simple indexing | simple indexing calculation | simple indexing numerical| Milling Machine
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Meaning of 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.
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. The spindle is then rotated by rotating the handle which is keyed to the worm-shaft
The following relation is used for simple indexing: T = 40/N, where T gives the number of turns or parts of a turn through which the index crank must be rotated to obtain the required number of divisions (N) on the job periphery.
Let us take an example of a gear blank on which 64 teeth are to be cut.
i.e., the worm is to be rotated by the handle through one complete rotation and two-third of the number of holes of any circle.
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