gear manufacturing processes can be grouped in two categories. Category one relates to teeth cutting, finishing and all necessary operations related to gear tooth profiles, such as hobbing, shaping, shaving, honing, etc. Category two relates to the rest of the conventional machining, such as, drilling, milling, grinding, etc.

GEAR CUTTING

Gear hobbing and shaping are the most commonly used cutting processes used for generating the gear teeth. Basis for selection of either of the two depends on application:

COMPARISON OF PROCESS CAPABILITIES OF HOBBING & SHAPING Features	Hobbing	Shaping
Accuracy Surface finish Versatility Limitation Production rate	Better with respect to tooth spacing and runout. Equal so far lead accuracy is required. Hobbing produces a series of radial flats based on feed rate of hob across the work. Can not be used for internal gears. Hob diameter determines the limitation of cutting gear with shoulder. For helical gears, only differential gearing is used which again can be eliminated in CNC hobbing Faster for gears with larger face width. Stacking can make hobbing faster than shaping even for gears with narrow face widths.	Better w.r.t. tooth form. Shaping produces a series of straight lines parallel to the axis of the gear. As the stroking rate can be varied independently of rotary feed, the numbers of enveloping cuts are essentially more than the same for a hobbed gear. Surface finish may be better. Can be used for internal gears Can cut upto shoulder with very little clearance. Each helix and hand requires a separate helical guide. No CNC system to replace helical guide is still developed. Time cycle will be 2-3 times of hobbing for wider gears. With high speed stroking, narrow width job can be finished in lesser time than by hobbing.

GEAR SHAPING

In the 'molding generating' process of gear shaping (Fig. 4.3), a gear of desired tooth profile with cutting capability can generate the similar tooth profile in a blank and produce a gear suitable for meshing with any gear of interchangeable series. The cutter with a particular number of teeth on its periphery rotates in the correct ratio required for generating the desired number of teeth on the rotating blank.

Fig. 4.3 Molding Generating Process of Gear Shaping

The rotations of the cutter and the workgear are in opposite direction for external gear and in same direction for internal gear. The cutter simultaneously reciprocates parallel to the tooth profile of the workgear. The distance between the cutter and workgear axes gradually reduces till the final size (pitch circle diameter) of the gear being generated is reached. Cutting may occur in downward stroke or in upward stroke. When cutting occurs in downward stroke, it is called down cutting or push shaping. When cutting occurs in upward stroke, it is termed as up cutting or pull shaping.

Machine Features

Over the years mechanical linkages have been gradually simplified reducing the effect of play and backlash in linkages. Machine structure has been improved to provide better static and dynamic rigidity and to dampen vibration originating from reciprocating cutter spindle and intermittent impact load at the start of each cutting stroke.

Cutter Head: An electro-mechanical or hydro-mechanical system provides the reciprocating motion to the cutter spindle. In most of the modern gear shaper, Fig. 4.4, a directly driven crankshaft links the cutter head with a connecting rod. In a hydro-mechanical system, Fig. 4.5, a servomotor drives the stroking linkage with provisions for adjustments for setting stroke length and appropriate quick return speeds. During the cutting stroke, the hydraulic pressure is directed to the large area of the spindle piston. The return force is applied on the small area of the spindle and thus accelerates the spindle to higher velocity during return stroke.

Fig. 4.4 A Second Generation Modern Gear Shaping Machine

The cutting force is concentric with the cutter spindle axis. High inertia of rotating or reciprocating parts is eliminated in hydromechanical system. In hydro-mechanical system of stroking, one cycle of the cutter spindle may use the cutting stroke at a lower velocity and the return stroke at a higher velocity. The cycle time is reduced. Hydromechanical system provides advantages for gears with wider faces (above 38 mm). However, the faster return stroke facilities are also available on machines with mechanical reciprocating system. In one system non-circular gears on the countershaft drive provide accelerated speed on the return stroke of the cutter spindle. In another system, this is accomplished by a special drive with a twin crank, Fig. 4.6. 

Cutter Head and Guide : An accurate index worm and worm gear drive unit with closely controlled backlash rotates the cutter head. A guide is essential to provide the compound motion of the cutter head. For a spur gear, a straight guide positively aligns the cutter, and

Fig. 4.5 Hydraulic-added Reciprocation

remains same for spur gears of different modules. For a helical gear, a helical guide, Fig. 4.7, imparts the twisting motion to the cutter as it reciprocates and rotates. For generating helical gears with different module and helix angle, generally different helical guides are required.

Fig. 4.6 Double Crank Stroking System System

Fig. 4.7 Principle of Helical Lead Guide  Fig. 4.8 A Hydrostatic Guide of Gear shaper

Almost all modern gear shapers are provided with hydrostatic bearings for guides, Fig. 4.8. A constant presence of a film of lubrication oil at about 60 bar pressure between the flanks is ensured. The possibility of wear is eliminated. The stroking rate can be increased even beyond 2000 strokes per minute. Even the extreme condition of high helix shaping does not present any trouble in operation. The diameter of the guide is kept large enough to withstand all torsional stresses generated in shaping. The hydrostatic bearing aligns the spindle and directs the cutter stiffly during cutting to attain an excellent tooth alignment tolerance. Pitch and base tangent tolerances of DIN class 6 are achievable on modern gear shapers.Almost all the modern gear shaping machines provide the back-off relief to the cutter spindle to avoid cutter interference. The cutter spindle backs off radially from the workpiece during the non-cutting return stroke.

Permissible radial and axial runout of the cutter mounting are within 0.005 mm (or better). With increasing trend for large diameter disc cutters, a lapped rear face of the cutter supports upto the extreme outside diameter to eliminate any possibility of deflection at very high cutting load. Automatic clamping system in the shaper spindle allows the use of preset shaper cutter. Fig. 4.9 shows a semi-automatic quick cutter clamping system for a modern gear shaping machine.

Work Table: The thrust load in shaping is taken on a wide peripheral area of the table. The ratio of the indexing worm wheel diameter to the maximum workgear diameter that can be shaped on the machine is kept high (above 1.5). High precision dual lead worms are used.

Fig. 4.9 A Semi-Automatic Quick Cutter Clamping System

Play on the flanks is kept constant by adjusting the backlash, when necessary. The accuracy of the worms and wheel drives of the indexing system is the deciding factor for the machining

accuracy of gear shaping. The work spindle or arbor used for work holding must run true within 5 micron or better.

Drive System Connecting Cutter Head to Work table: Index change gears maintain the required relationship of the rotation of cutter spindle and worktable that holds the workgear. In second generation machine, kinematics has undergone a major change mainly with fewer, short and torsionally stiff gear-trains. Separate motors of infinitely variable speed are used for reciprocation speed, rotary feed and radial feed. Desired combination of reciprocation rate with rotary and radial feed is possible to achieve better productivity and tool life. Ease of setting has been the main objective. Even on conventional machines, the replacement of change gears today is very easy in comparison with earlier models. For the same spindle size, the overall weight of the machine has increased by 2-3 times. The machines with more power and rigidity permit the very high cutting parameters that have greatly improved the productivity.