The Indexing or Dividing Head The indexing or dividing head is one of the most important attachments for the milling machine. It used to divide the circumference of a workpiece into equally spaced divisions when milling such items as gears, splines, squares, hexagons, etc. It may also be used to rotate the workpiece at a predetermined ratio to the table feed rate to produce cams and helical grooves on gears, drills, reamers, etc. The universal dividing head set consists of the headstock with index plates, headstock change gears and quadrant, universal chuck, footstock, and the center rest.
The Headstock A swivelling block, mounted in the base, n. The side of the base and the block are graduated to indicate the angle of the setting. Mounted in the swivelling block is a spindle, with a 40-tooth worm wheel attached, which meshes with a worm. The worm, at right angle to the spindles, is connected to the index crank, the pin of which engages in the index plate. A direx indexing plate is attached to the front of the spindle.
be threated into the end of the spindle. The footstock is used in conjunction with the headstosck to support work held between centers or the end of work held in a chuck. The footstock center may be adjusted longitudinally to accomodate various lenghts of work and may be raised or lowered off center. It may also be tilted out of parallel with the base when cuts are being made on tapered work. Long slender work held between centers is prevented from bending by the adjustable center rest.
Methodes of Indexing The main purpose of indexing or dividing head is to accurately divide the workpiece periphery into any number of divisions. This may be accomplished by the following indexing methodes: direct, simple, angular, and differential.
Differential Indexing When it is impossible to calculate the required indexing by the simple indexing methode, that is, when the fraction
can not be reduced to a factor of one of the available
hole circles, it is necessary to use differential indexing. With this method of indexing, the index plate must be revolved either forward or backward a part of a turn to attain the proper spacing or indexing. In differential indexing, as in simple indexing, the index crank rotates the dividing head spindle. The spindle rotates the index plate, after the locking pin has been disengaged, by means of change gears connecting the dividing head spindle and the worm shaft. The rotation of the plate may be either in the same direction (positive) or in the opposite direction (negative) of the index crank. This change of rotation is effected by an idler gear or gears in the gear train. When it necessary to calculate the indexing for a required number of divisions by the differential method, a number is chosen close to the required divisions which can be indexed by simple indexing. To illustrate the principle of differential indexing, assume that the index crank has to be rotated of a turn and there is only an 8-hole circle available. If the crank is moved of a turn, the index pin will contact the plate at a spot before the first hole on the 8-hole circle. The exact position of this spot would be the difference between and of a revolution of the crank. This would be than of a turn, or
of a turn less
of a turn short of the first hole. Since there is no hole at this point into
which the pin could engage, it is necessary to cause the plate to rotate backwards by means of change gears
of a turn in order that the pin will engage in a hole. At this point the index
crank will be locked at exactly of a turn. The method of calculating the change gears required to rotate the plate the proper amount is as follows: Change gear ratio
= (A - N)
=
A= approximate number of division
N= required number of division
When the approximate number of divisions is larger than the required number, the resulting fraction is plus and the index plate must move in the same direction as the crank (clockwise). This positive rotation is accomplished by using an adle gear. However, if the approximate number is smaller than the required number, the resulting fraction is minus and the index plate must move in a counter clockwise direction. This negative rotation requires the use of two idler gears. The numerator of the fraction represents the driving (spindle) gear or gears, while the denominator represents the driven (worm) gear or gears. The gearing may be either simple or compound and yhe rotation is as follows. Simple gearing:
1 idler for a positive rotation of the index plate, 2 idlers for a negative rotation of the index plate.
Compound gearing: 1 idler for a negative rotation of the index plate, 2 idlers for a positive rotation of the index plate. Example: Calculate the indexing and change gears required for 57 divisions. The change gears supplied with the dividing head are as follows: 24, 24, 28, 32, 40, 44, 48, 56, 64, 72, 86, 100. The available index plate hole circles are as follows: Plate #1
: 15,16,17,18,19,20
Plate #2
: 21,23,27,29,31,33
Plate #3
: 37,39,41,43,47,49
Procedure
=
1. Indexing =
Since there is no 57-hole circle and since it is impossible to reduce this fraction to suit any hole circle, it is necessary to select an approximate number close to 57, for which simple indexing may be calculated. 2. Let the approximate number of divisions equal 56. 3. Indexing for 56 divisions = 4. Gear ratio = (A - N) = (56 – 57) = -1 =-
= or 15 holes on the 21-hole circle.
Change gears = =Therefore, for indexing 57 divisions, a 40-tooth gear is mounted on the dividing head spindle, and a 56-tooth gear is mounted on the worm shaft. Since the fraction is negative, or counter clockwise, and two idlers must be used. After the proper gears are installed, the simple indexing procedure for 56 divisions should be followed.
