THE INDEXING MECHANISM OF GEAR

HOBBING MACHINE

GearsA gear is a wheel with teeth along its rim. It is used to transmit power from one shaft to

another.

Production of Gears Broaching Hobbing Shaping Milling Lapping Plastic Injection Molding Powder Metal Sintering Forging Casting

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.

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 runs

Gear Production Parameters Feed rate

Indexing head Index and feed change gears establish the

definite relationship for a specific gear.

Indexing The relationship between the rotation of the

hob, therotation of the work , and the amount and

direction offeed that enables the gear to be cut.

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

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

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

Indexing Movements

The movements of indexing head is controlled by

following to main components

Indexing Gearing

Feed Gearing

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 blank

Feed GearingControls 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 gearsNo.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.

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 constant

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 NK= machine index constantT= number of threads on hobN= number of teeth on gear

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 constant

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 bolts

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.

Types of indexing Direct indexing

Indirect indexing

Differential indexing

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 divisions

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, 61

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.

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.

Hob work rotation relationshipSpur 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 gears

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 hob

ApplicationApplication in Gear Hobbing MachineThe two shafts (Hob spindle and work piece

spindle) are rotated at a proportional ratioThis 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.

Thank You