What is the Dividing Head used For Milling Indexing

Milling Machine operations sometimes, required the rotation of jobs correct to the fraction of a minute, for each groove, slot, etc., to be cut equals on the job surface. The accuracy of the spacing of teeth is very significant especially when the work is of precision character e.g., shafts, gear teeth, cutter teeth, etc.

The operation of rotating the jobs through a required angle between two consecutive cuts is termed indexing. This is completed with the help of a milling attachment known as a dividing head that is an accessory to the milling machine. It helped to divide the job peripheries into a number of equal divisions, i.e., hexagonal, square, octagonal, etc.

Dividing Head:     

To rotate a job through the needed angle, one requires:

I. A device to rotate the job and

II. A source that can ensure when the job has been rotated through the wanted angle.

In the dividing head, the first requirement is met by an indexed crank and the second requirement is met by the index plate. The index plate has a number of holes arranged completely so that each circle has a number of holes equally spaced.

The crank has an appointment in connection with the plunger pin, which can slide through the slot, and the crank pivots at the center of a disc. This crank can be rotated about the axis and the plunger can be fixed at any desired hole.

The rotation of the crank is transmitted by a gear to the job so that the number of complete revolutions will result in certainly revolutions of the job. The ratio of the crank and the shaft on which the job is installed is 40: 1, i.e., when the index plate makes 40 revolutions, the job makes one revolution.   

For quick-placing of the plunger and in order to avoid the counting of holes, fixed arm sectors are provided that can be set apart at any number of holes desired.   

The following types of index plates having holes given against them are available.

Brown and Sharp:

Plate 1= 15, 16, 17, 18, 19, and 20.

Plate 2= 21, 23, 27, 29, 31, and 33.

Plate 3= 35, 37, 39, 41, 43, 47, and 49.

Parkinson:

Plate 1= 24, 25, 28, 30, 34, 37, 38, 39, 41, 42, and 43.

Plate 2= 46, 47, 49, 51, 53, 54, 57, 58, 59, 62, and 66.

Common Methods of Indexing:

There are five methods of indexing.

These are listed below:

1. Direct indexing,

2. Simple or plain indexing,

3. Compound indexing,

4. Differential indexing,

5. Angular indexing.

1. Direct Indexing:

In this case, the dividing head has an indexing plate, fit directed on the spindle. The intermediate use of worm and worm-wheel is avoided. The index plate has 24 holes and the periphery of the job can be divided into 2, 3, 4, 6, 8, and 12 equal parts directly. This type of indexing is most commonly used for indexing fixtures.

2. Simple or Plain Indexing:

In this case, different index plates with different numbers of holes are used to increase the range of indexing. The index is fixed in position by a pin called a lock pin.

3. Compound Indexing:

The principle of operation of compound indexing is the same as which of simple indexing, but the only difference is that compound indexing used two different circles of one plate and hence also sometimes referred to as the trial and hit method.

The principle of compound indexing is to obtain the required division in two stages:

I. By rotating the crank or handle in the usual way keeping the index plate fixed.

Ii.By releasing the back pin and then rotating the index plate with the handle.

For example, if a 27 teeth gear is to be cut, then T = 40/27 i.e., the rotation required for one tooth spacing is 40/27 which may be written as;

2/3 + 22/27 or 12/18 + 22/27

So for each tooth, the worm will be rotating by 12 holes of 18 holes circle with the help of the crank and then the index plate is rotated by 22 holes of the 27 holes circle.

4. Differential Indexing:

The available number of indexing plates with different hole circles sometimes limited the range of plain indexing. In such cases, differential indexing is found to be more appropriate. Between the indexing plate and spindle of the dividing head, a certain set of gears is involved in extra. Dividing heads are provided with a standard set of gears.

During the differential indexing, the index plate is un-locked and connected with a train of gears which received their Movements from the worm gear spindle. As the handle is turned around, the index plate also turns, but at a different rate and probably in the opposite direction. Differential indexing makes it possible to rotate the work by any fraction of revolution with the usual index plates furnished with the instrument.