What is Dividing Head used For Milling Indexing

What is Indexing ?

Milling operations sometimes, require the rotation of job correct to fractions of minutes, for each groove, slot etc., to be cut evenly on the job surface. The accuracy of spacing of teeth is very important particularly when the work is of precision character e.g., gear teeth, shafts, cutter teeth etc.

The operation of rotating the job through a required angle between two successive cuts is termed as indexing. This is accomplished with the help of a milling attachment known as dividing head, which is an accessory to the milling machine. It helps to divide the job periphery into a number of equal divisions, i.e., square, hexagonal octagonal, etc.

Dividing Head:

To rotate a job through required angle, one needs:

(i) A device to rotate the job and

(ii) A source which can ensure that the job has been rotated through the desired angle.

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

The crank has an arrangement in connection with the plunger-pin, which can slide through the slot and the crank is pivoted at the centre of a disc. This crank can be rotated about the axis and the plunger can be fixed at any desired hole.

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

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

The following types of index-plates having the 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 index plate, fitted directly on the spindle. The intermediate use of worm and worm-wheel is avoided. The index plate has 24 holes and the periphery of job can be divided into 2, 3, 4, 6, 8 and 12 equal parts directly. This type indexing is most commonly used for indexing fixture.

2. Simple or Plain Indexing:

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

3. Compound Indexing:

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

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

(i) By rotating the crank or handle in 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 rotated by 12 holes of 18 hole circle with the help of the crank and then the index plate is rotated by 22 holes of the 27 hole circle.

4. Differential Indexing:

Available number of index plates with different hole circles, sometimes confine the range of plain indexing. In such cases, differential indexing is found to be more suitable. Between the indexing plate and spindle of dividing head, a certain set of the gears is incorporated extra. Dividing heads are provided with such standard set of gears.

During the differential indexing, the index-plate is unlocked and connected to a train of gears which receive their motion from the worm gear spindle. As the handle is turned, the index plate also turns, but at a different rate and perhaps 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 equipment.