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Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.1
Laboratory for Manufacturing Systems and Automation
Department of Mechanical Engineering and Aeronautics
University of Patras, Greece
COMPUTER NUMERICAL CONTROL OF
MACHINE TOOLS
Dr. Dimitris Mourtzis
Assistant Professor
Patras, October 2013
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.2
Chapter 2:
Numerical Control Systems
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.3
Table of Contents
Chapter 2: Numerical Control Systems….………………………………………………………….......2
2.1 Types of Control Systems in use on NC Equipment………………………………..…………….10
2.2 Drive systems used on CNC machinery………………………………………………………..18
2.3 The Cartesian Coordinate System………………………………………..……..................29
2.4 Motion Directions of a CNC Milling Machine……..……………………………………...37
2.5 Absolute & Incremental Positioning..…………………………….……..……………..42
2.6 Setting the Machine Origin………………………………….….............................46
2.7 Dimensioning Methods…………………………………………………..............54
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.4
Objectives of Chapter 2
Describe the two types of control systems in use on NC equipment
Name the four types of drive motors used on NC machinery
Describe the two types of loop systems used
Describe the Cartesian coordinate system
Define a machine axis
Describe the motion directions on a three-axis milling machine
Describe the difference between absolute and incremental positioning
Describe the difference between datum and delta dimensioning
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.5
A CNC machine consists of two major components:
CNC components
The Machine-Tool Controller – Machine
Control Unit (MCU)
MCU is an on-board computer
MCU and Machine Tool may be manufactured by the same company
CNC Components
(W. S. Seames Computer Numerical Control: Concepts and Programming)
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.6
Figure 2-1(a): A Fanuc CNC Controller(Source: http://www.fanuc.co.jp/)
Fanuc
Bridgeport
Haas
Cincinnati Milacron
Mitsubishi
Siemens
Controllers
CNC controllers manufacturers
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.7
CNC controllers manufacturers market share
Figure 2-1(b): CNC controllers manufacturers market share survey, FANUC owns the largest share 26%(Source: cnccookbook.com)
Controllers
Haas
14%
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.8
Each MCU is manufactured with a standard set of build in codes
Other codes are added by the machine tool builders
Every CNC machine is a collection of systems coordinated by the
controller
Program codes vary somewhat from
machine to machine
Controllers
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.9
There are two types of control systems used on CNC machines:
Control Systems
Point – to – Point
Systems
Continuous – Path
Systems
Types of Control Systems
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.10
Point – to – Point machines:
I. Move in straight lines
II. They are limited in practical sense to hole operations:
Drilling
Reaming
Boring etc
III. Straight milling cuts parallel to a machine axis
IV. When making an axis move all affected drive motors run at the same
speed
Cutting of 45o angles is possible
BUT
not angles or arcs other than 45o angles
(W. S. Seames Computer Numerical Control: Concepts and Programming)
Types of Control Systems
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.11
Point – to – Point machines example
Move to (X1, Y1)
Move to (X2, Y1)
Move to (X3’, Y3) where X3’ < X3
Move to (X3, Y3)
Move to (X4, Y4’) and move to (X4, Y4)
Types of Control Systems
(W. S. Seames Computer Numerical Control: Concepts and Programming)
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.12
Figure 2-2:Point – to – point angles and arcs
Point – to – Point machines:
Angle and arc segments
must be programmed as a
series of straight line cuts
Types of Control Systems
(W. S. Seames Computer Numerical Control: Concepts and Programming)
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.13
Figure 2-4: Point – to – Point tool movement(Source: Manufacturing, Engineering & Technology, Fifth Edition,S. Kalpakjian and S. R. Schmid)
Movement of Tools
● Point-to-point: The drill bit drills
a hole at position 1, then is
retracted and moved to position
2 and so on
Types of Control Systems
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.14
Figure 2-3: Continuous – path angles and arcs
Continuous - Path machines:
Have the ability to move the drive
motors at varying rates of speed
while positioning the machine
The cutting of arc segments and
any angle can be easily
accomplished
Types of Control Systems
(W. S. Seames Computer Numerical Control: Concepts and Programming)
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.15
Figure 2-5:Continuous – Path tool movement(Source: Manufacturing, Engineering & Technology, Fifth Edition,S. Kalpakjian and S. R. Schmid)
● Continuous path by
a milling cutter
Movement of Tools
Types of Control Systems
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.16
Point – to – Point Machines where common
Their electronics where less expensive to produce
The machine tools where less expensive to acquire
Technological advancements have narrowed the cost difference between
point – to point and continuous – path machines
Most CNC machines now manufactured are
of continuous – path type
Types of Control Systems
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.17
STEPPER motors
DC (Direct Current) servos
AC (Alternating Current) servos
Hydraulic servos
Servomechanisms
The drive systems used on NC machinery:
(http://www.hsmworks.com)
Figure: The machine control sends a motion signal,
to a servomotor attached to each machine axis.
This causes the servomotor to rotate a ball screw
attached to the table or column, causing it to move.
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.18
STEPPER motors
Move a set amount of rotation (a step) every time the motor receives an
electronic pulse
DC and AC servos
Widely used variable-speed motors on small & medium continuous path
machines
A servo does not move a set distance
When current is applied the motor starts to turn and when the current is
removed the motor stops turning
The AC motor can create more power than a DC motor – used on CNC
Machining Centers
HYDRAULIC servos
Are variable-speed motors
Produce much more power than an electric motor
They are used on large CNC machinery with electronic or pneumatic system
attached
Servomechanisms
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.19
Virtual Model of Trajectory Generation and Axes Control
(Moriwaki T., “Multi-Functional Machine Tool”,2008)
Controllers
CL :"Cutter Location“
file
"Jerk" is the time rate
of change of
acceleration.
Jerk ramps the
acceleration to
smooth the velocity
Article
From: 9/9/2005 Modern
Machine Shop, Norman
Bleier, Siemens engineering
manager
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.20
Loop Systems
Loop systems are electronic feedback systems that send and receive
electronic information from the drive motors
Loop Systems
Open
Loop
Closed
Loop
The type of system used affects the overall accuracy of the machine
Open Loop use Stepper Motors
Closed Loop usually use Hydraulic, AC and DC Servos
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.21
Loop Systems For Controlling Tool Movement
MCU
Figure 2-6: An open-loop control system for a numerical-control machine(Source: Manufacturing, Engineering & Technology, Fifth Edition,S. Kalpakjian and S. R. Schmid)
Open – Loop System
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.22
Figure 2-7: An Open – Loop system
Open – Loop System:
The machine receives its information
from the reader and stores it in the
storage device
When the information is needed it is
sent to the drive motor (s)
After the motor has completed its
move a signal is sent back to the
storage device telling it that the move
has been completed and the next
instruction may be received
There is no process to correct for
error induced by the drive system
Machine Control Unit
ReaderStorage
Memory
Motor
Controller
Drive
Motor
Input
Media
Loop Systems For Controlling Tool Movement
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.23
An open loop system utilizes stepping motors to create machine
movements. These motors rotate a fixed amount, usually 1.8°, for each
pulse received.
Stepping motors are driven by electrical signals coming from the MCU.
The motors are connected to the machine table ball-nut lead screw and
spindle
Upon receiving a signal, they move the table and/or spindle a fixed amount.
The motor controller sends signals back indicating the motors have
completed the motion
The feedback, however, is not used to check how close the actual
machine movement comes to the exact movement programmed
Open – Loop System
Loop Systems For Controlling Tool Movement
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.24
Figure 2-8: A closed-loop control system for a numerical-control machine(Source: Manufacturing, Engineering & Technology, Fifth Edition,S. Kalpakjian and S. R. Schmid)
DAC : “digital-to-analog converter”
MCU
Closed – Loop System
Loop Systems For Controlling Tool Movement
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.25
Figure 2-9: A Closed – Loop system
Closed – Loop System:
The machine receives its information
from the reader and stores it in the
storage device
When the information is sent to drive
motor the motor’s position is
monitored by the system and
compared to what was sent
If an error is detected the necessary
correction is sent to the drive
system
If the error is large the machine may
stop executing the program for
correcting the inaccuracy
Most errors produced by the drive
motors are eliminated
Advanced Stepper Motors make
possible extremely accurate Open
– Loop Systems and less HW
Loop Systems For Controlling Tool Movement
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.26
Special motors called servos are used for executing machine movements
in closed loop systems
Motor types include AC servos, DC servos, and hydraulic servos.
Hydraulic servos, being the most powerful, are used on large CNC
machines. AC servos are next in strength and are found on many
machining centers
A servo does not operate like a pulse counting stepping motor. The speed of
an AC or DC servo is variable and depends upon the amount of current
passing through it
The speed of a hydraulic servo depends upon the amount of fluid passing
through it. The strength of current coming from the MCU determines the
speed at which a servo rotates
Closed – Loop System
(W. S. Seames Computer Numerical Control: Concepts and Programming)
Loop Systems For Controlling Tool Movement
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.27
Coordinate Systems
Geometrical means of communication between the operator and digitally
driven machine-tool
Univocal characterization of a point in the plane or in space relative to a
fixed point
Absolute coordinates
Relative coordinates
Figure 2-10:The simplest example of a coordinate system(en.wikipedia.org/)
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.28
The Cartesian Coordinate System
The Cartesian Coordinate System is defined by two orthogonal axes
which intersect at a point
X Axis: Abscissa axis
Y Axis: Ordinate axis
Figure 2-11 : The Cartesian Coordinate System definition, X&Y axis(Modern methods of material processing and programming with PC , D. Mourtzis et al.)
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.29
Z Axis: Applicate axis
The Cartesian Coordinate System
Figure 2-12 : The Cartesian Coordinate System definition, Z axis(Modern methods of material processing and programming with PC , D. Mourtzis et al.)
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.30
Figure 2-13: Cartesian coordinate quadrants
Cartesian Coordinate System:
Is divided into quarters
(quadrants): I, II, III, IV in
counter-clockwise direction
This is the universal way of
labelling axis quadrants
The signs of X and Y change
when moving from quadrant to
quadrant
The Cartesian Coordinate System
(W. S. Seames Computer Numerical Control: Concepts and
Programming)
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.31
Figure 2-14: Cartesian coordinates
Cartesian Coordinate System:
Points on a two-axis Cartesian
system:
Each of the points can be defined
as a set of coordinates (X, Y)
In mathematics this set of points is
called an ordered pair
In NC programming the points
are referred as coordinates
Cartesian coordinates will be used
in writing NC programs
The Cartesian Coordinate System
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.32
Quadrants
Quadrants in the spaceQuadrants in the plane
Figure 2-15:Positive and negative quadrants on a plane and in the space(Modern methods of material processing and programming with PC, D. Mourtzis et al.)
The Cartesian Coordinate System
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.33
Rule of right hand rotation
Figure 2-16: The Electronic Industries Association & Aerospace Industries of America established the
right hand rule of coordinates. The right hand rule determines positive direction of each axis
from the base of the finger to the tip
The Cartesian Coordinate System
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.34
Transferring coordinates
Scheduling cutting, based on the coordinate system with axes starting at the zero
point of the piece W
Transfer coordinates in the coordinate system of the machine-tool axes beginning
from the zero point of the machine M
Need to define the coordinate system of the user in connection with the zero point
or the reference point of the machine
Figure 2-17:Coordinate Systems Transfer(Modern methods of material processing and programming with PC , D. Mourtzis et al.)
The Cartesian Coordinate System
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.35
Rotation of a Coordinate System
Rotation can be made on any axis
If the rotation is made on more than one axis, or with different angles of the
90o, 180o and 270o using functions transformation of coordinates is required
Scheme 1 Scheme 2
Figure 2-18: Rotation around the Z axis by -90o
(Modern methods of material processing and programming with PC , D. Mourtzis et al.)
The Cartesian Coordinate System
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.36
Figure 2-19: Directions of movement on a machine
The basis for all machine movement is the Cartesian Coordinate system
On a machine tool an axis is a direction of movement
In a Two – Axis Milling Machine:
X is the direction of the Table travel
Y is the direction of the Cross travel
(W. S. Seames Computer Numerical Control: Concepts and Programming)
The Cartesian Coordinate System
(http://www.hsmworks.com/)
The Cartesian Coordinate System in machines
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.37
Figure 2-20: Three – Axis vertical mill
Three – Axis Milling machine:
In a Three – Axis Vertical Milling
Machine:
X is the direction of the Table
travel
Y is the direction of the Cross
travel
Z the Spindle travel up – down
The Cartesian Coordinate System in machines
The Cartesian Coordinate System
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.38
Figure 2-21: Six – Axis machine layout
Six – Axis Milling machine:
In a Six – Axis Vertical Milling Machine:
X is the direction of the Table travel
Y is the direction of the Cross travel
Z the Spindle travel up – down
A is the rotation around X – axis
B is the rotation around Y – axis
C is the rotation Z –axis (spindle)
The Cartesian Coordinate System
The Cartesian Coordinate System in machines
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.39
Positive and Negative Movement
Machine axis direction is defined in terms of spindle movement
On some axes the machine slides actually move; on other axes the spindle
travels
For standardization the positive and negative direction for each axis is
always defined as if the spindle did the travelling
The arrows saw the positive and negative direction of spindle movement
along axes
Example
To make a move in the +X direction (spindle right) the table would move to
the left
To make a move in the +Y direction (spindle toward the column) the saddle
would move away the column
The Z-axis movement is always positive (+Z) when the spindle moves
towards the machine head and negative (–Z) when it moves toward the
workpiece
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.40
The Polar Coordinate System
Figure 2-23 :The Polar coordinate system(Modern methods of material processing and programming with PC , D. Mourtzis et al.)
The polar coordinate system is a two-dimensional coordinate system in
which each point on a plane is determined by a distance from a fixed
point and an angle from a fixed direction
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.41
There are two ways that machines position themselves with respect to
their coordinate systems :
Positioning
Systems
Absolute
Positioning
Incremental
Positioning
Positioning Systems
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.42
Positioning Systems
Figure 2-24: Absolute positioning
Absolute Positioning:
All machine locations are taken from
one fixed zero point
All positions on the part are taken from
the (X0, Y0) point at the lower left
corner of the part
The 1st hole will have coordinates of
(X1.000, Y1.000)
The 2nd hole will have coordinates of
(X2.000, Y1.000)
The 3rd hole will have coordinates of
(X3.000, Y1.000)
Every time the machine moves the
controller references the lower left
corner of the part
(W. S. Seames Computer Numerical Control: Concepts and Programming)
ZERO REFERENCE POINT
FOR A MOVE TO ANY LOCATION
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.43
Figure 2-25: Incremental positioning
Positioning Systems
Incremental Positioning:
The (X0, Y0) point moves with the
machine spindle
Each position is specified in relation to
the previous one
The 1st hole coordinates are:
(X1.000, Y1.000)
The 2nd hole coordinates are
(X1.000, Y0)
The 3rd hole coordinates are
(X1.000, Y0)
After each machine move the current
location is reset to (X0, Y0) for the
next move
The coordinate system moves with
the location and the machine
controller does not reference any
common zero point
ZERO LOCATION FOR A MOVE
FROM HERE TO HOLE #1
ZERO LOCATION FOR A MOVE
FROM HOLE #1 TO HOLE #2
ZERO LOCATION FOR A
MOVE FROM HOLE #2 TO
HOLE #3
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.44
Positioning Systems
Figure 2-26(a):Relationship of the Cartesian
Coordinate system to the part
when using absolute positioning
Figure 2-26(b):Relationship of the Cartesian
coordinate system to the part
when using incremental
positioning
ZERO POINT FOR A MOVE
TO ANY LOCATION
INITIAL ZERO POINT,
(THIS POINT IS ZERO FOR A
MOVE FROM HERE TO
HOLE #1)
THIS POINT IS ZERO
FOR A MOVE FROM
HOLE #1 TO #2
THIS POINT IS ZERO
FOR A MOVE FROM
HOLE #2 TO #3
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.45
In conjunction with NC machinery there are two types of dimensioning
practices used on blueprints :
Dimensioning Methods
Datum
Dimensioning
Delta
Dimensioning
These two dimensioning methods are related to absolute and incremental
positioning
Dimensioning Methods
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.46
Dimensioning Methods
Figure 2-28: A datum dimensioned drawing
All dimensions on a drawing are
placed in reference to one fixed
zero point
Is ideally suited to absolute
positioning equipment
All dimensions are taken from the
corner of the part
Datum Dimensioning
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.47
Dimensioning Methods
Figure 2-29: A delta dimensioned drawing
Dimensions placed on a Delta
Dimensioned drawing are “chain-
linked”
Each location is dimensioned
from the previous one
Delta drawings are suited for
programming incremental
positioning machines
It is not uncommon to find the two
methods mixed on one drawing
Delta Dimensioning
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.48
Dimensioning Methods
Datum
Dimensioning
Delta
Dimensioning
Example of the two methods mixed on one drawing
Figure 2-30: I-phone drawing (osxdaily.com)
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.49
Setting the Machine Origin
Most CNC machinery have a default coordinate system assumed during
power-up the Machine Coordinate System
The origin of this system is called the Machine Origin
or Home Zero Location
Home Zero is usually located at
the Tool Change position of a Machining Center
Machine Coordinate System
(http://www.hsmworks.com/)
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.50
Setting the Machine Origin
A part is programmed independently of the Machine Coordinate System
The programmer can pick a location on the part or fixture becoming the
origin of the coordinate system for that part
The programmer’s coordinate system is called the Local or Part
Coordinate System
The Machine and Part Coordinate System will almost never coincide
Prior running the part program the coordinate system
must be transferred from the machine system to
part system,known as setting ZERO POINT
Programmer Coordinate System
(http://content.heidenhain.de)
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.51
There are three ways a ZERO POINT can be set on CNC machines
Manual Setting Work Coordinates
ZERO POINT Setting
Absolute Zero
Shift
Setting the Machine Origin
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.52
Setting the Machine Origin
The set-up person (technician) positions the spindle over the
desired part zero
Zero out the coordinate system on the Machine Control Unit (MCU)
console
The actual coding for accomplishing zero out varies
depending on the Machine Control Unit (MCU)
Manual Setting
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.53
(Send the spindle to home zero)
N010 G28 X0 Y0 Z0(Set the current spindle position)
(To X5.000 Y6.000 Z7.000)
N020 G92 X5.000 Y6.000 Z7.000
An Absolute Zero Shift is a transfer of the coordinate system inside the NC
program
First: the programmer commands the spindle to the Home Zero Location
Next: a command is given that tells the MCU how far from the Home Zero Location
the Coordinate System Origin is to be located
Line 0N10: Spindle moves to Home Zero
Line 0N20: The location of the spindle became
X5.0, Y6.0, Z7.0, for MCU
The machine will now reference the Part
Coordinate System
G28 - Return to reference point
G92 – Program absolute zero point
If more than a fixture is to be used on a machine, the programmer will use more
than one part coordinate system – send spindle back to home zero G28 X0, Y0, Z0 –
then G92 Line
Setting the Machine Origin
(W. S. Seames Computer Numerical Control: Concepts and Programming)
An Absolute Zero Shift is given as follows:
Absolute Zero Shift
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.54
Setting the Machine Origin
A work coordinate is a modification of the absolute zero shift
Work coordinates are registers in which the distance from home zero to
the part zero can be stored
The part coordinate system does not take effect until the work coordinate is
commanded in the NC program
When using G92 zero shifts the coordinate system was changed to the part
coordinate system when the G92 line was issued
When using work coordinates a register can be set at one place in the
program and called at another
If more than one fixture is used – a second part zero can be entered in
a second work coordinate and called up when needed
The work coordinate registers can be set manually by the operator or by
the NC programmer without having to send the spindle to the home zero
location
This saves program cycle time by eliminating the moves to home zero
Work Coordinates
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.55
Setting the Machine Origin
Work Coordinates Example
Figure 2-27 :A part gripped in a vise(Adapted by http://www.hsmworks.com/)
Figure 2-27 shows a part gripped in
a vise
The outside dimensions of the part
have already been milled to size on
a manual machine before being set
on the CNC machine
The CNC is used to make the
holes, pockets, and slot in this part.
The Work Coordinates are
located in the upper-left corner of
the block
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.56
Setting the Machine Origin
(Set work coordinate P1-which is G54)
(and work coordinate P2-which is G55)
N010 G10 L2 P1 X5.000 Y6.000 Z7.000
N020 G10 L2 P2 X10.000 Y3.000 Z15.000
(Call work coordinate G54 and move)
(To X1.000 Y1.000 Z0.500)
N100 G54 X1.000 Y1.000 Z.500
(Call work coordinate G55 and move)
(To X2.000 Y2.000 Z3.000)
N110 G55 X2.000 Y2.000 Z3.000
Are set and called in a program by commands called G-codes: G54, G55
and G56
An example of using the Work Coordinate System (WCS):
Line N010: the G54 WCS is set to X5.0, Y6.0,
Z7.0, from the zero home location
Line N20: the G55 WCS is set to X10.0, Y3.0,
Z15.0 from home zero
Line N100: the G54 WCS is called activating
the part coordinate system moving the spindle
to X1.0, Y1.0, Z0.5 as referenced from the
part activated part coordinate system
Line N110: the G55 WCS is called activating
the second part coordinate system moving the
spindle to X2.0, Y2.0, Z3.0 as referenced from
the part activated part coordinate system
Work coordinates remain active until cancelled
by another work coordinate
Work Coordinates
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.57
Summary 1/2
The two types of NC control systems are point-to-point and continuous
path
The four types of drive motors used on NC equipment are stepper motors,
AC servos, and hydraulic servos
Loop systems are electronic feedback systems used to help control
machine positioning. There are two types of loop systems: open and
closed. Closed-loop systems can correct errors induced by the drive
system; open loop system cannot
The basic of machine movement is the Cartesian Coordinate system. Any
point on the Cartesian coordinate system may be defined by X/Y or X/Y/Z
coordinates
An absolute positioning system locates machine coordinates relative to a
fixed datum reference point
In an incremental positioning system, each coordinate location is
referenced to the previous one
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.58
Summary 2/2
The machine coordinate system can be transferred to the part coordinate
system manually, by an absolute zero shift, or by use of work
coordinates
The positive or negative direction of an axis movement is always thought
of as spindle movement
Machine movements occur along axes that correspond to the direction
of travel of the various machine slides.
On a vertical mill, the Z axis of a machine is always the spindle axis. The
X and Y axes of a machine are perpendicular to the Z axis, with X being
the axis of longer travel
There are two dimensioning systems used on part drawings intended for
numerical control: datum and delta. Datum dimensioning references each
dimension to a fixed set of reference points; delta dimensioning references
each dimension to the previous one
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.59
Vocabulary Introduced in this Chapter
Absolute positioning
Absolute zero shift
Cartesian coordinate system
Closed-loop system
Continuous-path systems
Datum dimensioning
Delta dimensioning
Incremental positioning
Machine Control Unit (MCU)
Point-to-point systems
Open-loop system
Work coordinates
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.60
1. Ann Mazakas, Manager of Technical Communications, DP Technology, ‘’The Art of Automation’’, courses
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2. Chryssolouris G., «Manufacturing Systems: Theory and Practice», 2nd Edition, 2006, Springer-Verlag
3. http://blog.cnccookbook.com
4. http://www.dptechnology.com/
5. http://www.hsmworks.com
6. http://content.heidenhain.de
7. http://www.fanuc.co.jp
8. www.osxdaily.com
9. Kalpakjian S., «Manufacturing Engineering and Technology», 2nd Edition, 1992, Addison-Wesley Publishing
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10. Manufacturing, Engineering & Technology, Fifth Edition,S. Kalpakjian and S. R. Schmid
11. Mattson M., “CNC Programming, Principles and Applications”, Delmar, 2002
12. Moriwaki T., “Multi-Functional Machine Tool”, CIRP Annals Manufacturing Technology, Vol. 57/2, 2008, pp.
736-749
References
Laboratory for Manufacturing Systems and Automation
Director: Professor George Chryssolouris
Dr. Dimitris Mourtzis
2.61
13. Seams W., “Computer Numerical Control, Concepts & Programming”, 4th Edition, Delmar, 2002
14. Norman Bleier, Siemens engineering manager Article From: 9/9/2005 Modern Machine Shop,
15. Γ. Χρυσολούρης, «Συστήματα Παραγωγής Θεωρία και Πράξη» Μέρος Ι και ΙΙ, Εκπαιδευτικές Σημειώσεις,
Πανεπιστήμιο Πατρών, 2001,
16. Γ. Χρυσολούρης, Δ. Μούρτζης, Κ. Τσίρμπας, Σ. Καραγιάννης, “Ορθογωνική Κοπή”, Εκπαιδευτικές Σημειώσεις,
Πανεπιστήμιο Πατρών, 2000
17. Γ. Χρυσολούρης, Δ. Μούρτζης, και άλλοι, “Εργαστήρια Μηχανουργικής Τεχνολογίας Ι και ΙI”», Εκπαιδευτικές
Σημειώσεις για το εργαστήριο του αντιστοίχου μαθήματος, Πανεπιστήμιο Πατρών, 2008 (4η Έκδοση)
18. Δ. Μούρτζης, “Αριθμητικός Έλεγχος Εργαλειομηχανών” Εκπαιδευτικές Σημειώσεις, Πανεπιστήμιο Πατρών,
2011 (3η Έκδοση)
19. Πετρόπουλου Π.Γ., «Μηχανουργική Τεχνολογία – ΙΙ. Τεχνολογία κατεργασιών κοπής των μετάλλων», 1998,
Εκδόσεις Ζήτη
20. Σύγχρονες μέθοδοι κατεργασίας υλικών και προγραμματισμός με Ηλεκτρονικό Υπολογιστή (Η/Υ) ,Δ.
Μούρτζης ,Κ. Σαλωνίτης
References