Gear Cutting Process

Gear Cutting Process, Indexing, Direct or rapid indexing, Simple or plain indexing, Universal indexing head, Methods of indexing, Direct indexing, Simple indexing, Compound indexing, Differential indexing, Angular indexing,Gear shaping method, Rack type cutter generating process, Pinion type cutter generating process, Gear hobbing, Climb and conventional hobbing

Indexing:

                The indexing is the operation of dividing the periphery of a work piece into any number of equal parts. Indexing operation rotates the workpiece through a required angle between two successive cuts. The indexing operation is used for cutting spur gear, producing hexagonal and square headed bolts, cutting splines on shafts, fluting drills, taps and reamers and many other jobs, all requiring the periphery of the workpiece to be divided equally and accurately. Indexing performed by using a special attachment known as dividing head or index head. This index can be done by using following indexing heads,

• Direct or rapid indexing

• Simple indexing

• Universal indexing

• Optical indexing

Direct or rapid indexing:

                Direct or rapid indexing is the simplest method of indexing and is used only on work that requires a small number of divisions, such as square or hexagonal nuts, etc. In this indexing method, the spindle is turned through a given angle without interposition of gearing. The spindle is rotated through a given angle by turning the spindle by hand. The index plate is fastened directly to the spindle, so that one complete revolution of the index plate rotates the spindle to one complete revolution. During indexing, the latch pin is first taken out and then the spindle is rotate by hand, and after the required position is reached it is again locked by the latch pin.

                For example, an index plate with 24- holes can be used for any number of equal division divisible into 24. That is, the workpiece can be divided into 2, 3, 4, 6, 8, 12, and 24- parts directly. The index movement or the number of holes to be moved in the index plate can be calculated as,

                If indexing is to be carried out for square head screw, then the number of holes to move in the index plate of 24 holes is,

24/4 = 6 holes

Simple or plain indexing:

                The main parts of the plain indexing heads are spindle, worm wheel, worm, index plate, index pin and sector arm. The worm wheel is keyed to the spindle, so that the spindle turns with the wheel. The worm wheel has 4-0 teeth on most dividing heads. The worm meshes with the worm wheel. The ratio of the worm and worm wheel is 40:1. That is 40 revolutions of the worm are required to turn the worm wheel to one complete revolution. The index plate, mounted on the worm shaft, consists of several rows of holes-each circular row having a different number of holes. The Index plate, can be removed easily, and another plate is substituted as necessary for the desired spacing.

                The index pin is located on the end of the crank, which is attached to the worm shaft. The crank is used to rotate the spindle through the worm gearing. The arm length of the crank is adjustable, so that index pin can drop into any hole in any circular row of holes. The sector arms are used to eliminate the necessity of counting the number of holes for each movement of the index pin and saves the time. The work is mounted at the nose end of the spindle by a chuck or may be supported between the two centres. The live centre is fitted at the nose of the spindle and the dead centre is held by the tail stock. The tail stock is a separate assembly bolted to the machine table. The spindle may be rotated through the desired angle and then clamped by inserting the index pin into any one of the holes of the index plate. This type of dividing head is used for handling large number of work piece, which require a very small number of divisions on the periphery.

Universal indexing head:

                The universal dividing head is the most common type of indexing arrangement used in workshops. As the name implies, this type of index head can be used to execute all forms of indexing. In the universal dividing head, the spindle can be tilted, i.e, swivelled to any angular position in a vertical plane, within the angular range provided, in addition to the indexing mechanism provided in plain dividing head. The casting that carries the spindle is mounted in a circular guide, forming part of the universal head. The spindle can be tilted to any angular position by rotating the spindle casting, and can be clamped in any angular position by tightening the angular clamp bolt. The circular guide is graduated in degrees and fractional degrees. The universal dividing head is used for a greater range of work than the plain dividing head.

A universal dividing head is used:

• For setting the work in vertical, horizontal or in Inclined positions, relative to the table surface.

• For turning the workpiece periodically through a given angle to impart indexing movement.

• For imparting a continuous rotary motion to a workpiece for milling helical grooves.

Methods of indexing:

• Direct indexing

• Simple indexing

• Compound indexing

• Differential indexing

• Angular indexing

Direct indexing:

• Direct indexing for divisions which are factors of 24 only, i.e., 2, 3, 4,6, 8, 12 and 24.

• Index plate has 24 equal spaced holes and is located on the back of the head.

• These methods are used for indexing fixtures.

• Here indexing is achieved by rotating the spindle manually by Rule of indexing, 24/ N.

N = Number of divisions required.

Simple indexing:

• This method can index more number of divisions than direct method.

• Rotation of the crank turns worm meshing with worm wheel.

• The 40 turns of the crank are required for one complete revolution of the work.

• One turn of the crank rotates the work piece 1/40 of revolution.

• The standard sets of index plates are available such as Brown and Sharpe, Cincinnati and Parkinson.

• Index crank movement = 40 / N.

• '40’ denotes, the number of holes to be bypassed by index crank and ‘N’ denotes the holed circle on the index plate.

• If ‘40/ N’ gives a whole number, the crank should be rotated through a number of turns equal to a whole number.

Compound indexing:

• It is impossible to do simple index for all numbers.

• It is used for indexing numbers which are beyond the reach of simple indexing.

• Principle of operation is the same as that of simple indexing, but it uses two different circles in one index plate.

• Necessary crank movement is obtained by combination of two movements:

(i) Simple index movement of the crank.

(ii) Movement of the index plate itself with the handle.

• This can he also known as ‘hit and trail’ method.

• Rule for compound indexing;

• Consider +ve sign, for index crank and plate rotate in the same direction.

• For -ve sign, they are rotated in opposite direction.

a = Number of holes to be bypassed in ‘A’ circle

A = 'A’ number of hole-circle

b = Number of holes to be bypassed in ‘B’ circle

B = ‘B’ number hole-circle

N = Number of divisions required.

Differential indexing:

• Differential indexing is used to index almost all the numbers not obtainable by simple or compound indexing.

• This method is not very much different in principle than compound indexing. Hence, it can be said to be an automatic method of performing compound indexing.

• Differential indexing is carried in two stages;

(i) The crank is moved in a certain direction similar to the simple indexing.

(ii) In the second stage, either some movement is added to the above crank movement or subtracted from the same.

• The addition or subtraction of the crank movement is accomplished by moving the plate by means of a gear train, connecting the dividing head spindle to the worm spindle.

Angular indexing:

• The angular indexing is the process of dividing the periphery of a work in angular measurements and not by the number of divisions.

• Angular indexing is used when it is necessary to cut grooves or slots subtending a given angle at the centre of the circle upon which they are spaced.

• The indexing method is similar to the plain indexing. In earlier discussions we have seen that 40 crank rotations make the work rotate through 360°.

• Therefore, for each rotation of the crank the work will rotate through,

360/40 = 90°

Rule for angular indexing:

• To find the index crank movement, divide the angle by 9 if it is expressed in degrees, by 540 if it is expressed in minutes, and by 32,400 if it is expressed in seconds, the formula is;

• The said motion is added by rotating the index plate in the same direction as crank and is subtracted by rotating the plate in the opposite direction to that of the crank.

• In differential indexing, the index plate is made free to rotate, by taking out the locking pin.

• When the index crank is rotated, the workpiece spindle also rotates [through worm and worm wheel].

• As the workpiece spindle is connected to the index plate through the gears, the index plate will also start rotating.

• The direction of the movement of the index plate depends upon the gear train employed.

• If an idler gear is added between the spindle gear and the worm shaft gear in case of a simple gear train, then the index plate will move in the same direction as that of the indexing crank movement, In case of compound gear train, an idler gear is to be used when the index plate is to move in the opposite direction.

• The change gear set available [Brown and Sharpe dividing heads] is : 24, 28, 32, 40, 44, 48, 56, 64, 72, 86 and 100.

• The following relation is used for calculating the necessary gears to be placed between the spindle and the worm shaft,

Gear shaping method:

• In gear shapers, the cutters reciprocate rapidly.

• The teeth are cut by the reciprocating motion of the cutter.

• The cutter can either be ‘rack - type cutter’ or a rotary pinion type cutter.

Rack type cutter generating process:

                The rack cutter generating process is also called gear shaping process. In this method, the generating cutter has the form of a basic rack for a gear to be generated. The cutting action is similar to a shaping machine. The cutter reciprocates rapidly and removes metal only during the cutting stroke. The blank is rotated slowly but uniformly about its axis and between each cutting stroke of the cutter, the cutter advances along its length at a speed equal to the rolling speed of the matching pitch lines. When the cutter and the blank have rolled a distance equal to one pitch of the blank, the motion of the blank is arrested, then the cutter is withdrawn from the blank to give relief to the cutting edges and to return the cutter to its starting position. Then blank is indexed and repeated the same process to complete the whole gear.

Pinion type cutter generating process:

                The pinion cutter generating process is fundamentally the same as the rack cutter generating process, and instead of using a rack cutter, it uses a pinion to generate the tooth profile. The cutting cycle is commenced after the cutter is fed radically into the gear blank equal to the depth of tooth required. The cutter then gives reciprocating cutting motion parallel to its axis similar to the rack cutter. Then the cutter and the blank are made to rotate slowly about their axis at speeds which are equal at the matching pitch surfaces. This rolling movement blow the teeth on the blank to cut. The pinion cutter in a gear shaping machine may be reciprocated either in the vertical or in the horizontal axis.

Advantages:

• The gears produced by the method are of very high accuracy.

• Both internal & external gears can be cut by this process.

• Non - conventional types of gears can also be cut by this method.

Disadvantages:

• The production rate with gear shaper is lower than hobbing.

• There is no cutting on the return stroke in a gear shaper.

• Worm & worm wheels can’t be generated on a gear shaper.

Gear hobbing:

                Hobbing is the process of generating gear teeth by means of a rotating cutter called a hob. It is a continuous indexing process in which both the cutting tool and work piece rotate in a constant relationship, while the hob is being fed into work. The hob and the gear blank are connected by means of proper change gears.

                The ratio of hob and blank speed is such that during one revolution of the hob, the blank turns through one teeth. The teeth of hob cut into the work piece in successive order. Each hob tooth cuts its own profile depending on the shape of cutter. One rotation of the work completes the cutting up to certain depth. Then the hob is feed to required depth and above same procedure is repeated until the whole gear is hobbed.

Advantages:

• Hobbing process is its versatality, in that it can cover a variety of work including spur gears, helical gears, worms and worm wheels, splines and serrations and a variety of special forms.

• The indexing is continuous and there is no intermittent motion to give rise to errors.

• There is no loss of time due to non-cutting on the return stroke.

• It is also possible to generate internal gears, but the application is very limited and involves a special hob head on the machine and special cutting tools.

Limitations:

• Some gears are restricted by adjacent shoulders larger than the root diameter of the gear and close enough to restrict the approach or run-out of the hob.

• Such gears can be produced only by the gear shaping process.

• Splines and serrations are sometimes required with one tooth blocked or removed.

• This type of component however, is not suitable for hobbing, although it is ideal for gear shaping.

Climb and conventional hobbing:

                For spur or helical tooth forms where the hob swivel setting angle is relatively small, the type of cutting action can be either conventional or climb depending upon the rotation of the hob and the feed direction. Figure shows two conditions of hobbing and the type of chip removed. For helical tooth forms where the hob swivel setting angle is usually large, the type or cutting action is determined by the rotational direction of the hob in relation to the direction of workpiece rotation.

                In conventional hobbing, when the hob swivel setting angle is small, the hob is fed along the workpiece in a direction which is in agreement with the tangential vector, denoting the direction of hob rotation. This condition exists in the cutting zone as illustrated in the fig. Although the removed chips vary in size and shape, the centred hob tooth removes a chip which is comparable to that obtained in conventional milling. As indicated in the figure, the chip starts out thin and becomes increasingly thicker as the hob tooth sweeps through the cut. For components having larger helix angles, conventional type of hobbing exists when a component of hob rotation is opposed to the workpiece rotation. This holds true when the hand of the hob helix and the hand of the workpiece helix are alike. With a large workpiece helix angle, the hob setting angle is usually large, tending to position the axis of the hob parallel with the axis of the workpiece. Since the direction of the workpiece rotation depends solely upon the rotation of the hob and its hand of helix, the hand of the workpiece helix determines whether the hob rotation opposes the workpiece rotation. With small hob swivel setting angles, climb hobbing takes place when the hob is fed into the workpiece in a direction which opposes the tangential vector denoting the direction of hob rotation. This condition exists within the cutting zone. As with the conventional type of hobbing, the chip starts out thick and becomes increasingly thinner as the hob tooth sweeps through the cut. When the hand of the hob helix and the hand of the workpiece helix are dissimilar, resulting in a large hob swing angle setting, a component of the hob rotation agrees with the direction of workpiece rotation. This combination of unlike hands produces a ‘rolling’ effect which is essentially climb cutting, but in a tangential direction rather than in an axial one. Under these conditions, the hob tends to drive the workpiece, reducing the torsional load in the index drive of the machine. The choice of the climb type cutting over the conventional type cutting is usually based upon a comparison of surface finish obtained from each. There is no definite rule which can be used to determine the type of hobbing that produces the best finish for a specific workpiece under consideration. When conditions permit, the most satisfactory type of cutting can be determined from test runs on the specific workpiece. In many cases, climb type cutting has been applied to replace conventional type cutting because a better finish is obtained with equivalent hob life and workpiece accuracy.

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