2Gears A gear is a wheel with teeth along its rim. It is used to transmit power from one shaft to another.
3Production of Gears Broaching Hobbing Shaping Milling Lapping Plastic Injection MoldingPowder Metal SinteringForgingCasting
4GEAR 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.
5Process Characteristics Is a gear generating process that uses a hob cutterCutters and blanks rotate in a timed relationshipMaintains a proportional feed rate between the gear blank and the hobCuts several teeth on a progressive basis used for high production runs
6Gear Production Parameters Feed rateIndexing headIndex and feed change gears establish the definite relationship for a specific gear.
7Indexing The relationship between the rotation of the hob, the rotation 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.
8Indexing HeadThe cutter spindle mounted on swirling head is connected through toothed gearing of different combinations to the table upon which the blank is carriedFor each revolution of the table and blank, the cutter makes a definite number of revolution in the same period of time.
9Indexing Movements The movements of indexing head is controlled by following to main componentsIndexing GearingFeed Gearing
10Indexing GearingThe driving shaft and the table upon which the blank is mounted both fixed and change gear wheels are usually employed.Generally for spur gearsNo.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
11Feed 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 threadsin cutter=No.of teeth in driven wheels No of teeth to be cut in blank x Const.
12Relationship between index & feed To maintain desired relationship between index and feed there are two constant, to decide correct change gearsMachine index constantMachine feed constant
13Machine Index Constant The machine index constant is the number of revolutions of the hob spindle during one revolution of the work spindleFor spur gearsIndex gear ratio = KTNK= machine index constantT= number of threads on hobN= number of teeth on gear
14Machine Feed ConstantThe 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 1:1Feed gear ratio = FMF= feed in per revolution of the workM= machine feed constant
15INDEXING FIXTUREThe index fixture consists of an index head, also called a dividing headFootstock 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
17Index 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.
18Types of indexing Direct indexing Indirect indexing Differential indexing
19Direct 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 = NTN – No. of holes in Indexing PlateT – No. of required divisions
20Indirect 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 =N (number of required divisions)Indexing plates which are available:Plate , 30, 33, 36, 39, 51, 57, 63Plate , 24, 27, 29, 37, 43, 49, 59Plate , 25, 28, 31, 41, 47, 53, 61
21Differential 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.
22Indexing 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.
23Hob work rotation relationship Spur gearsFor 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
24Helical gearsFor 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
25Application Application in Gear Hobbing Machine The 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.