Milling Machine, Working principle of Milling Machine, Specifications of milling machine, Types of Milling Machine, Column and knee type milling machines, Vertical milling machine, Universal milling machine, Basic milling operations, Classification of milling cutter, Milling cutter nomenclature, Milling cutter materials, Cutting parameters
Introduction to Milling Machine:
Milling machine is a machine tool that form a work piece as
it is fed against a rotating multi point cutting tool called milling cutter.
The cutter rotates at a high speed and as it is provided with many cutting
edges, it removes the metal at a very fast rate. The milling machine can also
hold one or more number of cutters at a time. Milling machine finds wide
application in production work. Milling machine is one of the most important
machine tool widely used in tool room as nearly all the operations can be
performed on it with high accuracy and better surface finish. Milling machine
is used for machining flat surfaces, contoured surface, cutting key ways, slots,
grooves, cams, gears, thread cutting, drilling, boring and other kinds of
operation through a wide range of sizes.
Working principle of Milling Machine
In the milling process, metal is removed by moving the
workpiece against a rotating multipoint milling cutter. The milling cutter is
mounted on a rotating spindle called arbor. The workpiece is clamped on the
table. The cutter rotates at the required cutting speed. The work is fed slowly
past the milling cutter. The workpiece can be fed vertical, longitudinal or
crosswise. As the work is fed against the cutter, the cutting edges remove the
metal in the form of chips as shown in figure below.
Specifications of milling machine:
Milling machines can be specified in following ways,
•The size of the work table
•Maximum length of longitudinal, cross and vertical feeds
•Type of milling machine
•Power of the driving motor
•Number of spindle speeds
•Number of feeds
•Floor space required.
Types of Milling Machine
In order to meet the various requirements of present day
production, number of milling machines have been manufactured. According to the
general design, the milling machines are classified as,
1.Column and knee type milling machine:
•Hand milling machines
•Plain or Horizontal milling machine
•Universal milling machine
•Vertical milling machine
•Omniversal milling machine.
2.Fixed bed type milling machines:
•Simple milling machine
•Duplex milling machine
•Triplex milling machine.
3.Planer type milling machine.
4.Special purpose milling machine:
•Rotary table milling machine
•Drum milling machine
•Planetary milling machine
•Profile milling machine
•Pantograph milling machine
•Tracer controlled milling machine
Most milling machines used in metal processing are of the
column and knee type. It derives the name column and knee type from its
appearance. The work table is supported on a knee like casting, which can slide
in a vertical direction along a column. Here we will discuss about Column and
Knee type Milling Machine.
Column and knee type milling machines
•Hand milling machine:
In hand milling machine, the workpiece mounted on the table
is fed against the cutter by hand control. The cutter is mounted on a
horizontal arbor and is rotated by power. It is particularly used for light and
milling operation such as machining slots, grooves and key-ways.
•Plain milling machine:
In plain milling machine, a rotating cutter is mounted on a
horizontal spindle or arbor, and hence it is called as horizontal milling
machine.
Constructional details of Plain Milling Machine
In this machine, both the base and column are integral
casting The plain milling machine has following parts:
Base:
The base is made up of cast iron. It acts as the foundation
of the whole machine. It supports the entire structure of the machine. It gives
strength and rigidity to the machine. It should absorb vibrations, if generated
during machine. In some machines, the base is hollow and serves as a reservoir
for cutting fluid.
Column:
Column is the main supporting frame mounted vertically on
the base. The column houses all the driving mechanisms for the spindle and
table feed. The front vertical face of the column is provided with guideways,
on which the knee can slide up and down. The top of the column is accurately
finished to hold an overarm that extends outward at the front of the machine.
Knee:
The knee supports the saddle, table, workpiece and other
clamping devices. It can slide up and down on the vertical guideways of the
column. The height of knee can be adjusted by operating an elevating screw. The
knee houses the feed mechanism of the table. The top face of the knee forms a
slideways for saddle to provide cross travel of the table.
Saddle:
The saddle supports and carries the table. It is mounted on
the guideways provided on the top of the knee and can be moved by hand wheel or
by power.
Table:
The table is mounted on the saddle and it travels
longitudinally. The top surface of the table is accurately machined and T-slots
are provided, for clamping the workpiece and other fixtures on it. Table moves
perpendicular to the direction of saddle movement.
Arbor:
An arbor is an extension of the machine spindle on which milling
cutters are securely mounted and rotated. The arbor is provided with taper
shanks for proper alignment with the machine spindle. The position of the
cutter can be adjusted with spacer.
Spindle:
The spindle of the machine is located in the upper part of
the column and receives power from the motor through belts and gears and
transmits it to the arbor. The front end of the spindle projects from the
column face and is provided with a tapered hole into which arbors may be
inserted.
Working principle Plain milling machine:
In this machine, the workpiece is placed on the table, and
the table may be fed in longitudinal [back and forth], cross [in and out] and
vertical [up and down] directions. The feed is longitudinal when the table is
moved parallel to the spindle, and the feed is vertical when table is adjusted
in a vertical plane. The table may be fed by hand or power. The plain milling
machines are more rigid and heavier in construction than hand milling machine
and accommodates heavy workpiece. The multi-point milling cutter is mounted on
a rotating arbor. The cutter is rotating at the particular cutting speed. When
the work is fed the cutting edges of the cutter remove the metal from the workpiece.
Thus the milling is done by a milling machine.
Vertical milling machine:
In Vertical milling machine, the position of the spindle
head is vertical and the axis of the spindle is perpendicular to the work
table. In this both the base and column are integral casting. The vertical
milling machine may have fixed head or swiveling head or slidable head spindle.
The spindle head can be moved up and down over the guideways. The saddle is
mounted over the knee guideways and it can be moved on the guideway
horizontally either by hand or power. The knee can be moved up and down over
the guideways provided on the column face. The work table is mounted on the
saddle and can be moved longitudinally over the guideways provided on the top
of the saddle. The workpiece can be moved both in vertical and horizontal
plane. This machine is used to machine grooves, slots and flat surface using
end mill and face mill cutter.
Universal milling machine:
The universal milling machine is so named because it is
capable of handling a very wide range of jobs. It closely resembles with the
plain milling machine. The table of a universal milling machine is mounted on a
circular swiveling base which has degree graduations, and the table can be
swiveled to any angle upto 45 degree on either side of the normal position.
Thus, in a universal milling machine, in addition to three movements provided
on plain milling machine, the table may have fourth movement when it is fed at
an angle to the milling cutter. This additional feature makes it to perform
helical milling operation. The capacity of a universal milling machine is considerably
increased by the use of special attachments such as dividing head or index
head, vertical milling attachments, rotary attachment, slotting attachment,
etc. The machine can produce spur, spiral, bevel gears, twist drills, reamers,
milling cutters, etc.
Basic milling operations:
Milling is a process of removing metal with help of a rotary
cutting tool provided with multi cutting edges on periphery and face. Although
jobs of different shapes and sizes are performed on a milling machine, but the
basic milling operations performed on them are,
• Plain milling
• Face milling
• Angular milling
• Straddle milling
• Form milling
• Profile milling
• Saw milling
• End milling
• Gear milling
• Slotting
• Helical milling
• Cam milling
• Thread milling
• Gang milling.
Plain milling:
Plain milling can be otherwise called as slab milling.
Generally plain milling is used to machine flat and horizontal surfaces. Here
plain milling cutter is used, which is held in the arbor and rotated. The table
is moved upwards to give the required depth of cut. According to the relative
movement between the table and workpiece, the plain milling classified as
up milling and down milling.
• Up milling:
Up milling is the process of removing meta] by a cutter which is rotated against the direction of travel of the workpiece. Up milling process can be otherwise called as conventional milling. During up milling, workpiece material is removed by the resultant force. There is a risk that the workpiece may be pulled out of the mounting or that the milling table will buckle. This up milling can be otherwise called as conventional milling. Diameter of shank ranges from 10 mm to 50 mm.
• Down milling:
In clown milling the direction of milling cutter rotation is the same as the workpiece’s feed direction. The milling cutter approaches from the thickest part position of the chip.In down milling, the feed motion angle range from 90° to 180°. The resultant forces presses the workpiece against the base. In cases where the cutter arbor is insufficiently stiff, the milling cutter climbs onto the workpiece, and cutting edges breaks off. During down milling the resultant force direction coincides with feed motion direction. Resultant cutting force in downward leads to fixtures and holding devices simpler and less costly. Cutters with high rake angle can be used for down milling.
Advantages of down milling:
•Less power is needed.
•Chip disposal is easier and better surface finish.
•Chip thickness is maximum at the start of the cutter and minimum at the end.
•Resultant cutting force in downward leads to fixtures and holding devices simpler and less costly.
•Cutters with high rake angle can be used for down milling.
•Less tool wear.
•Preferred for slitting thin sheets.
•Preferred on rigid machines only.
•Higher speeds and feeds can be employed.
Face milling:
In face milling, the tool axis is perpendicular to the
surface to be generated. However, in face milling, the tool does not cut with
its face, as the name of the method indicates the face milling. But, as in
peripheral milling, peripheral the cutting edges remove metal primarily and
face cutting edges acts as secondary cutting edges. As the result, face milled
surface have a high surface quality.
Straddle milling:
Straddle milling is an operation in which a pair of side milling
cutters is used for machining two parallel vertical surfaces of a workpiece
simultaneously. The distance between the cutters is adjusted by the spacers.
This process is used to mill square and hexagonal surfaces.
Gang milling:
The gang milling is the operation of machining several
surfaces of a workpiece simultaneously by feeding the table against a number of
cutters having same or different diameters mounted on the arbor of the machine.
This method save much machining time and is widely used in repetitive works.
The cutting speed of a gang cutters are calculated from the cutters of the
largest diameter.
Form milling:
The form milling is the operation of producing irregular
contours by using form cutters. The irregular contour may be convex, concave,
or of any other shape. After machining the formed surface is checked by a
template gauge. The cutting speed for form milling is 20-30% less than that of
plain milling.
Profile milling:
Profile milling is the operation of reproducing outlines of
the templates or complex shape of a master die on a workpiece. Different
cutters may be used for different profile milling. An end milling is one of the
most widely used milling cutter in profile milling works.
End milling:
End milling is an operation of producing narrow slots,
grooves and key ways, which may be vertical, horizontal, or at an angle in
reference to the table surface. The cutter used is an end mill. A vertical
milling machine is most suitable for end milling operation.
Angular and T-slot milling
Angular milling:
The angular milling is the operation of producing an angular
surface on a workpiece other than at right angles to the axis of the milling
machine spindle. The angular groove may be single or double angle and may be of
varying included angle according to the type and shape of the angular cutter
used. One simple example of angular milling is the production of V-block.
T- slot milling:
In T-slot milling operation, first a plain slot is cut on
the workpiece by a side and face milling cutter. Then the T-slot cutter is fed
from the end of the workpiece.
Gear milling:
The gear milling operation is performed in a milling machine
by using a form relieved cutter. The cutter may be cylindrical type end mill
type. The cutter carries the profile on its cutting teeth corresponding to the
required profile of the gap between gear teeth.
Cam milling:
The cam milling is the operation of producing cams in the
milling machine by the use of a universal dividing head and a vertical milling
attachment. The cam blank is mounted at the end of the dividing head spindle
and an end mill is held in vertical milling attachment. The axis of the cam
blank and the end mill spindle should always remains parallel to each other
when set for cam milling. The dividing head is geared to the table feed screw
so that the cam is rotated about it axis while it is fed against the end mill.
The axis of the cam can be set from 0 to 90° in reference to the surface of the
table for obtaining different rise of the cam.
Saw milling:
The saw milling is the operation of producing a narrow slots
or grooves on a workpiece by using a saw milling cutter. The saw milling can
also be performed for complete parting-off operation. The cutter and the
workpiece are set in a manner so that the cutter is directly placed over one of
the T-slot of the table.
Milling cutters:
The milling cutters are revolving tools having several
cutting multi point edges of identical form equally spaced on the circumference
of the cutter. The cutting elements are called teeth which intermittently
engage the workpiece and remove material by relative movement of the workpiece
and cutter. Milling cutters are made of high carbon steels, high speed steels,
with sintered carbide tips, or of certain case non-ferrous alloys. High speed
steel cutters are generally used for milling operations, because they can
maintain a keen cutting edge at high temperatures. All cutters generally have
two types of cutting teeth, straight or spiral teeth and form relieved teeth.
Each milling cutter is identified by the outer diameter, thickness, number of
teeth and the bore diameter, apart from the material of the cutter.
Classification of milling cutter:
• Plain milling cutter
• Side milling cutter
• Metal slitting saw
• Angle milling cutter
• End-mills
• T-slot milling cutter
• Slot-drill
• Woodruff key slot milling cutter
• Fly cutter
• Form milling cutter.
Plain milling cutter:
• It has straight or helical cutting edges only on the
periphery.
• It produces flat surfaces parallel to the cutter axis.
• Do not carry cutting edges on its sides.
• With more width it becomes a slab mill.
Plain milling cutters are divided into;
(i) Light duty cutter:
• Removes less material
• Face width less than 20mm, straight teeth
• Parallel to the axis
• Face width more than 20mm, helical teeth with 25° helix
angle.
(ii) Heavy-duty plain milling cutter:
• With wider face width and helical teeth, removes more
material.
• Helix angle 25° to 45°, better shearing takes place.
• It gives a fine surface finish.
(iii) Helical plain milling cutter:
• It has as an helix angle between 45° to 60°
• Due to better shearing, provide better surface finish
• It is adapted for taking light cuts on soft materials.
Side milling cutter:
• They have the cutting edges not only on the face, but also
on both the sides of the cutter.
• It is used for slot milling and side milling.
• The diameter of the cutter ranges from 50 mm to 200 mm.
• Width range is 5 mm to 32 mm.
This side milling cutter can be classified as follows;
(i) Plain side milling cutter:
• In this, the cutting edges or teeth are straight on face
and both sides of the tool.
(ii) Half-side milling cutter:
• It has straight or helical teeth on its circumferential
surface and on one of its sides only.
• The peripheral teeth do the actual cutting, where as the
side teeth make them to size and finish the work.
(iii) Staggered teeth side milling cutter:
• In this, the teeth are arranged in an alternate helix
pattern.
• Provides greater chip space.
• These cutters are widely used for deep slots.
(iv) Interlocking side milling cutter:
• Cutter is formed out of two half side milling cutters or
two staggered teeth side milling cutters which are made to interlock to form
one unit.
• These are used for milling wider slots of accurate width.
• The heavy duty slotting in all types of materials, and can
make deeper cuts under coarser, width of the cutter may be varied by inserting
spacers of suitable thickness between the two halves of the cutter.
• The cutters are available in diameters ranging from 50 to
200 mm.
Metal slitting saw:
• Metal slitting saw resembles a plain milling cutter or a
side milling cutter in appearance but they are of very small width.
• These are used for parting off operation or slotting.
These metal slitting saw can be classified as follows;
(i) Plain metal slitting saw:
Plain metal slitting saw is thinner in construction and its
width is limited to 5 mm. The cutter is thinner at the center than at the edge,
to provide clearance and to prevent the cutter from binding in the slot.
(ii) Straggered teeth metal slitting saw:
Straggered teeth metal slitting saw resemble a staggered
teeth side milling cutter, but the width of the cutter is limited to 6.5 to 7
mm. These saws are used for cuts.
Angle milling cutter:
• It is designed to mill at an angle to the axis of
rotation.
• They are used for milling surfaces at various angles to
the axis of rotation, and are often used in making other milling cutters.
• The different types of angle milling cutters are;
(i) Single angle milling cutter:
• Single angle milling cutter has teeth on the conical or
angular face of the cutter and also on the large flat side. These cutters have
one side at an angle of 90° to the axis of the rotation, and the other side at,
usually, either 4-5° or 60°.
• Only one side of this cutter cuts at an angle other than
90° to the rotation axis.
• These are used to mill ratchet teeth or to mill dovetails.
(ii) Double angle milling cutter:
• Double angle milling cutter has V-shaped teeth with both
conical surfaces at an angle to their end faces.
• The two end faces on the outside diameter are neither
parallel nor perpendicular to the axis of rotation of the cutter body.
• They are available with included angles of 45°, 60° or
90°.
• These are used for cutting spiral teeth in milling
cutters.
End milling cutter:
• End mill has cutting teeth on the end as well as on the
periphery of the cutter. The peripheral teeth may be straight or helical and
the helix may be right hand or left hand.
• The end mills are used for light milling operations like
cutting slots, machining accurate holes, producing narrow flat surfaces and for
profile milling.
The different types of end mills are;
(i) Taper shank end mill:
• Taper shank end mill has a tapered shank or extension on
one end for mounting and driving the cutters.
• The tapered shank can be directly mounted in the spindle
with the help of the self- holding taper.
• If the taper is small compared to the spindle taper, then
an adopter accommodating both tapers are used.
• Diameter of shank ranges from 10 mm to 50 mm.
(ii) Straight shank end mill:
• Straight shank end mill has round shanks for mounting and
driving the cutters.
• The straight shank is used on end mills of small size and
is held in the milling machine spindle with the help of a suitable collet.
(iii) Shell end mill:
• Shell end mill is larger and is provided with a central
hole for mounting the cutter on a short arbor.
• The cutting edges are provided at the end and around the
periphery of the cutter.
• The teeth may be straight or helical and may be left or
right handed.
• Face milling operations are usually performed with these
cutters.
T-slot milling cutter:
• T-Slot milling cutter is special form of end mills for
producing T-slots.
• The tooth are provided on the periphery as well as on both
sides of the cutter.
Woodruff key slot milling cutter:
• These cutters are similar in appearance to a T-slot
cutter, and is used for the production of woodruff key slots.
• The cutter is provided with a shank and may have straight
or staggered teeth.
Fly cutter:
• Fly cutters has a single point cutting tool attached to
the end of an arbor.
• Set screws are used to hold the cutting tool in place.
• The cutting edges can be formed to any shape to produce
profiles on the workpiece.
• It is not very efficient, since it removes only one chip
per revolution of the spindle.
• Fly cutter is considered as an emergency tool when the
standard cutters are not available.
Form milling cutter:
• Form milling cutter has irregular profiles on the cutting
edges in order to generate irregular outline of the work.
• These form milling cutters can be classified as;
(i) Convex milling cutter:
• Convex milling cutter has teeth curved outwards on the
circumferential surface.
• The cutter produces a concave semi circular surface on
workpiece.
(ii) Concave milling cutter:
• Concave milling cutter has teeth curved inwards on the
circumferential surface.
• The cutters produce a convex semicircular surface on
workpiece.
(iii) Gear cutter:
• Gear cutter has formed cutting edges to produce the shape
of the cutter teeth on the gear blank.
• The shape of the cutter teeth may be involutes or cycloid
according to the gear tooth profile.
• The cutter tooth profile should be differently shaped for
each pitch and number of teeth, on the gear to be cut.
Milling cutter nomenclature:
Nomenclature of a milling cutter is shown in the figure
below.
• Outside diameter: The diameter of the circle passing
through the peripheral cutting edges.
• Root diameter: The diameter of the circle passing tangent
to the bottom of the fillet.
• Cutter face width: The cutter face is the surface at the
side or end of the cutter body which is perpendicular to the axis of the
cutter.
• Tooth face: It is that surface of the cutting tooth
against which the chip is forced in the metal cutting operation.
• Land: It is the part of the back of the tooth adjacent to
the cutting edge.
• Cutting edge: The cutting edge is the intersection of the
face of the tooth with the leading edge of the land.
• Flute: The flute is the chip space between the back of one
tooth and the face of the following tooth.
• Relief angle: The peripheral relief angle is the angle
between the surface formed by the land and a tangent to the cutter outside
circle passing through the cutting edge in a diametral plane.
• Radial rake angle: The radial rake angle of a milling
cutter is the angle formed in a diametral plane between the face of the tooth
and a radial line passing through the cutting edge. This may be positive,
negative or zero degree.
• Axial rake angle or Helical rake: When a milling cutter
has helical teeth, that is, when its cutting edge is formed along a helix about
the cutter axis, the resulting rake is called helical rake.
• Gash depth: Gash depth is the distance from the outside
diameter of the cutter to the fillet radius or root diameter.
• Fillet radius: The fillet radius is the curved surface at
the bottom of the flute which joins the face of one tooth to the back of the
tooth immediately ahead.
• Depth of recess: The distance from the cutting edge on the
land of the side tooth [or the hub which is the same width as the cutter] to
the recess is the depth of recess. This dimension is required to determine
width and angle modification limits.
• Hub diameter: The hub is the raised ground section between
the bore and recess. It is the same width as the cutter.
Tool signature:
The seven important elements comprise the signature of the
milling cutter and are stated in the following order:
• Axial rake angle
• Radial rake angle
• Face relief angle
• Peripheral relief angle
• Face cut edge angle
• Peripheral cutting edge angle
• Chamfer.
Milling cutter materials:
The common materials used for manufacturing milling cutter
are;
• High carbon steel
• High speed steel
• Cemented carbides
• Ceramics
• Stellite.
High carbon steel is not commonly used except for a few
cutters used for small scale production. High speed steel cutters containing
18% tungsten, are the most widely used cutters in general shop work. These
cutters are successfully used for plain milling or slotting operations where
the chip thickness is small. Cemented carbides are very commonly used when
milling cutters are to be run at higher speeds. Cemented carbide tipped cutters
has the following advantages;
• Their production capacity is high.
• They can produce surface finish of high quality.
• Hardened steels can be machined by them.
• Their use leads to reduction in machining cost.
The cemented carbide cutters are unsuitable for deep end
milling, deep narrow slotting and complicated forming operations. Stellite is
very useful material for milling cutters when particularly machining hard
metals, forgings and castings. Tips made of ceramics are also used in milling
cutters. Such cutters, because of their brittleness, they are used for
finishing operations and smaller cuts.
Cutting parameters:
Cutting speed:
The speed of milling cutter is its peripheral linear speed
resulting from rotation. It is expressed in meters per minute. The cutting can
be derived from the formula.
Where, v = Cutting speed in metres/min
d = Diameter of the cutter in mm
n = Cutter speed in r.p.m.
Feed:
It is defined as the rate with which the workpiece advances
under the cutter. The feed is expressed in a milling machine by the following
three different methods.
Feed per tooth:
The feed per tooth is defined by the distance the work
advances in the time between engagement by two successive teeth. It is
expressed in millimetres per tooth of the cutter.
Feed per cutter revolution:
The feed per cutter revolution is the distance the work
advances in the time when the cutter turns through one complete revolution. It
is expressed in millimetres per revolution of the cutter.
Feed per minute:
The feed per minute is defined by the distance the work
advances in one minute. It is expressed in millimeters per minute.
The feed per tooth, the feed per cutter revolution and the
feed per minute are related by the formula which is given below
= Feed per tooth
= Feed per cutter revolution
= Feed per minute.
Depth of cut:
The depth of cut in milling is the thickness of the material
removed in one pass of the work under the cutter. It is the perpendicular
distance measured between the original and final surface of the work piece, and
is expressed in mm.
Calculation of machining time:
The time required to mill a surface for any operation can be
calculated from the formula;
In fig. the length of the table travel ‘L’ is composed of
two parts. The length of the work ‘C’ and the approach length ‘A’ is the
distance through which the cutter must be moved before the full depth of cut is
reached.
Approach length for plain milling cutter:
The approach ‘A’ for a plain milling cutter can be
calculated from the equation.
Where, A = Approach length in mm
B = Depth of the cut in mm
D = Diameter of the cutter in mm.
END
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