Gear Hobbing

THE INDEXING MECHANISM OF GEAR HOBBING MACHINEGearsA gear is a wheel with teeth along its rim. It is used to transmit power from one shaft to another.

Production of GearsBroachingHobbingShapingMillingLappingPlastic Injection Molding Powder Metal Sintering Forging Casting

GEAR HOBBINGGear 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 CharacteristicsIs 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 ParametersFeed 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 HeadThe 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 MovementsThe movements of indexing head is controlled byfollowing to main components

Indexing Gearing

Feed Gearing

Indexing GearingThe 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 constantMachine feed constant

Machine Index ConstantThe machine index constant is the number of revolutions of the hob spindle during one revolution of the work spindleFor 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 MF= feed in per revolution of the work M= machine feed constant

INDEXING FIXTUREThe 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 PlateThe 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 indexingDirect indexing

Indirect indexing

Differential indexingDirect IndexingTo 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 TN No. of holes in Indexing PlateT No. of required divisions

Indirect IndexingThe 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 116, 30, 33, 36, 39, 51, 57, 63Plate 222, 24, 27, 29, 37, 43, 49, 59Plate 323, 25, 28, 31, 41, 47, 53, 61

Differential IndexingSometimes, 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 DegreesWorkpieces 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 gearsHelical 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.

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