Lecture # 15
Dr. N. VENKAIAH
Assistant Professor
Mechanical Engineering Department
NIT Warangal 506 004
Disclaimer
The content presented here is not entirely my own. Some portions are taken from
different sources with great regard. This content is solely for class room teaching and
not for any commercial use.
Don't measure yourself by what you have accomplished, but by
what you should have accomplished with your ability.
~ John Wooden
Types of dividing heads
Plain dividing
head
Used for
Used for
small number of
divisions on the
periphery
precise indexing and
for checking the accuracy
of various angular surfaces
Drawback:
cannot be used in milling
of helical gears.
Used for
1.Setting the work in
vertical, horizontal or in
inclined positions relative
to the table surface
2.Turning the w/p
periodically through a
given angle to impart
indexing movement
3.Imparting a continuous
rotary motion to the w/p
for milling helical grooves
This is the most widely used type of
dividing head.
Periodical turning of the spindle (3)
is achieved by rotating the index
crank (2), which transmits the
motion through a worm gearing 6/4
to the WP with a gear ratio 1:40;
(One complete revolution of the
crank
corresponds
to
1/40
revolution of the WP).
The index plate (1), having several
concentric
circular
rows
of
accurately and equally spaced holes,
serves for indexing the index crank
(2) through the required angle.
The WP is clamped in a chuck
screwed on the spindle (3). It can
also be clamped between two
centers.
1 index plate;
2 index crank;
3 spindle;
4 & 6 worm wheel and worm;
5 plunge;
7 sector arms
The dividing head is provided with either Brown and Sharpe type (three index plates) or
Parkinson type (two index plates) index plates
Simple Indexing
Work is positioned by means of index crank,
index plate, and sector arms
40 teeth on worm wheel
Single threaded worm
One complete turn on index crank causes spindle
and work to rotate 1/40th of a turn
Kinematic balance equation:
Number of index crank revolutions, n =
Z = Number of divisions on the job
1 Index plate
2 Index crank
3 Work spindle
4 Lock pin
40
Z
Determine the suitable index plates (Brown and
Sharpe) and the number of index crank revolutions
(n) necessary for producing the following spur gears
of teeth number 40, 30, and 37 teeth.
Solution:
40 Teeth
30 Teeth
37 Teeth
Select any index plate
Select index plate 1
with 18 holes
Select index plate 3 with
37 holes
To avoid errors in counting the number of holes, the adjustable sector on the
index plate should be used.
7 sector arms
Angular Indexing
Setup used for simple indexing may be used
Must calculate indexing with angular distance
between divisions instead of number of divisions
One complete turn of index crank turns work
1/40 of a turn
1/40 of 360o = 9 degrees
No. of degrees required
Indexing in degrees =
9
11
Angular Indexing
Calculate indexing for 45o
Calculate indexing for 60o
45
Indexing =
= 5
9
60
2 10
Indexing =
=6 =6
9
3 15
5 complete turns
6 full turns plus 10 holes
on 15 hole circle
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
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Angular Indexing
Calculate indexing for 2430'
First, convert angle into minutes
24o x 60' +30' = 1470'
9x60' = 540'
Divide
1470'/540' = 2 13/18
2 full turns and 13 holes on 18 hole circle
13
Angular Indexing
Calculate indexing for 24'
Divide
24'
24'
4
1
=
=
=
9x60 540' 90 22.5
1 hole on a 22.5 hole circle
The nearest is a 23 hole circle. Indexing would be 1 hole
on a 23 hole circle with a slight error (approximately 1/2
minute).
A need for higher accuracy requires differential indexing.
14
It is employed where simple indexing cannot be
effected;
That is, when an index plate with required number of
holes for simple indexing is not available.
Index plate must be revolved either forward or
backward as the index crank is turned to attain required
spacing
Change of rotation is effected by idler gear(s)
1 index plate
2 index crank
3 the work spindle
4 lock pin
5 - plunge
a, b, c, d differential
change gears
Z and Z Required and
approx. no. of teeth
from which
The change gears to match Brown and Sharpe index plates:
24(2), 28, 32, 40, 44, 48, 56, 64, 72, 86, and 100 teeth.
Simple gear train
One idler for positive rotation of index plate and
No or Two idlers for negative rotation
Compound gear train
One idler for negative rotation of index plate and
No or Two idlers for positive rotation
Select the differential change gears and the index plate (Brown and
Sharpe), and determine the number of revolutions of the index
crank for cutting a spur gear of Z = 227 teeth.
Solution:
Assume Z = 220
a = 64, b = 32, c = 28, and d = 44 teeth with an idler gear
(-ve sign)
After arranging these gears, Indexing would be 6 holes on a 33
hole circle
Index 83 divisions
Solution:
Let Z = 86
Index crank movement = 20/43
Change gears = (72/24)x(40/86) with no or two idlers
Setting the dividing head for milling helical grooves
The table is set to the spindle axis at an angle
Where
h = helix angle of the groove
thel = lead of helical groove (mm)
D = diameter of the W/P (mm)
The table is swiveled clockwise for left-hand grooves
and CCW for right-hand grooves
The kinematic balance equation is given by:
Change gears
Where, tls = lead of worktable lead screw (mm)
It is required to mill six right-hand helical flutes with a
lead of 600 mm; the blank diameter is 90 mm. If the
pitch of the table lead screw is 7.5 mm, give complete
information about the setup.
