Milling
Machine operations sometimes, required the rotation of jobs correct to the fraction
of a minute, for each groove, slot, etc., to be cut equals on the job surface.
The accuracy of the spacing of teeth is very significant especially when the work
is of precision character e.g., shafts, gear teeth, cutter teeth, etc.
The
operation of rotating the jobs through a required angle between two consecutive
cuts is termed indexing. This is completed with the help of a milling
attachment known as a dividing head that is an accessory to the milling machine.
It helped to divide the job peripheries into a number of equal divisions,
i.e., hexagonal, square, octagonal, etc.
Dividing Head:
.jpg)
To
rotate a job through the needed angle, one requires:
I. A device to
rotate the job and
II. A source that
can ensure when the job has been rotated through the wanted angle.
In
the dividing head, the first requirement is met by an indexed crank and the
second requirement is met by the index plate. The index plate has a number of
holes arranged completely so that each circle has a number of holes equally
spaced.
The
crank has an appointment in connection with the plunger pin, which can slide
through the slot, and the crank pivots at the center of a disc. This crank
can be rotated about the axis and the plunger can be fixed at any desired
hole.
The
rotation of the crank is transmitted by a gear to the job so that the number of
complete revolutions will result in certainly revolutions of the job. The
ratio of the crank and the shaft on which the job is installed is 40: 1, i.e., when the
index plate makes 40 revolutions, the job makes one revolution.
For quick-placing of the plunger and in order to avoid the counting of holes, fixed arm sectors are provided that can be set apart at any number of holes desired.
The
following types of index plates having 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 indexing plate, fit directed on the
spindle. The intermediate use of worm and worm-wheel is avoided. The index
plate has 24 holes and the periphery of the job can be divided into 2, 3, 4, 6, 8, and 12 equal parts directly. This type of indexing is most commonly used for
indexing fixtures.
2.
Simple or Plain Indexing:
In
this case, different index plates with different numbers of holes are used to
increase the range of indexing. The index is fixed in position by a pin called a lock
pin.
3.
Compound Indexing:
The
principle of operation of compound indexing is the same as which of simple
indexing, but the only difference is that compound indexing used two different
circles of one plate and hence also sometimes referred to as the trial and hit
method.
The
principle of compound indexing is to obtain the required division in two
stages:
I. By rotating the
crank or handle in the 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 rotating by 12 holes of 18 holes circle with
the help of the crank and then the index plate is rotated by 22 holes of the 27
holes circle.
4.
Differential Indexing:
The available number of indexing plates with different hole circles sometimes limited the
range of plain indexing. In such cases, differential indexing is found to be
more appropriate. Between the indexing plate and spindle of the dividing head, a
certain set of gears is involved in extra. Dividing heads are provided
with a standard set of gears.
During
the differential indexing, the index plate is un-locked and connected with a
train of gears which received their Movements from the worm gear spindle. As
the handle is turned around, the index plate also turns, but at a different
rate and probably 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 instrument.