differential indexing | Numerical Indexing for 97 teeth | Indexing Methods |Metal Cutting
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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.
DIFFERENTIAL INDEXING
Differential indexing is similar to compound indexing except that the index plate is turned during the indexing operation by gears connected to the dividing head spindle. Because the index plate movement is caused by the spindle movement, only one indexing procedure is required. The gear train--Differential Indexing.between the dividing head spindle and the index plate provides the correct ratio of movement between the spindle and the index plate.
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.
where N is the number of divisions to be indexed and n is a number slightly greater or less than N. The relation given by equation (1) will give a gear ratio to be placed on spindle (Driver) and the work shaft (Driven). The arrangement of gears can be in the form of simple wheel train or compound wheel train or compound wheel train depending upon the suitability and requirements.
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