KEJURUTERAAN MEKANIKAL III
KOD SUBJEK : BDA 2801
1.1 Learning Outcome
At the end of this module, the student should be
Apply and practice workshop safety
regulations during working in CNC workshop.
Apply the concept of CNC metal cutting
operation correctly in milling process.
Carry out CNC mill machining procedures
Utilize the important parameters in CNC mill
machining operation effectively according to
a given task
Milling is a cutting operation with a geometrically
specified cutting edge in which the tool makes the
rotating main movement, and the feed as well as the
infeed movement are generally made by the work part.
Milling operations are classified according to the
position of the milling axis towards the work part, i.e.
between face milling and peripheral milling. In case of
face milling, the milling axis is located vertically to the
The work part surface is machined by the main cutting
edges. Also, the work part surface is further finished
with auxiliary cutting edges
synchronous and conventional milling (Figure 1.4
and 1.5) are differentiated.
In case of conventional milling the rotation
direction of the milling tool is opposite to the feed
direction of the work part.
The milling tool chamfer edge starts with chip
The milling tool cutting edge slides in front of the
chip chamfer edge until the required minimum
chip thickness has been achieved for chip
For synchronous milling the rotation
direction of the milling tool and the feed
movement of the work part are parallel.
The tooth of the milling cutter immediately
penetrates into the work part.
Since the milling tool cutting edge is exposed to
impact forces the feed drive needs to be play
free. Several cutters should always be in
The surface quality is flatter and duller when
synchronous milling is used.
Compared with conventional milling higher feed
movements and cutting speeds within the same
cutting edge life can be achieved.
Due to the cutting path comma-form chips
are cut with a changing chip thickness
Figure 2.6 : Milling Plan
1.3 Theory of CNC Milling
1.3.1 Characteristics of CNC Milling
Work part machining on CNC machine
tools requires controllable and adjustable
infeed axes which are run by the servo
motors independent of each other.
CNC- milling machines (Figure 1.7) on the
other hand have at least 3 controllable or
adjustable feed axes marked as X, Y, Z.
Figure 2.7 : Controllable NC Axes on a Milling Machine
1.3.2 Basic Geometry for
a) Coordinate Systems On CNC
Coordinate systems enable the
exact description of all points on a
work plane or room. Basically there
are two types of coordinate systems
• Cartesian coordinate
A Cartesian coordinate system, also
called rectangular coordinate system
includes for the exact description of the
Two coordinate axes (two-dimensional
Cartesian coordinate system) or also.
Three coordinate axes (three-dimensional
Cartesian coordinate system), located
vertically to each other
• Polar Coordinate
In the Cartesian coordinate system a point is
described, for instance, by its X and Y coordinates. For
rotation symmetrical contours, such as circular boring
patterns, calculating the needed coordinates requires
In the polar coordinate system a point is specified by its
distance (radius r) to the point of origin and its angle (α)
to a specified axis. The angle (α) refers to the X axis in
the X,Y coordinate system.
The angle is positive, if it is measured
counterclockwise starting from the positive X axis
Figure 1.10 : Right-Hand-Rule
The specifications of the three axes as well as the three coordinates
are done as a so-called clockwise-rotating system and follow the right-
hand-rule (Figure 1.10). The fingers of the right hand always show to
the positive direction of each axis. This system is also called the
clockwise-rotating coordinate system.
b) CNC Milling Machine
Machine Coordinate System
The machine coordinate system of the CNC machine tool is
defined by the manufacturer and cannot be changed.
The point of origin for this machine coordinate system, also
called machine zero point M, cannot be shifted in its location.
Work Part Coordinate System
The work part coordinate system is defined by the programmer
and can be changed. The location of the point of origin for the
work part coordinate system, also called work part zero point
W, can be specified as desired.
Figure 2.11 : Machine Coordinate System Figure 2.12 : Work Part Coordinate System
The design of the CNC machine specifies the
definition of the respective coordinate system.
Correspondingly, the Z axis is specified as the
working spindle (tool carrier) in CNC milling
machines (Figure 1.13), whereby the positive Z
direction runs from the work part upwards to the
The X axis and the Y axis are usually parallel to
the clamping plane of the work part. When
standing in front of the machine, the positive X
direction runs to the right and the Y axis is away
from the viewer.
The zero point of the coordinate system is
recommended to be placed on the outer edge of
the work part.
Figure 1.13 : Milling Part In Three-Dimensional Cartesian
1.3.3 Zero And Reference Points On
CNC Machine Tools
a) Types Of Zero And Reference Points
Table 1.1 : Types of zero and references points
b) Machine Zero Point M
Each numerically controlled machine tool works
with a machine coordinate system.
The machine zero point is the origin of the
machine-referenced coordinate system.
It is specified by the machine manufacturer and its
position cannot be changed.
In general, the machine zero point M is located in
the center of the work spindle nose for CNC lathes
and above the left corner edge of the work part
carrier for CNC vertical milling machines.
c) Reference Point R
A machine tool with an incremental travel path
measuring system needs a calibration point
which also serves for controlling the tool and
work part movements.
This calibration point is called the reference point
R. Its location is set exactly by a limit switch on
each travel axis.
The coordinates of the reference point, with
reference to the machine zero point, always have
the same value. This value has a set adjustment
in the CNC control.
Figure 1.14 : Location Of The Zero And Reference Point For
1.3.4 Structure of a NC Milling
a) Structure of an NC-Block (Format)
Unlike the conventional milling machine, a
modern machine tool will be equipped with a
numerical control system. The machining of a
work part can be executed automatically,
provided that each machining cycle has been
described in a "language" (code) which can be
read by the control system.
The total of coded descriptions relating to a
work part is called an NC-program.
Each NC-program consists of a number of so-called
blocks, which contain the commands to be executed. The
blocks are consecutively numbered; each block number
consisting of a letter "N" plus a (e.g. three-digit) numeral.
Block numbers appear at the beginning of each program
* Words Address, Value
As a rule an NC block is comprised of several words.
Each word consists of an address (letter) and a value or
A numeral can either represent a code (e.g. G01:
Linear feed motion) or a real value (e.g. X+60 :
Approaching the target coordinate X=60).
Example of part program:
N110 F95 S850 M03
N115 G00 X+25 Y+30
N120 G01 Z-8
N110 A feed rate of 95 mm/min and a spindle speed of
850 U/min is programmed.
N115 The tool is moved in the rapid traverse motion
from its current position to the starting point
N120 Infeed in the Z-axis at the programmed feed rate (G01)
N125 Because G01 is a modal command, the tool will
continue to move at the programmed feed rate on a
straight line to the target position X=105
N130 The tool moves in the Y-axis to the target position
The technology data programmed in block N110 (feed rate,
speed and sense of cutter rotation) will be retentive and
take effect through blocks N120 to N130.
b) List Of G Codes
G00 Rapid move G0 X# Y# Z# up to 6 axis or G0 Z# X#
G01 Linear feedrate move G1 X# Y# Z# up to 6 axis or G1
G02 Clockwise move
G03 Counter clockwise move
G04 Dwell time
G08 Spline smoothing on, optional L# number of blocks to
G09 Exact stop check, spline smoothing Off
G10 Linear feedrate move with decelerated stop
G11 Controlled Decel stop
G17 X Y Plane
G18 X Z Plane
G19 Y Z Plane
G28 move to position relative to machine zero
G53 Cancel fixture coordinate offsets
G54-G59 fixture coordinate offsets 1 through 6
G70 Inch mode
G71 Millimeter mode
G80 Cancels canned cycles and modal cycles
G81 Drill cycle
G82 Dwell cycle
G83 Peck cycle
G84 Tapping cycle
G85 Boring cycle 1 bore down, feed out
G86 Boring cycle 2 bore down, dwell, feed out
G88 Boring cycle 3 bore down, spindle stop, dwell, feed out
G89 Boring cycle 4 bore down, spindle stop, dwell, rapid
G90 Absolute mode
G91 Incremental mode
G92 Home coordinate reset
G93 cancel home offsets
G98 - G199 User-definable G codes
c) Additional Functions (M-Functions)
With each NC-block a number of additional functions (M-Functions)
can be programmed, such as machine functions and switches, e.g.
to specify the feed rate, the spindle speed and the tool change.
List of M Codes
Feed Rate, F
The feed rate is programmed in millimeters per minute
(mm/min). Example: F080.000; Here the programmed
feed rate is 80 millimeters per minute.
Spindle Speed, S
The spindle speed is programmed in revolutions per
minute (RPM). Example: S500; Here the programmed
spindle speed is 500 revolutions per minute.
Tool Change, T
A tool change is programmed by a four-digit number at
the address T. The first two positions of that number
indicate the tool position in the magazine; the last two
positions indicate the tool compensation storage.
Example: T0808; This command effect the loading of the
tool to position No.8 of the current tool magazine and the
reading-in of the corresponding compensation value
In the CNC Simulator there is a maximum
of 99 magazine positions available, as well
as 99 compensation value registers. This
provides the opportunity, for example, to
assign the compensation value register No.
36 to the tool in the magazine position No.
12). The applicable NC-command would
then be programmed as follows: T1236
1.3.5 Clamping Devices For Milling
The following clamping variations can be distinguished for
The milling cutter machine table with its T-slots is the basis
for work part clamping. Depending on how the work part is
to be clamped, the following clamping devices can be
Mechanical clamping devices
Hydraulic clamping devices
Pneumatic clamping devices
Electric clamping devices
Universal machine vises can be horizontally as well as
vertically turned. Furthermore, there are also vises that
pneumatically generate clamping power.
Figure 2.21 : Precision Sine Vise
Magnetic Clamping Devices
Work parts made of iron can be clamped with
electromagnetic devices. The work part is drawn to the
clamping plate after a current is switched on. It can be
easily removed after the current is switched off.
Figure 1.22 : Electromagnetic Clamping Plate
1.3.6 Cutting Values
Milling is a cutting operation with a rotating tool,
whereby the cutting edges are not in operation all
the time. The cutting movement is caused by the
rotation of the tool. Feed direction and cutting
direction do not depend on each other. It is
realized either by the tool or by the work part or by
both of them (Figure 2.23).
The Cutting Speed (Vc) and the Feed Speed (Vf) is
overlap to each other and results in a continuous
Cutting Speed (Vc)
The cutting movement is the movement between the tool
and the work part, generating only one nonrecurrent chip
cut during one rotation without a feed movement. Cutting
speed corresponds to circumferential speed of the milling
tool on the current cutting edge. It is expressed as Vc and
m/min. Under consideration of the number of rotations of
the spindle n the following formula is received
Vc = π * d * n in m/min
The cutting speed of a cutting tool depends on the number
of the rotations. The direction constantly changes however
during cutting operation.
Feed Speed (V f )
The feed movement together with the
cutting movement enables a constant chip
removal during several rotations. In milling,
the feed can be indicated in three ways:
Feed speed (Vf) in mm / min
Feed per tooth (fz) in mm
Feed per milling rotation (f ) in mm
The calculation of the feed speed Vf is based on the feed
fz , i.e. the feed path per milling tooth. Under consideration
on the number of rotations n and the number or teeth z the
formula is as follows:
Vf= fz * n * z in mm / min
The feed speed can be expressed with the following
formula as well with reference to the feed per milling
Vf = f * n in mm / min
Consequently, the following equivalence is valid:
Vf = f * n = fz * n* z in mm / min
Calculate the cutting speed for milling if the
milling tool diameter, d = 50 mm and the
number of rotations, n = 520 1/min
Calculate the number of rotations, n of an end
mill with a diameter of d = 12 mm and cutting
speed of Vc = 120m/min.
In plain milling with a face milling cutter a cutting
speed of Vc = 180 m/min has been scheduled
and the number of rotations should not exceed
400 1/min. What is the maximum diameter, d of
the face milling cutter so that these values are