Milling
Milling cutter
Gear blank
Gear blank
Gear cutting
Figure 1.14 : Gear Cutting Operation
Helical Milling Operation
Helical milling produces helical flutes or grooves on the periphery of a cylindrical
or conical workpiece. This is performed by swiveling the table to the required
helix angle, then rotating and feeding the workpiece against revolving cutting
edges of milling cutter. Helical gears and drills and reamers are made by this
operation.
Cam Milling Operation
The operation cam milling is used to produce the cam on milling machine. In this
operation cam blank is mounted at the end of the dividing head spindle and the
end mill is held in the vertical milling attachment.
Thread Milling Operation
The operation thread milling produces threads using thread milling centres. This
operation needs three simultaneous movements revolving movement of cutter,
simultaneous longitudinal movement of cutter, feed movement to the workpiece
through table. For each thread, the revolving cutter is fed longitudinal by a
distance equal to pitch of the thread. Depth of cut is normally adjusted equal to the
full depth of threads.
1.10 INDEXING
Indexing is the operation of dividing the periphery of a workpiece into any number of
equal parts. For example if we want to make a hexagonal bolt. Head of the bolt is given
hexagonal shape. We do indexing to divide circular workpiece into six equal parts and
then all the six parts are milled to an identical flat surface. If we want to cut n number
of teeth in a gear blank. The circumference of gear blank is divided into n number of
equal parts and teeth are made by milling operation one by one. The main component
used in indexing operation is universal dividing head.
Universal Dividing Head
It is most popular and common type of indexing arrangement. As indicated by its
name universal, it can be used to do all types of indexing on a milling machine.
Universal dividing head can set the workpiece in vertical, horizontal, or in inclined
position relative to the worktable in addition to working principle is explained
below with the help of illustration in Figure 1.15. The worm gear has 40 teeth and
the worm has simple thread. Crank is directly attached with the worm. If we
revolve crank by 40 revolutions the spindle attached with worm gear will revolve
by only one revolution and one complete turn of the crank will revolve the spindle
only by 1/40th revolution (turn). In order to turn the crank precisely a fraction of a
revolution, an indexing plate is used. An indexing plate is like a circular disc
having concentric rings of different number of equally spaced holes. Normally
indexing plate is kept stationary by a lock pin. A spring loaded pin is fixed to the 19
Manufacturing Processes-III crank which can be fixed into any hole of indexing plate. The turning movement
of the workpiece is stably controlled by the movement of crank as explained
below.
Worm gear
Worm
Worm piece
Spindle Work carrier
Change
gear
Worm
shaft
Driver
Driven
gear
Indexing Indexing
plate crank
Spring
loaded pin
Figure 1.15 : Working Principle of Indexing Mechanism
If the pin is moved by one hole on the indexing plate in the circle of 20 holes, the
1 1 1
spindle will revolve by th turn of one revolution.
40 20 600
1.11 INDEXING METHOD
There are different indexing methods in popularity. These are :
(a) Direct indexing
(b) Simple indexing
(c) Compound indexing
(d) Differential indexing
Direct Indexing
It is also named as rapid indexing. For this direct indexing plate is used which has
24 equally spaced holes in a circle. It is possible to divide the surface of workpiece
into any number of equal divisions out of 2, 3, 4, 56, 8, 12, 24 parts. These all
numbers are the factors of 24.
In this case fist of all worm and worm wheel is disengaged. We find number of
holes by which spring loaded pin is to be moved. If we want to divide the surface
24
into 6 parts than number of holes by which pin is to be moved for 6 parts
N
N = 6.
26
So number of holes 4 holes that is after completing one pair of milling
6
whole surface of workpiece we have to move the pin by 4 holes before next
milling operation, that is to be done for 5 number of times for making hexagonal
bolt.
Simple Indexing
It is also named as plain indexing. It over comes the major limitation of direct
indexing that is possibility of dividing circumference of workpiece into some fixed
20
number of divisions. In this case worm and worm gear is first engaged. So one Milling
1
complete turn of indexing crank revolves the workpiece by th revolution.
40
Three indexing plates are used. These plates have concentric circles of holes with
their different numbers as described below :
Plate No. 1 15 16 17 18 19 20
Plate No. 2 21 23 27 29 31 33
Plate No. 3 37 39 41 43 47 49
These are the standard indexing plates followed by all machine tool
manufacturers.
Indexing Procedure
(a) Divide 40 by the number of divisions to be done on the circumference
of workpiece. This gives movement of indexing crank.
40
Indexing crank movement
N
N is the number of divisions to be made on the circumference of
workpiece.
(b) If the above number is a whole number, then crank is rotated by that
much number of revolutions after each milling operations, till the
completion of the work.
For example, if we want to divide the circumference into 10 number
of parts.
40
Indexing crank movement 4 revolutions.
10
That is the indexing crank is given 4 revolutions after each of milling
operation for 9 more milling operations.
40
(c) If indexing crank movement calculated by is not whole number,
N
it is simplified and then expressed as a whole number and a fraction.
(d) The fractional part of the above number is further processed by
multiplying its denominator and numerator by a suitable common
number so that the denominator will turn to a number equal to any
number of holes available on the any of indexing plates.
(e) That particular holes circle is selected for the movement of crank pin.
(f) The numerator of the process fraction stands for the number of holes
to be moved by the indexing crank in the selected hole circle in
addition to complete turns of indexing crank equal to whole number
40
part of .
N
Let us do the indexing to cut 30 teeth on a spur gear blank that means
we need to divide the circumference of gear blank into 30 identical,
parts. Crank movement is calculated s given below.
40 40
Crank movement
N 30
Here, N = 30.
10 1
1 1
30 3 21
Manufacturing Processes-III Let us multiply both numerator and denominator by 5.
5
1
15
Denominator becomes 15 so we will select 15 hole circle of plate 1.
Action 1
After each milling operation we will rotate indexing crank by one
complete turn and 5 holes in 15 holes circle. This way we do milling
total 30 times.
In this case we can multiply numerator and denominator by 7 a the
place of 5 as described below.
40
Indexing crank movement ( N 30 teeth)
N
40 10 1 7 7
1 1 1
30 30 3 7 21
Action 2
We will select the hole circle of 21 holes. After each milling
operation indexing crank will be rotated by 1 complete circle and
7 holes in 21 holes circle. This way milling operation will be done by
total 30 times.
Both the answers determined in the above problem are correct and
substitute of each other.
Limitations
This method can used for indexing upto 50 for any number of divisions
after 50 this method is not capable for some numbers like 96, etc.
Compound indexing overcomes the limitations.
Compound Indexing
The word compound indexing is an indicative of compound movements of
indexing crank and then plate along with crank. In this case indexing plate is
normally held stationary by a lock pin, first we rotate the indexing crank through a
required number of holes in a selected hole circle, then crank is fixed through pin.
It is followed by another movement by disengaging the rear lock pin, the indexing
plate along with indexing crank is rotated in forward or backward direction
through predetermined holes in a selected hole circle, then lock pin is reengaged.
Following steps are to be followed for compound indexing operation. The
procedure is explained with the help of numerical example.
Example 1.1
Let us make 69 divisions of workpiece circumference by indexing method.
(Using compound indexing)
Solution
Follow the steps given below :
(a) Factor the divisions to be make (69 = 3 23) N = 69.
(b) Select two hole circles at random (These are 27 and 33 in this case,
both of the hole circles should be from same plate).
(c) Subtract smaller number of holes from larger number and factor it as
(33 27 = 6 = 2 3).
22
