In this article we will discuss about:- 1. Meaning of Indexing 2. Methods of Indexing 3. Helical of Spiral Milling.
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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.
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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.
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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.
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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.
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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. The spindle is then rotated by rotating the handle which is keyed to the worm-shaft as shown in Fig. 16.61.
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.
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.
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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.
For making the necessary calculations and to find the change of gears to be placed between the spindle and the worm shaft, use the following relation:
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.
The difference of N and n causes the index plate to rotate itself in a proper direction relative to crank. If (n — N) is positive, the index plate will rotate in the direction in which crank is rotated and if (n — N) is negative, it will rotate in opposite direction to that of crank.
5. Angular Indexing:
Instead of rotating the job through certain division on its periphery, sometimes it may be needed to rotate the job through certain angle. Angular indexing is used for this purpose. Since the crank and spindle ratio is 40 : 1 and hence when the crank moves through one revolution, the spindle or the job moves through 1/40 of revolution, i.e., the job will revolve through an angular movement of 9°.
If it is desired to index a job by 35 degree, then the index head movement required to perform the operation will be = (35/9) = 3(8/9) = 3 + ((8 X 3) / (9 X 3)) = 3 + (24/27), i.e., the crank must be turned three complete revolutions plus 24 holes in the 27-holes circle.
Helical of Spiral Milling in Indexing:
One of the important indexing operations to be performed on milling machine is the helical or spiral milling. Generation of flutes on twist drills, milling of helical and spiral gears, milling of worms and cutter, etc. are some of the examples of this class.
In the case of helical milling, the job is rotated and side by side it is moved linearly due to movement of table under the rotating cutter fixed in one position. This is done by connecting the worm shaft to the milling table feed screw with the help of a set of gears. Lead of the helix depends upon the rate at which the job is rotated with respect to table movement.
While performing helical milling, the following points must be taken into consideration:
a. The table of the milling machine must be set at an angle (equal to the helix angle) to the normal position of the table. This is done so that when the job advances and at the same time revolves also, the impression left by the cutter in the job will be identical to the contour of the cutter. The direction in which the table is swiveled determines the hand of spiral.
b. There must be a proper relation between the movement of job and table.
c. The job is to be fed to the cutter by the table movement.
Lead of helical milling machine is the distance through which the table moves when the spindle of dividing head moves through one revolution without any change in velocity ratio between the dividing head spindle and the table feed screw.
Sometimes it is necessary to introduce change gears between the worm shaft and the table feed screw, because when the table travels a distance equal to the lead of helix, the job must have completed one revolution. This can be achieved by introducing change gears. The formula given below holds good for the change gears.
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