Indexing methods

INDEXING METHODS

There are different methods of indexing. The selection of indexing depends on the number of divisions required and the type of dividing head used. The common methods of indexing are given below.

1. Direct or rapid indexing

2. Plain or simple indexing

3. Differential indexing

4. Compound indexing

5. Angular indexing.

1. Direct or Rapid Indexing

This method of indexing is used for indexing a large number of identical pieces by very small divisions. The operation is performed on a plain dividing head and universal dividing head. When a universal dividing head is used, the worm and worm gear are disengaged. The indexing is done with the help of a direct index plate. This index plate is fitted to the front end of the spindle nose.

The rapid index plate having 24 slots is fitted to the frame. The spindle can be rotated by hand and locked at any position by engaging the front indexing pin with the hole provided on the rapid indexing plate. A spring loaded pin is pushed into any one of the holes to lock the spindle with the body while indexing. This lock pin is released and the spindle is rotated by a hand wheel. Thus, the index plate is turned through the required revolution. The dividing head spindle and the workpiece are also turned through the same amount. To find the index movement, the total number of holes in the rapid index plate is divided by the number of divisions required on the work i.e.

Required ratio = N/n

where

N = Number of slots on the periphery of the index plate i.e. 24

n = Number of divisions required on the workpiece.

2. Plain or Simple Indexing

This method of indexing is used for divisions which could not be indexed by direct or rapid indexing. Simple indexing can be carried out by either a plain indexing head or a universal dividing head. This method of indexing involves the use of crank, worm, worm wheel and index plate. The worm wheel generally carries 40 teeth and the worm is single threaded. Hence, one turn of the index crank turns the index head spindle 1/40 of a complete turn or through 9o. Otherwise, 40 turns of the crank will move the spindle by one complete revolution.

Parkinson and Cincinnati dividing heads are supplied with a single plate only. It carries holes on both sides as given below:

Front side: 24, 25, 28, 30, 34, 37, 38, 39, 41, 42, 43

Back side: 46, 47, 49, 51, 53, 54, 57, 58, 59, 62, 66

The crank movement for dividing the periphery of the workpiece into the number of equal divisions will be as follows.

Index crank movement = 40/N

where

N = Number of divisions required.

3. Compound Indexing

The method of compound indexing is used when the number of divisions required on the job is outside the range of simple indexing. The operation involves the use of two separate simple indexing movements.

1. By turning the crank to a definite amount in one direction in the same way as in simple indexing, and

2. By turning both index plate and crank in the same or reverse direction, thus adding further movement to or subtract from the first stage.

Principle of compound indexing:

The principle of compound indexing can be illustrated by taking an example. Let us consider, the crank is turned 3 holes on a 15 holes circle and the index plate and crank both turned further 4 holes on 12 holes circle. These two movements will turn the worm through:

Since 40 turns of the worm turn the workpiece through 1 revolution, 8/15 turn of the worm will turn the work through 8/15 × 1/40

i.e.= 1/75 of a revolution.

This will enable 75 divisions on the work.

Another example is wherein the second operation the index plate and the crank are rotated in a reverse direction to that adopted in the first operation. Suppose, the crank is turned 6 holes on the 18 holes circle in one direction and then the index plate along with the crank, turned 4 holes on 16 holes circle in a direction opposite to the former. On account of these two movements, the worm will be turned through

Now, the corresponding movement of the work will be

Thus, 480 divisions can be obtained on the work.

The following procedure for compound indexing is to be followed.

1. Factorise the number of divisions required.

2. Factorise the standard number 40.

3. Select any two circles on the same plate and on the same side for trial. Factorise their difference.

4. Factorise the number of holes of the circle.

5. Factorise the number of holes of the other circle.

After obtaining these factors place them as follows.

Suppose, we get 1/x after simplification and if a and b are the number of holes on the two circles, the indexing movement is given by

After finding the values of a and b, the algebraic sum of the two movements is checked by the formula

4. Differential Indexing

Differential indexing principle is very much similar to the compound indexing. This process is also carried out in two stages.

Stage 1:

The crank is moved in a certain direction.

Stage 2:

Movement is added or subtracted by moving the plate by means of a gear train. The dividing heads are supplied with standard sets of change gears. Brown and Sharpe supply the dividing head with the following change gears.

24 (2 Nos.), 28, 32, 40, 44, 48, 56, 64, 72, 86, 100

The simultaneous movement of the index plate is obtained by a gear train. The gear train connects the spindle and the bevel gear shaft. The index plate can be made to rotate in the same direction as the crank or opposite to the crank rotation. By this rotation of the index plate, the actual movement of the index crank at every indexing is automatically increased or decreased i.e., when the index plate rotates in the same direction of the crank, the resulting movement of the crank is increased. When the index plate rotates in the opposite direction of the crank, the resulting actual movement is decreased.

During differential indexing, the index plate locking pin should be taken out to make the plate free to rotate. The bevel gear shaft and the dividing head spindle are connected by a gear train as shown in Figure 3.139.

When the crank is turned, the drive is given to the spindle through worm and worm gear. Thus, the spindle rotates. The motion is transmitted to index plate through change gears. A bevel gear slowly rotated in one direction or other. Index plate is rotated slowly in one direction or other depending on the gearing.

Rules for differential indexing:

where

A - Assumed number of divisions that can be indexed by a plain or simple indexing.

N - Required number of divisions to be indexed on the workpiece.

2. Index crank movement = 40/A

The index crank has to be moved for N number of revolutions for one division of work. The movement of index plate with respect to crank movement depends on the type of gearing and standard number A.

If (A - N) is positive, the index plate must be rotated in the same direction as the crank.

If (A - N) is negative, the index plate must be rotated in the opposite direction to the crank.

To achieve these conditions, the number of idle gears used depends on the following factors.

a) If the gear train is simple and (A - N) is positive, only one idle gear is used. 

b) If the gear train is compound and (A - N) is positive, no idle gear is used.

c) If the gear train is simple and (A - N) is negative, two idle gears are used. 

d) If the gear train is compound and (A - N) is negative, only one idle gear is used.

5. Angular Indexing

Angular indexing is dividing the circumference of the workpiece in angular movements. The procedure is similar to plain or simple indexing. In a dividing head, the gear train is such that 40 revolutions of the crank rotate the workpiece through one revolution. In other words, 40 revolutions of the crank rotate the workpiece through 360°. Thus, one turn of the crank rotates the workpiece through 360/40 = 9°.