THE INDEXING MECHANISM OF GEAR HOBBING MACHINE - PowerPoint Presentation

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THE INDEXING MECHANISM OF GEAR HOBBING MACHINESlide2

Gears

A gear is a wheel with teeth along its rim.

It is used to transmit power from one shaft to another.Slide3

Production of Gears

Broaching

Hobbing

Shaping

Milling

Lapping

Plastic Injection Molding

Powder Metal Sintering

Forging

Casting Slide4

GEAR HOBBING

Gear hobbing is a multipoint machining process in which gear teeth are progressively generated by a series of cuts with a cutting tool known as hob. Both the hob and the workpiece revolve constantly as the hob is fed across the face width of the gear blank.

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Process Characteristics

Is a gear generating process that uses a hob cutter

Cutters and blanks rotate in a timed relationship

Maintains a proportional feed rate between the gear blank and the hob

Cuts several teeth on a progressive basis used for high production runsSlide6

Gear Production Parameters

Feed rate

Indexing head

Index and feed change gears establish the definite relationship for a specific gear.Slide7

Indexing

The relationship between the rotation of the hob, the

rotation of the work , and the amount and direction of

feed that enables the gear to be cut.

Speed change gears can be changed without affecting the relationship between hob, work and feed.Slide8

Indexing Head

The cutter spindle mounted on swirling head is connected through toothed gearing of different combinations to the table upon which the blank is carried

F

or each revolution of the table and blank, the cutter makes a definite number of revolution in the same period of time.Slide9

Indexing Movements

The movements of indexing head is controlled by

following to main components

Indexing Gearing

Feed GearingSlide10

Indexing Gearing

The driving shaft and the table upon which the blank is mounted both fixed and change gear wheels are usually employed.

Generally for spur gears

No.of teeth in driving wheels No.of theads in cutter x Constant quantity.

=

No.of teeth in drivenwheels No.of teeth to be cut in blankSlide11

Feed Gearing

Controls the feed of the cutter relatively to the blank,it is defined as the movement of the cutter with respect to the blank per revolution ot the blank and table,in direction parallel to axis of latter.

For spur gears

No.of teeth in driving wheels Feed per rev. of blank x No.of threads

in cutter

=

No.of teeth in driven wheels No of teeth to be cut in blank x Const.Slide12

Relationship between index & feed

To maintain desired relationship between index and feed there are two constant, to decide correct change gears

Machine index constant

Machine feed constantSlide13

Machine Index Constant

The machine index constant is the number of revolutions of the hob spindle during one revolution of the work spindle

For spur gears

Index gear ratio = KT

N

K= machine index constant

T= number of threads on hob

N= number of teeth on gearSlide14

Machine Feed Constant

The machine feed constant is the distance in inches which the hob slide will advance during one revolution of the work spindle when the ratio of the feed change gear is 1:1

Feed gear ratio = F

M

F= feed in per revolution of the work

M= machine feed constantSlide15

INDEXING FIXTURE

The index fixture consists of an index head, also called a dividing head

Footstock which is similar to the tailstock of a lathe.

The index head and footstock attach to the worktable of the milling machine by T-slot boltsSlide16
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Index Plate

The indexing plate is a round plate with a series of six or more circles of equally spaced holes.

The index pin on the crank can be inserted in any hole in any circle.

The interchangeable plates regularly furnished with most index heads, the spacing necessary for most gears, boltheads, milling cutters, splines, and so forth can be obtained. Slide18

Types of indexing

Direct indexing

Indirect indexing

Differential indexingSlide19

Direct Indexing

To perform this type of indexing,the worm shaft must be disengaged from the worm gear wheel.

Most direct indexing plate have 24 holes, all divisions of which 24 is a divisible (24, 12, 8, 6, 4, 3, 2) can be produced with this plates.

Indexing data = N

T

N – No. of holes in Indexing Plate

T – No. of required divisionsSlide20

Indirect Indexing

The indexing data or the number of turns of the crank for most division, it is necessary only to divide 40 by the number of divisions to be cut.

Indexing data = 40

N (number of required divisions)

Indexing plates which are available:

Plate 1 16, 30, 33, 36, 39, 51, 57, 63

Plate 2 22, 24, 27, 29, 37, 43, 49, 59

Plate 3 23, 25, 28, 31, 41, 47, 53, 61Slide21

Differential Indexing

Sometimes, a number of divisions is required which cannot be obtained by simple indexing with the index plates regularly supplied.

To obtain these divisions, a differential index head is used. The index crank is connected to the wormshaft by a train of gears instead of a direct coupling as with simple indexing. Slide22

Indexing In Degrees

Workpieces can be indexed in degrees as well as fractions of a turn with the usual index head.

There are 360 degrees in a complete circle and one turn of the index crank revolves the spindle 1/40 or 9 degrees.

Therefore, 1/9 turn of the crank rotates the spindle 1 degree. Workpieces can therefore be indexed in degrees by using a circle of holes divisible by 9. Slide23

Hob work rotation relationship

Spur gears

For spur gears with a single thread hob,the blank moves one tooth space while the hob rotates once the rotation is timed by means of change gearsSlide24

Helical gears

For helical gears,the rotaion of the work is silightly retarted or advanced in relationship to the rotation of the hob,and the feed is also held in a definite relationship with the work and the hobSlide25

Application

Application in Gear Hobbing Machine

The two shafts (Hob spindle and work piece spindle) are rotated at a proportional ratio

This determines the number of teeth on the blank; for example, if the gear ratio is 40:1 the hob rotates 40 times to each turn of the blank, which produces 40 teeth in the blank.Slide26

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