UNIT-V
MILLING MACHINES
Milling is another basic machining process by which surface is generated progressively by the
removal of chips from a work piece as it is fed against a rotating cutter. Milling operations can
be classified into two broad categories
1. Peripheral Milling 2. Face Milling
Peripheral Milling:
Peripheral Milling Operation performed by milling cutter to produce a machined surface
parallel to axis of rotation of cutter Cutting force is not uniform throughout the length of the
cut by each tooth Quality of surface generated and shape of the chip formed depends upon the
rotation of cutter relative to direction of feed movement It can be either UP milling or DOWN
milling
Up milling:
 Up milling Also called conventional milling
 Wheel rotation opposite of the feed
 The chip formed by each cutter tooth starts out very thin and increases its thickness
 The length of the chip is relatively longer
 Tool life is relatively shorter
 Need more clamping force to hold the work part still.
Down milling: :
 Down milling: Also called climb milling.
 Wheel rotation is parallel to the feed.
 The chip formed by each cutter tooth starts out thick and leaves out thin.
 The length of the chip is relatively short.
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Tool life is relatively longer.
Need less clamping force to hold the work part still.
FACE MILLING :
Face Milling The generated surface is at right angles to the cutter axis It is the combined
result of actions of the portions of the teeth located on both periphery and the face of
the cutter Most of the cutting is done by the peripheral portions of the teeth, with the
face portions provides the surface finish.
MILLING MACHINES:
Milling machines can be classified into general and production type. In
Engineering Workshop there are two types of machines available which are horizontal
and vertical milling machine.
Column & Knee Milling Machine:
Column & Knee Milling, Machine Column, spindle, cutter, table, knee,
base are the common parts of the vertical and horizontal milling machines. The milling
machines having only the three mutually perpendicular table motions (x-y-z axes) are
called plain column and knee type. Vertical type is especially well suited for face and end
milling operations.
Horizontal Milling Machine:
The most common type of milling machine is the horizontal knee type; so called, because of
the over-hanging ‘‘knee’’ which can slide up and down the front of the machine and which carries the
cross slide and the table. Horizontal milling machines may be either plain or universal type. The main
difference between the two is that the table of the universal type is mounted on a turn table and may
be swiveled in a horizontal plane. This feature permits the cutting of helix. In addition, the standard
accessories provided on the universal machine include a ‘dividing head’ for indexing. There are some
other minor refinements, which make the universal horizontal machine very useful for tool room work.
The plain version of the horizontal machine is much more robust and more suitable for production work.
In the diagram, the arbor on which peripheral cutters are mounted is not shown. It is fitted in the
spindle nose ‘C’ and extends a little beyond arbor supporting bracket ‘B’. Even end mills, face milling
cutters and drills etc. can be used with this machine. In such a situation, arbor is removed and the taper
shank of these cutters is fitted into the hollow spindle ‘C ’.With such an arrangement the vertical faces
of the work piece may be milled without any difficulty. The table of the horizontal milling machine can
be given either hand feed or auto feed. It is also capable of being traversed at high speed. With these
features, the machine proves really useful.
Vertical Milling Machine:
In vertical, milling cutters can be mounted directly in the spindle Housing
(Head) can be fixed or Swiveling type. Fixed Type-Head is always vertical and can be
adjusted up and down. Swiveling Type-Head can be swiveled to any desired angle.
Universal Milling Machine:
Has a table that can be rotated in a horizontal plane to any specified angle.
Bed Type Milling Machine:
Designed for mass production Achieves heavier feed rates and depth of cuts,
high MRR Work table is directly fixed on the bed of the machine tool. The cutter
mounted in a spindle head that can be adjusted vertically along the machine column
Three types are available w.r.t . the count of spindles available.
Simplex :
Fixed bed replaces saddle and knee. The table has longitudinal travel only
Spindle head carries spindle to which arbor is fitted to carry cutter that moves up and
down along column ways.
Duplex:
Has two spindles, permitting simultaneous milling of two surfaces at a single pass.
Triplex:
Has three spindles, permitting simultaneous milling of three surfaces at a single
pass.
Planer type milling machines:
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Consists of a fixed bed carrying a table having longitudinal movements.
Consists of 2 vertical columns on each side of the bed Cross rail fitter across columns can
be raised or lowered.
Each column carries a milling head (side/horizontal) moving upwards or downwards.
All cutters can be operated simultaneously to machine 4 surfaces at a time.
MILLING OPEATIONS:
Plain milling , face milling, side milling, straddle milling, angular milling, gang milling, form
milling, profile milling, end milling, saw milling, gear milling, helical milling, cam milling, thread
milling, Milling Keyways, Grooves & Slots
Plain Milling(Slab Milling) :
Operation of production of plane flat surface parallel to axis of rotation of plain milling
cutter. Cutter width extends beyond the work piece on both sides
Face Milling :
The face milling cutter is rotated about an axis perpendicular to the work surface
. End Milling
:
Combination of peripheral and face milling Cutter has teeth both on end face and
periphery Vertical milling machine is most suitable for end milling operation
side Milling :
Operation of production of flat vertical surface on side of work piece by using side milling
cutter Depth of cut adjusted by rotating vertical feed screw of the table
Straddle Milling :
Operation of production of flat vertical surfaces on both side of the workpiece by using
two side milling cutters mounted on the same arbor
Angular Milling :
Operation of production of angular surface on a workpiece other than at right angles to
the axis of milling machine spindle
Gang Milling :
Operation of milling several surfaces of the workpiece simultaneously by feeding the
table against a number of cutters having same or different diameters
Form Milling :
It is the operation of production of irregular contours using form cutters The contours
maybe convex, concave or any other shape
Profile Milling :
It is the operation of reproduction of an outline of a template or complex shape of a
master die on a workpiece
Milling Keyways, Grooves & Slots :
Operation of production of keys, grooves and slots of varying shapes and size by using a
plane milling cutter, a metal slitting saw, an end mill or by side milling cutter
Thread Milling :
Operation of production of threads by using a single or multiple thread milling cutter. It
is performed in special thread milling machine to produce accurate threads
Milling cutters:
1. According to the Construction of Milling Cutter
(a) Solid milling cutter
(b) Inserted teeth cutter
(c) Tipped solid cutter
2. According to Relief Characteristics of the Cutter Teeth
(a) Profile relieved cutter
(b) Form relieved cutter
3. According to Method of Mounting the Cutters
(a) Arbor type (b) Facing cutter (c) Shank cutter
4. According to Direction of Rotation of the Cutter
(a) Right hand rotational cutter
(b) Left hand rotational cutter
5. According to the Direction of Helix of the Cutter Teeth
(a) Parallel straight teeth
(b) Right hand helical
(c) Left hand helical
(d) Alternate helical teeth
6. According to Purpose of Use of the Cutter
(a) Standard milling cutter
(b) Special milling cutter
Solid cutter
Solid cutter consists of teeth integral with the cutter body, in tipped cutter, teeth
are made of cemented carbide or satellite, teeth are brazed to steel cutter body
called shank.
Inserted cutter
Inserted teeth cutter are larger in diameter, teeth of hard material are inserted
and secured in the shank.
Tipped cutter
Similar to solid cutter but the teeth are made up of cemented carbide or stellite
tips.
profile relieved cutter
In case of profile relieved cutter, a relief to cutting edges is provided by grinding a
narrow land at their back.
Form relieved cutter
In case of form relieved cutters a curved relief is provided at the back of the
cutting edges.
Arbor type
Arbor type cutters have a central hole and keyways for their mounting on arbor
Facing cutter
Facing type milling cutter are used to produce flat surfaces. These are balled or
attached to the spindle nose or the face of a short arbor (stub arbor).
Shank cutter
Shank type cutters are provided with straight or tapered shanks inserted into the
spindle nose and clamped there.
Right hand rotational cutter
A right hand rotational cutter rotates in an anticlockwise direction when viewed
from end of the spindle
Left hand rotational cutter
left hand rotational cutter rotates in a clockwise direction.
Parallel straight teeth
Parallel or straight teeth cutter consists of teeth parallel to axis of rotation of the
cutter with zero helix angle.
Right hand helical & left hand helical teeth
In case of right hand and left hand helical teeth cutters, teeth cut at an angle to
the axis of rotation of the cutter. Teeth have opposite inclination in both the
cutters.
Alternate helical teeth
Alternate helical teeth cutter has alternate teeth of right hand and left hand
helical teeth cutters
Standard milling cutter
These are conventional type of milling cutters whose dimensions such as cutter
diameter & width, diameter of center hole, width & depth of key ways , etc, are
standardized.
Special milling cutters
Designed to perform special operations which may be the combination of serval
standard operation.
Methods of indexing:
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
work piece 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 into 6 parts than number of holes by which pin is to be moved
24/N for 6 parts
N = 6.
So number of holes 26/6=4 holes that is after completing one pair of
milling whole surface of work piece 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 work piece into
some fixed number of divisions. In this case worm and worm gear is first engaged.
So one complete turn of indexing crank revolves the work piece by1/40th
revolution.
Three indexing plates are used. These plates have concentric circles of
holes with their different numbers as described below :
15
Plate No. 1
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 work piece. This gives
movement of indexing crank.
Indexing crank movement 40/N
N is the number of divisions to be made on the circumference of work piece
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
Indexing crank movement 40/10 =4 revolution
That is the indexing crank is given 4 revolutions after each of milling operation for 9
more milling operations
c) If indexing crank movement calculated by 40/N is not whole number, 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 part of 40/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.
Crank movement=40/N=40/30
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.
CUTTING PARAMETERS
There are three major cutting parameters to be controlled in any milling operation. These
three parameters are cutting, speed, feed rate and depth of cut. These parameters are
described below
Cutting Speed
Cutting speed of a milling cutter is its peripheral linear speed resulting from operation. It is
expressed in meters per minute. The cutting speed can be derived from the above formula
V= dn/1000
where d = Diameter of milling cutter in mm,
V = Cutting speed (linear) in meter per minute, and
n = Cutter speed in revolution per minute.
Spindle speed of a milling machine is selected to give the desired peripheral speed of cutter.
Feed Rate
It is the rate with which the work piece under process advances under the revolving milling
cutter. It is known that revolving cutter remains stationary and feed is given to the work piece
through worktable. Generally feed is expressed in three ways.
Feed per Tooth
It is the distance traveled by the work piece (its advance) between engagement by the two
successive teeth. It is expressed as mm/tooth (ft).
Feed per Revolution
Travel of work piece during one revolution of milling cutter. It is expressed as mm/rev. and
denoted by f(rev).
Feed per Unit of Time
Feed can also be expressed as feed/minute or feed/sec. It is the distance advances by the work
piece in unit time (fm).
Above described three feed rates are mutually convertible
fm n frev
where n = rpm of cutter.
It can be extended further as
fm n frev z n ft
where z = Number of teeth in milling cutter
Depth of Cut
Depth of cut in milling operation is the measure of penetration of cutter into the work piece. It is
thickness of the material removed in one pairs of cutter under process. One pairs of cutter means when
cutter completes the milling operation from one end of the work piece to another end. In other words, it is
the perpendicular distance measured between the original and final surface of work piece. It is measured
in mm.