ME 2252 MANUFACTURING TECHNOLOGY – II

ABRASIVE PROCESSES AND GEAR CUTTING

By

Dr. V.S.SENTHIL KUMAR,

Associate Professor

Department of Mechanical Engineering

CEG Campus, Anna University,

Chennai - 25

UNIT IV ABRASIVE PROCESSES AND GEAR CUTTING  

• Abrasive  processes:  grinding  wheel  – specifications  and  selection,  types  of grinding  process  –  cylindrical  grinding, surface  grinding,  centreless  grinding  – honing,  lapping,  super  finishing, polishing  and  buffing,  abrasive  jet machining  -  Gear  cutting, forming,generation, shaping, hobbing.

GEAR MANUFACTURING -INTRODUCTION

•Gear manufacturing can be divided into two categories namely forming and machining as shown in flow chart in Fig 1.•Forming consists of direct casting, molding, drawing, or extrusion of tooth forms in molten, powdered, or heat softened materials and machining involves roughing and finishing operations.

Fig 1. Categories of gear manufacturing process

FORMING GEAR TEETH•In all tooth-forming operations, the teeth on the gear are formed all at once from a mold or die into which the tooth shapes have been machined. •Accuracy of the teeth is entirely dependent on the quality of the die or mold. •Most of these methods have high tooling costs making them suitable only for high production quantities.

1. Casting Sand casting, die casting and investment casting are the casting processes that are best suited for gears and are shown in fig 2.

Fig 2. Casting processes

1a. Sand Casting

Characteristics:

Cheaper, low quality gear in small numbers

 

The tooling costs are reasonable. Poor Surface finish and dimensional accuracy

 

Due to low precision and high backlash, they are noisy.

They are suited for non- critical applications

Applications: (without finishing operation)

Toys, small appliances, cement-mixer barrels, hoist gearbox of dam gate lifting mechanism, hand operated crane etc.,

Materials:

C I, cast steel, bronzes, brass and ceramics.

1b. Die casting Characteristics:

Better surface finish and accuracy (tooth spacing and concentricity) High tooling costs

Suited for large scale production Applications.

 Applications:

Used in instruments, cameras, business machines, washing machines, gear pumps, small speed reducers, and lawn movers.

 Materials: Zinc, aluminium and brass..

1c. Investment casting

Characteristics:

 Reasonably accurate gears

 Applicable for a variety of materials

 Refractory mould material

 Allows high melt-temperature materials

Accuracy depends on the original master pattern used for the mold.

Materials: Tool steel, nitriding steel, monel, beryllium copper.

2. Sintering or P/M process The powder metallurgy technique used for gear manufacture is shown in fig 3.

 

Characteristics:

 Accuracy similar to die-cast gears

Material properties can be Tailor made

Typically suited for small sized gears

Economical for large lot size only

Fig 3. Process chart for P/M gear manufacture

The components manufactured by P/M technique are shown in Fig 4.

Fig. 4. Components manufactured by sintering

3. Injection Molding•Injection molding is used to make nonmetallic gears in various thermoplastics such as nylon and acetal. •These are low precision gears in small sizes but have the advantages of low cost and the ability to be run without lubricant at light loads.

 

Injection Molding Applications:

Used in cameras, projectors, wind shield wipers, speedometer, lawn sprinklers, washing machine.

Materials:

Nylon, cellulose acetate, polystyrene, polyimide, phenolics.

4. Extruding •Extruding is used to form teeth on long rods, which are then cut into usable lengths and machined for bores and keyways etc. •Nonferrous materials such as aluminum and copper alloys are commonly extruded rather than steels. •This result in good surface finishes with clean edges and pore free dense structure with higher strength.

Extruding Materials:

Aluminum, copper, naval brass, architectural bronze and phosphor bronze.

Applications:

Splined hollow & solid shafts, sector gears are extruded

5. Cold Drawing

•Cold drawing forms teeth on steel rods by drawing them through hardened dies. •The cold working increases strength and reduces ductility. •The rods are then cut into usable lengths and machined for bores and keyways, etc.

6. Stamping •Sheet metal can be stamped with tooth shapes to form low precision gears at low cost in high quantities. •The surface finish and accuracy of these gears are poor.  

Applications:

Toy gears, hand operated machine gears for slow speed mechanism.

6 a. Precision stampingIn precision stamping, the dies are made of higher precision with close tolerances wherein the stamped gears will not have burrs.

Applications:

Clock gears, watch gears etc.

 7. Preforming For close die forging the feed stock has to be very near to the net shape and this is obtained by performing.

8. ForgingThe steps in forging process are represented in fig 5.

Fig 5. Procedure for forging of gears

Machining This is most widely used gear manufacturing method.

Gear blank of accurate size and shape is first prepared by cutting it from metal stock.

The gear blank can also be the metal casting.

Gear is prepared by cutting one by one tooth in the gear blank of desired shape and size along it periphery.

Different gear cutting methods are used in this category.

Two  principal  methods  of  gear manufacturing include

- gear forming, and- gear generation.

Each  method  includes  a  number  of machining processes

Gear formingIn gear form cutting, the cutting edge of the cutting  tool  has  a  shape  identical with  the shape of the space between the gear teeth.

Two  machining  operations,  milling  and broaching  can  be  employed  to  form  cut gear teeth.

Form milling: Forming is sub-divided into milling by disc cutters and milling by end mill cutter which are having the shape of tooth space.

Form milling by disc cutter:

The disc cutter shape conforms to the gear tooth space.

Each gear needs a separate cutter. However, with 8 to 10 standard cutters, gears from 12 to 120 teeth can be cut with fair accuracy.

Tooth is cut one by one by plunging the rotating cutter into the blank as shown in fig 6.

Fig 6. Form milling by disc cutter & milling of a helical gear.

Form milling by end mill cutter: The end mill cutter shape conforms to tooth spacing.

Each tooth is cut at a time and then indexed for next tooth space for cutting.

A set of 10 cutters will do for 12 to 120 teeth gears. It is suited for a small volume production of low precision gears. The form milling by end mill cutter is shown in fig 7.

Fig 7. Form milling by end mill cutter

To reduce costs, the same cutter is often used for the multiple-sized gears resulting in profile errors for all but one number of teeth. Form milling method is the least accurate of all the roughing methods.

BroachingBroaching can also be used to produce gear teeth and is particularly applicable to internal teeth.

The process is rapid and produces fine surface finish with high dimensional accuracy.

However, because broaches are expensive-and a separate broach is required for each size of gear-this method is suitable mainly for high-quantity production.

Broaching the teeth of a gear segment by horizontal external broaching in one pass.

Gear generationIn gear generating, the tooth flanks are obtained (generated) as an outline of the subsequent positions of the cutter, which resembles in shape the mating gear in the gear pair:

In gear generating, two machining processes are employed, shaping and milling.

There are several modifications of these processes for different cutting tool used,

milling with a hob (gear hobbing),

gear shaping with a pinion-shaped cutter, or

gear shaping with a rack-shaped cutter.

Gear generating is used for high production

runs and for finishing cuts.

Gear Cutting by Gear Shaper This process uses a pinion shaped cutter carrying clearance on the tooth face and sides and a hole at its centre for mounting it on a stub arbor or spindle of the machine.

The cutter is mounted by keeping its axis in vertical position.

It is also made reciprocating along the vertical axis up and down with adjustable and pre-decide amplitude.

Gear Cutting by Gear Shaper The cutter and the gear blank both are set to rotate at very low rpm about their respective axis.

The relative rpm of both (cutter and blank) can be fixed to any of the available value with the help of a gear train.

This way all the cutting teeth of cutter come is action one-by-one giving sufficient time for their cooling and incorporating a longer tool life.

Generating action of a gear-shaper cutter

The principle of gear cutting by this process as explained above is depicted in the Figure 8. The main parameters to be controlled in the process are described below.

Figure 8: Process of Gear Cutting by Shaper Cutter

Cutting Speed •The downward movement of the cutter is the cutting stroke and its speed (linear) with which it comes down is the cutting speed.

•After the completion of cutting stroke, cutter comes back to its top position which is called return stroke.

•There is no cutting in the return stroke.

Indexing Motion • Slow rotations of the gear cutter and workpiece provide the circular feed to the operation. • These two rpms are adjusted with the help of a change gear mechanism. The rpm are relatively adjusted such that each rotation of the cutter the gear blank revolves through n/N revolution.

 where n = Number of teeth of cutter, and

N = Number of teeth to be cut on the blank.

Depth of Cut

• The required depth is maintained gradually by cutting the teeth into two or three pass.

• In each successive pass, the depth of cut is increased as compared to its previous path.

• This gradual increase in depth of cut takes place by increasing the value of linear feed in return stroke.

The whole of this process is carried out in a gear shaping machine which is shown in Figure 9.

Figure 9. Setup for Gear Shaping Machine

Advantages of Gear Shaping Process

(a) Shorter product cycle time and suitable for making medium and large sized gears in mass production.

(b) Different types of gears can be made except worm and worm wheels.

(c) Close tolerance in gear cutting can be maintained.

(d) Accuracy and repeatability of gear tooth profile can be maintained comfortably.

Limitations

(a) It cannot be used to make worm and work wheel which is a particular type of gear.

(b) There is no cutting in the return stroke of the gear cutter, so there is a need to make return stroke faster than the cutting stroke.

(c) In case of cutting of helical gears, a specially designed guide containing a particular helix and helix angle, corresponding to the teeth to be made, is always needed on urgent basis.

Gear Shaping by Rack Shaped Cutter • In this method, gear cutting is done by a rack

shaped cutter called rack type cutter. The principle is illustrated in Figure 10.

• The working is similar to shaping process done by gear type cutter.

• The process involves rotation (low rpm) of the gear blank as the rack type cutter reciprocates along a vertical line.

• Cutting is done only in the downward stroke, the upward stroke is only a return movement.

• The main difference of this method with the previous one is that once the full length of the rack is utilized the gear cutting of operation is stopped to bring the gear blank to its starting position so that another pass of gear cutting can be started.

• So this operation is intermittent for cutting larger gears having large number of teeth over their periphery.

Figure 10. Gear Cutting by Rack Shaped Cutter

Rack Planning Process • This process is used for shaping of spur and

helical gear teeth with the help of a rack type cutter.

• In this process the gear blank is mounted on a horizontal aims and rotated impertinently.

• At the same time the gear blank is kept in mesh with a reciprocating rack type cutter.

• The process is shown in Figure 11.

• The teeth cutter gradually removes material to cut the teeth and to make the required profile.

• The whole operation includes some important operations. These are feeding cutter into the blank, rolling the blank intermittently and keeping cutter in mesh with the rolling gear blank.

• After each mesh the gear blank is rolled by an amount equal to one pitch of gear tooth.

After each cutting, the rack is withdrawn and re-meshed after the rotation of gear blank.

Figure 11. Gear Shaping by Rack Type Cutter

Gear Hobbing Process • Gear hobbing is done by using a multipoint

cutting tool called gear hob. • It looks like a worm gear having a number

of straight flutes all around its periphery parallel to its axis.

• In gear hobbing operation, the hob is rotated at a suitable rpm and simultaneously fed to the gear blank.

• The gear blank is also kept as revolving.

• Rpm of both, gear blank and gear hob are so synchronized that for each revolution of gear hob the gear blank rotates by a distance equal to one pitch distance of the gear to be cut.

• Motion of both gear blank and hob are maintained continuously and steady.

• A gear hob is shown in Figure 12 and the process of gear hobbing is illustrated in Figure 13.

Figure 12. Gear Hob. Fig 13. Process of Gear Hobbing

• The hob teeth behave like screw threads, having a definite helix angle.

• During operation the hob is tilted to helix angle so that its cutting edges remain square with the gear blank.

• Gear hobbing is used for making a wide variety of gears like spur gear, helical, hearing-bone, splines and gear sprockets, etc.

Figure 14. (a) Schematic illustration of gear cutting with a hob. (b) Production of worm gear through hobbing.

• Three important parameters to be controlled in the process of gear hobbing are indexing movement, feed rate and angle between the axis of gear blank and gear hobbing tool (gear hob).

• If a helical gear is to be cut, the hob axis is set at an inclination equal to the sum of the helix angle of the hob and the helix angle of the helical gear.

• Proper gear arrangement is used to maintain rpm ratio of gear blank and hob.

• The arms of hob are set at an inclination equal to the helix angle of the hob with the vertical axis of the blank.

Figure 15. Setup for Gear Hobbing Machine

• The process of gear hobbing is classified into different types according to the directions of feeding the hob for gear cutting.

Hobbing with Axial Feed

• In this process the gear hob is fed against the gear blank along the face of the blank and parallel to its axis. This is used to make spur and helical gears.

Hobbing with Radial Feed • In this method the hob and gear blanks

are set with their axis normal to each other.

• The rotating hob is fed against the gear blank in radial direction or perpendicular to the axis of gear blank.

• This method is used to make the worm wheels.

Hobbing with Tangential Feed • This is also used for cutting teeth on

worm wheel.

• In this case, the hob is held with its axis horizontal but at right angle to the axis of the blank.

• The hob is set at full depth of the tooth and then fed forward axially. The hob is fed tangential to the face of gear blank.

Advantages of gear hobbing process• (a) Gear hobbing is a fast and continuous process so it is

realized as economical process as compared to other gear generation processes.

• (b) Lower production cycle time, i.e. faster production rate. • (c) The process has a larger variability’s in the following of

sense as compared to other gear machining processes.

(i) Capable to make wide variety of gears like spur gear, helical gears, worms, splines, sprockets, etc.

(ii) Process of required indexing is quite simplified and capable to make any number of teeth with consistent accuracy of module.

(iii) Harringbone gear cam be generated by gear hobbing exclusively.

(iv) Wide variety of batch size can be accommodated by this process.

• (d) Several gear blanks, mounted on the same arbor, can be processed simultaneously.

• (e) Lots of time is available to dissipate the generated heat. There is no over heating of cutting tool.

• Limitation of the process of gear hobbing is manufacturing of internal gears is not possible.

GEAR FINISHING OPERATIONS

• Surface of gear teeth produced by any of the generating process is not accurate and of good quality (smooth).

• Dimensional inaccuracies and rough surface generated so become the source of lot of noise, excessive wear, play and backlash between the pair of gears in mesh.

GEAR FINISHING OPERATIONS • In order to overcome these problems

some finishing operations are recommended for the produced gears.

• Sometimes poor quality of finish and dimensional inaccuracies occur due to hardening of a produced gear.

• So finishing operations are to be done at last.

GEAR SHAVING • Gear shaving is a process of finishing of

gear tooth by running it at very high rpm in mesh with a gear shaving tool.

• Tool is of a type of rack or pinion having hardened teeth provided with serrations.

• These serrations serve as cutting edges which do a scrapping operation on the mating faces of gear to be finished.

• Both are gears in mesh are pressed to make proper mating contact.

Shaving operation is shown in fig 16.

Fig 16. External gear being shaved

GEAR BURNISHING • The gear to be finished is mounted on a

vertical reciprocating shaft and it is kept in mesh with three hardened burnishing compatible gears.

• The burnishing gears are fed into the cut gear and revalued few revaluations in both the directions.

• Plastic deformation of irregularities in cold state takes place to give smooth surface of the gear.

• In burnishing, a specially hardened gear is run over rough machined gear.

• The high forces at the tooth interface cause plastic yielding of the gear tooth surface which improves finish and work hardens the surface creating beneficial compressive residual stresses.

Gear Grinding • In this operation abrasive grinding wheel of

a particular shape and geometry are used for finishing of gear teeth.

• Gear to be finished is mounted and reciprocated under the grinding wheel.

• Each of the gear teeth is subjected to grinding operations this way.

• In grinding, a contoured grinding wheel is run over machined surface of the gear teeth using computer control.

Fig 17. shows grinding operations and dressing of the wheel.

Fig 17. (a) Grinding the flanks only, (b) Grinding root and flanks, (c) Grinding each flank separately with twin grinding wheels

and (d) Pantograph dressing of the wheel

Fig 18. Finishing gears by grinding: (a) form grinding with shaped grinding wheels; (b) grinding by generating with two wheels

• Grinding is used to correct the heat-treatment distortion in gears hardened after roughing.

• Improvement in surface finish and error correction of earlier machining are added advantages.

• Grinding operation for gears can be done by profile grinding or form grinding as shown in figs 19. and 20.

Fig 19. (a) Maag zero pressure angle profile grinding and (b) Maag profile grinding

Fig 20.David Brown form grinding of worm threads

Lapping of a Gear • The process of lapping is used to improve

surface finish of already made teeth. • Gear to be lapped is run under load in

mesh with cast iron toothed laps. • Abrasive paste is introduced between the

teeth. It is mixed with oil and made to flow through the teeth.

• One of the mating members (gear/ lapping tool) is reciprocated axially along with the revaluations.

Fig 21. Lapping operation for bevel gears

HONING • It is used for super finishing of the generated

gear teeth. • Honing machines are generally used for this

operation. • The hones are rubbed against the profile

generated on the gear tooth. • Gear lapping and gear honing are the lost

finishing operations of a gear generation process.

In the above gear finishing operations some operations are based on metal cutting by removing very small size of chips like gear shaving, gear grinding, lapping and honing and some other operations like gear burnishing, roll finishing and based on finishing by plastic deformation of metal.

CUTTING BEVEL GEARSStraight bevel gears are roughed out in one

cut with a form cutter on machines that index automatically.

The gear is then finished to the proper shape on a gear generator.

The reciprocate across the face of the bevel gear as shown in fig 22 and 23.

The machines for spiral bevel gears operate on essentially the same principle.

The spiral cutter  is basically a face-milling cutter that  has  a  number  of  straight-sided  cutting blades protruding from its periphery.

Figure 22. (a) Cutting a straight bevel-gear blank with two cutter. (b) Cutting a helical bevel gear

Fig 23: (a) Cutting a straight bevel-gear blank with two cutters. (b) Cutting a spiral bevel gear with a single cutter.

Gear Manufacturing Cost as a Function of Gear Quantity

Figure 24. Gear manufacturing cost as a function of gear quality. The numbers along the vertical lines indicate tolerances